URBANA ILLINOIS STATE GEOLOGICAL SURVEY 3 3051 00000 2133 STATE OF ILLINOIS DEPARTMENT OF REGISTRATION AND EDUCATION A. M. SHELTON. Director DIVISION OF THE STATE GEOLOGICAL SURVEY M. M. LEIGHTON, Chief BULLETIN NO. 49 GEOLOGY AND MINERAL RESOURCES OF THE DIXON QUADRANGLE BY RUSSELL STAFFORD KNAPPEN PRINTED BY AUTHORITY OF THE STATE OF ILLINOIS URBANA, ILLINOIS 1926 fLLFNOfS GEOLOGICAL SURVEY LIBRARY Digitized by the Internet Archive in 2012 with funding from University of Illinois Urbana-Champaign http://archive.org/details/geologymineralre49knap £ en Vs-6 . A^ Xv-X^ d.\ STATE OF ILLINOIS DEPARTMENT OF REGISTRATION AND EDUCATION A. M. SHELTON, Director DIVISION OF THE STATE GEOLOGICAL SURVEY M. M. LEIGHTON, Chief Committee of the Board of Natural Resources and Conservation A. M. Shelton, Chairman Director of Registration and Education Charles M. Thompson Representing the President of the Uni versity of Illinois Edson S. Bastin Geologist Schnepp & Barnes, Printers Springfield, III. 1926 56288— 3M CONTENTS PAGE Chapter I — Introduction 11 Location and area 11 Previous work 11 Field work 12 Acknowledgments 12 Geography of the quadrangle 13 Physiographic province 13 Climate and vegetation 13 Topography 13 Elevation and relief 13 Upland plain 13 Valleys 14 Till valleys 14 Lim'estone valleys 14 Sandstone valleys 15 Valleys in sandstone with overlying limestone 15 Flood-plains and terraces 16 Drainage 16 Culture 18 Distribution of population 18 Industry IS Transportation and communication 18 Chapter II — Geologic terms and principles 19 Topographic map 19 Land divisions 20 General geologic principles 21 Rocks 21 Weathering 22 Erosion 22 Marine deposition 23 Stratification 23 Clastics 24 Limestone 24 Dolomite 24 Continental deposition 25 Eolian deposits 25 Stream deposits 25 Lake deposits 26 Glacial deposits 26 Eskers 27 Drift 27 Consolidation of sediments 27 Ground water 2S 5 PAGE Chapter II — Geologic terms and principles — Concluded. Historical geology 2S Geologic time table 28 Correlation 28 Physiographic cycle 30 Chapter III — Stratigraphy 32 Pre-Cambrian rocks 34 Crystallines 34 Keweenawan ( ? ) sandstone 34 Paleozoic group 36 Cambrian system 36 Croixan series 36 Ordovician system 39 Prairie du Chien series 39 Oneota dolomite 39 Name 39 Lithology 40 Areal distribution 40 Correlation 40 "New Richmond" sandstone 40 Name 40 Lithology 41 Topographic expression 42 Thickness 42 Areal distribution 42 Correlation and relation to adjacent formations 42 Shakopee dolomite 43 Name and lithology 43 Topographic expression, thickness and areal distribution. 45 Paleontologic character 46 Correlation 47 Relations to adjacent formations 47 Middle Ordovician series 48 St. Peter sandstone 48 Name 4S Lithologic character 48 Topographic expression 50 Thickness and areal distribution 50 Paleontologic character and correlation 51 Relations to adjacent formations 51 Glenwood shale 52 Name and character 52 Topographic expression 52 Thickness 52 Areal extent 53 Age. correlations and relations 53 Platteville limestone 53 Name 53 Lithologic character 53 Topographic expression 55 Thickness and details of section 55 6 PAGE Chapter III — Stratigraphy — Concluded. Areal distribution 58 Paleontologic character 58 Correlation 60 Relations to adjacent formations 60 Galena dolomite 61 Name 61 Lithologic character 61 Topographic expression 62 Thickness 62 Areal distribution 64 Paleontologic character 64 Correlation 65 Relations to adjacent formations 65 Younger Paleozoic formations 65 Cenozoic group 66 Pleistocene system 66 Pre-Illinoian deposits 66 Illinoian and Iowan (?) tills 67 Introduction 67 Age of the till 70 Grand Detour esker 73 Loess 74 Lithology 74 Topographic expression 75 Thickness 75 Area 75 Age 75 Early Wisconsin valley train 76 Late Wisconsin valley train 77 Backwater deposits 78 Recent sediments 78 Flood-plain alluvium 78 Peat and muck 78 Sand dunes 79 Chapter IV — Geologic history SO Introduction 80 Pre-Cambrian eras 80 Paleozoic era 80 Cambrian period 80 Ordovician period 81 Lower Ordovician or Prairie du Chien epoch SI Oneota stage 81 "New Richmond" stage 81 Shakopee stage 82 Middle Ordovician epoch 85 St. Peter stage 85 Glenwood stage 86 Platteville stage 87 Galena stage 89 7 PAGE Chapter IV — Geologic history — Concluded. Later Paleozoic record 90 Cenozoic era 90 Tertiary peneplanation 90 Pleistocene period 94 Pre-Illinoian time 94 Illinoian glaciation 95 Sangamon interglacial epoch 96 Iowan glacial epoch 96 Peorian interglacial epoch 96 Wisconsin epoch , 97 Post-Illinoian drainage development -. 98 Recent history 101 Asymmetrical valley slopes, stream displacement and exposure to sun and wind 102 Human activities and their erosional effects 104 Chapter V — Structural geology 107 General statement 107 Structure-contour map 107 Structure of the quadrangle 109 Origin of the structure 110 Pre-St. Peter structure Ill Faulting Ill Chapter VI — Mineral resources 112 General statement 112 Water 112 Surface water 112 Ground water 113 Water for vegetation 113 Springs 113 Shallow wells in till 114 Wells in limestone 115 Wells in the St. Peter sandstone 116 Artesian wells 117 Cement materials 119 Limestone and limestone products 122 Glass sand 125 Sand and gravel 128 Potash 129 Petroleum 131 Natural gas 132 Ore minerals 132 Sulphides 133 Limonite 134 Copper and gold 135 ILLUSTRATIONS PLATE PAGE I. General and economic geology of the Dixon quadrangle Pocket II. Topographic map of the Dixon quadrangle Pocket III. Index mlap of Illinois, showing location of the Dixon quadrangle and the meridians and base lines 10 IV. Changes in Rock River and its tributaries, resulting from Illinois glaci- ation 98 V. Areal geology map of the Dixon quadrangle with structure contours drawn on the top of the St. Peter sandstone 108 FIGURE 1. Diagramatic section of alluvial and rock-defended terraces in Rock River alluvium 17 2. Geologic column for Dixon quadrangle 32 3. Bar cross-bedding in the "New Richmond" sandstone 41 4. Typical exposure of Shakopee dolomite 44 5. Weathered bluff of St. Peter sandstone 49 6. Bluff of St. Peter sandstone east of Green Rock 49 7. Outcrop of Platteville limestone 55 8. Galena dolomite showing typical massive beds which weather to thinner strata 62 9. Close view of exposure in figure 8 63 10. Gentle folding of the Shakopee dolomite 84 11. Sharp folding and fracturing of the Shakopee dolomite 85 12. Development of cuestas and their relations to a peneplain 92 13. Rock River valley and the Tertiary peneplain 93 14. Maximum extension of North American ice-sheets 94 15. Conditions producing an artesian water supply for the Dixon quadrangle .118 16. Quarrying of the Blue limestone at the plant of the Sandusky Cement Company . 120 17. Quarry of the Sandusky Cement Company 121 18. The mill at the plant of the Sandusky Cement Company 122 19. Sand crushing and washing plant of the National Silica Company 126 20. Sand pit of the National Silica Company 126 TABLES PAGE 1. Geologic formations 33 2. Fossils collected from the Platteville limestone 59 3. Wells not reaching rock in the southwestern part of the quadrangle 69 4. Wells in the southwestern part of the quadrangle with depth to rock only 70 5. Sink holes in the quadrangle 102 6. Elevations of top of St. Peter at points outside Dixon quadrangle used in preparing structure map 108 7. Analysis of water in 200-foot well at Dixon State Hospital 116 8. Analysis of Croixan water from Dixon 119 9. Analyses of surface and washed sand from quarry of National Silica Company 127 10. Potash content of samples from deposits in the quadrangle 130 10 OF THE >n quadrangle, see Plate III) / and 89° 30', It is approxi- :mare miles. eology of Illi- f the area or le preparation 34, pp. 134-161, :rom Oregon to lois, vol. 5, PP. ois, vol. 5, pp. Geol., vol. 5, p. e in northwest- ey Seventeenth Illinois: Amer. issippi Valley: 18, 798 pp., 1899. Survey Bull. 1, Dl. Survev Bull. 10. Geologic formal Fossils collectec Wells not reach Wells in the soi Sink holes in t Elevations of tc preparing str Analysis of wat Analysis of Crc Analyses of su Company . . . Potash content GEOLOGY AND MINERAL RESOURGES OF THE DIXON QUADRANGLE By Russell Stafford Knappen CHAPTER I— INTRODUCTION Location and Area The area described in this report is known as the Dixon quadrangle. It is situated in Lee and Ogle counties in northern Illinois (see Plate III) and is limited on the east and west by the meridians of 89° 15' and 89° 30', and on the north and south by the parallels 42° and 41° 45'. It is approxi- mately 17.3 miles long and 12.9 miles wide and covers 223 square miles. Previous Work In addition to papers dealing in general terms with the geology of Illi- nois, the following publications describe specific portions of the area or special problems relating to it and have been consulted in the preparation of this report. Shepard, C. U., Geology of upper Illinois: Amer. Jour. Sci., vol. 34, pp. 134-161, 1838. Everett, Oliver, Geology of a section of the Rock River valley from Oregon to Sterling: Illinois Nat. Hist. Soc. Trans., vol. 1, pp. 53-58, 1861. Shaw, Hon. James, Geology of Lee County: Geol. Survey of Illinois, vol. 5, pp. 124-139, 1873. Geology of Ogle County: Geol. Survey of Illinois, vol. 5, pp. 104-123, 1873. Tiffany, A. R., Record of a deep well at Dixon, Illinois: Amer. Geol., vol. 5, p. 124, 1890. Hershey, Oscar H., The Elkhorn Creek area of St. Peter sandstone in northwest- ern Illinois: Amer. Geol., vol. 14, pp. 169-179, 1894. Leverett, Frank, Water resources of Illinois: U. S. Geol. Survey Seventeenth Ann. Rept., Pt. 2, pp. 695-849, 1896. Hershey, Oscar H., Preglacial erosion cycles in northwestern Illinois: Amer. Geol., vol. 18, p. 71, 1896. Physiographic development of the upper Mississippi Valley: Amer. Geol., vol. 20, p. 246, 1897. Leverett, Frank, The Illinois glacial lobe: U. S. Geol. Survey Mon. 38, 798 pp., 1899. Weller, Stuart, The geological map of Illinois: Illinois State Geol. Survey Bull. 1, 1906. Geologic structure of the State: Illinois State Geol. Survey Bull. 2, 1906. 11 VI DIXON QUADRANGLE Geological map of Illinois: Illinois State Geol. Survey Bull. 6, pp. 1-34, 1907, (especially map). Udden, Jon A. and Todd, J. E., Structural materials in Illinois: Illinois State Geol. Survey Bull. 16, Dixon vicinity, pp. 362-368, 1910. (analyses). Bleininger, A. V., et al, Portland-cement resources of Illinois: Illinois State Geol. Survey Bull. 17, p. 87, 1912. Cady, G. H., The structure of the La Salle anticline; Illinois State Geol. Survey Bull. 36, pp. 85-179, 1920. Field Work The field work for this report occupied July, August and half of Sep- tember, 1919, four weeks in August, 1920, and three weeks in May and June, 1923. While the general distribution of formations could be deter- mined readily, the heavy cover of glacial drift in many places obscured boundaries and relations of formations. Much time was spent in finding good exposures in critical areas. The general geologic map (Plate I) in- dicates the actual outcrops ; additional information needed in compiling the map showing areal geology and structure contours (Plate V) was obtained from well records. Acknowledgments This study was proposed by Mr. F. W. DeWolf, former Chief of the Illinois State Geological Survey and the late Dr. R. D. Salisbury, Consult- ing Geologist. The author desires to express his sincere appreciation of their cordial assistance in the carrying on of the work. Dr. Gilbert H. Cady, of the Survey, spent two days in the field; and two field conferences, covering six days, were held with Dr. M. M. Leigh- ton, Pleistocene Geologist, and present Chief of the Survey, regarding the Pleistocene geology of the quadrangle. Dr. J. J. Galloway of Columbia University, New York City, spent three days in field consultation on strati- graphic problems. In addition, Dr. Leighton has critically read the sec- tions of the report dealing with Pleistocene problems, making many helpful suggestions, and Drs. Johnson, Ogilvie and Galloway of Columbia Uni- versity have criticized the major portion of the report. The author is glad to acknowledge his indebtedness to all of them for their cordial assistance. The unfailing helpfulness, courtesy and good will of the citizens of the area made the field work a pleasure and contributed much to its accuracy and completeness. It is hoped that in addition to adding some facts to gen- eral geologic knowledge, this report may explain many local features and lead to a better understanding of and greater interest in the geologic history and phenomena of the area. INTRODUCTION 13 Geography of the Quadrangle PHYSIOGRAPHIC PROVINCE The Dixon quadrangle lies in what is known physiographically as the Till Plains section of the Central Lowlands Province. Essentially flat-lying beds of sedimentary rock underlie the area, but the present topography is chiefly the product of glacial deposition and stream erosion, which are characteristic of the Till Plains section as a whole. CLIMATE AND VEGETATION The climate is continental. Summer is hot, with many thunderstorms ; winter is cold and invigorating, but not severe. The temperature ranges from 100° F. in summer to -10° in winter. The mean annual precipitation amounts to about 33 inches. Vegetation consisted originally of prairie grasses on the uplands and a moderate growth of deciduous trees along the valleys. Little of the original sod now remains, corn, wheat, oats, clover and other farm crops having re- placed the prairie grasses. Around the farm houses, shade and hardy fruit trees have been planted. TOPOGRAPHY ELEVATION AND RELIEF The quadrangle as a whole is a gently rolling plain, rising from an ele- vation of ?80 feet at the south to slightly more than 900 feet above sea level at the northern boundary. The highest points in the area are simply parts of this upland plain. Ridge Road has an elevation of 901 feet in eastern Pine Creek Township, and Devils Backbone in Oregon Township stands 905 feet above sea level. Many of the surrounding areas approach the same elevation. As is to be expected, the lowest region lies along the master stream, Reck River. This stream enters the quadrangle at an eleva- tion of 658 feet and, falling 18 feet in 23 miles, leaves it at an elevation of 640 feet. The total relief of the quadrangle is 265 feet. A relief of 200 feet within a half mile is not uncommon along Rock River. UPLAND PLAIN A very gently rolling till plain which rises northward about 8 feet per mile forms the upland of the quadrangle, and extends beyond it in all di- rections. Its original surface is so nearly level that many sections do not have a relief of 20 feet. Rock outcrops are few on this plain, and where rock occurs, its surface blends perfectly with the upland and does not affect the topography. For instance, the rock now exposed in quarries in sections 28 and 30, South Dixon Township, was discovered in plowing. Originally, the upland extended across the entire area with minor interruptions, but it 14 DIXOX QUADRANGLE has been severely dissected by Rock River and its tributaries. Rock River valley is now a great trough extending southwest through the upland. The valley itself averages two miles in width, but the numerous tributaries have so dissected the upland that it now covers only about 10 per cent of the northwestern half of the quadrangle. Traces of the old upland persist on the stream divides, especially where limestone is present ; but these remnants are small. The southern and eastern parts of the quadrangle have not been severely eroded, and the original gently rolling till surface is preserved over large areas in Bradford, China. Nachusa and South Dixon townships. VALLEYS The upland valleys are of four principal types, each a resultant of the varying resistance of underlying rocks to weathering and erosion. So pro- nounced are the characteristics of these different valleys, that the kind of material in which each has been excavated usually can be inferred from the topography. The four classes of valleys are those developed in (a") till (b) limestone (c) sandstone and id) sandstone with overlying limestone. TILL VALLEYS Glacial till is very easily eroded, and Avhere conditions are favorable, narrow valleys develop rapidly. Gullies six to eight feet deep, and more than 100 feet long, have been cut in a single season. The typical till valley is not. however, narrow and steep-sided, because slope-wash readily erodes the valley sides, often carrying down to the bottom of the valley more clay and silt than the temporary wet-weather stream can remove. As a conse- quence, till valleys are broadly open, with gently sloping sides, which merge almost imperceptibly into both the flat alluvial bottoms and the upland plain above. Such valleys often appear shallow and of slight importance, and only after considerable contact with them does one realize that they are fre- quently deeper and much wider than the sharper and more impressive rock vallevs. Franklin Creek, southeast of Franklin Grove, and Threemile and Fivemile branches in South Dixon Township occupy till valleys. LIMESTONE VALLEYS Limestone and soft sandstone are the only important rocks exposed in this quadrangle. Their distribution and characteristics are described in Chanter 111 and the general geologic map (Plate E) shows their surface extent. Each type of rock produces a distinctive valley form. In a limestone region, valleys arc deepened slowly because the streams cannot readily erode the solid rock of their channels. Valley sides are worn back slowly by weathering, and in time their slopes become gentle. Since glaciation, the streams in this region have deepened, but have not materially widened the limestone channels. Weathering has been slow, and INTRODUCTION 15 consequently the valleys in limestone have rather steep-sided slopes with little or no flood-plain areas. The valley sides commonly meet the upland at rather pronounced angles, instead of merging gradually into the original surface, as do the till-valley slopes. Pine Creek has a typical limestone valley from Stratford east to The Pines, and several of the short tributaries of the lower Rock River have developed similar limestone valleys. SANDSTONE VALLEYS The chief sandstone of the area — the St. Peter — is a slightly cemented, pure-quartz sandstone. Water passes through it readily ; bare fingers can dig in a fresh exposure, but weathering neither dissolves nor readily disin- tegrates the rock. Streams erode it easily, cutting deep valleys quickly and widening them by removing the soft sandstone of the lower valley sides. Wind erosion, growth of plants, and movement of animals combine to round off the upper slopes of the valley wall, but do not materially affect the lower parts, which are kept steep or vertical by the undercutting of the stream at their base. The normal sandstone valley, accordingly, has a wide flood- plain, and valley sides which are steep or vertical below, but more gentle above, passing gradually into the rounded slopes of the sandstone hills. Characteristic sandstone valleys are those of lower Franklin and Chamberlin creeks. VALLEYS IN SANDSTONE WITH OVERLYING LIMESTONE Where the valley has been cut through limestone into sandstone below, a broad flood-plain with steep lower slopes is formed, as in a normal sand- stone valley. Stream erosion widens the valley floor in the sandstone more rapidly than weathering removes the capping limestone above, and as the sandstone is cut away, the unsupported limestone breaks off along vertical joint planes. These valleys, then, have rather wide flood-plains, vertical or very steep walls in both sandstone and limestone, and practically no grada- tion from the valley wall into the upland surface, except where the over- lying till is thick. Such "box" valleys are typical of western tributaries of Rock River above Grand Detour, of Pine Creek, and of Franklin Creek for two miles north of the Chicago and Northwestern Railway. Few valleys belong exclusively to one type. Rock River valley belongs to all four types, although the till valley is poorly developed. At the north- ern boundary and for two miles near Grand Detour, there is a till valley ; from Devils Backbone to a point three miles below Grand Detour, there is a normal sandstone valley with the exception of two miles near Grand De- tour ; at Devils Backbone and from Pine Creek to the cement plant, two miles above Dixon, the valley is of the sandstone-with-overlying-limestone type, and from the cement plant westward, Rock River has a limestone valley. 16 DIXOX QUADRANGLE From Daysville, near the north boundary of the quadrangle, to Dixon, the valley is alternately narrow and wide. Sometimes the river flows be- tween steep, unscalable sandstone cliffs, which rise sheer to the castellated limestone rim above. Elsewhere, the bluffs draw back, and sometimes al- most disappear, while rich agricultural land lies on the flood-plain. Some of the variation in width and ruggedness is due to the fact that the valley consists alternately of preglacial and postglacial sections, but the greater part of the irregularity is due to the varying kinds of rock in which the valley lies. With the possible exception of Illinois Valley in the vicinity of Starved Rock State Park, no more beautiful scenery is found in northern Illinois than along the deep, almost gorge-like valley of Rock River. Grand Detour, lying in the center of the "gorge," is one of the popular summer resorts of the State. FLOOD-PLAIXS AND TERRACES While most of the streams are still actively deepening their channels, many of the larger streams are developing, or flowing upon, flood-plains, the extent of which are shown on Plate I by the deposits of alluvium. Rock and Kyte River valleys were once filled with sand and gravel to approximately 45 feet above the present streams. The origin and character of these valley trains are discussed on page 76. When these valleys were buried in alluvium, the tributary valleys were either filled to this same level, or were ponded, forming temporary lakes, remnants of which persist as marshes in lower Kyte and Franklin valleys. Ever since the valley trains were formed, Rock River and its tributaries have been removing the filling. As the streams erode this material, they SAving back and forth across the alluvium, forming new flood-plains at lower le\*els. Remnants of the higher flood-plains are left as terraces or "second bottoms" on the valley sides. In the western part of Dixon, four distinct terraces are recognizable, each rep- resenting a step in the removal of the valley train. Usually these terraces result from the river swinging back before completing a traverse across the entire flood-plain; sometimes, as in the case of the terrace on which the milk condensery stands in North Dixon, rock interrupts the swing and a rock- defended terrace results. Figure 1 shows diagrammatically the develop- ment of these two types of terraces, common in the broader parts of Rock and Kyte valleys. Ordinary alluvial terraces are present in all the wider parts of the Rock River valley. DRAINAGE The entire quadrangle is well drained by Rock River and its tributaries, of which Kyle River and Pine Creek arc perennial streams throughout their courses in the quadrangle. Other permanent streams have their headwaters INTRODUCTION 17 within the area, and consequently are intermittent in their higher reaches. Gagings of Rock River have not been made in this area, so far as the writer can learn, but accurate records are available for the four years 1914 to 1918 at Rockford, 35 miles upstream, and at Lyndon, the same distance downstream 1 . From these records, it appears that the average flow of Rock River in this quadrangle is approximately 6,000 second-feet (cubic feet per second), the average flood discharge is 25,000 second-feet, and average low-water discharge, 1,300 second-feet. The lowest water during the four years was about 500 second-feet, and the greatest flood discharge was 35,000 second-feet in March, 1916. High-water stage normally comes in March, when melting snows and spring rains are adding to the stream flow, with a second, but much lower, high-water stage in September or October, following the fall rains. Low-water normally occurs in July or August, when precipitation is low and evaporation of ground and stream water is greatest, and a second low-water stage occurs in January, when precipitation accumulates on the surface as snow, and frozen soil retains the ground water. Fig. 1. Diagramatic section of alluvial and rock-defended terraces in Rock River alluvium. In general, only the deeper and larger valleys have permanent streams. If the underlying rock is sandstone, the valley must be deeper, and the drainage basin larger, in order to support a permanent stream, than if the valley is in limestone. This condition is caused by the greater perviousness of the sandstone, and the more ready flow of water through it, resulting in more gentle slopes of the ground- water table and a more ready absorption of water in the St. Peter areas. A very small portion of the area is drained by sub-surface streams through limestone. This drainage has produced sink holes in section 27 and adjacent sections of Dixon Township and in section 18, LaFayette, and sec- tion 19, Ashton Township. Two small depressions, neither exceeding live feet in depth or three acres in area, lie south of Chicago and Northwestern Railway in sees. 1 1 Surface water supply of tli<> l T niWd States, 1918: r. S. Geol. Survey Water-Sup- ply Paper 475, p. 98, 1921. 18 DIXON QUADRANGLE and 11, South Dixon Township. In the lower parts of Kyte and Franklin valleys are marshes which have resulted from the aggrading of Rock River (page 78). CULTURE DISTRIBUTION OF POPULATION The Dixon quadrangle is essentially an agricultural region. About 29 per cent of the population live on farms. Dixon, which had 8,201 inhabit- ants in 1920, is the only city, and contains about 58 per cent of the popula- tion of the quadrangle. Approximately 13 per cent live in the villages of Franklin Grove, Grand Detour, Nachusa, Eldena, Daysville, Honey Creek, Stratford, and part of Lee Center. INDUSTRY Farming, including cattle raising, is the principal industry of the area. The newly- formed glacial soil of the till plains makes Illinois one of the richest agricultural regions of the country. The till-covered areas are the most fertile; limestone provides a rich but often stony soil, while areas of exposed sandstone are normally dry and less productive than the rest of the quadrangle. Dixon is an important manufacturing city producing a wide variety of products, chief of which is cement. Of the mineral resources developed in the area, those used in the manufacture of cement rank first in importance. Next in importance is the production of sand for glass making and other purposes, in Oregon Township. Mineral resources are described in detail in Chapter VI. TRANSPORTATION AND COMMUNICATION The quadrangle is well served by railroads and highways. The Chi- cago and Northwestern Railway, the Chicago, Burlington and Quincy Rail- road, the Illinois Central Railroad, the Lincoln Highway, a state highway along Rock River, and limestone macadam roads provide excellent means of transportation. The dirt roads are naturally good, since the loess-silt which covers most of the area is too granular and pervious to form mud, and too coherent to produce a sandy surface. A moderate amount of at- tention, including frequent dragging, keeps these roads in excellent condi- tion. CHAPTER II— GEOLOGIC TERMS AND PRINCIPLES This chapter has been prepared for the non-technical reader, and in- troduces the more important conceptions upon which the rest of the report is based. In the succeeding chapters, evidence is presented for the inter- pretations offered, but a knowledge of general geologic and geographic prin- ciples is assumed. Detailed information on the subject matter of the pres- ent chapter may be found in any text-book of general geology. Topographic Map The topographic map (Plate II) is one of the series being prepared by the Illinois State Geological Survey in cooperation with the United States Geological Survey. The scale used is 1 to 62,500 ; that is, one inch on the map represents 62,500 inches on the face of the earth, or approximately one mile. On the map, the works of man are printed in black. Public roads are indicated by double, continuous lines ; for improved highways, one of the lines is heavier; private roads are indicated by double broken lines. Houses are shown by small squares, and school houses by the same symbol with a triangular flag attached. Churches have a cross added to the square and large buildings are mapped with their approximate shapes. Where roads do not follow section lines, the latter are indicated by light dotted lines. A series of long dashes represents township boundaries, and long and short dashes, county boundaries. Surface forms are printed in brown. Elevations of many points are shown by brown numbers, and where these points have been permanently marked by a brass cap, set on a pipe embedded in cement, or attached to a building, a black cross and the letters B M (bench mark) are used. Exact elevations of these points are given in a government bulletin 1 which may be obtained upon request to The Director, U. S. Geological Survey, Wash- ington. Another paper 2 gives the exact geographic positions of many points. Contour lines show the topography of the area. A contour is a line passing through points of equal elevation, so that a person walking along such a line would walk on a level and go neither up nor down hill. The con- tour may be considered as a potential shoreline, for if the ocean were raised 1 Marshall, R. B., Spirit Leveling in Illinois for the years 1914-1917, inclusive, U. S. Geol. Survey Bull. 672, pp. 31-35, 1918. 2 Marshall, R. B., Results of triangulation and primary traverse for the years 1911 and 1912, U. S. Geol. Survey Bull. 551, pp. 135-136, 1914. 19 20 DIXON QUADRANGLE 800 feet above its present level, for instance, the shoreline would cross the Dixon quadrangle along the irregular course of the 800-foot contour. On the Dixon map, the contours are drawn at intervals of 20 feet, and in pass- ing from one contour line to another at any place in the area, one would either rise or descend 20 feet, regardless of the horizontal distance. Where the siope is steep, the contour lines are close togther, and where it is gentle, they are far apart. Examples of steep slopes are shown along Rock River and Franklin Creek where the contours are so crowded that they almost merge. The gently rolling surface in the southern third of the area is in- dicated by the very wide spacing of the contours. Water features are shown in blue. Where a stream flows throughout the year, a solid line is used, while intermittent streams are shown by dash-and-double-dot lines. Springs are represented by small circles, usually at the sources of streams. A large spring is mapped in sec. 12, T. 22 N., R. 10 E. Marshes are shown by short horizontal lines with radiating lines above, suggesting tufts of grass. The only marsh indicated on the map is north of Kyte River in sec. 11, T. 23 N., R. 10 E. Land Divisions Most of the land west of the Appalachian Mountains belonged at one time to the United States Government, and was surveyed into townships practically six miles square. If time and funds had permitted, it would have been desirable to begin the surveys at the east boundaries of the public lands and carry them continuously w T est to the Pacific. It was necessary, however, to keep the surveys abreast of settlement, and as the pioneers did not occupy the land progressively westward, but moved into the more de- sirable areas first, surveys were initiated at many points. Whenever a new survey was started, a north-south line called a principal meridian was estab- lished and townships were laid off to the east and west of it. From the initial point of the survey an east-west line, called a base line, was also run. Townships were numbered east and west from the principal meridian and north and south from the base line. For example, the township in which most of Dixon is located is Township 21 North, Range 9 East, (T. 21 N., R. 9 E.), which means that it is the 21st township north of the base line and is located in the ninth range of townships east of the principal meridian. The entire Dixon area was surveyed from the Fourth Principal Meridian (Plate III). The Third Principal Meridian lies about four miles east of the Dixon quadrangle. Mile posts along this meridian are not on the prolongation of section lines surveyed from the Fourth Meridian, since each survey started from an arbitrary point. Accordingly, section lines east and west of the Third Principal Meridian do not meet on that line and east-west traffic GEOLOGIC PRINCIPLES 21 must jog north or south on passing from the area of one survey to that of the other. The eastern and western boundaries of townships converge towards the north because all north-south lines become closer together as they ap- proach the poles. In order to keep the townships nearly six miles wide, it is necessary to introduce correction lines. Along a correction line, the southern boundaries of townships are made full six miles in length. North- south roads are offset on correction lines in the same way that the Third Principal Meridian offsets east- west roads. A correction line lies about lj4 miles north of this area. The township is divided into 36 sections, practically a mile square, num- bered from the northeast corner westward and alternately eastward and westward. (See Plate II.) General Geologic Principles ROCKS The rocks of the earth's surface may be divided into three main classes : igneous, sedimentary and metamorphic. Igneous rocks are those which have solidified from a molten or paste- like condition. Where solidification proceeded slowly deep beneath the sur- face of the earth, various minerals formed, producing crystals of different sizes. The various types of crystallized igneous rocks, of which granite and diorite are the most common examples, are distinguished by their tex- ture and their mineral components. Where the rock flowed out on the surface as a lava, it cooled much more quickly, forming very fine-grained or glassy rocks. No igneous rocks are known in place in this quadrangle, but many igneous boulders were brought in by glaciers. Sedimentary rocks are deposited by oceans and lakes, running water, winds, and glaciers. These rocks consist principally of material which has been obtained from other rocks, and which is deposited directly by these agents. Less commonly, various substances have been extracted from the water by plants or animals, included in their structures, and their organic remains have accumulated on their death and decay. Most sedimentary rocks have been deposited as gravel, sand, clay or lime muds, which on solidification have formed conglomerate, sandstone, shale or limestone. Less common types of sediments are rock salt, coal and gypsum. In this quadrangle, sandstone, shale, limestone and dolomite are present. The conditions of their formation are discussed under marine deposition. Metamorphic rocks are formed from igneous or sedimentary rocks, by change of form, composition, and crystallization under intense heat, or pres- 22 DIXON QUADRANGLE sure, or both. The more common types of metamorphic rocks are marble, gneiss, schist, and slate, none of which is known in place in this area. There are, however, metamorphic boulders in the glacial drift. It is possible that both igneous and metamorphic rocks underlie the quadrangle at depths greater than 2000 feet (see Chapter III), but their position, character, and structure are entirely unknown, and could be determined only by deep drill- ing. WEATHERING At or near the surface, the breaking down of all types of rocks is ef- fected by exposure to various geologic processes, collectively known as weathering. These processes may be classified as mechanical weathering, or disintegration, and chemical weathering, or decomposition. Mechanical weathering includes the various processes that break up the rock without affecting its composition. These are principally the freezing of water in openings in the rock, which pries its constituents apart ; the expansion and contraction of the surface as the rock is heated and cooled by daily or seasonal temperature changes ; the wedge work of growing plant roots ; the burrowing of animals, and attrition of material which is blown, rolled, thrown or rubbed against the solid rock. Chemical weathering involves a change in the composition of the rock. Its most common type is solution of certain rock constituents. Other important forms are oxidation, notably of iron, and the addition of carbon dioxide or water to the original substances of the rock. In this quadrangle, mechanical weathering is commonly illus- trated by the spalling off of pieces of stone buildings and the occasional pry- ing apart of rocks by tree roots. Chemical weathering has produced the crumbling surface zone of several limestone outcrops and has changed the loess and till of the glacial deposits from their original blue-gray to the present yellow or brown color. EROSION Erosion includes both weathering and the removal of material by wind, running water, waves or glaciers. In dry or barren areas, wind blows dust and sand from place to place, removing the finer material and piling up the remaining sand in irregular hills called dunes. Examples of such dunes are discussed on p. 79. Part of the rain water runs over the surface, carrying with it varying amounts of clay, silt and sand. This is called slope wash, and where vege- tation does not interfere, it may remove weathered rock very rapidly. In streams, the erosive power of water is frequently greater, and not only sand and sill are carried, but pebbles and even boulders may be rolled along, grinding the bed of the stream and breaking away other fragments, which in turn are rolled, and form tools for the river. With the help of these tools, the streams deepen and widen their channels. GEOLOGIC PRINCIPLES 23 In regions where the winter's snow does not entirely melt during the summer, snow accumulates, and under pressure forms ice. With further accumulation, the weight of overlying ice becomes too great for the lower portion to support, and the ice spreads outward from the central area where it formed. Such moving ice bodies are called glaciers, well-known examples of which occur in the Alps and in Glacier National Park, Montana. In the cold region near the poles, glaciers cover large areas and are not confined to valleys or mountain sides. In Greenland, for instance, there is a glacier covering over 400,000 square miles. Such a glacier is called a continental- glacier. As the ice moves over the country, it erodes the surface vigorously, sweeps away loose material, freezes to rock and pulls it along, and grinds rock fragments in the bottom of the ice against the underlying rock surface. In this way, glaciers remove great quantities of both weathered and origi- nally solid rock, and often carry the material long distances before deposit- ing it. The portions of North America covered by glaciers during the last period of glaciation are indicated in fig. 14. MARINE DEPOSITION STRATIFICATION Nearly all of the eroded material is again deposited to form sedimentary rocks, of which the most common types are those deposited in the ocean. Because sediments are usually found in well-defined beds or strata, they are often called stratified rocks. The strata are distinguished from one another by changes in kind of material, size of grain, or color of cement. In the marine strata, bedding planes are formed nearly parallel to the ocean floor, and any change in material represents a change in oceanic conditions at the time of deposition. For instance, the change from coarse to fine material indicates either that the waves or currents sweeping the ocean floor became less vigorous and brought finer sediment, or that there was a reduction in the size of material contributed to the ocean by streams. Because these strata were deposited on a nearly flat floor, any variation from the hori- zontal indicates that the rocks have been deformed ; consequently, the struc- ture of a region can be determined from the attitude of its beds. In any such studies, however, only originally flat-lying sediments can be used. For example, in this area, the limestone strata were probably originally prac- tically horizontal. In the St. Peter sandstone, however, many beds were deposited by strong currents with sloping or curving surfaces. Such beds which lie at an angle with the main, originally flat, strata are called cross beds. Cross-bedding may often be used to determine the source of mate- rial or the agent which deposited the rock, but its attitude is deceptive and is usually ignored in structural studies. 24 DIXON QUADRANGLE CLASTICS Materials which ape moved in solid form, as gravel, sand and clay, are called elastics. The kind and size of material indicates the conditions in the ocean at the time of deposition. Sand, for instance, is evidence of mod- erately strong currents which spread out the coarse material. Where the ocean floor is practically flat, the sand may extend over hundreds of square mile- : but where the bottom slopes steeply, the water may be too deep near the shore for active wave or current work, and accordingly the zone of sand deposition is very limited. Like the sand, clay and silt are secured from the land. Currents and waves carry clay farther from the shore than sand. and when the bottom slopes gradually, the clay is washed man}" miles out to sea. On the other hand, in deep salt water, clay is quickly precipitated. and is not carried far from land unless currents -weep it over the ocean floor. LIMESTONE Animals live in nearly all parts of the ocean, and after death their shells. bones, and other hard parts settle to the bottom. The accumulation of these animal remains is slow, and if sand or clay is being deposited rapidly, the animal material may form an inconspicuous portion of the strata : on the other hand, if the ocean is clear and sand is not supplied by rivers or shore- line erosion, animal structures may accumulate close to or even at the shore. as in the case of coral reefs. Most of the animal structures consist of cal- cium carbonate which has been extracted from the water in the same way that man builds his own bone.- from the mineral content of the water and food that he uses. A few plants also extract calcium carbonate from the ocean and when they die this calcium carbonate is contributed to the accu- mulating sediments. If these calcareous organic remains settle in quiet water, they may form a mass in which most of the original structure- can be recognized: but where the water is shallow, waves and currents ma}" roll and grind the sediment to a fine mud. in which few or no original struc- tures can be determined. Such animal and subordinate plant deposit- are abundant in the Dixon area, and form the limestones of the Prairie du Chien. Plarteville and Galena formation-. Because the}- usually have re- sulted from accumulation in clear water, any .-tonus which brought mud into the ocean are represented by bedding plane-, and stratification is easily recognizable. dolomite If the ocean water contain- much magnesium, dolomite may be formed contemporaneously with the deposition of the mud. In such case, the mud will not suffer further change on induration, and the dolomite will be dense, solid, and free from solution cavities and channel-. Dolomite is also often formed by the substitution of magnesium for calcium after the limestone GEOLOGIC PRINCIPLES 25 has been indurated. This change develops a porous rock which frequently contains solution cavities and channels. In the process of dolomitization, most of the original animal structures are entirely destroyed. CONTINENTAL DEPOSITION EOLIAN DEPOSITS In many cases, the products of erosion are deposited on the land, and are then called continental sediments. In this area, eolian or wind-formed deposits are represented by sand dunes and loess. A sand dune is formed where an obstruction interferes with the movement of sand-laden wind, so that its velocity is decreased and the sand is dropped. A pile or drift is formed, to which additional sand brought by the wind is added. Wind blow- ing up the windward side of the dune rolls or carries sand to the top. Some of this sand falls down onto the leeward side, causing that side of the hill to travel forward gradually, and perhaps to override and bury the obstacle which originally caused its formation. Such moving sand-hills are called live dunes; those covered with vegetation are said to be dead. Dunes may be killed by planting, or by building obstructions, such as fences, on their windward slopes to prevent the movement of sand. A dead dune in sec. 33, T. 21 N., R. 10 E., has been revived by over-pasturing, which has des- troyed the vegetation and removed the hindrances to wind activity. Loess is a fine silt which has been deposited by wind over all the upland of this area. It probably was blown from areas of glacial till soon after deposition, and from the flood-plains of streams flowing from those areas. The material consists of fine-grained rock fragments and minerals, such as can be found in the glacial till of this and the adjacent areas. Where fresh, it contains much original limestone material. Weathering removes this cal- careous matter, and the loess then is said to be leached. One criterion of the age of a loess is the amount of oxidation and leaching which it has under- gone. STREAM DEPOSITS Where tributaries bring more material to the main stream than it can carry, the excess load is deposited on the valley floor. Similarly, where a stream gradient decreases sharply, the carrying power diminishes and much sediment may be deposited. Slope wash and tributaries have overloaded many streams in this area and produced nearly flat bottom lands or Hood- plains along the streams. In time of high water, the stream often covers this flood-plain, and because it flows there more slowly than in the main channel, it deposits additional alluvium, building up its flood-plain still higher. 26 DTXOX QUADRANGLE In the same way. streams flowing from g-laciers commonly cariw great quantities of sand and gravel, much of which is deposited in their channels, unless the gradient continues as high as that which the stream had on leav- ing the ice-sheet. If the stream deposits this sediment, it raises its channel, and, shifting back and forth across the valley, may till it to a great depth. Deposition continues until the gradient of the stream is sufficient to give it the velocity required to carry all the material supplied by the melting ice. Such an accumulation of sand and gravel is called a valley train. In this area, valley trains fill the Rock and Kyte River valleys. Ever since the dep- osition of the valley train. Rock River has been removing this material bv erosion, producing the terraces described on p. 16 (fig. 1). Other types of river deposits are unimportant in this area. LAKE DEPOSITS In large lakes, sediments similar in many respects to those of the ocean are formed. In this area, however, no lakes of importance are known ever to have existed, and lake deposits are confined to the accumulations of plant material in swampy areas along Kyte River, Franklin Creek, and a few low areas on the upland. In these places, the contribution of streams has been unimportant, and the work of the wind negligible. Rushes and cat-tails have deposited great masses of vegetable fiber, and have entirely filled the swamps with soft muck or peat. Accumulation is still going on and the material is not yet solid enough to bear the weight of a man. These muck deposits are very small-scale examples of certain conditions typical of coal- forming swamps. Coal beds are the result of accumulation in clear water of plant material which later formed peat, and being buried by other sedi- ments, was metamorphosed to coal with great loss of water and gases. None of the muck in this area ever could make satisfactory coal, however, because the streams have brought much silt to the accumulating sediment, and this silt would constitute too high a percentage of ash in any coal that might result. GLACIAL DEPOSITS The material which glaciers erode from the overridden areas is de- posited in part beneath the glacier, and in part at its margin, where the rate of melting balances the rate of advance of the ice. At such a place the edge of the ice is standing still, although all of the ice is advancing and its trans- ported material accumulates in irregular ridges, called terminal moraines. The material deposited beneath the glacier and during the rapid retreat of its margin is the (/round moraine. Its composition is similar lo that ot the terminal moraine, but prominent topographic features arc not produced. Ground moraine covers most of the upland areas in the quadrangle, and GEOLOGIC PRINCIPLES 27 while it has been eroded from many valleys, it occurs on the side slopes and in the bottoms of others. ESKERS Where streams flow over the surface of glaciers or follow channels through or under the ice, they frequently form a deposit of sand and gravel, in every way analogous to the valley trains of streams that leave the glacier. If the ice-sheet melts away without further movement, these sand and gravel accumulations will be left on the surface of the underlying rock or till, and may form large, often fairly well-bedded bodies of sand and gravel called eskcrs. During the melting of the Illinoian ice-sheet, such an esker was de- posited in this area from Nachusa to a point a mile southwest of Grand Detour. DRIFT All of the material moved by a glacier is included under the general name of drift. The unstratified, unwashed, unsorted portion of the drift that accumulates in masses of heterogeneous material in ground and terminal moraines is called till. Till may consist of any and every kind of rock over which the ice has traveled, with the exception of extremely soluble types, such as rock salt. Large boulders, coarse gravel, fine sand and finely crushed rock, often called rock flour, are all indiscriminately mingled in till. CONSOLIDATION OF SEDIMENTS When marine sediments have been deposited, the weight of the over- lying material compacts the beds beneath and consolidates the mud to hard clay or shale. The lime muds are similarly compressed and their particles often adhere, making solid rock. Similarly, the sand grains under pressure are brought into close contact and may form rock because of pressure alone. Water in the rock dissolves mineral matter, especially from the points where individual grains are in contact, and deposits it between the grains to form a cement. In limestone, the cement is normally the calcium carbonate of the rock. In sandstone, calcium carbonate or iron oxide is commonly in- troduced and serves as a cement ; or silica, of which, the rock principally consists, is dissolved and precipitated to form a more or less hard sandstone. In this area, the sandstones have been only very slightly cemented and in most cases will crumble between one's fingers. The limestones are well- consolidated and the green clay of the Glenwood type has formed a low- grade shale. Among the glacial deposits little cementation has occurred, but in a few places some gravel has been thoroughly consolidated. In the NW. J /[ sec. 29, T. 22 N., R. 9 E., a small body of glacial gravel has been bound together by calcium carbonate to form a weak conglomerate. In none of the commercial gravel pits is the material so consolidated or cemented that it cannot readily be removed with a shovel. 28 DIXOX QUADRANGLE GROUND WATER Water which reaches the earth in the form of rain or snow either runs away to join the streams and so becomes part of the run-off, soaks into the ground and becomes part of the run-in, or evaporates from the surface into the air once more. The water which runs in is called "ground water". It travels in a generally downward direction until it reaches a surface beneath which the rock is saturated with water. This surface which separates the saturated from the unsaturated rocks above is called the ground-water table. The water beneath the ground-water table obeys the same laws as surface water, flows down slopes, and reappears at the surface in seepages, springs and artificial openings such as wells. The surface of a river or pond marks the elevation of the ground-water table. The rock underneath must be saturated or else the water would seep away from the pond or stream and leave a dry surface. The economic importance of the ground water is dis- cussed in Chapter VI. HISTORICAL GEOLOGY GEOLOGIC TIME TABLE The geologic history of the earth is divided into live main eras which in turn consist of two to seven periods. In general, each period embraces the interval between two important, wide-spread changes in life or in land-and- sea relations on the earth's surface. Periods are often divided into cpoclis and these into stages. The smaller units are recognizable only over limited areas, in most cases, although some are distinctly marked over much of the earth. The sediments formed during a period constitute a system, and those of an epoch form a series. In general, continuous beds of one type of rock or repeatedly alternating beds of two or more rocks are called a for- mation. A table of the geologic periods, epochs and formations or stages referred to in this report is given on p. 33. CORRELATION Formations of one area may be proved to be the same in age as those of another region. The determination of the age equivalence is called correlation. Various methods are available for correlation, of which the most important are continuity of the formations, identical lithologic char- acter of one or several beds or formations, fossil content, or similar relations to some definitely datable event in geologic history. Because of the depth of glacial till over this and surrounding areas, the formations here cannot be correlated by tracing them to points where they were first described. Also with two possible exceptions, they are cov- ered by younger beds so that they could not be traced to the type outcrops even if there were no glacial deposits. Correlation by lithology is very GEOLOGIC PRINCIPLES 29 unsatisfactory, because the minerals of sedimentary rocks have come largely from pre-existing formations, and those from a common source may be present in two or more formations in the same area. A good example of this fact is offered by the outcrop of "New Richmond" sandstone in Frank- lin Creek valley. The sand grains in this rock are scarcely distinguishable from those of the St. Peter formation above or the Croixan beneath, and it is probable that all the sand came from the same source in central Wiscon- sin or Canada. The formations cannot be distinguished definitely by dif- ferences in their constituents. It was this practically identical lithology that led James Shaw in his reconnaissance of the area fifty years ago to re- gard the "New Richmond" sandstone as the St. Peter, and accordingly to correlate the overlying Shakopee with the Platteville limestone which over- lies the St. Peter. In this case, it is possible by carefully following the out- crops, to show that the St. Peter formation of Rock Valley is continuous with a thin sandstone overlying the Shakopee, which Shaw had correlated with the Platteville. Similarly, a careful study of the lithology of the two limestones indicates that their correlation is an error, even though the two sandstones cannot be distinguished with certainty. The evidence from historical geology in different areas is difficult to secure, and usually involves a large amount of uncertainty as to the exact equivalence of the formations. No correlations in this report are made on such a basis. Fossils are evidences of life contained in the rocks. Such life traces may be portions of the animals or plants themselves, as shell or bone frag- ments, impressions, molds or casts of organic matter, material which has replaced the original substance but retained its structure, such as petrified wood, trails and footprints of animals, or even structures made by them, as the worm-borings in the St. Peter sandstone of this area. Paleontology is the study and interpretation of these fossils. Paleontologic evidence is the most valuable means of correlation, for the development of life throughout geologic time can be traced in the fossil record. Plaving determined the interval during which a given form existed, it is evident that any formation containing this fossil must have been de- posited between the earliest and latest times when the creature lived. Free- swimming animals with distinctive hard parts are most useful, for their remains will be wide-spread and will make correlations possible over long- distances, In correlating glacial deposits, the problem is rendered very difficult by the mode of their formation. Contemporaneous life is rarely buried in the till, and forms distinctive of the various glacial invasions are not known. Correlations are based largely upon the length of time the material has been exposed to weathering and erosion. So, for instance, over two areas of 30 DIXOX QUADRANGLE till equally distant from the main drainage line of the region, the presump- tion is that the one which has suffered most erosion is the older. This pre- sumption must be verified very carefully, however, for changes in original topography, variation in climate and in interference by bed rock with drain- age development, and differences in original character of material may all affect the result. One must also be certain that no later deposits have covered either of the areas which are to be compared. Similarly, where erosion of the surface has not occurred, the depth to which percolating ground water has leached and oxidized the material often gives strong evi- dence of the length of exposure and of the age of the deposits in question. It must be remembered that the depth of oxidation varies with the amount of material to be oxidized. If the rate of oxidation is the same, a till which contains four per cent of ferrous iron will require twice as much time for oxidation to a depth of one foot as would a similar till containing only two per cent. It is obvious that the same condition affects the depth of leaching of limestone material. If the porosity of the till is not the same, the less porous material retains the ground water longer and so favors more rapid leaching, unless the porosity is so very low that the water cannot readily travel through the material. In that case, a much smaller amount of water may pass a given point and so the amount of leaching may be much less than in more porous substances. Other things being equal, the liner the material, the more rapidly it will be weathered, for the smaller particles have more surface ex- posed per unit of volume. The line grain of the loess has undoubtedly favored deeper leaching and oxidation than that produced in gravel in the same length of time. In such studies, one must be certain that the leached material has been continuously above the ground- water table, for the weathering processes go on very slowly in the presence of the standing water. Any error in assuming uninterrupted weathering will vitiate the conclusions drawn from the field evidence. Within a small area, differences in climate, such as rainfall, temperature, amount of wind, season, and rate of precipitation will not be appreciable ordinarily, but they must be con- sidered carefully where the distance between formations to be correlated is great. PHYSIOGRAPHIC CYCLE The tendency of practically all processes of weathering and erosion is to lower the land surfaces and to till the ocean basins. In particular, streams remove the soil and rock from regions through which the)' tlow. Given time enough, they will reduce their drainage basins practically to sea level. The proportion of the work accomplished in a given region is indicated by de- scribing the area as young, mat it re or old. A young region has had little GEOLOGIC PRINCIPLES of its upland dissected and removed by stream development. It frequently contains lakes and swamps on its surface, due to poor drainage. In matu- rity, the upland is all drained and largely removed, flood-plains have devel- oped, and the relief, or difference in elevation between uplands and valley floors, is the greatest of any time in the cycle. In an old region, the valley sides slope gently, the hills are low and rolling, and the drainage of the flood- plains is poor. If erosion continues, the region becomes nearly flat as the hills are removed. This flat or very gently rolling surface is called a pene- plain, and the series of topographic changes through which a region passes during erosion from youth to old age and peneplanation is known as the physiographic cycle. Similarly, a young stream has accomplished little of its ultimate work. It is cutting its valley deeper, developing tributaries and destroying lakes and falls. In maturity, the stream has stopped its rapid downward erosion, has developed a flood-plain and is maintaining a balance between material supplied by tributaries and material removed by the main stream. In old age, the stream is no longer vigorously eroding, but carries most of its load in solution, has become sluggish, and meanders widely on its flood-plain. The Dixon area was reduced to a peneplain in the period before glaci- ation. It was later uplifted and the rejuvenated streams dissected the flat surface until it was perhaps midway between extreme youth and maturity, or in the stage called late youth. (See fig. 13.) CHAPTER III— STRATIGRAPHY The following table lists the geologic groups, systems, series and rock formations to which reference is made in this report. A graphic repre- FEET 800 - -Q U r_r n: T^T II, ii i ii i r.i.i i ii. i.i k I , I ? I !■=» I I ol 0-15 Loess 20 150+ Galena dolomite Platteville Lowell Park Blue Buff -o-7 Glenwood "shale 30-180 St Peter sandstone 35-110 Shakopee dolomite 0-32 New Richmond sandstone 160 Oneota dolomite L480+ Croixan Fig. 2. Geologic column for Dixon quadrangle. sentation of the formations outcropping in the quadrangle is given in the geologic column, fig. 2. 32 GEOLOGIC TIME TABLE 33 Table 1. — Geologic Formations Group System Cenozoic Mesozoic Paleozoic Recent Pleistocene Tertiary (sub- group) Cretaceous C Permian Pennsylvanian Mississippian Devonian Silurian Ordovician Series Wisconsin (glacial) Late Wisconsin Early Wisconsin Peorian (interglacial) Iowan (glacial) Sangamon (interglacial) Illinoian (glacial) Yarmouth (interglacial) Kansan (glacial) Aftonian (interglacial) Nebraskan (glacial) Formation Cambrian Proterozoic Archeozoic f Niagaran I Alexandrian Upper Ordovician Middle Ordovician Lower Ordovician or Prairie du Chien Upper Cambrian or Croixan (Potsdam) ' Keweenawan Upper Huronian Middle Huronian Lower Huronian Niagaran limestone Maquoketa shales Galena dolomite Platteville limestone Lowell Park Blue Buff Glenwood shale St. Peter sandstone Shakopee dolomite New Richmond sandstone Oneota dolomite Jordan sandstone Trempealeau formation Mazomanie sandstone Pranconia formation Dresbach formation Eau Claire formation Mt. Simon formation 34 DIXON QUADRANGLE Pre-Cambrian Rocks CRYSTALLINES The oldest rock outcropping in this State is of Lower Ordovician age and is exposed in this quadrangle along Franklin Creek. Knowledge of pre- Cambrian and Cambrian formations in Illinois is obtained entirely from well records and from outcrops and drillings in adjacent states. In northern and central Wisconsin, a complex series of pre-Cambrian igneous and metamorphic rocks crops out. At the close of the pre-Cambrian a fairly well-developed peneplain beveled these formations. This peneplain was preserved by burial under the heavy sandstones of Croixan (Upper Cambrian) age, and has been traced southward beneath these sandstones in numerous wells in southern Wisconsin. Presumably the pre-Cambrian complex and its peneplain continue into northern Illinois and underlie this quadrangle. These crystalline rocks have not, however, been penetrated by any well in this state, and their character and distribution are unknown. Weidman 1 has prepared a structure-contour map of this peneplain surface in Wisconsin, which, projected into northern Illinois, indicates that at Dixon the pre-Cambrian crystalline rocks should be found about 700 feet below sea level. The Dixon Water Company's deepest well, however, pene- trated 1203 feet below sea level without encountering crystalline rock, and three other wells in Dixon Township were drilled more than 1000 feet below sea level without finding either igneous or metamorphic rock. The bottom of the Amboy city well, 1600 feet below sea level, is in sandstone. Evi- dently the southward dip of the peneplained crystalline surface is consider- ably steeper than has been previously recognized. The depth of the crys- talline rocks, as well as their character, is uncertain. Available evidence indicates only that they lie more than 1200 feet below sea level and are buried under more than 1800 feet of Paleozoic sediments throughout this quadrangle. KEWEENAWAN (?) SANDSTONE Keweenawan red sandstone underlies the Croixan sandstones in north- western Wisconsin and eastern Minnesota. 2 Several wells in northern Illi- nois have penetrated a red sandstone which Thwaites 3 suggests may be Keweenawan, beneath lighter-colored, typical Croixan sandstones. 1 Weidman, S., and Schultz, A. R., Water supplies of Wisconsin: Wisconsin Geol. and Nat. Hist. Survey Bull. 35, PL I, 1915. 2 Hall, C. W., Meinzer, O. E., and Fuller, M. L., Geology and underground waters of southeastern Minnesota: U. S. Geol. Survey Water-Supply Paper 256, pp. 32, 48, 1911. Thwaites, F. T., Sandstones of the Wisconsin coast of Lake Superior: Wiscon- sin Geol. and Nat. Hist. Survey Bull. 25, pp. 58-61, 1912. :| Thwaites, F. T., Paleozoic rocks in deep wells in "Wisconsin and northern Illinois: Jour. Geol., vol. 31, p. 555, 1923. I'RE-CAMBRIAN ROCKS 35 In this quadrangle, knowledge of Cambrian and possible pre-Cambrian formations is obtained from three well logs. The Dixon Water Company has four wells near its pumping station in the southwest corner of sec. 33, T. 22 N., R. 9 E. These wells are 1610, 1720, 1765 and 1860 feet deep, respectively, but only the log of the 1765-foot well is available (p. 37). No samples of drill cuttings were saved. Two wells have been drilled on the Dixon Epileptic Colony farm near the southwest corner of sec. 21, T. 22 N., R. 9 E. Samples from these wells were saved and studied for the State Geological Survey by C. B. Anderson, whose identifications are shown in the log on p. 38. At Amboy, three miles south of this quadrangle, the 2220-foot city water well was drilled to 1600 feet below sea level. Although the surface elevation is about 160 feet lower than at the Dixon Colony, the well started about 100 feet higher stratigraphically in the Galena dolomite, and accord- ingly penetrated only about 200 feet of strata lower than those in the Dixon Colony well No. 1. No written log of this well is available, but it is re- ported that the bottom of the well was in a distinctly pink, but not red, sand. 4 Water of good quality and abundant quantity has been obtained from all these wells. Dissolved matter is moderate and suspended matter or sediment is negligible. Thwaites 5 made the tentative suggestion that the lower 76 feet of the Colony No. 2 well may be Keweenawan. If this is so, the lower 218 feet of the adjacent well No. 1 must also be Keweenawan. Assuming the dis- tance from the top of the St. Peter to the Keweenawan as the same in all wells in this region, the lower 100 feet of the deepest Dixon city well and the bottom 418 feet of the Amboy well would be in Keweenawan sediments. This assumption cannot be proved, however, in the absence of carefully kept drill records and of identifiable horizon markers. Descriptions of the Keweenawan sandstones 6 have emphasized the large amount of shale interbedded with the sandstones, the dark red color rarely varied by pink or white beds, and the brackish or saline character of the water. It is notable that none of these characteristics are found in the possible Keweenawan beds of this area. In the deep Colony well No. 1, there are only 21 per cent of red sandstone and 4 per cent of shale in this doubtful zone; in No. 2 there is 20 per cent of red sandstone and no shale; and the Amboy well is stated to have found clean pink sand, similar to the overlying Mt. Simon. In none of the wells is the water noticeably brackish. Correlation based on lithology is notoriously dangerous, but since it is upon 4 Jonas Stultz, driller, Amboy. 5 Thwaites, F. T., Paleozoic rocks in deep wells in Wisconsin and northern Tlli- nois: Jour. Geol., vol. 31, p. 534, fig. 1, 1923. 6 Hall, C. W., et al, op. cit. Thwaites, F. T., Wisconsin Geol. and Nat. Hist. Survey Bull. 25, pp. 58-61, 1912. 36 DIXON QUADRANGLE this very basis that the Keweenawan age is suggested, the use of lithologic evidence seems justified in the present instance. There is a possibility that the deep wells penetrated the Keweenawan, but the additional evidence here presented makes this seem very doubtful. The writer considers the bottoms of all the wells as Cambrian. How much deeper the Keweenawan sand- stone, if present, and the pre-Cambrian crystallines lie, must remain a matter of conjecture until more drilling has been done. Paleozoic Group CAMBRIAN SYSTEM CROIXAN SERIES Upper Cambrian formations of the upper Mississippi valley were origi- nally grouped together as the Potsdam formation, thus correlating them with the Upper Cambrian rocks of Potsdam, New York. Because further study proved that the two series were not of identical age, Winchell 7 in 1873 pro- posed the name "St. Croixan" for the formations typically developed along St. Croix River on the Minnesota- Wisconsin boundary. The term "Croixan" is now used by the Illinois State Geological Survey. The near- est outcrop of the Croixan series is approximately 42 miles northeast of this quadrangle, near Beloit, Wisconsin. Within the quadrangle, the Croixan has been penetrated by the four city water wells at Dixon, the two wells at Dixon Epileptic Colony, and by a well near Honey Creek, which was sunk more than TOO feet in search for oil. The logs of three of the Dixon Water Company wells and of the oil-test well are not available. Possibly a log published in 1890 of a "deep well near Dixon, Illinois," 8 represents the first well drilled for the company. The record shows only 10 changes in rock penetrated; depths are stated in round numbers, and the total depth as published is 30 feet greater than that shown on the company's books. The formations can be correlated only in the most general terms, and the log is not considered reliable. The log of the 1765-foot well which follows on page 37 was furnished by the company. Formation names have been supplied by the writer. 7 Winchell, N. H., General sketch of the geology of Minnesota: Geol. and Nat. Hist. Survey of Minnesota First Ann. Rept. 1873, pp. 7-72. 8 Tiffany, A. S., Record of a deep well near Dixon, Illinois: Amer. Geol., vol. 5, p. 124, 1890. CAMBRIAN SYSTEM Si Driller's log of well of the Dixon Water Company, in the SW. corner sec. S3, T. 22 N., R. 9 E. Elevation— 657 feet Description of strata Thickness Depth Feet Feet Surficial material 9 9 Platteville Limestone 95 104 Glenwood and St. Peter (glauconitic) Shale, sandy 56 160 St. Peter Sand rock 178 338 Prairie du Chien Shakopee Marl, red 71 409 Oneota Limestone 114 523 Croixan Marl, red 17 540 Lime rock 172 712 Shale, blue 94 806 Sand rock 183 989 Shale, blue 131 1120 Sand rock 645 1765 It is unfortunate that accurate samples from every 10 feet of drilling- are not available for study. Because of the importance to people who are about to drill for water, coal, oil, or other purposes, the Illinois State Geo- logical Survey is endeavoring to secure and preserve samples from all im- portant drill holes. Those who are doing drilling may obtain sample bags, and instructions for taking samples, free upon request to the Survey. A detailed log based on the examination will be furnished whenever desired. Much unnecessary drilling can be avoided by having on file an adequate collection of such drill records. In drilling the Dixon Epileptic Colony well No. 2, samples of cuttings were taken at frequent intervals and the resulting log shows by contrast with that of the Dixon Water Company the value of such samples. Only the Cambrian portion of the log is presented here. The correlations with the Wisconsin section are those of Thwaites 9 except that 76 feet of doubt- ful Keweenawan rocks have been included by the writer in the Mt. Simon formation. 9 Thwaites, F. T., Paleozoic rocks in deep wells in Wisconsin and northern Illi- nois: Jour. Geol., vol. 31, p. 534, fig. 1, 1923. 38 DIXON QUADRANGLE Partial log of Dixon Epileptic Colony well No. 2, in the SW. % sec. 21, T. R. 9 E. N., Elevation — 780 feet Formation Prairie du Chien Oneota dolomite Cambrian C'roixan series Jordan sandstone Trempealeau formation Franconia formation Dresbach formation Eau Claire formation Mt. Simon formation Description of strata Thickness Depth Feet Feet Sandstone, sandy dolomite, siliceous oolite, and dark red shale 18 460 Dolomite, gray, fine-grained to sub-crystalline ; practi- cally no sand or shale... 184 644 Dolomite, slightly sandy, greenish-gray, with glau- conite grains 80 724 Sandstone, colorless, well- rounded grains 145 869 Dolomitic sandstone, shale, gray, and dolomite, gray to buff 211 1080 Sandstone, various colors but chiefly white, pink and gray; fine to very coarse-grained 700 1780 This well extended to a depth of only 1780 feet, but well No. 1, situated near No. 2, reached a depth of 1922 feet. The following entries are from the log of well No. 1 : Thickness Depth Feet Feet Sand and shale, chocolate-color 8 1783 Sand, pinkish-gray, in clean, large and medium grains 42 1825 Sand, reddish, medium to coarse-grained 46 1871 Sand, gray-pink, large and mediun. grains 51 1922 The above log is generalized from the original record, which describes 148 samples. The detailed log is on file in the State Geological Survey offices, and is available to anyone who wishes to study it. The correlations are based entirely upon lithology since no fossils were obtained in any drill cuttings collected. Formations have been traced with some degree of certainty from the outcrop in Wisconsin through various wells to Dixon. Thwaites 10 has presented in some detail the evidence sup- ()i>. cit. ORDOVICIAN SYSTEM 39 porting the correlations, and since no new data are available, the reference of these beds to their respective formations need not be discussed further. The thickness of the Croixan series is here more than 1480 feet, which is greater than any previously recorded in Illinois. Probably it was pene- trated 200 feet farther in the Amboy well, but a satisfactory record is not available. Since the bottom of the Croixan is not known in Illinois, there is no means of determining its total thickness. ORDOVICIAN SYSTEM All the indurated rocks which outcrop in this area are of Ordovician age, in the usual sense of the term. Ulrich 11 and others have assigned the lower member of the Prairie du Chien, the Oneota dolomite, to the Canadian system, and the two upper members to the Ozarkian. These systems have not been recognized by this Survey and the entire formation is here treated as Ordovician. THE PRAIRIE DU CHIEN SERIES In the earliest geologic reports on the upper Mississippi valley, the dom- inantly magnesian limestone series between the Croixan and the St. Peter formations was called the Lower Magnesian 12 and the rocks from the Galena to the Niagaran, inclusive, were named the Upper Magnesian. Finding of the thick, calcareous Maquoketa shales in this series led to the early drop- ping of Upper Magnesian as a formation name. Lower Magnesian re- mained in common use until recently, because it described an outstanding characteristic of the formation. Grant and Burchard 13 in 1907 proposed Prairie du Chien as a geographic name for the series, which is well exposed in the Mississippi River bluffs near Prairie du Chien, Wis. Bain 14 proposed the division of the series into the Oneota dolomite below, the New Richmond sandstone, and the Shakopee dolomite above. ONEOTA DOLOMITE Name. The lowest member of the Prairie du Chien was named Oneota by McGee 15 from Oneota (now called Upper Iowa) River which enters Mississippi River near the northeastern corner of Iowa. 11 Ulrich, E. O., Revision of the Paleozoic systems: Bull. Geol. Soc. Amer., vol. 22, pi. 27, 1911. 12 Owen, David Dale, Ex. Doc. 239, 26th Cong., 1st sess., p. 17, 1S40. "Grant, U. S., and Burchard, E. P., U. S. Geol. Survey Geol. Atlas, Lancaster- Mineral Point folio (No. 145), p. 3, 1907. Bain used Prairie du Chien in 190G (U. S. Geol. Survey Bull. 294, p. 18, 1906), citing the manuscript of the Lancaster-Mineral Point folio as proposing the name. 14 Bain, H. F., Zinc and lead deposits of northwestern Illinois: U. S. Geo! Survey Bull. 246, p. 17, 1905. 15 McGee, W. J., Pleistocene history of northeastern Iowa: U. S. Geol. Survey Eleventh Ann. Rept., pt. 1, p. 331, 1891. ■iO DIXON QUADRANGLE LitJwIogy. At its outcrops, the Oneota dolomite is commonly described as a thin to thick-bedded, roughly stratified magnesian limestone, contain- ing considerable amounts of clay, both in shaly layers and intimately inter- mingled m the limestone, with very subordinate amounts of chert, sand, and crystalline quartz. In this quadrangle, the Oneota dolomite is known only by well records. Cuttings from the Dixon Epileptic Colony well Xo. '2 show a predominantly gray to pink, subcrystalline to crystalline dolomite. White chert is com- mon, pink or yellow chert is less common, and sand grains are rare. In 55 samples, shale was noted only twice. In several of the lower bed-, geodes lined with quartz crystals were found. This rock extended from 294 feet below the surface to d<30 feet below, making a thickness of 166 feet. The normal thickness of the Oneota formation reported in northeastern Iowa is £00 to 350 feet. At the nearest outcrops in Wisconsin, the thickness was reduced by erosion before St. Peter deposition to less than 100 feet. A real distribution. The Oneota dolomite probably underlies the entire quadrangle. Where the rock crops out in Wisconsin, Iowa and Minnesota, it is usually described as conformable with the underlying Cambrian. Its uniformity, its freedom from sand and shale, and its regular appearance in deep wells throughout northern Illinois suggest that it was deposited over the Dixon quadrangle with a thickness of more than 150 feet. Deep erosion preceded the deposition of the St. Peter, but the top of the sand was approxi- mately plane in this area. Its thickness here is not known to exceed 200 feet, although one log; of doubtful accuracy records 340 feet of sandstone above 55 feet of Oneota. With an additional mantle of 100 feet of Shako- pee and "New Richmond", it seems improbable that the Oneota was entirely removed at any place before the St. Peter sand was laid down. _ '-elation. Xo fossils were found in the cuttings described above, and the correlation oi this formation with the Oneota depends upon its stratigraphic position and normal lithologic character. The Prairie du Chien formation is usually correlated with the Beekmantown series of New York. Ulrich, 16 however, regards the Prairie du Chien as older than most of the Beekmantown. and calls it Ozarkian. The Little Falls dolomite (^Di- vision A and part of B i of the Beekmantown. is correlated by him with the Oneota and part of the upper Croixan series. "NEW RI< HMOND" -AXDSTOXE Name. The "New Richmond"" sandstone member of the Prairie du Chien formation was named by Wooster 17 at New Richmond. St. Croix County. Wisconsin. 16 Ulrich, E. O., Revision of the Taleozoie svstems: Bull. Geol. Soc. Amer., VOL 22. p. 640 and pi. 27. 1911. "Wooster, L. C. Geology of the lower St. Croix district: Geology of Wiscon- sin, vol. IV. p. NEW RICHMOND SANDSTONE 41 Lithology. In this quadrangle, the "New Richmond" is a massive, poorly bedded, slightly cemented and remarkably pure quartz sandstone. Fresh exposures do not show bedding, but weathering develops traces of horizontal beds and sweeping curves of cross-bedding whose dip decreases from 15° at the top to 0° where the curve is tangent to the main or true bed- ding. (Fig. 3.) The sand varies from 0.03 mm. to 1 mm. in diameter. Large grains are beautifully rounded and "frosted"; the smaller grains show less wear and some are quite angular. Aside from quartz, the rock contains a Fig. 3. Bar cross-bedding in the "New Richmond" sandstone, SW. M sec. 34, T. 22 N., R. 10 E. The exposure is about 15 feet high. fraction of one per cent of white chert fragments. These are dull, porous, and thoroughly weathered. They are sharply angular, however, and the weathering is probably due to solution since deposition, as they are now too soft to be transported. Cement is almost entirely lacking, and hand specimens are very difficult to obtain because of the very "tender" character of the rock. 42 DIXON gUADKAXGLE Topographic expression. Being very easily eroded, but overlain by the resistant Shakopee dolomite, the formation is exposed in a box canyon. Lateral erosion is rapidly pushing back the foot of the canyon walls, but the contrasting resistances of the two formations maintain the vertical bluffs. In one place, this canyon is deeper than its width. Thickness. At the only recognized outcrops in this quadrangle, the rock is exposed in two places to a height of '2d feet above Franklin Creek. Excavations for the abutments of the Lincoln Highway bridge across Franklin Creek reached the Oneota dolomite eight feet below water level, giving a total thickness of 32 feet for the formation at that point. Idle for- mation is strictly homogeneous and cannot be divided into beds or zones. As noted below, it is missing in all available deep well logs. A real distribution. This formation is known to outcrop only in a nar- row strip along Franklin Creek in sec. 2. T. 21 X.. R. 10 E.. and in sees. 33 and 3d, T. 22 X.. R. 10 E. I See Plate I.) None of the deep-well logs of the quadrangle record the sand. It is possible that some of the sand- stone poorly exposed beside the Shakopee outcrops in Rock River valley represents this formation. Xone of those beds are clearly underneath the Shakopee. and much sandstone is unmistakably resting upon it. The sand is of nearly uniform size, which is characteristic of the St. Peter rather than "Xew Richmond." The available evidence indicates St. Peter age. and the exposures are so mapped, although the other alternative cannot be defi- nitely disproved. The known exposures probably represent a bar or bars in the Prairie du Chien ocean because (1) all clay is thoroughly removed from the sand, (2) the sizes of sand grains are too varied for wind transportation. (3) the gently curving cross-beds are not steep enough for dune deposits, (d) the cross-beds dip regularly to the west, and are not interrupted and chan- neled as in river bars. I 5 I the tops of the cross-beds are planed off smooth- lv at three horizons, whereas wind or stream erosion would more probably have left irregular surface-, and I 6 | at each exposure the Shakopee forms a gentle anticline with beds parallel to the surface of the sandstone, as would be the case with lime muds consolidating and settling over an incompres- sible sand lens. This interpretation accords with the absence of the "New Richmond" in well logs in the vicinity. It is improbable that the sandstone ever covered the area a- a whole. rrelation and relation to adjacent formations. No fossils have been found in the sandstone. It is here doubtfully correlated with the New Richmond of Wooster 18 , Bain 13 and others on the basis ni its stratigraphic ] » Wooster, L. C, op. cit. 19 Bain. H. F.. Zinc and lr; ; .l (h-posits of Illinois: U. S. Heel. Survey Bull. LMtf, p. iv SHAKOPEE DOLOMITE 43 position and lithological character. However, sandstone is found at various horizons in the Oneota and Shakopee. Upham 20 for instance, reports three beds of sandstone in a 65-foot section of the Shakopee. Other geologists have reported sandstones at various horizons in the Prairie du Chien. The shaly lower Shakopee beds immediately overlying this sand in sec. 2 contain sands or sandstones at three horizons. One bed of sandstone is two feet thick and another dominantly arenaceous zone is 5^ feet thick. There is no evidence of a continuous sandstone body either in this quadrangle or in the areas to the northeast. Southeastward, in the Hennepin and La Salle quadrangles, the "New Richmond" is a sheet sand, reaching a maximum thickness of 188 feet. 21 In southwestern Wisconsin and northeastern Iowa, the sandstone is very irregular in thickness, but there are usually one or more sandstones in the Prairie du Chien series. There is no information about the contact of this sand with the under- lying Oneota. It is apparently conformable with the Shakopee dolomite above, and is approximately equivalent to the type "New Richmond," but an exact correlation cannot be made. SHAKOPEE DOLOMITE Name and lithology. The upper member of the Prairie du Chien was named Shakopee by N. H. Winchell 22 from the town of that name in Scott County, Minnesota. While it is predominantly a fine-grained, porous, buff, argillaceous dolomite, it contains considerable amounts of variously colored shales and some sandstone. The typical dolomite is nearly massive, but weathering develops beds 6 to 10 inches thick. The weathered rock is buff, light brown or yellow-brown, but the weathered surface is usually gray or dirty white. When fresh, the rock is light buff or gray. Sand is abundant in the lower beds and at several horizons in the more shaly portions, but the massive dolomite is nearly free from sand. Its weathered surface appears very sandy, due to projecting dolomite crystals which resist weather attack better than the fine-grained ground mass. The shales are normally calcitic or dolomitic, buff or yellow, and well-laminated. Red, purple and green shales are not uncommon, however, and these are usually free from carbon- ates. Much of the clay shale is sandy ; and sandstone beds, up to two feet in thickness, are interbedded with the shale. The sand grains, whether in the carbonate or argillaceous rock, are well-rounded ; many are "frosted" and dulled by wind action. The sand is identical in appearance with that of the "New Richmond" below and both probably came from the same 20 Upham, Warren, Geology of Minnesota, vol. 1, p. 429, 1884. 21 Cady, G. H., Geology of the Hennepin and La Salle quadrangles: Illinois State Geol. Survey Bull. 37, p. 34, 1919. -Winchell, N. H., Minnesota Geol. and Nat. Hist. Survey Second Ann. Rept., p. 138, 1873. 44 DIXON QUADRANGLE source. White, yellow and a few pink cherts appear in the more massive layers. The cherts are lenticular masses ranging up to six inches in diam- eter; re-entrant angles and depressions with quartz-crystal linings are found on some masses. These are not geodes although they approach those forms. Oolitic chert is rare, but is important since it identifies this formation as Prairie du Chien. 23 There are also thin beds of a peculiar, stringy or ropy, coarse-grained dolomite which appears as though the material had been worked over by large earthworms, and which is called "wormy" dolomite in the absence of a better term. The shaly dolomite is abundantly mud-cracked, marked with shallow water ripples, and in many places broken and crumpled into an edgewise Fig. 4. Typical exposure, 10 feet high, of Shakopee dolomite, sec. 34, T. 22 N., R. 10 E. A mass of brecciated material with slightly curved, overlying beds is shown right of the center of the photo- graph. conglomerate, in which there are cryptozoon fragments. The shaly beds lying immediately above these brecciated masses curve over them ; but higher beds are not deformed. The broken fragments are not weathered or rounded by attrition, and there is no evidence of weathering or transporta- tion. Possibly these peculiar masses resulted from slumping on the flanks of algal reefs. (Fig. 4.) 28 Thwaites, F. T., Paleozoic rocks in deep wells in northern Illinois: Jour. Geol., vol. 31, p. 542, 1923. SHAKOPEE DOLOMITE 45 Topographic expression, thickness and a/real distribution. The massive dolomite resists erosion, and forms cliffs where streams are cutting then- channels deeper, as in the Franklin Creek box canyon (sees. 33 and 34, T. 22 N., R. 10 E.), and along tributaries of Clear Creek, near Tealls Corners (sees. 4 and 9, T. 22 N., R. 10 E.). Where the streams have reached grade, valley sides are more gentle and are covered with a rich but stony soil. The thickest section of the Shakopee found in this quadrangle is 63 feet, measured on the west bluff of Rock River in the SE. % sec. 30, T. 23 N., R. 10 E. Neither top nor bottom of the member is exposed at this place, however. In the cuttings from Dixon Colony well No. 2, C. B. An- derson reports "Dolomite, gray, subcrystalline, in many cases containing embedded sand grains 83 feet Dolomite, chert and some siliceous oolite 29 feet" making a total of 112 feet. The thickest section that can be studied in detail is exposed in a ravine on the south side of Franklin Creek. Although the St. Peter lies on top, the section represents only the lower portion of the Shakopee. More mas- sive beds, representing the upper Shakopee, outcrop near Tealls Corners and along Rock River. Section of lower Shakopee member, Prairie clu Chien formation, in the NW. 14 SW. % sec. S.'i, T. 22 N., R. 10 E. Description of strata Thickness Ft. In. St. Peter sandstone Shakopee member Green clay and clay shale 8 Buff -brown, earthy limestone , 1 Mottled buff and blue-gray or green-gray, fucoidal and "wormy" dolomite, porous in many places, irregular, lumpy beds 2 inches or less in thickness, some floating sand grains in lower beds. ... 8 Green, buff and ash-gray, sandy shales, well-laminated 6 Buff to light brown, dense, uniform, fine-grained dolomite. Pew mottled, ripple-marked and mud-cracked irregular beds inter- calated; base containing some brecciated dolomite 7 6 Gray or ashen, laminated shale with thin sandstone layers 2 Buff, "wormy" and fucoidal dolomite, alternating coarse and fine- grained, weathering chalk-white, alternately even-bedded' with conchoidal fracture and lenticular beds, with granular, hackly fracture, weathering with a lumpy surface 3 Massive, poorly bedded dolomite with many floating sand grains in lower part, shaly layers ripple-marked and mud-cracked'; ap- proximately 17 "New Richmond" sandstone Pure white, crumbling quartz sandstone Total 39 8 46 DIX0X QUADRANGLE \\ hile the above section is fairly typical, none of these beds can be identified at a point a mile upstream, where, in a 12-foot exposure, there is much red and purple shale, more dolomitic shale and less massive dolomite. In this exposure, several shale beds pinch out within a distance of 300 feet. To show the rapid horizontal variation of this formation, the following section located two miles upstream from the first one is described. Section or lower Shakopee member. Prairie du Chien formation, in the SE. V± XW. y± sec. 2. T. 21 N., R. 10 E. Description of strata Thickness Ft. In. St. Peter sandstone Shakopee member Buff dolomite, with floating sand grains and thin, argillaceous limestone layers lithologically similar to "water lime" 12 White. 0.5 mm. -grained sandstone, many beds with dolomite ce- ment, sandy dolomite, and sanely dolomitic thin-bedded shale.. 5 6 Buff to gray dolomite and interbedded ■"water-lime" layers 3 "Water lime'* shaly dolomite 6 White, saccharoidal sandstone 8 "Water lime" shaly dolomite 1 Buff, sandy dolomite and "water-lime" layers with small amount of red shale 1 6 White sandstone and dolomitic sandstone 2 Sandy dolomite 6 Buff, thin-bedded "water lime" or "cotton rock" 1 6 Crystalline buff dolomite, with much floating sand 2 "Water lime" 8 Green, gray and purple clay shales, interlaminated and containing a bed of sand one inch thick 3 "New Richmond" sandstone Total Shakopee member 33 10 The distribution of the Shakopee exposures follows in general the axis of the La Salle anticline across the quadrangle. As shown in Plate I. the Shakopee is discontinuously exposed along Franklin Creek from a point two miles southeast of Franklin Grove to a point four miles northeast of that town. It also outcrops near Tealls Corners in Clear Creek and tributary valleys, at several places northward along Rock River valley, and two miles east of Lighthouse Point in sec. 30, T. 23 X.. R. 11 E. Paleontologic character. Because of the dolomitization of the Shako- pee member, its fossil content has been almost entirely destroyed. The clay shales are non-fossiliferous ; the dolomitic shales originally contained numer- ous gastropod shells, but none of these is sufficiently preserved to be specif- ically identifiable. The following fossils were collected from the Shakopee in the Dixon quadrangle. Identifications have been checked by Dr. J. J. ( rallowav. SHAKOPEE DOLOMITE 47 Fossils from the Shakopee member, Prairie dn Chien formation Cryptozoon minnesotense Winchell Hormotoma sp. Lingulepis acuminata Conrad Liospira sp. Ophileta sp. Fucoids are abundant in the shaly limestones, and the "wormy" dolomite is probably to be classed with the other fucoids. Correlation, The usual correlation of the Shakopee has been with the Beekmantown (lower Ordovician) of New York. Ulrich considers the formation older than most of the Beekmantown and puts it in the upper part of a new system, the Ozarkian, which he places definitely below the Ordo- vician. The presence of Lingulepis acuminata suggests a correlation with the Hoyt limestone of New York, in which this form is prominent. Ulrich and Cushing 24 consider the Hoyt distinctly older than most of the Beek- mantown, and place it in the upper Ozarkian. Lacking more conclusive evidence, the writer adopts the usual correlation with the Beekmantown and does not limit it to the Hoyt member alone. Relations to adjacent formations. The base of the Shakopee is entirely conformable with the top of the "New Richmond" at the only certain ex- posures in Franklin Creek. On the other hand, a sharply marked erosional unconformity always separates the Prairie du Chien from the St. Peter. This unconformity is also angular at several places in Franklin Creek valley and in the NW. % sec. 16 and the SW. ]/ A SE. y A sec. 9, T. 23 N., R. 10 E. Flat-lying St. Peter lies on Shakopee beds with dips ranging up to 45°. (NE. y A NE.>4 sec. 33, T. 22 N., R. 10 E.) Erosion cut deeply and steep- ly into the Shakopee before the St. Peter was deposited. At several places where the two formations are horizontal, the contact dips over 20°. No residual soil or weathered dolomite is found at the contact. Fragments of fresh, angular dolomite and even of shale are found in the St. Peter near the steep contacts. As an example of these relations, the hill west of Rock River in sec. 30, T. 23 N., R. 10 E., may be cited. Sandstone forms the northwest and south sides of the hill, but dolomite makes the extreme top and part of the east face. The total height of the buried hill is unknown ; but dolomite and sandstone exposures prove a minimum of 63 feet. The contact is largely hidden, but outcrops show that its dip is more than 18° and less than 27°. Fragments of dolomite are found in the sandstone near the Shakopee, but they are entirely missing at a distance of 200 feet. The low- est dolomite is exposed 10 feet above the river. Sandstone forms the bank 75 feet north. It is possible that this sandstone is "New Richmond," under- 24 Ulrich, E. O., and Cushing, H. P., Age and relations of the Little Falls dolo- mite: New York State Mus. Bull. 140, pp. 130-136, 1010. 48 DIXOX QUADRANGLE lying the dolomite, but there is no evidence for this suggestion. If it is St. Peter, the Shakopee hill was more than 7 5 feet high. Similar relations but showing less erosion are observable at all Prairie du Chien outcrops except in Franklin Creek, where the St. Peter rests upon and appears conform- able to beds of lower Shakopee. Half a mile north, the maximum angular unconformity and sharp erosion are found. Obviously, the apparent con- formity results merely from parallel bedding on both sides of a flat erosion surface. MIDDLE ORDOVICIAN SERIES ST. PETER SAXDSTOXE Name. This formation was originally defined by D. D. Owen 25 and was named from St. Peter's (now called Minnesota) River, near St. Paul, Minnesota. It does not occur near the town of St. Peter. LitJiologic character. This well-known formation has its usual char- acteristics in this region. It is a white, medium-grained, poorly cemented, thick-bedded, pure quartz sandstone. Its color is strikingly white, except at a few places where it is stained brown by ferric oxide or green with a glau- conitic clay. Its grains are uniformly well-rounded quartz sand, varying from 0.2 mm. to 1.0 mm. in diameter. The sand is better sorted, more rounded and more uniformly worn by eolian transportation than are the grains in the "New Richmond'' sandstone. Some of the sand has been enlarged by a secondary growth of quartz. These grains glisten as light is reflected from their perfect crystal faces, and contrast sharply with the usual dull or milk-white appearance of the "frosted" sand. Cement is so nearly absent that it is commonly difficult to collect a specimen of the rock, and its crumbling character permits rapid erosion wherever the rock is exposed to friction. In spite of its softness, the rock does not weather rapidly. (Figs. 5 and 6.) Bedding is usually evident on a large exposure, but is never striking, and may be difficult to recognize on small surfaces. The strata range from six inches to three feet in thickness and are more prominent where the rock is weathered. Bedding is marked by change in size of grain, rather than by change in color or character of material. Most of the bed- ding is normal, but cross-bedding appears at many places. The cross-beds have the long, sweeping curves of the standing- water type, and are not at all suggestive of wind or stream deposits. In no case do the cross-beds dip at an angle exceeding 18° ; eolian strata commonly have dips of 2T° to 32°. The sand is here, as throughout the Mississippi Valley, a pure quartz sand. Most analyses of samples from this quadrangle show more than 98 per cent silica, and some have been reported with over 99 per cent. The most common impurity is iron in the form of limonite. This is disseminated through the rock in a few places, giving it a faint yellowish tint, but is more 25 Owen, D. D., Sen. Exec. Doc. No. 2, 30th Cong. 1st sess., p. 16i». IS 17. ST. PETER SANDSTONE 49 often deposited between the grains along joint planes, and less often along bedding planes. Along these channels, it cements the rock for thicknesses of one to four inches into a hard, heavy, dark-brown sandstone. Weather- ing removes the softer sandstone, and leaves the iron-cemented sandstone projecting as dark ribs from the general white surface. At present, water Fig. 5. Weathered bluff of St. Peter sandstone, showing etching by wind and frost action along softer beds and some joint planes. Fig. 6. Bluff of St. Peter sandstone, east of Green Rock, SE. corner, NW. 14 sec. 11, T. 22 N., R. 10 E. The highest sandstone exposed is about 85 feet above river level. in the sandstone does not contain iron, for exposures remain white, and iron is not deposited by springs coming from the St. Peter. Next to iron, the most common impurity is a glauconitic green clay which coats the sand grains and fills the spaces between them. Hand speci- mens appear to be green sand, but the clay coating can readily be removed. 50 DIXON QUADRANGLE One analysis showed that the clay constituted about 5 per cent of the rock and that the potash content of the specimen was .22 per cent. This is 4.4 per cent of the coating, or nearly the theoretical potash content of glauconite. Prolonged boiling with hydrochloric acid does not destroy the green color nor extract the iron content. The color, potash content, insolubility of the iron, and coating relation are normal for non-foraminiferal glauconite, such as is sometimes deposited in shallow water. The best exposure of this green sand is at "Green Rock," a cliff a mile northwest of Grand Detour, where the upper 22 feet of the St. Peter consists of this sand. Although the green sand is found at various horizons throughout the formation, it is most abun- dant near the top. Topographic expression. Because of its softness, streams easily erode the St. Peter to grade and then by lateral planation widen the valleys and develop the flood-plains. Where the stream is widening the valley, the sides are steep. This is especially the case where the overlying Platteville lime- stone protects the top of the slope. Typical box canyons result, with flat floors and vertical sides. Where the limestone is missing, after the stream reaches grade the valley sides become more gentle, and the divides between the streams are slowly reduced to rounded, sandy hills. Vertical exposures are slowly destroyed by wearing off the top of the cliff and by talus burying its foot. Weathering of faces protected from friction is very slow. Thickness and areal distribution. Measured thicknesses of this forma- tion vary from 30 to 180 feet. A direct measurement cannot be made at any point in the area, except in a well, but both the maximum and minimum measurements are taken where the St. Peter is apparently horizontal and the distance from Platteville to Shakopee outcrops is less than a quarter of a mile. The formation is thinnest in the NW. ]/ A sec. 27, T. 22 N., R. 10 E., and its greatest thickness is south of Devils Backbone in Oregon Township. Along Rock River from the northern boundary of the quadrangle to a point a mile west of Grand Detour, the thickness is certainly over 100 feet, except where Shakopee dolomite hills extend up into the St. Peter formation; and from sec. 17 to sec. 30, T. 23 N., R. 10 E., the sandstone is normally over 160 feet thick. Yet in section 30, on top of the Shakopee hill referred to on page 47, the thickness cannot have exceeded 80 feet. In each case of estimated thickness, the base of the section is the top or side of a Shakopee dolomite hill. The dolomite is not sufficiently exposed to permit an estimate of its average relief, but hills over 60 feet high are known on the Shakopee surface. Two well logs are available, but both are unreliable. Each reports about 50 feet of shale from the Glenwood horizon ; yet at no exposure in the quadrangle or adjacent areas is this shale over 8 feet thick. Possibly it caved and contaminated the cuttings in the Dixon Water Company's well (p. 37). Although the Colony well records the shale as 50 feet be- ST. PETER SANDSTONE 51 neath the surface, from excavations near the well and sections measured half a mile northeast, it is known that the shale is at least 95 feet beneath the surface. The Water Company's well record shows 175 feet of St. Peter, and the Colony well, 82 feet. Probably the average thickness over the en- tire area is about 160 feet, although this estimate is believed to be too small rather than too large. The best continuous exposure in the area is at Green Rock (fig. 6) where 83 feet of normal sandstone is overlain by 22 feet of green sand with minor amounts of interbedded white sand. In the lower 83 feet, no distinction could be made between beds, a description of any one bed being applicable to any other. This section shows much more green sand than is normal. On the whole, the green sand probably does not constitute 4 per cent of the formation. The St. Peter covers the entire quadrangle except for the limited areas of Prairie du Chien outcrop. The sandstone is the surface formation over most of the La Salle anticline, and therefore outcrops in a broad belt ex- tending from the southeastern corner of the area to the center of the north- ern boundary. In addition, it occurs along Rock River almost to Dixon and occupies most of the northeastern portion of the quadrangle. (Plates I and V.) It is best exposed along the larger streams where erosion is con- tinually uncovering fresh surfaces. Paleontologic character and correlation. The only fossil found in the formation in this area is a worm boring, Scolithns minncsotensis Hall. This was found about 65 feet below the top of the formation at the south end of the Rock River bridge at Grand Detour, and also about 10 feet from the top of the sandstone in the NE. ]/ A sec. 26, T. 23 N., R. 10 E. Shells were probably buried in the sand as it was deposited, but the formation is so porous that freely circulating water could easily dissolve the calcareous matter completely. The loose, unconsolidated sand could not preserve casts of the shells and no trace of them was left. Marine fossils have been found by Winchell, 26 Sardeson, 27 and Trowbridge. 2S On the basis of these fossils and its stratigraphic position, the formation is usually correlated with the upper Chazy series of the Ordovician. Relations to adjacent formations. As already described, the St. Peter-Shakopee contact is an erosional unconformity of marked relief. The St. Peter-Glenwood contact is gradational, for the lower Glenwood is very sandy ; but sand is not abundant more than two feet above the contact. The 26 Winchell, N. H., Minnesota Geol. and Nat. Hist. Survey Fourth Ann. Rept., p. 41, 1875. 37 Sardeson, F. W., Fossils in the St. Peter: Minnesota Soc. Nat. Sci. Bull., vol. 4, pp. 64-87, 1896. 2S Trowbridge, A. C, Origin of the St. Peter sandstone: Iowa Acad. Sci. Proa, vol. 24, p. 173, 1917. 52 DIXON QUADRANGLE surface of the St. Peter was nearly plain when the Glenwood was deposited, The most conspicuous exception is a mound of sandstone, rising about 20 feet above the general level of the St. Peter, and offsetting the St. Peter- Glenwood boundary nearly half a mile in sees. 22 and 23, T. 22 N., R. 10 E. The top of the St. Peter was also probably higher than the general plain in the neighborhood of Devils Backbone, Oregon Township, for the Glenwood is very thin and locally absent, while the basal Platteville is 10 to 15 feet thinner than usual. GLENWOOD SHALE Name and character. A gray-green to olive-green sandy shale over- lying the St. Peter has been named Glenwood 29 at its outcrop in Glenwood Township, near Decorah, la. Occurrence of a green shale above the St. Peter has commonly been reported, but it is normally thin and generally has been regarded as the lowest bed of the Platteville, or less often as the top of the St. Peter. In this area, however, the shale is different from any part of either adjacent formation 30 ; it is conspicuous at several localities ; it causes practically all the springs of the area, and its economic possibili- ties are so interesting that it is separately described. The Glenwood is intermediate between clay and shale, is characteristic- ally sandy in its lower beds, and is even an argillaceous sand in some places. The clay is soft, greasy, plastic and adherent when wet. Where fresh, it is grass green, but on exposure, it slowly fades and becomes gray or buff and finally weathers to a dark brown. Its potash content is high. Possible utilization of the Glenwood is discussed in Chapter VI. Topographic expression. Because of its soft character, the tendency of the overlying limestone to slide on the greasy clay and the great amount of glacial drift in the area, outcrops are rare, being limited to bluff faces or rock-bedded ravines. Presence of the shale is indicated on many hillsides by springs which occur along the outcrop. Water seeping through the lime- stone follows the surface of the impervious Glenwood and appears in springs and seepages, often high on a hillside. Being less resistant to erosion than the St. Peter, the shale with the sandstone forms a cliff under a protecting limestone cap. Thickness. Where normally developed, the shale varied from 2^ to 7 feet in thickness. Part of this variation may be due to slumping down of overlying limestone, but drillers report similar irregularities in wells. In general, the lower portion of the shale is quite sandy, in places being an argillaceous sand. The sandy zone is limited to the lower two feet, above 29 Calvin, S., Geology of Winneshiek County: Iowa Geol. Survey, vol. XVI, p. 61, 1906. :!0 Bevan, Arthur, The Glenwood beds as a horizon marker at the base of the Platteville formation: Illinois State Geol. Survey Report of Investigations No. 9, 1926, is interesting in this connection. GLENWOOD SHALE PLATTEVILLE LIMESTONE 53 which the clay is quite uniform in its freedom from sand, even lamination, and regular grass to olive-green color. The shale is thin or entirely miss- ing on the crest of the La Salle anticline and reaches its maximum thickness along the west flank of that structure. The amount of shale varies with the original depth of clay deposited, rather than being affected by later erosion, and since the top of the St. Peter was nearly a plane surface, the Glen wood thickness varies only slightly over limited areas. Excellent ex- posures of the shale may be found in ravines entering Rock River between Pine Creek and the Sandusky Cement plant at Dixon, but the best exposures are north of the Dixon-Grand Detour road in sees. 15 and 22 and on the south side of sec. 22, T. 22 N., R. 9 E. Areal extent. The shale outcrops between the Platteville and St. Peter formations along both sides of Rock River from the cement plant to Grand Detour and along the west side of the river northward to the Chicago, Bur- lington and Quincy Railroad in Oregon Township. Similar outcrops follow both sides of Pine Creek valley, beyond the northern edge of the quadrangle. The shale is not well known in the eastern and south central parts of the area, in part because of the heavy mantle of glacial till, but also because apparently it was not deposited continuously over the axis of the La Salle anticline. Age, correlations and relations. No fossils have been found in this shale, but it is correlated on the basis of its position and lithology with the Glenwood and other green shales known between the St. Peter and the Platteville. For the same reasons, it is correlated with the Joachim lime- stone of Missouri. It marks a complete change in sedimentary conditions, and usually is regarded as the first of the Platteville deposits. The shale appears conformable with both the St. Peter and the Platteville, and seems to indicate a change in sediment brought to the ocean rather than a dias- trophic event. PLATTEVILLE LIMESTONE Name. The Platteville limestone was named by Bain 31 from the town of Platteville, Wisconsin, where the formation is well developed. It in- cludes the Buff and Blue limestones of the first state survey, 32 and is equiv- alent to the ''Trenton" of most early workers and to the still earlier St. Peter Shell Limestone. Others have used "Trenton" to include both Platteville and Galena formations. Since the Galena alone is approximately equivalent to the type Trenton of New York, the name Platteville has been substituted. Lithologic character. In this area, the Platteville consists of three dis- tinct divisions; the basal part, or Buff limestone; the middle, or Blue lime- 31 Bain, H. F., Zinc and lead deposits of northwestern Illinois: U. S. Geol. Survey, Bull. 246, p. 18, 1905. 32 Shaw, James, Geol. Survey of Illinois, vol. V, pp. 104-13!*, 1873. 54 DIXOX QUADRANGLE stone, and the upper member, for which the name Lowell Park is here pro- posed from its typical development in Lowell Park and along the road north of the park. The Buff limestone is a buff to yellow-brown, fine to coarse- grained, well-bedded, magnesian limestone, which carries some sand grains in a carbonate matrix in the lower beds, and contains much clay but no inter- bedded shale. The purer beds weather to a light brown, rough, porous, granular rock, which in hand specimens is identical with some of the dolo- mite in the Shakopee. The more argillaceous strata break down to thin, regular laminae and produce a light brown clay, which often feels and ap- pears sandy because of included dolomite grains. White chert is common in this part of the Platte\ T ille. The Blue limestone consists of a lower, highly fossiliferous, thick- bedded, blue limestone, and an upper, sparingly fossiliferous, crypto-crystal- line, very heavy-bedded, blue limestone or "glass rock," which contains brown dolomitic nodules and branching masses. The lower Blue limestone is deep blue, fine-grained, somewhat argillaceous, free from sand and chert, and, when fresh, has strata 8 to 10 inches thick. Unlike the Platteville to the north and west, no interbedded shale occurs. As a result of weather- ing, residual clay appears between the irregular limestone laminations. Pro- longed weathering produces a gray, or in some beds, a buff color, and breaks up the beds into lumpy, lenticular masses ranging up to two inches in thick- ness and six inches in length. The surface of the weathered rock is covered with a characteristic chalk- white coating of calcium carbonate, so that the outcrop looks as though it had been whitewashed. The glass rock is a crypto-crystalline, very pure, blue to buff, brittle limestone containing numerous nodules and irregular masses of coarsely crystalline dolomite. The texture, conchoidal fracture, and brittleness give the rock its name. It spalls off like glass under impact, but is a dense, hard stone which would be valuable for building purposes if it were free from the magnesian masses, which resemble worm borings in timber. Because the dolomite is porous and crystalline, and is readily attacked by ground water, the comparison to worm work may be carried further, for the disintegrated dolomite sand corresponds to the half-eaten "saw dust" left by the worms in timber. On the whole, the glass rock is less easily weathered than any othe r rock which outcrops in the area. The Lowell Park consists of interbedded gray and buff, argillaceous, but heavy-bedded limestones, and coarse-grained, deep yellow-brown, porous dolomites. The dolomite is in every way typical of the next succeeding formation, the Galena, and in small outcrops cannot be distinguished litho- logically from it. The chalky, earthy limestones are more like the impure portions of the Buff member. The chalky beds carry abundant fucoids, and bedding planes are marked by numerous dendrites. Weathering reduces the PLATTEVILLE LIMESTONE 55 dolomitic beds to a dark yellow-brown sand of dolomite grains. The chalky beds split up into a thin-bedded, shaly mass, which is easily eroded, and accordingly rarely outcrops. Topographic expression. The members of the Platteville all resist weathering processes very well, the Blue being the most resistant and the Lowell Park least. The formation produces narrow, steep-sided valleys; where a stream cuts through the limestone into the soft shale and sandstone below, a vertical bluff is commonly formed, which persists as the dominant feature of the topography even after the valley is widened by lateral plana- tion. Since the Platteville protects the underlying sandstone from rapid erosion, vertical cliffs are formed, which are the distinctive and alluring features of the landscape along Rock River and its tributaries. Solution and frost action open vertical joints and horizontal bedding planes (fig. 7), Fig. 7. Outcrop of Platteville limestone in NW. *4 sec. 20, T. 22 N., R. 11 E., showing character of bedding, jointing, and overlying till. producing a good imitation of masonry walls with towers, pinnacles and bat- tlements. This characteristic appearance was called "castellated topography" by Owen, the first geologist to study this formation in the Mississippi valley. He grouped the Platteville and overlying Galena together as the "Cliff lime- stone," and at first correlated them with a younger formation of similar aspect in Ohio. Thickness and details of section. The thickness of the Platteville lime- stone ranges from 100 to 125 feet, distributed as follows : Lowell Park member 20-30 feet Blue limestone Glass rock 25-30 feet Fossiliferous 40-45 feet Buff limestone 0-20 feet 56 DIXON QUADRANGLE The Buff member varies from a foot or two to a maximum thickness of 20 feet, except on top of the La Salle anticline, where it is usually about 6 feet thick. The following section is typical. Section of the Buff limestone member, Platteville limestone, in ravine in the NE. 1,4 SW. % sec. 22, T. 22 N., R. 9 E. Description of strata Thickness Ft. In. Blue limestone Typical "white-washed" nodular beds Covered 3 6 Buff limestone Very ferruginous, strongly dolomitic, yellow-brown limestone, ap- pearing sandy on surface because of coarse dolomite grains .... 1 3 Similar to above, with %-inch sand laminations intercalated 4 Light brown, dolomitic limestone, poorly exposed 3 8 Light buff to deep brown, somewhat sandy dolomite 1 Well-bedded, rounded-grain quartz sandstone with dolomite cement . . 2 Buff, dense limestone, slightly dolomitic, much sand scattered through limestone; clay in wavy bedding 2 2 Very fossiliferous, buff to light brown, slightly dolomitic limestone . . 10 Buff dolomitic limestone with scattered sand grains which are abundant in lower part 5 2 Glenwood shale Total, Buff limestone 14 7 The thickness of this section probably should be increased by adding the covered strata, making a total of 18 feet. The Blue limestone consists of 40 to 45 feet of the very fossiliferous limestone and 25 to 30 feet of glass rock. The following section is typical of the fossiliferous zone. Section of lower, fossiliferous zone of Blue limestone member, Platteville lime-' stone. Abandoned quarry in the SW. % NW. % sec. 22, T. 22 N., R. 9 E. Description of strata Thickness Ft. In. Top of cliff, covered Buff, very fine-grained limestone, weathers white on surface, develops lenses or lumps on weathering, coarser-grained than beds below.... 11 Thin-bedded, argillaceous, very fine-grained, blue limestone, weathers to nodular white-coated surface. Much clay along surfaces between lenses or nodules, which are % to 2 inches thick. This bed forms the most distinctive castellated bluffs along Rock River from Dixon to Devils Backbone 10 Blue, fine-grained limestone, weathering buff, but not brown, beds 2 to 6 inches, no dolomite 6 2 PLATTEVLLLE LIMESTONE 57 Section of lower, fossiliferous zone of Blue limestone member, Platteville lime- stone. Abandoned quarry in the SW. % NW. % sec. 22, T. 22 N., R. 9 E. — Concluded Description of strata Thickness Ft. In. Blue, glass-rock type, sparingly fossiliferous limestone, weathering into angular, thick blocks instead of usual nodules 6 4 Extremely fossiliferous, fine-grained, blue limestone, weathering to usual "white-washed" beds 2 6 Irregular-bedded blue limestone, with 50 per cent of buff dolomitic irregular masses; weathers to a rough, honey-combed face 5 8 Buff limestone Total fossiliferous Blue limestone 41 8 The glass rock is not readily divisible into zones. In the cement plant quarry, 28 feet of typical glass rock is exposed. The original thickness has been reduced by glacial erosion. The following section indicates the com- mon relations and characteristics of the glass rock. Section of glass rock and Lowell Park members of Platteville limestone. Aban- doned quarry, in the NE. % NE. y± sec. 20, T. 22 N., R. 9 E. Description of strata Thickness Lowell Park member Ft. In. 7 Chalky, argillaceous, soft, buff limestone 1 6 6 Alternating fine and coarse-grained, buff to yellow-brown, porous dolomite and dolomitic limestone containing many fucoids and numerous fossils 10 5 Slight erosional unconformity Blue limestone member Glass rock zone 5 Buff to light brown, tough, massive, fine-grained, slightly magnesian limestone, with numerous dolomite nodules or masses; weathers into six to ten-inch thick, well-jointed beds 6 9 4 Typical, deep blue, crypto-crystalline, conchoidal fracturing, dense, brittle limestone, called "glass rock"; contains dolo- mitic nodules, increasing in amount upward; weathers to solid, angular, gray blocks 8 6 3 Heavy-bedded, dense, blue glass rock, with a few irregular zones which weather to "whitewashed" material; most of this zone turns gray and breaks into angular blocks on weathering; rare fossils 12 6 2 Fossiliferous zone, lower Blue limestone, typical, to river level 32 1 Buff limestone, estimated from outcrops upstream 15 Total Glass rock (3, 4 and 5) 27 9 Total Platteville (1-7) 86 8 58 DIXON QUADRANGLE The following is the best section of the Lowell Park member of the Platteville, and contains only 14 feet more than the section immediately preceding. Section of Lowell Park member, Platteville limestone. Roadside exposures, east side NE. % SE. % sec. 18, T. 22 N., R. 9 E. Description of strata Thickness Feet Galena dolomite Platteville limestone Lowell Park member Buff to yellow-brown, fine-grained, argillaceous, chalky, soft limestone in well-defined beds 4 to 8 inches thick, about.... 12 Yellow-brown to brown, coarse-grained, porous, dolomitic, fucoidal, wormy limestone, lithologically very similar to Galena limestone 14 Blue member Total, Lowell Park member 26 Areal distribution. The Platteville limestone crowns all the higher hills between Rock River and Pine Creek and occurs in the valley sides farther west. From Pine Creek southward, it occupies a narrowing zone along the right bank of the main stream and extending under the valley train to the western edge of the quadrangle. It covers most of the upland south of Rock River, west of Chamberlain Creek and north of the Chicago and Northwestern Railway. Because of a heavy cover of glacial till, it is not well known south of the railroad, but it covers a wide belt extending south- eastward from Dixon to Lee Center. It also underlies most of the upland northeast, east and southeast of Franklin Grove. Details of its distribution are shown in Plates I and V. Paleontologic character. The following list indicates the fossils which were collected from the Platteville limestone in this area. As in the case of the other fossil lists, the determinations have been checked by Dr. J. J. Galloway. The small number of forms obtained from the Lower Buff and the Lowell Park is not evidence of the small amount of life existing during the early and late Platteville time, but results from the partial dolo- mitization which has affected much of each member. There is no reason to postulate a faunal difference between the different members, but their contents are indicated in separate columns for the sake of an accurate record. PLATTEVUXE LIMESTONE 59 Table 2. — Fossils collected from the Platteville limestone, Dixon quadrangle Fossils Lower Buff Blue Lowell Park Arthropora simplex Ulrich Batostoma cf. magnopera Ulrich Batostoma winchelli Ulrich Carabocrinus cf. radiatus Billings Clathrospira subconica Hall Columnaria halli Nicholson Constellaria varia Ulrich Crania trentonensis Hall Cyrtoceras sp. Cyrtodonta obliqua Meek and Worthen Dalmanella testudinaria Dalman Dekayella praenuntia Ulrich Dinorthis pectinella Emmons Encrinurus vannulus Clarke Endoceras proteiforme Hall Eridotrypa aedilis Eichwald Eridotrypa aedilis minor Ulrich Escharopora subrecta Ulrich Eurychilina reticulata Ulrich Gonioceras occidentale Hall Hemiphragma irrasum Ulrich Homotrypa minnesotensis Ulrich Illaenus punctatus Raymond Leperditia f abulites Conrad Leptaena charlottae Winchell and Schuchert. Lichenaria typa Winchell and Schuchert Lingula elderi Whitfield Liospira obtusa Ulrich and Scofield Liospira progne Billings , Liospira vitruvia Billings Lophospira bicincta Hall Lophospira obliqua Ulrich Ophiletina sublaxa Ulrich and Scofield Orthis tricenaria Conrad Orthoceras sp Orthoceras cf. sociale Hall Pachydictya acuta Hall Phragmolithes fimbriatus Ulrich and Scofield Pianodema subaequata Conrad Plectambonites sericeus Sowerby Rafinesquina alternata Emmons Rafinesquina minnesotensis N. H. Winchell... Rhinidictya exigua Ulrich Rhinidictya grandis Ulrich Rhinidictya mutabilis Ulrich 60 DIXON QUADRANGLE Table 2. — Fossils collected from the Platteville limestone, Dixon quadrangle- Concluded Fossils Lower Buff Blue Lowell Park Rhinidictya trentonensis Ulrich Salpingostomy buelli Whitfield Scenidium anthonense Sardeson Scolithus sp Spyroceras bilineatum Hall Streptelasma corniculum Hall Streptelasma profundum Conrad (Owen) Strophomena emaciata Winchell and Schuchert. . . . Strophomena incurvata Shepard Strophomena scofieldi Winchell and Schuchert Strophomena trentonensis Winchell and Schuchert. Strophomena winchelli Hall and Clarke Subulites regularis Ulrich and Scofield Thaleops ovatus Conrad Trochonema beachi Whitfield Trochonema umbilicatum Hall Zygospira nicoletti Winchell and Schuchert Zygospira recurvirostris Hall Correlation. The Platteville limestone was deposited in Middle Ordo- vician time. The Buff and Blue members are correlated with the Upper Stones River series of Tennessee and the Chazy-Lowville of New York. The Lowell Park member is the same in age as the Decorah shale of Iowa. A local term has been used here in place of Decorah, since a limestone, in- stead of shale, is present in this quadrangle, and also because the original definition of Bain 33 and later of Calvin 34 who defined the Decorah shale, was that the Platteville included all limestone between the St. Peter and Galena formations. The Black River formation of New York is correlated with the Decorah and with the Lowell Park member of the Platteville. Relations to adjacent formations. As already stated, there is no evi- dence in this quadrangle of erosion following St. Peter deposition, but the Glenwood shale and the Buff member of the Platteville are thin or entirely absent on top of the La Salle anticline. The Buff does not consistently ex- tend farther up the flanks than the Glenwood ; both may be present and both thin. It seems that conditions on the anticline were unfavorable for deposi- tion until the Blue limestone was formed. Cady 35 found evidence of slight 33 Bain, H. F., U. S. Geol. Survey Bull. 246, p. IS, 1905. 3i Calvin, S., Geology of Winneshiek County: Iowa Geol. Survey, vol. XVI, p. 81, 1906. 35 Cady, G. H., Geology of the Hennepin and La Salle quadrangles: Illinois State Geol. Survey Bull. 37, p. 39, 1919. GALENA DOLOMITE 61 erosion at this horizon 30 miles farther south along the anticline. The boundary between the Buff and Blue members is gradational without a sharp change in material or evidence of disconformity. Between the Blue and Lowell Park members, an erosional unconformity of slight relief is found at many places. This is well exhibited in the quarry beside Ravine Road, 300 feet south of Rock River, in Dixon. (SE. % SW. }i sec. 33, T. 22 N., R. 9 E.) Depressions of six to eight inches developed in the top of the Blue before the Lowell Park was deposited. No residual soil or chert marks this unconformity within the quadrangle, and there is no evidence of important erosion. The Galena formation succeeds the Lowell Park with- out sharp change in sediments or apparent erosion. Argillaceous matter decreases and magnesia increases at the contact, but sedimentation was prob- ably continuous. GALENA DOLOMITE Name. The Galena dolomite was so named by James Hall 36 because it was the principal source of lead in Mississippi Valley, galena being the name of the common lead-ore mineral. Lithologic character. Except the St. Peter, the Galena is the most uni- form of the formations in the area. It is a thick-bedded, porous, cherty, coarsely crystalline, buff to yellow-brown dolomite. In fresh exposures, the strata average 30 inches in thickness ; weathered outcrops show beds 6 to 10 inches thick. Originally a limestone, the formation has been thor- oughly dolomitized with accompanying decrease in volume. This decrease in volume was accomplished without perceptible slumping or subsidence, by increase of pore space, so that the rock is very porous, yields water read- ily to wells, and is generally deeply weathered in outcrops. White and buff-colored cherts are found throughout the formation. They are not common in the basal 30 feet, but are abundant above that level. Bryozoa are rather numerous in the white chert. The entire formation consists of crystalline dolomite, the crystals rang- ing from microscopic size to 2 mm. in diameter. Weathering removes the cement between the crystals or disintegrates the rock, so that a sandy residue of dolomitic grains results. Where very fresh, the rock is gray-buff, but on all natural exposures it is buff to deep yellow-brown. There is not suffi- cient iron to cement the residual soil or to segregate into masses of limonite, as in the Shakopee, but the disintegrated rock is always colored a deep brown. Well-developed vertical joints occur throughout the rock. Bluff faces are vertical, the rock having broken free along a joint plane, and on such faces, the rock has a rough, granular, rugged surface, marked by numerous solution cavities. Where there is much vegetation, the rock sur- 36 Foster, J. W., and Whitney, J. D., Geology of the Lake Superior Land Dis- trict: 32d Cong., spec, sess., Senate Doc. 4, p. 146, 1851. 62 DIXON QUADRANGLE face is -gray, probably because organic matter reduces and helps to dissolve the iron oxide from the surface. In the absence of plant life, the surface weathers a deeper brown than the interior of the dolomite. Figures 8 and 9 illustrate the normal aspect of the Galena dolomite. Topographic expression. This formation has less resistance to solu- tion and freezing water than any of the other carbonate rocks of the area. Accordingly its valley sides are readily reduced to gentle slopes, bluffs occur- ring only where downward erosion is unusually rapid, as along Rock River in the city of Dixon. In the uplands, the Galena outcrops have been reduced until they merge imperceptibly into the prairie. Quarries have been opened in sees. 28 and 30, South Dixon Township (T. 21 N., R. 9 E.) and in sec. 20, Ashton Township (T. 22 N., R. 11 E.), where the rock was discovered in plowing. Fig. 8. Galena dolomite in a quarry in NE. ^4 sec. 8, T. 21 N., R. 9 E„ showing typical massive beds which weather to thinner strata. Thickness. At no place is more than 60 feet of this formation ex- posed. It is so uniform that individual beds cannot be correlated from one section to another. For this reason, a general section with closely estimated thickness cannot be built up from the limited exposures. The Platteville- Galena contact at the Rock River dam in Dixon is 680 feet above sea level. This surface cannot be traced southwestward underground because well logs do not distinguish between the Galena and Plattcville. It will be found at about 630 feet under the center of section 8, provided the dip continues the same as in the area northeast of Dixon. On this assumption of uniform dip, the Galena may be said to have a thickness of about 150 feet in this area. Southward the surface slopes approximately parallel to the dip and GALENA DOLOMITE 63 may be nearly the original top of the Galena, from which the overlying Maquoketa shales have recently been removed. The only evidence, how- ever, for this hypothesis is the parallelism between bedding planes and sur- face topography. The only identifiable horizons in the Galena are a zone of Receptaculites oweni about six feet above the Lowell Park member of the Platteville; a marked increase in chert about 30 feet above the base, and another Recep- taculites zone about 45 feet higher, or 75 feet above the base. Hand speci- -v;€>'-*;.:V^:\ *■ Fig. 9. Close view of exposure in figure 8, show- ing cellular structure produced by solution. mens from various horizons cannot be differentiated. The oil rock and interbedded shales which are conspicuous in northwestern Illinois are en- tirely absent in this area. It is probable that this absence of impervious beds favored the dolomitization of the formation, which is here much more complete than at places north and west. 37 87 Calvin, Samuel, and Bain, H. F., Geology of Dubuque County vey, vol. 10, pp. 402-411, 1900. Iowa Geol. Sur- 64 DIXON QUADRANGLE Areal distribution. The Galena is the highest indurated rock in this area and has been removed from the higher parts of the regional structure. Although the area underlain by Galena is less than that covered by any other formation except possibly the "New Richmond" sandstone, its out- crop is larger than that of any formation except the St. Peter and the Platte- ville. As shown on Plate V, the Galena covers most of the western part of the quadrangle. North of Rock River, its outcrop is dissected by numer- ous tributaries, and the dolomite is confined chiefly to the uplands. South of the Rock River and Chicago Road from Dixon to Lee Center, the Galena covers practically all the surface. The line between the Platteville and Galena, which extends southeastward from Dixon, is the least accurately mapped of all the boundaries on the map, for here the drift cover is heaviest and well records do not distinguish between the two formations. Pahontologic character. Because of the thorough dolomitization of the rock, the original fossil content is largely destroyed. Such forms as remain are usually preserved only as molds of coarse-grained dolomite and most of them cannot be identified. The following is a list of species found during this study. As in the case of the Platteville fauna, Dr. J. J. Gallo- way has checked and criticized the determinations. Fossils collected from the Galena dolomite in the Dixon quadrangle Amplexopora sp. Bellerophon troosti D'Orbigny Bucania nashvillensis Ulrich Fusispira angusta Ulrich and Scofield Hebertella sp. Holopea excelsa Ulrich and Scofield Homotrypa similis Foord Homotrypa sp. Hormotoma gracilis Hall Hormotoma major Hall Illaenus americanus Billings Ischadites iowensis Owen Liospira americanus Billings Lophospira augustina Billings Lophospira bicincta Hall Lophospira obliqua Ulrich Rafinesquina alternata Emmons Rafinesquina minnesotensis N. H. Winchell Receptaculites oweni Hall Rhinidictya sp. Rhynchotrema increbescens Hall Sinuites cancellatus Hall Sinuites cancellatus trentonensis Ulrich and Scofield Streptelasma corniculum Hall Strophomena trilobata Owen Trochonema umbilicatum Hall Vanuxemia wortheni Ulrich YOUNGER PALEOZOIC FORMATIONS 65 Correlation. The Galena is regarded as the Mississippi Valley equiv- alent of the Trenton in New York. "Trenton" was used for some time in this region as the name for the Platteville, and reports on Mississippi Valley geology which apply Trenton to a limestone or shale group are describing Platteville limestone and Decorah shale, or some part of them. Relations to adjacent formations. In this area, there is no evidence of any interruption of deposition between the Lowell Park member of the Platteville and the Galena. The dolomitic portion of the Lowell Park is similar in every way to typical Galena, and is repeatedly interbedded with less magnesian limestones which resemble the Buff member of the Platte- ville. So striking is this relation that Shaw 38 described the Lowell Park member as "beds of transition." Trowbridge and Shaw 39 have described and figured an erosional unconformity at the base of the Galena, but no evi- dence of such a relation was found in this quadrangle. Erosion has re- moved any rocks that formerly overlay the Galena in this area, and there is no evidence of the relation of the Galena to any higher beds, except the glacial deposits. YOUNGER PALEOZOIC FORMATIONS The Galena dolomite is the youngest indurated formation now known in this quadrangle. Above the Galena, the Maquoketa shales were origi- nally deposited. These outcrop in all directions from this area, except due north, and presumably were once present here. The Maquoketa is known in outcrops or wells in Polo, Woosung, and Nelson townships, west of this quadrangle, and in Harmon and Marion townships to the south. There may be remnants of the Maquoketa within the Dixon quadrangle in South Dixon or Marion Township, but if such beds occur they are buried beneath the glacial drift and no available logs record their presence. Certainly no im- portant amount of these shales remains. The Niagaran limestone probably once covered this area also, for its outcrops surround the Dixon quadrangle just as the Maquoketa exposures do, but at a greater distance. Weathering and stream erosion removed most or all of it before glaciation. Outliers of the formation may have persisted here until glaciation, just as "mounds" capped by Niagaran lime- stone outliers are prominent topographic features in the Driftless Area to- day. If there were such remnants, they were destroyed during the glacial invasions, and the Niagaran and Maquoketa rocks were ground up and mingled with other glacial debris in the till. Pebbles and angular frag- ments of these formations have been identified here, but may have been carried in from the northeast by the ice. 38 Shaw, James, Geology of Lee County: Geol. Survey of Illinois, vol. 5, p. 130, 1873. 39 Trowbridge, A. C. and Shaw, E. W., Galena and Elizabeth quadrangles: Illi- nois State Geol. Survey Bull. 26, pp. 54, 61, 1916. 66 DIXOX QUADRANGLE There is no evidence that later Paleozoic or Mesozoic formations ever covered the region. Cenozoic Group pleistocene system As shown on Plate I. glacial deposits cover most of the npland areas. Rock and Kyte valleys have heen deeply tilled by valley trains, which neces- sitated deposition in most tributary valleys also. rRE-ILLIXOTAX DEPOSITS Xo pre-Illinoian soils are now exposed beneath the till. Where the base of the till is not weathered, the surface rock normally is fresh, and in many places is polished. An augur-boring beside the Lincoln Highway a mile and a half east of Dixon encountered residual soil on the Galena dolo- mite, under six feet of calcareous till. This was a dark red-brown, com- pact, coherent clay, such as is typical of residual soils from limestone. Wells in Ashton Township (T. 22 N., R. 11 E.") and one in sec. 18, South Dixon Township ^T. 21 X.. R. 9 E.) have penetrated the Illinoian till and encountered a forest-soil bed containing decayed tree fragments and yielding a dark-colored, disagreeable-tasting water. In each case the well was abandoned, and no evidence was obtained of an underlying till, if one exists. The character of the material does not. as popularly supposed, in- dicate pre-Illinoian swamps at these places, for any buried plant matter may assume this apparent swamp-debris character after long standing in a saturated condition in or under the till. No clear proof is known of X^ebraskan or Kansan glacial deposits in this area. A few thoroughly weathered glacial boulders occur in unleached and unoxidized light yellow or blue Illinoian till. So complete is the weathering of some of the boulders that they crumble readily between the lingers, although they had originally been gabbro, diabase, greenstone, and granite. The sides of the boulders are faceted, striated and ground smooth by the ice. Subsequent exposure developed concentric weathering bands in the basic rocks, paralleling the glaciated faces. These boulders accord- inglv represent an earlier period of glaciation, in which the fresh rock was abraded to its present form : they were weathered during one or more in- terglacial epochs, and while frozen solid, were picked up by the Illinoian ice and incorporated in the till of that epoch. Although the boulders indi- cate a pre-Illinoian glaciation. there is no satisfactory evidence that they were in this area before Illinoian time. If they could be gathered by the ice, they could be transported an indefinite distance, but the large number of these boulders suggests that they have not travelled far and accordingly suggests pre-Illinoian glaciation of this region. ILLINOIAN AND IOWAN (?) TILLS 67 ILLINOIAN AND IOWAN (?) TILLS Introduction. Covering most of the uplands is a typical glacial till, con- sisting chiefly of material from the local limestone and sandstone formations and the soil which formerly covered the area. Mixed with it is a consider- able amount of material which may belong to formations entirely destroyed by the glacier, such as fragments of the Niagaran limestone. As outlined above, this formation possibly outcropped in this area in rounded hills, as it does today in northwestern Illinois, and southwestern Wisconsin, although it may have been entirely removed before the coming of the glacier. If the latter is the case, the Niagaran fragments found in the area came from the east. Less important than either of these sources, but much more prominent because of their unusual color and texture, are the igneous and metamorphic rocks from Canada. Such rocks attracted the attention of the first travelers across the prairies because they were so different from any visible masses of bed rock. In fact, the Indians first noticed the pres- ence of unusual boulders, and one striking example in Minnesota was singled out by them for special reverence, including offerings and ceremonial paint- ings with vermilion. The rock gave its name to the present village of Red Rock, Minnesota, although under its paint it is a gray granite. 40 In a series of careful counts of boulders and pebbles in the till, the average showed 12 per cent of crystalline rocks. If it were possible to study all sizes of material, this proportion would be greatly reduced, for the soft sediments of the Paleozoic contributed a much larger proportion of the sand and rock flour than they did of the pebbles and larger material. Conversely, of the large boulders, a much larger percentage is crystalline, for the sedi- ments break rather easily along their stratification planes and do not form large masses. A list of the different types of crystallines found includes practically every common type of rock. There are granites, syenites, dio- rites, gabbros and peridotites of the coarse-grained igneous rocks ; rhyolites, andesites and basalts of the lavas ; and gneisses, schists, quartzites and slates of the metamorphic series. The red rock in Dixon Park is red felsite porphyrite, very similar to much of the lava in the Michigan copper dis- trict ; and the gray gneiss in Oregon, marking the site of the Lincoln-Douglas debate, is typical of the Grenville series of Ontario. A petrologist ac- quainted with the Ontario crystallines could find specimens of most of them in the unique collection assembled about the Reed home in Watertown. An unusual constituent of the till is coal, which is found in quarter-inch pebbles at several places in the area, notably in a ravine in sec. 16, South Dixon Township (T. 21 N., R. 9 E.). ^"Keating, W. H., Narrative of an expedition to the source of St. Peter's River, performed in 1823, vol. 1, pp. 200, 263, 281-288. Quoted by Martin, Lawrence, riiysi- ography of Wisconsin: Wisconsin Geol. and Nat. Hist. Survey Bull. 36, p. 73, 1916. 6S DIXOX QUADRANGLE The till when deposited was a blue-gray, sticky clay, such as is seen today in the beds of Fivemile Branch where it crosses the south line of sec. 27, South Dixon Township and Threemile Branch in sees. 19, 21 or 14. Blue till is commonly found in well drillings, and less often, in cellar exca- vations. On exposure, the till is oxidized to a light yellow-brown, and the car- bonates are dissolved from the limestone pebbles and powder. Fresh till is popularly called boulder clay because weathering has not removed any of its original constituents. Weathered till is less coherent; the limestone boulders have been dissolved, and many of the crystalline pebbles have dis- integrated more or less. The Illinoian till commonly develops a rough cleavage or fracture on weathering which divides the till into well-denned blocks. The Illinoian till was spread out in a remarkably smooth plain, cover- ing all of this area. It had little of the "knob and kettle" topographv often shown by fresh till. Those areas which have surtered least erosion since glaciation are today the areas of least relief. Only two. small undrained depressions are known in this original surface. They are in sees. 10 and 11. South Dixon Township, and are so shallow that they are not represented on the topographic map. The Iowan ( ?) till forms a low. rolling ridge across the southern end of the quadrangle. The northern side of this ridge was taken as the approximate limit of Iowan ( ?) till, since the relations of the two tills are masked everywhere by a heavy cover of loess. As a result of its mode of deposition, the thickness of the till originally varied from the thinnest possible deposit to more than 1S5 feet. A well in the XE. : _ sec. "29. South Dixon Township, penetrated 1ST feet of till with- out reaching bed rock, but within both a mile to the east, and a mile and a quarter to the west, rock outcrops through the till. Depth of till shown in deep wells in the southern part of the area is indicated by tables 3 and 1. Average depth of glacial till can be estimated from well logs, but these give only a minimum estimate of the total depth of hlling. Averages of depths to rock in the wells over a given area give an idea of the total amount of tilling, but again the ligure is too low. because any well that stops short of rock reduces the average below its proper value, and there are few deep wells in the area, as water is commonly obtained within 75 feet of the sur- face. The maximum known depth of Illinoian till tilling is 1ST feet. South of the Chicago and Northwestern Railway, the depths of 39 wells in the Illinoian till area are known. The average depth to rock, or to the bottom of the well where rock was not reached, is 65 feet. Assuming an average thickness for the loess of 6 feet ever this area where erosion of the till lias been slight, the Illinoian till must have averaged at least 60 feet in thick- Ill the area of Iowan ( ?) drift, seven wells encountered rock at an ILLINOIAN AND IOWAN (?) TILLS 69 average depth of 75 feet, and twenty- four wells that did not pass through the till have an average depth of 78 feet. If the loess was 6 feet thick on the average, the combined Illinoian and Iowan ( ?) till sheets averaged at least 70 feet in thickness. Originally the till covered practically the entire area, but it has been Table 3. — Wells not reaching rock in the southwestern part of Dixon quadrangle Location of well Part of sec. NE. % NE. % SW. corner .... SW. 1,4 ... SE. corner SW. 14 Center E. line.. Center N. line.. W. line NW. % W. line NW. % SW. cor. SE. y± sw. 14 sw. 14 Center W. line . . NW. corner . . . . NW. corner NE. corner .... NE. 14 NW. 14 NE. 14 NW. corner .... SW. 14 NW. 14 NW. corner .... NW. % N. line SE. 14 . . SE. 14 NW. 14 NE. 1/4 Center W. % . . . SW. 14 sec. T.N. 19 29 29 32 32 33 33 34 34 34 34 35 36 3 3 3 4 5 5 3 3 4 4 4 6 31 31 32 32 33 35 21 21 21 21 21 21 21 21 21 21 21 21 21 20 20 20 20 20 20 20 20 20 20 20 20 21 21 21 21 21 21 R. E. 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 10 10 10 10 10 10 10 10 10 10 10 10 Total depth Feet 58 187 40 40 90 + 90 40 70 78 80 97 130 50 43 90 45 65 103 100 + 103 83 72 30 96 75 66 75 40 90 75 103 removed from a strip roughly three miles wide along Rock River and from similar areas adjacent to the larger streams. As shown on Plate I, till al- most entirely covers the southern third, and about half covers the eastern third of the area. In the central and northwestern areas, it has been eithcr removed entirely or greatly reduced in thickness. 70 DIXON QUADRANGLE Age of the till. The till of the quadrangle belongs to at least two dif- ferent epochs, the Illinoian and the Iowan (?). There is only slight evi- dence of a pre-Illinoian drift, as cited on page 66. The Illinoian covers the greater part of the area, the Iowan (?) being confined to the extreme southern part. The separation was made by Leighton after a broad study of northwestern Illinois, on the basis of (1) the degree of weathering and erosion of the till, (2) the relations to the loess, and (3) the existence of a low marginal ridge in this quadrangle, limiting the area of fresher drift. These lines of evidence are considered in order. 41 1. Sixty-six borings, made within the Iowan (?) area of this quad- rangle and to the east and south to the outwash along Green River, showed Table 4. — Deep wells in the southwestern part of Dixon quadrangle with depth to rock only Location of well Part of sec. Center SE. corner . . . SB. corner . . . NW. !/4 SW. % Center E. line. NE. % NW. 14.. NE. 14 NW. % . . SW. % NE. corner Center W. line. . NW. cor. SW. 14 E. line SE. & . . . S. line SW. U.. sec. T.N. R. E. 20 21 9 20 21 9 25 21 9 28 21 9 29 21 9 35 21 9 36 21 9 36 21 9 36 21 9 3 20 9 5 20 10 29 21 10 30 21 10 30 21 10 31 21 10 Total depth Feet 50 60 65 37 48 45 100 110 102 60 135 100 140 105 50 an average of 3.9 feet of soil and leached loess-like silt over 1.3 feet of leached till. In contrast with this, 24 borings in the area of Illinoian drift immediately to the north and west, which succeeded in passing through the loess and into the leached till beneath, gave an average of 5.6 feet of soil and leached loess or loess-like silt, and 5 feet of leached till, totaling 10. 6 feet. The borings were made on flat spots or surfaces so gently undulat- ing that surface wash would be a negligible factor. 2. In the younger drift area, in cases where the loess is thick enough so that the base is calcareous and till was encountered below, the till showed 41 A summary of the data for northwestern Illinois is given by Dr. M. M. Leighton, in Journal of Geology, vol. 31, 265-281, 192:?, but specific data on this area were contributed by him from his field notes. ILLINOIAN AND IOWAN (?) TILLS 71 no leaching but contained limestone pebbles to the top. This of course proves that deposition of the drift was followed immediately by deposition of the loess. North of the margin of the ridge, on the Illinoian plain, several widely separated borings revealed an old soil or humus muck sep- arating the unweathered loess from the weathered till. In other places, an old, brown, loess-like silt, 2 to 3 feet thick, non-calcareous and containing manganese pellets, was also found beneath the yellow loess and on the un- derlying Illinoian till, but was not encountered in the area of shallow leaching. 3. The ridge which bounds the younger drift area is gently undu- lating and has a glacial contour, and while not of great relief like the Bloom- ington moraine to the east and south, it is high enough to be shown on the topographic map with a 20-foot contour. The average height of the ridge is about 30 feet. It is composed primarily of till, although some gravel deposits occur, as in the west half of sec. 4, Marion Township (T. 20 N., R. 9 E.). Boulders are fairly numerous south of the Kelley School, but elsewhere the mantle of loess-like silt seems to be sufficient to cover them. A low terrace of sandy gravel occurs along Fivemile Branch about half a mile south of the Kelley School, but elsewhere there is but little evi- dence of outwash which is in keeping with the absence of kames. The amount of erosion shown by the ridge is small. Earlier workers 42 mapped the drift of most of this quadrangle as Iowan, with less than a square mile of Illinoian till exposed in the ex- treme northwest corner of the quadrangle. The Iowan drift was de- scribed as overlying the Illinoian in many hills. Railroad cuts were noted where Leverett believed the Iowan was superimposed on Illinoian till. In a later publication, Leverett 43 mapped all the Dixon area as Illinoian. That map was generalized and the glacial history of the Dixon area was not germane to the discussion, so that possibly it was not intended to classify all the drift of this area as Illinoian. In 1922, he 44 reaffirmed his earlier differentiation of the two tills. After a careful study of the field evidence, the writer believes that the till in the central and northern parts of the area, which Leverett and Hershey differentiated into Illinoian and Iowan, is actually all contempora- 42 Leverett, Frank, The Illinois glacial lobe: U. S. Geol. Survey Mon. 38, pp. 131-139, 1899. O. H. Hershey is cited as first making the differentiation, but no reference is given. 43 Leverett, Frank, The Pleistocene of Indiana and Michigan: U. S. Geol. Sur- vey Mon. 53, pi. 5, 1915. 44 Leverett, Frank, Discussion of paper by M. M. Leightcn, Further data on the differentiation of the glacial drift sheets of northern Illinois: Bull. Geol. Soc. Amor., vol. 33, p. 116, 1922. 72 DIXON QUADRANGLE neons, and is of Illinoian age. The reasons for this conclusion are as follows : 1. Both the supposedly Illinoian and Iowan tills of Leverett and Hershey are covered by loess, which they regarded as Iowan. In each case, the till beneath the loess is leached to an average depth of five feet and oxidized to about ten feet, even where the base of the loess is calcareous. If the loess is Iowan and covers Iowan till, the till should not be greatly weathered beneath any calcareous loess, but the Illinoian till should show marked weathering. If the loess is Early Wisconsin, the Iowan till should be much less weathered than the Illinoian. As stated above, there is no difference in the amount of weathering of the supposedly distinct tills. 2. The erosion-cycle age of the Iowan should be less than that of the Illinoian. Instead of being more maturely dissected than the Iowan. as it should be under Leverett's hypothesis, his Illinoian till is less eroded. Con- sidering the greater distance of the Illinoian till of Leverett from the Rock River, it ought to show less erosion, if. as the present writer be- lieves, the two till deposits are contemporaneous. 3. Leverett reported exposures which he regarded as showing two tills in cuts along the Chicago and Northwestern Railwav in Dixon and along the Chicago, Burlington and Ouincy Railroad east of Stratford. The present writer was unable to confirm this observation. In addition to study of the exposures, trenches were cut with a spade through the sod and slumped material to the original, undisturbed, glacial deposits in each cut. Augur borings were also made at these cuts, and like the trenches, showed weathered loess, fresh loess, leached and oxidized till, fresh till, and solid rock, in most instances. In two cuts there is no calcareous loess ; m one. the loess lies directly on the Galena limestone. Waste from the cuts overlies the loess in several places, but is readily recognized as arti- ficially deposited, by the absence of a soil and weathered zone at the top. L Gumbotil occurs on a fiat till plain in the southeast corner of the XE. % sec. 14. T. 21 X.. R. 10 E.. and has been found by M. M. Leighton at a number of points east and northeast of this quadrangle within Lever- ett's "Iowan" area. Gumbotil is a fine, sticky or gummy gray clay, practicallv free from pebbles, which forms on the surface of the till under extreme weathering. It is characteristic of the Illinoian till surface, but has not been reported on the Iowan. Presence of gumbotil does not prove that there is no Iowan in the region, but its repeated presence on uneroded areas is difficult to explain on the assumption of later deposition of the Iowan. Much more gumbotil would probably be found, if the heavy loess mantle did not conceal the till surface. For these reasons, it is believed that the Iowan ice-sheet did not in- vade the region here mapped as Illinoian, and that its effects in this area are principally confined to the possible production of the loess. GRAND DETOUK ESKER 73 On the other hand, the age of the moraine along the southern boundary of the Illinoian area is not so definitely determinable. The uncertainty about this age is emphasized in a recent paper. 40 The greater thickness of the loess, the soil zone underlying the loess, and the deep weathering of the till all distinguish the Illinoian till from the Iowan (?). The depth of leaching found by Leighton was somewhat less in this drift than in the Iowan deposits of Iowa but possible differences in rainfall, drainage, ground-water level, and original calcareous content make it difficult to compare leaching in such widely separated areas. All available evidence indicates that the formation of loess in this area was continuous (p. 76), and the greater thickness of loess over the Illinoian till shows that part of the loess deposition preceded the accumu- lation of the Iowan (?) till. Heavy loess deposition accompanied and im- mediately followed the Iowan glaciation in Iowa, but some also followed the recognized Early Wisconsin glaciation in Illinois. 46 GRAND DETOUR ESKER Besides the till of the ground moraine just described, the Illinoian glacier formed an esker in Dixon and Nachusa townships as indicated on Plate I. In addition to its own interest, the esker is important because it forced Rock River to the northwest making the "grand detour" which gave its name to the neighboring village. The esker consists of well- washed sand and gravel containing pebbles up to two inches in diameter. As is to be expected from its origin, bedding planes in the esker were largely destroyed in the slumping which followed the melting of the ice. Stratification is preserved in at least four places : in gravel pits in the NW. ]/ A sec. 31, T. 22 N., R. 10 E., and NW. % sec. 23, T. 22 N., R. 9 E., and in prospecting pits in the NE. *4 sec. 25, and the NE. )/\ sec. 36 of the same township. In each case, the bedding planes dip from 15° to 20° in a direction varying from due west to N. 75° W. The maximum thickness of gravel exposed is 9 feet, which was shown in the pit in sec. 31. This face did not extend to the bottom of the gravel. The esker extends northwest from the vicinity of Emmert School, Nachusa Township, paralleling Chamberlain Creek on the northeast, and terminating on the Dixon-Grand Detour Road, northeast of Bend School, Dixon Township. It forms a well-marked ridge in parts of sees. 14, 15 and 25, Dixon Township. Between sees. 14 and 25 it crosses a high sandstone hill where it is represented only by scattered gravel, and in the valley of western sec. 24, Dixon Township, it has been destroyed by erosive work 45 Leighton, M. M., The differentiation of the drift sheets of northwestern Illi- nois: Jour. Geology, vol. 31, p. 280, 1923. 46 Cady, G. H., Geology and mineral resources of the Hennepin and La Salle quadrangles: Illinois State Geol. Survey Bull. 37, p. 83, 1919. 74 DIXON QUADRANGLE and later filling of Rock River and its tributaries. The southeastern end is not well defined, but it originates in a region of sand and gravel with confused stratification northeast of Nachusa. This esker makes the high dam of gravel blocking a preglacial valley about half a mile wide, and 100 feet deep in the S\Y. Y\ sec. 14, Dixon Township. This is the largest unfilled preglacial valley which has not been reoccupied and reexcavated by a stream. Although the gravel is unconsoli- dated and easily eroded, it was high enough so that when drainage was established along what is now Rock River, the water flowed around the "grand detour'' and adopted a course five miles longer than otherwise would have been necessary. If the gravel were removed, a 20-foot flood today would send water through the old channel. LOESS Lithology. The loess of this area consists of mineral and rock par- ticles intermediate in size between sand and clay. Except in the coarser material, the naked eye cannot distinguish individual grains. Uniformity in composition, color and size of grain characterizes the loess. It generally has no bedding, and the only structure is a vertical parting, which is due to the fact that the silt enclosed vertical plant stems as it accumulated or which has developed on drying faces. Because of this parting, the loess stands for many years in vertical cliffs without serious slumping or crum- bling. Scattered fossils are found in the loess which were buried in the accumulating silt. Roughly ellipsoidal calcareous concretions, formed by circulating ground water, have grown in the loess to a maximum diameter of an inch. Where fresh, the loess is blue-gray and so highly calcareous that a drop of hydrochloric acid causes the surface to swell and puff up sharply as the carbon dioxide gas escapes. On exposure, the iron content is oxidized and the carbonates leached out, leaving a buff to yellow-brown, sticky, plastic, fine-grained clay, similar to the residual soil from a limestone. Leached loess in this area differs from limestone soils chiefly in its large content of quartz fragments. Oxidation and leaching of the loess extend five to six feet below the surface on uplands ; where erosion has occurred, the present depth of leaching naturally is less. Loess has been regarded as a silt, blown from desert areas, from flood- plains of streams carrying their normal load or burdened with out- wash from glaciers, or from glacial-till areas recently uncovered by a retreating glacier; as an outwash product of a glacier which settled in standing or slowly running water, or as a product of earthworms. Each suggested process has undoubtedly produced loess in some places. In this quadrangle, the loess is a rock Hour formed by crushing limestones and shales with lesser amounts of sandstone and crystalline rocks. The silt has not resulted from weathering or attrition by wind or running water. Glaciers supplied the material, and it must have been spread over the up- lands by the wind, for some of the grains show a slight rounding off of points and edges, such as would result from wind transportation. There is no evidence of lakes from which the material could settle, no elevated land to furnish shores for the lakes, and none of the lamination which lake clays commonly show. Streams that flooded this area and deposited the silt would have left large bodies of sand and gravel intermingled with the finer material, and, unless the general slope of the area was greatly re- duced, the streams would have flowed too fast to deposit the loess. Earth- worms bring much fine material to the surface, but they seem quantitatively incapable of producing so thick a body, since their operations are chiefly confined to the two or three feet of soil immediately below the surface. In addition, the underlying till is deeply weathered, while the overlying loess, where thick, is calcareous and unoxidized. No source for the loess is ap- parent in the subjacent till. The upland loess is accordingly believed to be eolian, although some of the loess on the lower slopes of valleys may be a product of slope-wash from the main body above. Topographic expression. The loess mantles the till and has probably softened its contour considerably. Loess areas are notably smooth or very gently rolling. Along roadsides and other excavations, the loess stands in vertical faces, but similar natural exposures are found only where erosion is rapid, as on the outside of curves of upland streams. Thickness. The average thickness of loess in this area is about six feet, but it is known to reach a maximum of 15 feet in the cement quarry north- east of Dixon. The loess seems somewhat thicker immediately east of Rock River, than either west of the river or several miles east. Undissected loess- ial areas west of the river are so limited that this suggestion cannot be tested by enough measurements to prove it. Area. Loess overlies all the undissected, till-covered area and also covers some rock in the uplands where till is missing. Where vigorous erosion is now in progress, there is little loess on the lower slopes, but in older valleys, loess mantles the slopes and sometimes the flood-plain. Some of this loess has been deposited by the wind since the valleys were formed ; but probably more of it has been contributed by creep and slope-wash from the uplands. Age. Loess is accumulating to a slight extent in this area today, but by far the greater part of the Mississippi Valley loess is either late Iowan and early Peorian, or Early Wisconsin. Before the loess was deposited, the Illinoian till was leached to an average depth of live feet, which is practical- 76 DIXON QUADRANGLE ly the amount of leaching of the Iowan ( ?) till and its overlying loess to- gether. Since the loess on the Illinoian till is not separable into older and younger portions, it must have been deposited without any interruption which permitted important weathering ; also since loess overlies the Iowan (?) till, which is unweathered where the base of the loess is calcareous, loess deposition must have followed immediately after the retreat of the Iowan (?) ice-sheet. So far as the Dixon area supplies evidence, the loess could be entirely of Iowan and early Peorian age. This is in accord with the evidence in nearby areas. Thick loess deposits underlie the Early Wis- consin till in Bureau County, 47 25 miles south of the quadrangle. At a point 17 miles northeast of this area, Leighton 48 has described thrusting of a rock ledge by the Early Wisconsin ice-sheet over fossiliferous loess. After a study of the relations of the loess to the Iowan drift in north- eastern Iowa, Alden and Leighton 49 concluded that the great bulk of the loess in that state is early Peorian in age, although some was probably de- posited during the closing stages of the Iowan. The evidence for the inter- glacial date is the existence of fossil land-snail shells in the loess, which Shimek has found are chiefly the species that live in the climate prevailing today in the Upper Mississippi Valley region. EARLY WISCONSIN VALLEY TRAIN A fairly well-developed valley train of Early Wisconsin age fills Kyte River valley. Much material of the same age is undoubtedly present in the valley train of Rock River, but it is not separable from the Late Wiscon- sin valley train. When the Early Wisconsin glacier reached its maximum development, its terminus in Kyte Valley was about 10 miles east of this quadrangle. Water from the melting ice carried well-washed sand and gravel into the river. Overloaded, the river filled its valley 18 feet above the present stream level with clean, sharp sand, angular gravel and rounded pebbles ranging up to two inches in diameter. All kinds of rock are repre- sented in the filling, but limestone predominates. Kyte River has removed about 35 per cent of the train which originally stood above the present stream level. No information as to the depth of the filling is available, but it probably extends 60 to 75 feet below the river level, just as does the gravel under Rock River. The remainder of the valley train forms high, well-drained, fertile terraces along the length of the river in this quadrangle. (See Plate I.) 47 Leverett, Prank, The Illinois glacial lobe: U. S. Geol. Survey Mon. 38, p. 187, 1899. 48 Leighton, M. M., in Bretz, J H., Geology and mineral resources of the Kings quadrangle: Illinois State Geol. Survey Bull. 43, pp. 239-241, 1923. 49 Alden, W. C., and Leighton, M. M., The Iowan drift: Iowa Geol. Survey, vol. 2G, p. 158, 1917. EARLY AND LATE WISCONSIN VALLEY TRAINS 77 LATE WISCONSIN VALLEY TRAIN The Green Bay lobe of the Late Wisconsin ice-sheet overran the divide between Green Bay and Rock River basins and advanced southwestward to Janesville, Wisconsin. Water from the melting ice swept u tremendous quantity of sand and gravel into Rock River. This filled the channel, raised the river level and finally filled the entire valley with well-washed, roughly stratified sand and gravel. A similar valley train forming in Mississippi River tended to pond Rock River, causing still further deposition, until the channel of the Rock was aggraded to the level of the Mississippi. The valley train consists chiefly of sand, with less than 15 per cent of gravel over one-fourth inch in diameter. Some pits have no gravel, but most exposures contain flat-topped lenses of gravel, ranging up to three inches in diameter. The sand is very clean and fairly sharp. The grains that are rounded are either limestone or else the "frosted" sand from St. Peter or lower sandstones. All the gravel is rounded, as is natural after being rolled more than 50 miles. Limestone pebbles are most abundant in spite of their softness, for probably 98 per cent of the original gravel was limestone, and much of it has not yet been crushed and eliminated by trans- portation. In spite of its porosity and susceptibility to weathering, at no place is the valley filling leached and oxidized to a greater depth than two feet. The following section is typical of the valley train, although probably none of its members continues for a quarter of a mile. Section of a gravel %>it in West Dixon in the SW. % NE. % sec. 6, T. 21 N., R. 9 E. Description of strata Thickness Feet Dune sand and loam Sand, with some interbedded gravel. 12 Sand, 0.05 to 2 mm. in diameter; thin, nearly horizontal laminations.. 6 Gravel and sand, up to one inch in diameter 10 Sand, 0.5 to 1.0 mm., with thin, perfect cross beds dipping 12° SW 7 Gravel and sand, up to 1.5 inches in diameter; bedding indistinct 8 River level Total 43 Like the Early Wisconsin valley train in Kyte River, this filling forms high, dry, prominent terraces which extend along the entire course of Rock- River through this quadrangle. Originally, the surface of the train must have been nearly flat, but Rock River in removing it has sculptured it into many terraces. (See p. 16 and fig. 1.) The terraces are not systematic, but represent purely accidental interruptions in the side-swings of the en- trenching river. As shown on Plate I, the valley train has an average width of half a mile across the quadrangle. 78 DIXON QUADKANGLE Originally, the valley was filled about 45 feet above present stream level. Depth of sand and gravel below the stream is uncertain. Several holes drilled just below the Illinois Central bridge in Dixon penetrated more than 60 feet of sand and gravel, indicating a total thickness for the original train of approximately 105 feet, if these test holes were located in the form- er main channel. BACKWATER DEPOSITS Deposits of alluvium of local origin formed in valleys tributary to Rock River as a result of the growing valley train. Just as Rock River had to aggrade its channel as fast as Mississippi River filled its valley, so Pine, Chamberlain and Franklin creeks were forced to raise their beds in order to flow into the Rock. Consequently, these creeks deposited sedi- ment in their upper courses and gradually built up to the level of Rock River. It is probable that their lower courses were temporarily ponded and that delta deposits from Rock River extended up into these streams, for cross-bedding dipping upstream is exposed along Pine Creek in a ter- race north of the farm house in the NE. % SE. Y^ sec. 3, Grand Detour Township. A similar situation exists near the southwest corner of sec. 24, Dixon Township. The backwater deposits are well-bedded sand, car- bonaceous silt and clay, and they now form distinct terraces above the present alluvium. A similar deposit resulting from the Early Wisconsin filling of Kyte River is found along the unnamed stream in sec. 30, Pine Rock Township. RECENT SEDIMENTS FLOOD-PLAIN ALLUVIUM All permanent streams in this area have alluvial flood-plains on which they deposit their surplus load in flood time. During low water, the stream follows a channel a few feet below the level of the alluvium. This al- luvium has been deposited in greatly increased quantities within the last 75 years. As a result of the breaking of the prairie sod, rain water runs away more quickly over the surface and can erode the soil more vigorously than before. In deep stream channels in the alluvium, a zone of very black, carbonaceous silt can be seen one to three feet below the present surface. Above this, the silt is uniformly lighter in color, usually more sandy, and often laminated with several black soil zones. The surface al- luvium, similarly, is lighter in color than this distinctive lower horizon which marks the alluvial surface before agriculture commenced in this region. PEAT AND MUCK In the lower courses of Kyte River and Franklin Creek, peat and vegetable muck are accumulating over considerable areas, as shown on Plate I, gradually filling in depressions which formed behind the Rock RECENT DEPOSITS 79 River valley train. Backwater deposits did not fill these depressions and the sluggish streams bring in little sediment. The slow process of vegetal deposition is destroying these marshes and forming exceedingly rich farm land. SAND DUNES Small sand-dune areas are indicated on Plate I in sees. 27 and 33, T. 21 N., R. 10 E., and along Rock River in sees. 21 and 22, Dixon Township, sees. 11, 12 and 13, Grand Detour Township, sees. 5, 6 and 7, Taylor Township and sees. 20 and 21, Nashua Township. The dunes near Temperance Hill School (T. 21 N., R. 10 E.) have formed on the sandy surface of the Iowan ( ?) till. Other dunes nearby are too small to be indicated on the map. The north side of the dune area in sec. 33 has been overpastured, vegetation has been destroyed, and some sand moves during high winds. With this exception, the dunes are dead. The dunes along Rock River are formed chiefly of sand from the valley train but contain a much larger proportion of St. Peter-type sand grains than does the valley filling. Probably they have received an ap- preciable addition of sand from the St. Peter outcrop upon which all of them rest, except those in sec. 7, Taylor Township. CHAPTER IV— GEOLOGIC HISTORY Introduction The history of the area is partly written in the rocks described in the preceding chapter. Many portions of the record, however, are in- complete, and others are ambiguous or present knowledge does not furnish the explanation of existing features. Pre-Cambrian Eras Because no rock of this age has been found in place in this area, the history must be inferred from evidence in neighboring areas. In Wisconsin, a series of pre-Cambrian sedimentary rocks is known. They were intruded by various igneous rocks, folded, compressed, and metamor- phosed., forming what has been called the ''crystalline complex''' of igneous and metamorphic rocks. Upon it long continued erosion developed a par- tial peneplain which has been traced by surface exposures and wells south- ward across Wisconsin, as described in Chapter III. Presumably, a similar series of metamorphosed igneous and sedimentary formations underlies the Paleozoic rocks of this area. In northern Wisconsin and northeastern Min- nesota, a series of Keweenawan red sandstones and shales covers the crys- tallines, and it has been suggested that rocks of this series have been reached in deep wells in Dixon. The evidence was considered in the last chapter and reasons were stated for believing that, although Keweenawan rocks may underlie this area, they have not been proved by any drilling up to this time. Paleozoic Era CAMBRIAN PERIOD The red color of the Keweenawan sediments is due to the iron oxide which thev contain and indicates that little or no organic matter was de- posited with those sands and clays. Similarly, the pink and. in a few places. red color of some of the Croixan sands found in this area indicates that the sand was deposited with little included organic matter. At present, the absence of life and the red color seem to suggest desert conditions. The fact that many of the very small sand grains are either rounded or "frosted" offers further indication of their origin in a barren area where wind trans- portation was dominant, where soft and cleavable minerals were ground to dust and blown away, and where hard minerals other than quartz were elim- inated by thorough chemical weathering, which, followed by sorting, pro- SO GEOLOGIC HISTORY 81 duced a pure-quartz sand. Some of the sand may have come from the crystalline rocks in this area, but much of it was probably carried from the northern Wisconsin and Canadian crystalline areas by southward-flowing streams. 1 From the study of Cambrian deposits in other areas, it is known that the ocean advanced from the south or southeast. As the shoreline moved northward, at least the upper part of this well-prepared sand was worked over, washed and deposited in strata on the ocean floor. At certain times, less sand was laid down, either because streams did not supply it or be- cause the water was too deep to permit the waves to roll it to this area, and the shells of animals and calcareous structures of plants accumulated, forming lime muds. These lime muds were changed to dolomite by sub- stituting magnesia for part of the lime either while still in the form of mud or after they had solidified to limestone. So, with alternating deposition of sand and of carbonates, possibly interrupted by withdrawal of the sea and exposure to the air, more than 1480 feet of sediments accumulated In the Cambrian sea on the present site of Dixon. ORDOVICIAN PERIOD LOWER ORDOVICIAN OR PRAIRIE DU CHIEN EPOCH ONEOTA STAGE According to the generally accepted geologic chronology, Croixan de- position was continuous with that of the Beekmantown epoch which is rep- resented in this area by the Prairie du Chien. The ocean water was clearer, but not necessarily deeper, and carbonate muds accumulated which consoli- dated to form the 150 to 200 feet of Oneota dolomite. Minor amounts of sand and clay were swept into the ocean and buried in the carbonate ac- cumulation. "NEW RICHMOND" STAGE The "New Richmond" sandstone supplies the first field evidence from this area. Up to this horizon the history has been interpreted from well cuttings and from outcrops in neighboring states. In the valley of Franklin Creek, the oldest formation outcropping in this state is exposed. Evidence has already been summarized which indicates that this outcrop is part of a bar formed on the Ordovician ocean floor. The sand is beauti- fully rounded into spheres and ellipsoids of limpid, frosted quartz. While it probably was polished and ground up by wind action on land, it clearly was deposited here by the ocean. It is apparently similar to the Croixan in its place and mode of origin, and manner of transportation and depo- sition, except that the "New Richmond" is thinner and less continuous. 1 Dake, C. L.., The problem of the St. Peter sandstone: Missouri Sch. Mines and Met. Bull., vol. 6, No. 1, pp. 216-218, 1921. 82 DIXON QUADRANGLE The coming of this sand may have been due (1) to an uplift of the land to the north, which enabled the faster flowing streams to carry sand into the ocean, (2) to an uplift of the ocean floor, permitting the waves to wash the bottom and roll sand to this area, (3) to the shifting of the mouth of a stream pouring sand into the ocean at a new location, or (4) to some change in general wind direction which either blew sand to the ocean or, by shifting the shore currents, changed the site of sand deposi- tion. In the absence of good exposures over a large area to the north, these possibilities cannot be investigated and the changes which actually took place cannot be determined. In all probability the "New Richmond'' simply represents a sandy phase of the Shakopee deposition. The bedding of the sandstone and limestone is conformable wherever the contact is exposed in this or neigh- boring states. At many places, the "New Richmond" is missing and the Shakopee and Oneota unite. No evidence of erosion has been reported from such places, and the absence of the "New Richmond" indicates that no sand deposits interrupted the formation of the more common carbonate and argillaceous muds. SHAKOPEE STAGE The Shakopee ocean was very shallow. Waves molded the muds into ripple marks. Fresh, soft sediments were repeatedly exposed to the air by low tides or oscillations of the ocean level. Upon drying out, the lime- stone checked and cracked open. The following mud layer not only cov- ered the surface, but settled down in the cracks and preserved their record. The thin-bedded, shaly limestones or dolomites solidified quickly and in many places were broken and disarranged before the overlying beds formed. What caused the crumpling that formed these "edgewise con- glomerates" ? Possibly cryptozoon reefs expanded too widely, became unstable and collapsed. Fragments of the algae commonly occur in the brecciated masses. Possibly dolomitization caused shrinkage and collapse of underlying layers. Wave action is not a probable explanation, for the breccia is not sorted, wave-worn or arranged in horizontal layers. Higher beds extend over the irregular masses and follow their outlines without fracturing, proving that these brecciated masses were formed during Shakopee deposition. The ocean was usually muddy. Clay settled with the animal structures, making all the dolomite very argillaceous. Sand was probably supplied from the same source as that of the "New Richmond", and sand grains were freely embedded in the muds or spread out in thin layers over the ocean floor. Conditions varied from place to place so that pure clay was deposited near an area where more agitated water was spreading sand over GEOLOGIC HISTORY 83 the bottom. As a consequence, individual beds cannot be traced long dis- tances and neighboring sections can be correlated only in general terms. At times, clay was deposited so rapidly that no organic matter ac- cumulated with it. Red and purple shales formed where the well-weathered, red, yellow or brown soil from the land areas was deposited by the ocean. When accumulation was less rapid, enough plant and possibly animal mat- ter was buried in the clay to react with its iron content and produce a blue, green or gray color. All the calcareous and fossiliferous strata originally contained enough organic matter to reduce the iron and to yield a light- colored rock. Gastropods, of which the snails are modern examples, crawled on the bottom, while peculiar calcareous algae built the laminated, hemi- spherical masses which were long so little understood that they received the name, Cryptozoon, meaning "hidden animal." There were other plants which left the twig-like fucoids as their record, and worms burrowed through the mud. All the chemical sediment of the Shakopee is dolomite. The usual origin of dolomite is the substitution of magnesia for part of the lime in un- consolidated sediments that otherwise would have formed limestone. The alteration of the Shakopee to dolomite was completed as deposition occurred. No evidence of later alteration, such as irregular or high porosity, solution channels or cavities, or especially thorough dolomitization along bedding or joint planes, is found in the fresh rock. The evidence is unusually clear in the brecciated masses, where the fragments are dolomite and are sharply separable from the enclosing dolomite matrix. If alteration followed the slumping, a blending of the materials and blurring of the contact would have resulted. The upper Shakopee was deposited after conditions became more stable, when the ocean was clearer, and less clastic sediment was accumulating. The thick, uniform beds of pure dolomite of the Clear Creek basin (sees. 4 and 9, Taylor Township) are typical of the sediments formed at that time. Then the ocean withdrew. Deep erosion and slumping or folding of the Shakopee followed. Exposures are too limited to determine the amount and extent of the folding. All of the folds and steep dips exposed could have resulted from slumping of the partially consolidated, deeply- eroded sediments. Such, for instance, might have been the origin of the little syncline shown in fig. 10. The anticline shown in fig. 11 has a dip of 45° on one limb while the other limb is flat. This dip is not dupli- cated in any of the outcrops in the vicinity. Shattering of the limestone without bending and the absence of slickensides indicate that the folding- was accomplished under very slight cover. There is no parallelism be- tween the attitudes of strata in adjacent exposures. Several nearby out- crops show irregular variations in dip similar to that of fig. 10. Appar- 84 DIXOX QUADRANGLE ently, the steep dip is purely local. The longest outcrop showing steep dip is located about a mile southeast of the National Silica Company's plant, in a ravine in sec. 9, T. 23 N., R. 10 E., where a well-defined bed dips 12° NE. throughout an exposure about 120 feet long. There are no Shakopee outcrops within half a mile with which to compare this unusual attitude. The two examples last cited have been described because thev are the only cases where the slump origin of the distortion seems at all doubtful. The writer believes that the deformation of Shakopee sediments is entirely a local and surface feature and without diastrophic implications, because (1) there is no system in the attitudes of various outcrops, (2) the folds cannot be traced from one outcrop to another, (3) the -bulk of the out- crops show practically horizontal strata, which would be improbable if regional folding had crumpled some beds, (-1) close folding of the Shakopee has not been found in any other area, (5) the formation is shown by its Fig. 10. Gentle folding of the Shakopee dolomite, NE. V± NE. V± sec. 33, T. 22 N, R. 10 E. contemporaneous edgewise conglomerate to have been very unstable, and (6) the deep erosion with steep cliffs favored such slumping. Valleys at least 65 feet deep were cut in the Shakopee. Xo cliffs are recognizable, but in two places in sec. 30, Oregon Township, outcrops prove that the contact slope must exceed 25°. Similarly, exposures in ravines on the west side of Clear Creek in sec. 4, T. 22 N., R. 10 E. prove a dip exceeding 20°. Contacts are poorly exposed because of slumping down of the St. Peter and because the sandstone is easily stripped away where the contact is unprotected. Where the St. Peter rests on the Shakopee there is no evidence of residual soil or weathered chert, such as have been found in other areas. In Franklin Creek a green clay lies at the contact, but it probably repre- sents post-St. Peter leaching of the dolomite, for it is not oxidized, does not grade up into the sandstone, is not granular as many residual soils are. GEOLOGIC HISTOKY 85 and analogous clays follow joints and bedding planes deep below the zone of oxidation in the Shakopee and Platteville today. A few fresh, angu- lar chert and rare dolomite fragments are found in the St. Peter within 50 feet of the steep Shakopee contacts, indicating that erosion was proceeding under water while the St. Peter was accumulating. Fig. 11. Sharp folding and fracturing of the Shakopee dolomite, 300 feet east of fold shown in fig. 10. (Photograph by G. H. Cady.) MIDDLE ORDOVICIAN EPOCH ST. PETER STAGE Following the erosion just described, the sea returned, advancing from the south over this surface of comparatively high relief, and the St. Peter sandstone was deposited on the ocean floor. Grabau 2 believed that the lower St. Peter was deposited during the 2 Grabau, A. W., Types of sedimentary overlap: Bull. Geol. Soc. Amer., vol. 17, p. 618, 1905. S6 DIXOX QUADRANGLE withdrawal, or regression, of the ocean which closed the Prairie du Chien epoch, and that the tipper St. Peter was spread over the area in the absence of the water and during its return. This was not the case in this area or in adjacent areas in Wisconsin, for dolomite pebbles and fragments would have been mingled with much sand in the valleys during the period of erosion, and unless all the sand were removed before the later deposition, a marked erosional unconformity would separate early and late sandstones. Neither of these phenomena is present. Like the "New Richmond" sandstone below, the St. Peter sand is well-rounded and frosted, but is more uniform in size. Its purity, rounded grain and thorough sorting are due to wind transportation, but its bedding is horizontal, and its cross beds are curved and never dip steeply. Near the middle of the formation (at the south end of the Grand Detour bridge). it shows oscillation ripple marks which are formed in standing water; it contains marine worm borings ; it carries glauconite ; and some of its sand grains are often '20 times the size of adjacent grains, which variation is much greater than that of dune sands. Accordingly, it seems clear that while the St. Peter was originally a dune sand, it was here deposited under marine conditions. The advance of the ocean was probably from the south and the sand was washed down the slope of the ocean floor as the shoreline transgressed northward across the crystalline rocks which previous- ly had supplied the sand for the Croixan and "New Richmond" strata. Pos- sibly the outcrop of the Croixan sandstone itself supplied part of the sand. 3 The sand buried the erosion surface on the Shakopee. gradually over- whelming the hills and reaching a thickness of nearly '200 feet in many places. Greater thicknesses occur outside this area at points not on the La Salle anticline. Where Shakopee hills project high into the St. Peter. the sand was often less than 40 feet deep. The La Salle anticline is the dominating structure of northern Illinois. Its growth began during St. Peter deposition and as a result the sand thins down to less than 40 feet along the crest of the structure near Franklin Creek. Waves swept the top of the St. Peter keeping it practically plane. Such a marked reduction in thickness indicates a corresponding uplift, and probably the La Salle anticline had been raised at least 100 feet before the deposition of the Glenwood shale. GLEN WOOD STAGE The St. Peter deposition closed with the influx of a great quantity of green clay from the northwest. The thickest described occurrence of this clay is in its type locality in northeastern Iowa, growing thinner to the east. Dake, C. I-.. The problem of the St. Feter sandstone: Missouri Sch. Mines and Met. Bull., vol. 6, No. 1. p. 21S. 1921. GEOLOGIC HISTORY 87 southeast and south, and practically disappearing in the Hennepin-La Salle area. At first the waves churned up clay and sand together, forming a transition zone between the two formations. As the Glen wood grew thicker, storms no longer agitated the bottom deep enough to reach the sand, and several feet of argillaceous, green, glauconitic shale were formed. Condi- tions attending the mud deposition were apparently most unsuitable for existing animal life, or at least there was no fauna in the region adapted to living in this muddy sea. Since clay furnishes ideal conditions for the preservation of shells and other structures, it is improbable they have since been destroyed by solution. The uplift of the La Salle anticline continued. Shallow water above the anticlinal axis favored wave work and prevented deep deposition of the clay. A thin bed of clay and sand accumulated on much of the axis, while along the western limb of the structure, clay was laid down to a depth of seven feet or more, carrying valuable potash in the form of glauconite. PLATTEVILLE STAGE The inflow of Glenwood mud decreased greatly and the ocean became clearer. Sand was washed across the ocean floor and mingled with the argillaceous muds. At first conditions for fossilization were not very fa- vorable, and an argillaceous, somewhat sandy, magnesian limestone resulted. Probably the sand came from the St. Peter exposure on the La Salle anti- cline, since the grains are identical. The sandy Buff limestone is thin and discontinuous over the anticline and the Blue limestone which first cov- ered the anticline is not arenaceous. The total uplift of the anticline by this time probably had amounted to at least 125 feet. Non-deposition and minor erosion on the anticline, with simultaneous heavy deposition on the adjacent lower parts of the ocean floor, reduced the topographic promi- nence of the anticline. A deficiency of 100 feet or more of the St. Peter, of 5 feet of Glenwood shale and of 15 feet of the Buff limestone indicates part of the total movement. There is no way to estimate the elevation of the anticlinal axis above the neighboring region, but probably it was slight, since the overlying beds always appear conformable with the Glenwood shale. The Buff was partly altered to dolomite before the deposition of the Blue. Like the Shakopee dolomite, it does not show shrinkage, alteration along structural planes, or high porosity. It was not dolomitized by water later circulating through the St. Peter, for where the Glenwood shale is missing, the Blue is often in contact with the sandstone, and silicification has resulted, but not dolomitization. Conditions favorable for the preservation of fossils, and probably an environment stimulating animal growth, produced the Blue limestone. Here 88 DLXOX QUADRANGLE in a gigantic funeral mound are heaped the remains of the myriad animals that thronged the Platteville ocean. Clear water alternated with muddy, for layers of shell limestone or compacted coquina an inch or two thick are separated by irregular, shaly laminae which are rarely half an inch in thickness. These interlayered materials do not form distinct, parallel beds, since they have warped and wrinkled greatly during compacting. On cliff exposures, weathering has etched away the shaly laminations, leav- ing small bodies of purer limestone in relief on a lumpy surface of thin, irregular, lenticular beds. The list of Platteville fossils indicates the variety of the animal life. Their number is proportionate to their variety. Every cubic inch of the fossiliferous Blue limestone contains fragments of one or more an'mal structures. They were buried in the mud of a quiet ocean, unmarred by rolling. Dolomitization has not affected them and they preserve their original features perfectly. From the 10-foot long orthoceras to the deli- cate, lacy bryozoa. they preserve the best record of the contemporary life that any formation in this area affords. Sponges were abundant at times, and the fine collection of Dr. Everett 4 was made in this area. A duplicate of much of this collection is pre- served in the Dixon Public Library. Brachiopods and gastropods are the most abundant forms, but representatives of all phyla except vertebrates and protozoa are found. Accumulation of all these forms with very thin interlaminated clays produced a mass of fossils which is now about 45 feet thick, but probably was more than twice as thick before compacting and con- solidating. The character of the deposits changed sharply; and slightly fossilifer- ous, very fine-grained, calcareous muds were deposited to form the Glass Rock. Fossils are comparatively rare in this rock. Animals or plants, or both, must have been present, however, in great quantity as this material accumulated, for fresh rock, when freshly broken, has a strong odor of petroleum. This distinctive odor was noted in only one other rock in the area — a dense limestone bed in the Lowell Park. Irregular, often branching, masses of dolomite penetrate the Glass Rock at all horizons. These masses have the size and shape of fillings of very large worm-borings, or the cavities left by decaying plants that were buried upright in the lime mud. They never contain fossils. Whatever their origin, these masses mark the beginning of the return of magnesian sedi- ments. Either the ocean floor rose above the surface, or, less probably, it rose high enough so that waves rubbed and scraped away the top of the 4 Ulrich, E. O., and Everett, Oliver, Descriptions of Lower Silurian sponj Geol. Survey of Illinois, vol. 8, pp. 253-2S2, 1890. GEOLOGIC HISTORY 89 Glass Rock. A slight erosional unconformity appears at some places be- tween this and the overlying Lowell Park member. Residual soil and worn, rolled chert and limestone pebbles are not found in this area, but mark the erosion line in a quarry in the north part of Ashton, three miles e:ist of the Dixon quadrangle. This unconformity has not been reported in surrounding areas, and it may be a local feature marking a temporary ex- posure when elevation along the La Salle anticline was more rapid than the gradual subsiding of the ocean floor over the whole Mississippi Valley. It does not indicate any important break in the sedimentation of this region. The Lowell Park member of the Platteville, formed in Decorah time, marks the transition from true limestone to the pure dolomite of the Galena. It is more argillaceous than either the underlying or overlying formations, and thereby indicates the muddy ocean in which it was formed. Streams poured great quantities of mud into the ocean to the northwest of this region, forming the thick Green Shales of Minnesota and the some- what thinner Decorah shale of northwestern Iowa; and the clay which was carried farthest from shore in the Dixon area mingled with the lime muds to make the argillaceous Lowell Park. At first the sediment was highly magnesian, being almost a dolomite and containing a moderate percentage of clay. The clay content of the ocean varied greatly during Decorah time, but increased, on the whole, to the end of the Platteville. Magnesian deposition varied inversely, the more argillaceous beds being less magnesian. Life in the ocean was less flourishing, or at least its remains are less numerous. A few new animals developed, notable among which is the coral, Columnaria halli, a typical fossil of Decorah time. Plants are repre- sented, for the first time since the Shakopee, by abundant fucoids in the more shaly layers. Dolomitization was completed in each bed before the next bed was deposited, for dolomite and limestone are here interbedded. and in general the lower strata of the Lowell Park are much more dolomitic than the upper ones, indicating that the magnesia could not have been introduced later by waters descending from the Galena. GALENA STAGE Without interruption, so far as is known, the clear waters of Galena time succeeded the muddy Decorah-Lowell Park ocean. Abundant plant material constitutes the oil rock at the base of the Galena in northwestern Illinois, but no trace of plant life, either as casts or in oil rock, appears in this quadrangle. In the beginning of the Galena stage, a clear ocean apparently inhabited chiefly by gastropods, corals, and sponges, deposited a lime mud which has recrystallized to form the characteristic coarse- 90 DIXON QUADRANGLE grained, porous dolomite. This type of sedimentation went on to the close of the sedimentary record in this quadrangle. The high porosity of the Galena suggests that it was altered to dolo- mite after consolidation. No other evidence for this suggestion is known in this region. Calvin and Bain 5 have presented evidence that the Galena in Iowa has been dolomitized since deposition and that the depth of dolo- mitization varied greatly at different localities. At times, sponges were especially abundant, covering the entire ocean floor and producing two dis- tinct horizons which can be used as markers for determining distance from the base of the formation. Again, the amount of silica deposited increased greatly and in the resulting chert the only Galena bryozoa of the quadrangle are preserved. Presumably, the silica was extracted from the water by animals or plants, for there is no sand or appreciable increase of other clastic sediment at this horizon. LATER PALEOZOIC RECORD Surrounding areas show that following the Galena stage, the ocean withdrew. When it returned, it was more muddy and the Maquoketa series of calcareous shales and limestones covered the area. In a clearer water, the Niagaran limestone formed above the Maquoketa. During the Devonian period, the relations of continent and ocean were entirely reversed, and the open sea lay to the northwest and land to the southeast. Black shales and near-shore deposits mark this period in Ohio, Indiana and southern Illinois. Limestone formed in clearer water at Rock Island, and in North Dakota and Manitoba. Probably, Devonian sediments once covered this area, but they have been entirely removed. There is no evidence of the presence of later Paleozoic formations in this area, although it is entirely possible that the coal fields originally ex- tended at least this far north. Cenozoic Era TERTIARY PENEPLANATION Aside from some thoroughly weathered gravels of probable Tertiary age, no sediments exist in this or neighboring areas to indicate the history of events from the Pennsylvanian to the Pleistocene. During this time all post-Galena sediments were removed from this area, and in the late Tertiary a broad peneplain developed over northern Illinois and southern Wisconsin. In the Cretaceous-Tertiary interval, several partial peneplains were cut in the Appalachian mountains. Similar surfaces may have been formed here; 5 Calvin, Samuel, and Bain, H. F., Geology of Dubuque County: Iowa Geol. Sur- vey, vol. 10, pi.. 407-412, 1900. GEOLOGIC HISTORY 91 but, if so, they were entirely removed in the development of the existing peneplain. Renewed erosion in the late Tertiary or early Pleistocene dissected the peneplain considerably before Illinoian glaciation. Since that interruption, further stream work has cut deep valleys, and along Rock and Kyte rivers the peneplain is entirely destroyed. If the glacial deposits could be stripped away and the valleys filled to the level of the rock in the intervening uplands, the peneplain would, be restored as a gently rolling plain rising from about 760 feet above sea level at the south to an elevation of 900 feet in the north. The following cross sections illustrate this surface. In a strip four miles wide across the southern end of the quadrangle, the highest rock points either outcropping or encountered in wells are an outcrop of Galena dolo- mite in sec. 28, South Dixon Township at 750 feet ; a quarry in the Galena in sec. 19, Nachusa Township with a top elevation of 780 feet; limestone, which is probably Platteville, in a well in sec. 28, Nachusa Township at 780 feet ; St. Peter sandstone in a well in sec. 29, Bradford Township at 778 feet, and a Platteville outcrop in sec. 32, Bradford Township at 785 feet. The rock surface shows a uniform elevation at its higher points of about 780 feet, while the thickness of the exposed beds is over 250 feet. A similar section two miles north of Dixon would show the Galena dolomite at 789 feet on Pennsylvania Avenue and at 780 feet in Rock River bluffs ; the vSt. Peter at 800 feet two miles east of the river and at 790 feet a mile east of Franklin Creek ; and in the next two miles, the Shakopee, St. Peter and Blue member of the Platteville all outcrop between elevations of 790 and 820 feet. Along this section the peneplain elevation is about 800 feet. Another east-west section a mile north of Pennsylvania Corners would show the Galena dolomite in a well at the southeast corner of sec. 19, Pine Creek Township, at 840 feet ; a mile and a half east, the Galena outcrops on the south side of sec. 21 at 8-10 feet ; east of Oak Ridge Road the Blue member of the Platteville outcrops at 860 feet on the south side of sec. 24; the wSt. Peter makes a remnant of the upland in sec. 22, Nashua Township at about 850 feet. In this section the peneplain surface is fairly well outlined at 850 feet above sea level. Martin has argued that the land forms of southern Wisconsin and northern Illinois are not peneplain remnants, but rather are the normal de- velopments of erosion on a series of gently dipping hard and soft forma- tions. His position may be summarized briefly as follows : The entire dis- trict consists of six formations gently dipping southward. Erosion remove the Potsdam (St. Croix), St. Peter and Richmond (Maquoketa) strata more readily than the Lower Magnesian (Prairie du Chien), Trenton 6 Martin, Lawrence, Physiography of Wisconsin: Wisconsin Geol. and Nat. Hist. Survey Bull. 36, pp. 63-70, 1916. 92 DIXOX QUADRANGLE (Platteville) -Galena, and Niagaran, leaving these formations outcropping in cuestas or unsymmetrical hills, each consisting of a nearly horizontal re- sistant formation underlain by a softer rock. Erosion of the non-resistant formation undermines the harder one. and makes one side of the hill a bluff, or steep slope, while the other slope approximates the dip slope of the upper formation (fig. 12, line A-B). These limestone formations are supposed to make three north- facing cuestas with a lowland, or valley lying on the northern side of each, where the sandstones and shales outcrop. Martin's cross sections 7 show that the tops of all the hills closely ap- proach a line, which in three dimensions would he a peneplain, rather than a series of steps such as characterize a repeated cuesta topography. The limestones capping the cuestas form wedge-shaped masses, due to the bevel- ing of this peneplain instead of maintaining their normal thickness nearly to the cuesta face and then thinning rapidly toward the bluff. In time the normal cuesta features of a belted coastal topography will develop and eliminate all trace of the Tertiary peneplain. The present upland of the Fig. 12. Diagram showing cuestas, AB, resulting from erosion of gently inclined rocks of varying resistance; a peneplain CD later beveling these formations; cuestas, EFGHK. developed by dissection of the peneplain; and cuestas, EMGNK, result- ing from complete removal of the peneplain. The Dixon area had reached stage EFGHK before glaciation. region is interpreted as a peneplain because, (1) the hills approach a single plane surface which occurs on all formations, (2) the limestones are bev- eled by this surface into wedge-shaped masses. (3) conversely, the back slopes of the cuestas do not approach parallelism with the dip of the strata, and (4) in the limited area of the Dixon quadrangle where there is less opportunity for error owing to long-distance projections, a plane surface ap- pears on both the cuesta-forming limestones and the less resistant sandstone. The age of the peneplain is commonly stated as Tertiary. There are no sediments in the region by which it can he precisely dated. Its age is determined in part by correlation with gravels in southern Illinois and in part by correlation with more distant topographic features. Op. cit., figs. 12, 14. GEOLOGIC HISTORY 93 Following the development of the peneplain, the region was uplifted relatively, and the rejuvenated streams dissected the old surface to a con- siderable extent before Illinoian glaciation. In areas of St. Peter outcrop the peneplain was largely destroyed and steep-sided valleys were developed, while in the limestone areas, wider and shallower valleys were eroded. The resulting topography was buried by the Illinoian till, and in most places can be reconstructed only from well logs and a few exposures where post-Illi- noian valleys intersect the preglacial valley systems. The Rock River valley has been greatly enlarged since the glacial invasion, as shown by the absence of glacial deposits from most of its walls and the rapid erosion it is now experiencing. In its present form, the valley presents a topography similar in most respects to that sculptured in the peneplain before glaciation (fig. 13). The comparison is not entirely accurate, for Rock Valley is largely underlain by one formation, the St. Peter, and accordingly it pre- Fig. 13. Rock River valley and the Tertiary peneplain. Looking northeast along Rock River from Castle Rock, SE. % NE. 14 sec. 19, T. 23 N., R. 10 E. Cultivated terraces of the valley train on either side of the river, timbered slopes of St. Peter sand- stone along the bluffs, and remnants of the Tertiary peneplain on the sky line. At the left, the peneplain bevels the Glass Rock member of the Platteville; in the center, the Buff member outcrops on the Devils Backbone and at the right, St. Peter sandstone and Buff lime- stone support the peneplain. In the distance, the peneplain may be seen north of the Oregon basin, where it cuts the Platteville and Galena formations. sents few examples of the limestone-bluff topography that pre-Illinoian ero- sion produced in many areas. In the Driftless Area in the northwestern corner of the State, the present surface resembles more closely the Dixon landscape of early Pleistocene time. 94 DIXON QUADRANGLE PLEISTOCENE PERIOD At the close of Tertiary time, climatic changes caused the development of the continental glaciers which characterized the Pleistocene (fig. 14). Of the five glacial invasions of the United States, at least two and pos- sibly three entered the Dixon quadrangle. Fig. 14. Map of area covered by the North American ice- sheets of the glacial epoch at their maximum extensions, showing the approximate southern limit of glaciation, the three main centers of ice accumulation, and the driftless area within the border of the glaciated region. (U. S. Geol. Survey.) PRE-ILLINOIAN TIME Weathered glacial boulders in the Illinoian till clearly represent an earlier glaciation and must have been incorporated in the Illinoian till while frozen. There is no positive evidence that the boulders were in the Dixon GEOLOGIC HISTORY 95 area when they were picked up by the Illinoian ice, and it is possible they were transported a long distance from the site of their earlier deposition and thorough weathering. Cady 8 described some till that is quite definitely older than Illinoian about 25 miles south of this quadrangle. Leverett 9 and Alden 10 discussed evidences of pre-Illinoian glaciation in neighboring areas, but had less definite evidence than Cady, and arrived at the conclusion which applies to the Dixon area, namely, that while there is much evidence suggest- ing a pre-Illinoian glaciation of this area, it is not proved. Certainly only the Illinoian and later invasions have appreciably affected the present topog- raphy and soils. ILLINOIAN GLACIATION The Illinoian ice-sheet advanced into this area from the southeast, overriding and displacing the drainage systems and burying the southern part of the quadrangle at some places more than 180 feet deep in till. Evidence of the movement from the southeast is found in glacial stria- tions which have been uncovered in the stripping of till from the Sandusky Cement Company's quarry northeast of Dixon. Seven sets of grooves or striations were found on the surface of the Platteville glass rock. Their direction varied from N. 75° W. to N. 84° W., averaging N. 80° W. "Stoss and lee" phenomena developed on a small scale by harder fossils show clearly that the movement was westerly, and not easterly. Further evidence of the northwesterly movement of the ice is given by the occur- rence of coal in the till, and the northwesterly dip of all cross-beds in the Grand Detour esker. The amount of erosion by the ice-sheet cannot be approximated because of lack of knowledge of the preglacial surface. Filling of valleys can be approximated if sufficient records of deep wells are available. It has been shown that the glacial till averaged more than 60 feet in that portion of the till plain which is still undissected. By filling valleys and eroding uplands, glaciation greatly reduced the relief and ruggedness of most parts of the area. Outcrops and wells in the southern part of the area indicate a marked smoothing of the surface by till deposition. The extreme case is presented by sec. 28, South Dixon Township, where a relief of at least 187 feet and an original slope of at least 250 feet per mile are shown by the buried rock surface. In no case where the original till plain is preserved, does it slope over TO feet per mile, although this slope probably was exceeded in some of the partially filled 8 Cady, G. H., Geology and mineral resources of the Hennepin and LaSalle quadrangles: Illinois State Geol. Survey Bull. 37, pp. 70-72, 1919. 9 Leverett, Frank, The Illinois glacial lobe: U. S. Geol. Survey Jlmi, 38, pp. L05-118, 1899. 10 Alden, W. C, The Quaternary geology of southeastern Wisconsin: U. S. Geol. Survey Prof. Paper 106, p. 153, 1918. 96 DIXON QUADRANGLE valleys which have since been re-excavated. A good example of the change in topography produced by glaciation is the contrast between the parts of Franklin Creek valley north and south of the Chicago and Northwestern Railway. The headwaters of the stream flow through a slightly dissected, gently rolling till plain. In the middle reaches of the valley. post-Illinoian erosion of the sandstones and limestones has developed below the till plain a rugged, ravine topography, which is probably similar to the sharper val- leys of the early Pleistocene surface. The climate changed: melting of the ice exceeded its late of accumu- lation ; the glacier dwindled, and its margin retreated. In the final stages of the glacier, the Grand Detour esker was formed. SANGAMON INTERGLACIAL EPOCH When the ice disappeared, fresh blue-gray till was exposed to the attack of various weathering processes. Carbonates were dissolved from the upper four to six feet of the till, while oxidation reddened its surface and changed its color to bull for an average depth of 10 feet. In extreme cases, where the ground-water table was low. oxidation extended as much as "2-3 feet into the till. Black soil is commonly present beneath the loess, showing that vegetation mantled the surface during this stage. A light- gray, sticky, thoroughly leached material, called gumbotil, resulted from the Sangamon weathering, and later was buried in many places by the lcess. In this quadrangle, it was found beneath ,8 inches of loess, which was calcareous at the base, in sec. 14. T. 21 X.. R. 10 E. Leighton has also found gumbotil at various points outside the Dixon area on the weathered surface of the Illinoian till. Many of the original depressions on the till surface were drained, and a stream pattern established which is essentially that of today. IOWAX GLACIAL EPOCH While evidence is not conclusive, it appears that during the Iowan stage, the ice invaded the Dixon area as a part of the lobe which pushed into the Green River basin from the east. In the opinion of Leighton. the Iowan sheet did not remain within the quadrangle long, for the terminal ridge which lies across the southern part of the area is only moderately developed and there are some spots northeast of Amboy where gumbotil is exposed, in striking contrast to the relatively fresh drift elsewhere. PEORIAN IXTERGLACIAL EPOCH Just as the larger part of the Inc.-- in northern Illinois and eastern Iowa appears to lie of late Iowan and early Peorian age. so does deposi- tion of the loess in the Dixon quadrangle appear to have begun soon after the margin of the Iowan ice-sheet commenced its retreat and to have con- GEOLOGIC HISTORY 97 tinued into Peorian time. Wherever the base of the loess overlying Iowan till is calcareous, the drift is fresh, unleached, and blue-gray, showing no signs of erosion or exposure. For a time, the rate of loess accumulation was comparatively rapid, and exceeded the rate of leaching, but eventually the relation was reversed, and leaching and oxidation began to produce the present weathered zone. Where the loess is more than five feet thick, its base is commonly calcareous, but above that, the calcium carbonate has been dissolved away. Erosion has removed much of the loess from the valley slopes and probably all of it from the valley bottoms. Loess formation is continuing at a slow rate today, but most of the silt is oxidized and leached, and the new loess is indistinguishable from the surface loam. WISCONSIN EPOCH The Peorian interglacial stage was brought to a close by the Wisconsin glacier which approached the area, but did not reach it. The Lake Michi- gan lobe of the glacier followed the depression now occupied by Lake Michigan and spread south and southeast into Illinois, covering the area outlined by the Bloomington moraine, which runs about 10 miles east and 18 miles south of the area. In late Wisconsin time, the glacier re-invaded the border area of Lake Michigan and developed the Green Bay lobe, which pushed down Green Bay and across the low divide into Rock River basin, finally stopping near Janesville, Wisconsin, about 75 miles (measured along the river) north of the Dixon quadrangle. When it entered Green Bay, it dammed Fox River and formed a lake which found an outlet south- west to Rock River. This new supply of clear lake water enabled the Rock to erode vigorously and remove much of the valley train that had filled Kyte River in Early Wisconsin time and extended down the channel of tbe Rock. As the glacier advanced, the lake grew smaller, until the ice overtopped the divide and water from the melting ice swept great quantities of sand and gravel into Rock River. The supply of material was too great for the stream to move and it deposited the surplus sand and gravel, forming the Late Wisconsin valley train. By continued depo- sition, Rock River built up its channel to keep pace with Mississippi River which was being filled with outwash from the Superior lobe. Deep filling at Janesville increased the gradient and consequently the velocity of Rock River, until equilibrium was established between the sup- ply and removal of outwash. The Wisconsin glacier melted away, and since that time Rock River has been removing the overload that it dropped to form the valley train. When the glacier retreated, Rock River was flowing through Dixon about 45 feet above its present level, and its total filling amounted to more than 105 feet. Tributary streams that carried much sediment built up to the level of Rock River, while others were ponded 95 DIXOX QUADRANGLE and much line sand was swept into these temporary lakes from the main river. POST-ILLINOIAN DRAINAGE DEVELOPMENT When the Illinoian ice-sheet melted away, it left this quadrangle nearly completely buried under an irregular, thick blanket of till and other drift. Many valleys, especially the smaller ones, were completely obliterated ; oth- ers were partially rilled or dammed with drift. Surface water collected in shallow depressions to form ponds and small lakes, or drained away along irregular, crooked courses. The new channels were independent in man}" places of the preglacial drainage, and in some instances utilized parts of two or more old valleys. Rains and melting ice and snow provided water which scoured out new channels. In no case is the present drainage materially different from that which developed on the withdrawal of the Illinoian ice. but marked departures from the pre-Illinoian system are known. In studying the drainage changes, trustworthy comparisons and con- clusions as to ages of valleys usually can be made only between channels in similar rock. Rapid erosion of the sandstone has developed many broad valleys since glaciation. while comparatively little erosion of lime- stone has occurred. Pine Creek illustrates the combination of old and new drainage. In sees. T. 5. 10. 11. and IT. T. "23 X.. R. 9 E.. small tributaries occupy broad, gentle-sloped, preglacial valleys, now partly rilled with till, while the main stream flows through a narrower, steeper-sided valley in sees. 9. 16. 22. and 27, which has been excavated since the Illinoian invasion, in the same limestone as the preglacial valleys. A depression in the surface and an absence of rock outcrops indicate that the old valley in the northern part of sec. 9 formerly continued southeastward through sec. 10 to the broad valley in sec. 15. The connection of valleys in sees. T. S. and IT is not clearly established, but absence of outcrops and occurrence of till in valley walls suggest a drainage line through sec. "21 to the main channel in northeastern sec. £8 and across the present divide into the southeastern part of the same section. The changes in this drainage system appear to have resulted from damming and filling of the valleys by till. A comparatively broad sandstone valley in sec. 22 and a similar one in sec. -4 of the township to the south (T. 22 X.. R. 9 E.) are not occupied, while the stream uses adjacent, narrower, younger valleys excavated in the same formation. The abandoning of these old valleys is not necessarily due to damming bv till, for it may have resulted from the rilling connected with the Wisconsin valley trains raising the stream until it flowed over narrow, low divides between tributaries of the main stream. If the stream still occupied this position when it resumed its downward erosion after the filling, the new courses could then have been established. T 99 ^.ock VI staj^ .6£ .oM MiTajj* ount uad- ation lstn- rec- 3wed 2 old and *nne- near com- Mis- River Qorth route nbian ^ban- 9 E., Z reek •esent tiwest bwest c. 21. flows ty of lown, earn?, The road River valley slopes irgely 3 and Creek rupied •1. XII, 98 and mud river. Whe complete] Many va ers were in shallo along irr many pla of two ( water wl material! Illinoian known. In 5 elusions - in simil; broad v; stone ha drainage occupy 1 while th sees. 9, ] in the i surface ; northern the broa 17 is no in valley in north part of have res A ( in sec. 4 while th same fc due to ( with th' narrow, stream after t! GEOLOGIC HISTORY 99 The most important drainage change in this region is that of Rock River. Plate IV is modified from Leverett 11 and the following account of changes in river course is a review of his work outside the Dixon quad- rangle. Within this area, the writer agrees with the earlier interpretation except as to Chamberlain and Pine creeks. From a careful study of the present drainage system, of the distri- bution of abandoned and till-nlled valleys, and numerous deep well rec- ords, it is known that before Illinoian glaciation, Rock River followed a course similar to the present one south as far as Rockf ord ; but the old valley leaves the present stream a short distance south of that city and continues on a southerly course to the present Illinois valley near Henne- pin. Leverett believes that preglacial Mississippi River turned east near Clinton, Iowa, joined Rock River northwest of Princeton and the com- bined stream flowed down the present Illinois valley to the present Mis- sissippi River, near Alton. Kyte River flowed east to join Rock River southwest of Rochelle. Pine Creek entered the Dixon quadrangle north of The Pines, but turned southeast from sec. 9 into sec. 10 along the route of the Chicago, Burlington and Quincy Railroad, south past Columbian School and then followed nearly the present course to Rock River. Aban- doned valleys in sec. 22, T. 23 N., R. 9 E., and sec. 4, T. 22 N., R. 9 E., have been mentioned. Sevenmile Branch was a tributary of Pine Creek from the west. Chamberlain Creek had a course similar to the present one to the Rock River, then flowed westward to a point a mile southwest of Grand Detour (sec. 14, T. 22 N., R. 9 E.) where it turned southwest through sees. 14, 15, and 22 to join Pine Creek near the middle of sec. 21. Preglacial Clear Creek drained much of the area that its successor flows through today, and probably joined Chamberlain Creek in the vicinity of Grand Detour. The exact course of Clear Creek is not definitely known, but the broadly open valleys, carrying till as low as the present streams, indicate a mature preglacial valley southeast of Tealls Corners. The stream probably flowed northwest into sec. 9, roughly paralleled the road southwest to sec. 8 and turned northwest again to the present Rock River valley. A tributary from the north may have been excavating a valley later occupied by Rock River, but the absence of till far down its slopes indicates that this valley has been greatly enlarged and perhaps largely developed since Illinoian glaciation. From the width, side slopes and occurrence of till near the flood-plain, it seems probable that Clear Creek valley was more mature before glaciation than was the valley now occupied by Rock River north of Taylor Township. 11 Leverett, Frank, The Illinois glacial lobe: U. S. Geol. Survey Mon. 38, PI. XII, pp. 484-492, 1899. 100 DIXON QUADRANGLE This statement differs from Leverett's only in that he believed that Pine Creek flowed east along the present course of Rock River to Chamber- lain Creek, and that the latter stream originally flowed southeast to Rock River through the southeastern part of the quadrangle. The absence of outcrops immediately southeast of Chamberlain Creek accords with his hypothesis, but recent drilling of water wells in Nachusa and China town- ships has shown the St. Peter sandstone within 50 feet of the surface across the postulated valley. From a point a mile south of Nachusa, east to Franklin Creek, the greatest distance between wells which have rock bottom is three-quarters of a mile and the lowest rock elevation is 750 feet, which is the depth of the sandstone in the NW. % sec. 16, T. 21 N., R. 10 E. (PI. IV). The top of the sandstone in the valley dammed by the esker at Grand Detour has an elevation of less than 680 feet. Till crops out for more than a mile in lower Pine Creek valley at 680 feet, and the buried valley partly exposed by Clear Creek in sec. 4 has its channel below present stream level, which is 720 feet. These streams, therefore, could not have flowed southeast along Chamberlain Creek valley. Positive evi- dence of western, rather than southeastern drainage from Grand Detour is offered by the esker-dammed valley. Rock surface at the eastern end of the valley is below 680 feet but above 665, while the western end of the valley does not contain rock outcrops, but the valley filling is less than 660 feet above sea level. The fact that Rock Valley narrows downstream does not indicate that the constricted portion is newer than that upstream, for all the wider parts of the valley in this quadrangle occur where the easily eroded St. Peter outcrops and all the narrow reaches are found where limestone outcrops in the valley floor. The same relation of valley width to kind of rock is even more strikingly shown in Pine Creek valley. Still further evidence is the fact that limestone is found at the Illinois Central Railroad bridge over Rock River in Dixon at 575 feet, or 175 feet lower than the supposed Pine Creek course southeast through Chamber- lain valley. Some of the depth may have been attained in the periods of erosion which followed the Illinoian glaciation and preceded the last Wisconsin period of valley filling. The limestone channel at the railroad bridge is twice as wide as the narrowest part of the postglacial channel of Rock River through the St. Peter sandstone, sec. 30, T. 23 N., R. 10 E. Since the sandstone is much more easily eroded than limestone, it seems probable that the limestone valley is largely preglacial. When the Illinoian ice-sheet advanced from the east, it filled preglacial Rock Valley east of this quadrangle with drift to a depth exceeding 300 feet in some places and forced the river to seek another route to the south. The ponded water rose until it flowed over the Leaf River divide into Kyte River, overtopped the divide to the Chamberlain Creek tributary, flowed GEOLOGIC HISTORY 101 southwest to the esker dam with an elevation of over 720 feet, crossed a low divide to the northwest into Pine Creek and followed its course south- westward to join Mississippi River, which had been pushed westward by the ice and did not return to its original location when the glacier melted. Kyte River valley was filled with glacial drift to the east, and found its outlet to the southwest with Rock River. It is the only clearly reversed stream in this quadrangle. RECENT HISTORY Since the deposition of the Rock River valley train, the age-old proc- esses of weathering and erosion have been active. The loess has been oxidized and leached; where till was close to the surface it has been similarly weathered, as was the Illinoian till before loess deposition. Leach- ing of the valley train gravels since Late Wisconsin time has extended 12 to 18 inches beneath the surface. The loess overlying the Iowan (?) till is leached to an average depth of 3.9 feet, but the figures are not strictly comparable, because the porous gravel permitted water to pass through it rapidly without accomplishing as much leaching as if it had stood longer in contact with the pebbles. In a few places, the top of the Galena dolomite has broken down to a red-brown sand of dolomite rhombohedra. There is no way to determine how much of this weathering is pre-Illinoian. The denser Platteville limestone rarely shows thorough postglacial weather- ing, but its surface is commonly shattered and hackly from frost attack and solution. With three exceptions, original depressions on the till plain have been destroyed by natural processes. Lakes and ponds occupied the deeper basins when the ice first retreated. Loess, accumulated plant matter and sediment brought in by streams have filled some of these, and erosion of the outlet by flood waters has probably drained a great many others. Two original depressions nearly filled with decaying vegetation remain south of the Chicago and Northwestern Railway in sees. 10 and 11, T. 21 N., R. 9 E. The largest swamp remnant on the upland is located in sees. 18 and 19, T. 22 N., R. 11 E. Depressions have been formed since glaciation by the piling up of sand dunes and by solution of limestone by ground water. Sand dunes have blocked a drainage line in sec. 5, T. 22 N., R. 10 E., forming a small lake; a quarter of a mile north in sec. 32, a sand-dune dam has produced another pond. These are the only bodies of open standing water in the area. Ground water dissolves limestone and produces caves and under- ground channels. Where the roof of a cave collapses, the surface falls and the resulting depression is called a sink hole. Less spectacular is the for- mation of a sink hole by slow surface subsidence as the rock immediately beneath the soil is dissolved. 102 DIXON QUADRANGLE The following table summarizes the data on sink holes in this area: Table 5. — Sink holes in the Dixon quadrangle Location Surface rock Number Diameter Depth Part of sec. sec. T.N. R. E. Feet Feet Galena dolomite 2 60 8 NE. cor 19 22 9 Galena dolomite 3 30-60 4-10 3E. y 4 26 21 9 Platteville limestone.. 25 10-100 5-18 8E. 14 27 22 9 NE. % 34 22 9 Platteville limestone.. 2 20-30 4-10 SE. 14 16 22 9 Platteville limestone.. 2 25 10 NE. % 35 23 10 St. Peter sandstone... 1 40 8 NE. % 25 23 9 Shakopee dolomite . . 2 20 5 SE. 1/4 30 23 11 The sink hole in the St. Peter undoubtedly results from solution of the underlying Shakopee. Southwest of Britton School (sec. 27, T. 22 N., R. 9 E.) the sink holes are elongated in a northeast-southwest direc- tion, parallel to the strike of the best-developed joint system of the Platte- ville limestone. Probably most of the holes are connected underground. One hole has a large opening leading into a channel three feet high which was followed a distance of 350 feet past the entrance of a second to a third hole. The channel was blocked by a rock fall a short distance beyond the third hole. Quarrying operations have destroyed Fuller's Cave, which once underlay the present site of the Sandusky Cement Company's quarry. Local tradition reports that this was once a bandits' rendezvous. None of the other sink holes are known to be connected with caves. Sand dunes have been heaped up near Temperance Hill School (sec. 27, T. 21 N., R. 10 E.) ; in three places on the valley train, namely, north of Grand Detour, southwest of Daysville and north of Prairieside School (sec. 7, T. 22 N., R. 10 E.) ; and also in three places on the Rock River bluffs — a mile northwest of Grand Detour, north of Prairieside School, and in sees. 20 and 21, T. 23 N., R. 10 E. All of these dunes have formed or moved since loess deposition was practically complete, for there is no loess superimposed. The sands show little rounding; and sorting and siz- ing of material are poor. Probably none of the dunes have moved a mile from the place of origin. Vegetation covers them completely, except where overpasturing has destroyed the plants and a little sand has recently been blown into gentle ripples. ASYMMETRICAL VALLEY SLOPES, STREAM DISPLACEMENT AND EXPOSURE TO SUN AND WIND An interesting feature of the drainage in this area is a tendency of the streams to follow the south and west sides of the valleys, and, by under- GEOLOGIC HISTORY 103 cutting, to make those sides steeper than the north and east sides. There are many exceptions to this statement, but where streams are not affected by rock outcrops, they usually obey this rule. It should be noted that the topographic map indicates the position of the flood-plains accurately, but the stream locations within those plains are generalized and this eccentric situation of the streams is not so marked on the map as in the field. At first, it was believed that this was an interesting case of Ferrel's law that moving bodies in the northern hemisphere tend to be deflected to the right. A south-flowing stream would accordingly be thrown against its right, or west bank, and an east-flowing stream against the south side of its valley. But north-flowing streams also follow the west side, instead of the east as Ferrel's law requires, and west-flowing streams are more often on the south than on the north sides of their valleys. Ferrel's law thus does not explain the situation. The north sides of valleys are more exposed to the sun than the south sides ; and the afternoon sun shines directly on the east valley wall when the air is warmer than in the morning. Consequently, the east and north slopes dry out more quickly than the opposite sides of the valleys. In Illinois the northeast wind is a cool rain-bearer, while the southwest winds are usually warm and drying, removing moisture from the north and east sides of valleys. Thus the wind and sun both tend to dessicate the north and east slopes, which consequently have poorer, sparser vegetation. With reduced plant protection and drier material, wind blows more dust from these slopes ; run-off finds the soil more accessible for erosion and meets less interference from vegetation, and creep is more active. All factors combine to erode the north and east sides of the valleys which have gentle slopes. The washed or creeping material on reaching the flood- plain tends to displace the streams to the opposite side of the valley. There- fore, relative exposure to drying conditions is believed to be the cause of the stream locations on the south and west sides of their valleys. Demangeon 12 has noted this greater erosion of the east and north sides of valleys in Picardy, France, and concluded that it was due to west- erly rain-carrying winds precipitating more water on the east and north sides of the valleys, so that slopewash eroded them more rapidly, built up the flood-plains on them, and forced the streams across to the south and west sides of the valleys. In Illinois, the rain-bearing winds are easterly, yet the erosion is most marked on the east and north sides. This fact suggests that the direction of rain-bearing winds is of minor importance. Differences of vegetation between north and south sides of valleys are well known and abundantly 12 Demangeon, Albert, La Picardie, Paris, p. 77, 1905. 104 DIXON QUADKANGLE evident in this area. Erosional differences normally are closely related to the amount and luxuriance of the vegetation. Accordingly, since the amount of drying exposure is correlated with the amount of erosion, and the correlation with wind-bearing rains is not shown in this area, the writer believes that the position of the streams and the gentle slopes of the north and east sides of valleys have resulted from the exposure of those sides to drying by sun and wind. HUMAN ACTIVITIES AND THEIR EROSIONAL EFFECTS At present, erosion is more rapid than it has been at any time since vegetation established itself after the last glacial invasion. Processes of weathering and erosion are the same as usual, but man has greatly aided the erosional effects of running water by destroying the prairie sod, clear- ing the timber, plowing annually, quarrying, draining the land and min- ing. So great has been the increase in erosion and transportation of ma- terial that the Rock and all its tributaries are silt-laden streams running through muddy channels in which carp thrive, whereas when the region was first settled, Rock River had a gravel bottom which could be seen at depths of four or five feet and the mud-loving carp was unknown. The resulting increase in silt deposition is described in the section on alluvium. Mining has had practically no effect on this area, since only one small prospect has ever been opened in it. Artificial drainage is somewhat more important, although its effects are less notable here than in swampy areas like the Inlet and Winnebago swamps of Green River. In the Dixon quadrangle, little land has been drained by ditches, so that drainage has not hastened the run-off. Tiling has changed much upland soil from a normally wet, sticky condition to a dry, granular substance of improved tilth, but at the same time a much more easily eroded material. Quarry- ing has hastened erosion in limited areas by steepening the gradients of wet-weather streams and by forming piles of wasted overburden, subject to easy erosion. Clearing of brush and timber from the slopes of Rock River valley and its tributaries has been more important. On these steep slopes much soil and disintegrated rock have been released by the removal of the gross vegetation, and erosion by slope wash, creep and landslide has been greatly facilitated. Accelerated erosion is noted especially where the St. Peter formation underlies the slope, for the sandstone soil is so dry that sod does not form readily. Gullies and ravines have been cut back into the upland as much as 250 feet as a result of clearing timber in the last 50 years. In some untimbered ravines along Rock River, the St. Peter sandstone is being removed rapidly while adjacent ravines which are well-timbered show little removal of soil and rock. Since the timber-land slopes are not GEOLOGIC HISTOKY 105 generally arable and serious erosion of valuable upland may result from clearing, the timber should not be taken off. From the standpoint of accelerated erosion, man's most important ac- tivities are the breaking of the prairie sod and the annual plowing. Early settlers tell graphic stories of the difficulties encountered in breaking the land for the first time. The heavy prairie sod was a thick, tough mat of living and dead vegetation with roots extending deep into the subsoil. This com- plicated tangle of grass and roots protected the entire upland from erosion. Run-off was sluggish, for water cannot flow rapidly through such vege- tation and the slow-moving water had neither the velocity to carry much load nor the opportunity to obtain a load because of the covering of plant fibers. Because the water flowed slowly, there was more opportunity for it to soak into the soil and less water ran off. Streams were not as full as now and hence were slower and less powerful in attacking their banks and beds. The larger run-in provided more water for plant growth during the dry season and insured a more steady flow of the streams. Moist soil is eroded less easily than dry and increased vegetation hinders erosion. The steady flow of streams favored vegetation along their banks and many creeks flowed on a bed of grass. In flood time, the streams removed all the load which was brought by slope wash. Except for those streams which were filled by or aggraded because of the Wisconsin valley trains, the flood-plains were small. When the sod was destroyed, these conditions were changed. The protecting cover was removed and run-off quickly eroded the soil and overloaded the streams. Run-off was more rapid and was greater because less water ran into the ground or evaporated. Greater run-off gave the streams greater velocity, and greater eroding and transporting power. In some places, slope-wash overloaded the smallest wet-weather streams and the resulting deposition formed a flood-plain ; the smaller streams carried too much silt and sand to the larger streams, and they overflowed their banks and aggraded their channels and flood-plains. In practically all the larger valleys these deposits are still being formed. During high water, the stream overflows its banks and spreads clay and silt over the surface. The grass blades rise through the mud, and the new deposit becomes a part of the alluvium of the flood-plain. The change in alluvial deposits may be seen in many gullies where the older fine-grained alluvium is black with plant matter, while the recent overlying silt is often sandy, and is brown instead of black, because its rapid deposition does not per- mit burial of enough plant matter to blacken it. The annual plowing produces results similar to those following the original breaking of the sod. It destroys the year's growth of vegetation and exposes fresh, loose soil to erosion. Much of the erosion can be pre- 106 DIXON QUADRANGLE vented by contour plowing instead of plowing straight furrows down steep slopes. Plowing around the hill provides a series of depressions in which the run-off must pause and deposit much of the soil it is carrying, whereas the furrow running down the slope forms a ditch or channel aiding the re- moval of the valuable black soil. Gullies develop on steep slopes and work back into the upland by ero- sion of the head of the gully. A vertical fall of five feet or more is not uncommon where the run-off from the upland enters the gully. The falling water undermines the steep slope and erodes it as it plunges down. A one- day rain cut a gully 40 feet back into the upland east of Ridge Road. Fur- ther development of such ravines can be checked only by preventing erosion at their heads. This may be done by sodding the gully slopes, by diverting water from their heads, by care in plowing, or by building a dam. A dam may fail because the water is allowed to fall over it, scouring out a pot hole below which may be so large that the dam itself will tumble into it, or be- cause the soil settles down from the uphill side of the dam and the wet- weather stream runs underneath the structure. Such failure can be pre- vented by making the water flow down an inclined concrete face, or by let- ting it fall upon a large and well reenforced concrete platform at the foot of the dam. Whatever process is used, the purpose is to prevent erosion at the danger-point, which is the head of the gully or ravine. CHAPTER V— STRUCTURAL GEOLOGY General Statement The structure of the Dixon area is controlled by the La Salle anticline, which extends through the quadrangle from the center of its north side to its southeastern corner, and by the Savanna-Sabula anticline, which extends east- west immediately north of the quadrangle. Cady 1 has described these features in their general relations to the structure of the State. The present discussion is an elaboration and slight modification of his work in the Dixon area. Throughout most of its length, the La Salle anticline is asymmetrical, with a much steeper and longer limb on the west than on the east. In fact, the eastern limb dips so gently that in mo;-t places the anticline is properly describable as a monocline, using this term in its original sense of a steeply dipping series of strata connecting two nearly flat series. From Freeport to the oil fields in Crawford and Lawrence counties, the anticline trends about S. 20° E. The Savanna-Sabula anticline has been described along Mississippi River by McGee, 2 Savage 3 and Carman. 4 Cady 5 has traced it eastward through the Oregon Basin, which, for the most part, lies immediately north of this quadrangle, although a portion occupies Kyte River valley and Rock River valley north of Devils Backbone. The basin is a result of the erosion of the dome formed at the intersection of the axes of the La Salle and Sa- vanna-Sabula anticlines. Tertiary peneplanation exposed the St. Peter throughout the basin, and subsequent erosion has removed much of the sandstone, leaving the outward-dipping Platteville limestone standing in an encircling series of hills. This escarpment includes Devils Backbone, and its continuation southeastward across Rock River to Lighthouse Point, near Lighthouse School, and then eastward to the south branch of Kyte River. The remainder of the basin rim lies north and northeast of this quadrangle. Structure-contour Map The structure map (PI. V) indicates graphically the detailed structure of the quadrangle by means of contours drawn on the original surface of 1 Cady, G. H., The structure of the La Salle anticline: Illinois State Geol. Sur- vey Bull. 36, pp. 89-179, 1920. 2 McGee, W. J., Pleistocene history of northeastern Iowa: U. S. Geol. Survey Eleventh Ann. Rept., pt. 1, p. 340, 1891. 3 Savage, T. E., Geology of Jackson County, Iowa: Iowa Geol. Survey, vol. 16, p. 640, 1905. 4 Carman, J. E., The Mississippi Valley between Savanna and Davenport: Illi- nois State Geol. Survey Bull. 13, p. 10, 1909. 5 Op. cit., p. 132. 107 108 DIXON QUADRANGLE the St. Peter sandstone. These structure contours are similar to topo- graphic contours, each line passing through points of equal elevation on top of the formation. Where lines are crowded the dip or slope is steep ; where they are more widely separated, the dip is more gentle. The vertical inter- val between the structure contours is 20 feet. A dip of one degree amounts to 92 feet per mile and hence is represented by approximately four and a half intervals in a mile. Over almost all of the area, the dip is less than 40 minutes, or 60 feet per mile. Elevations of the St. Peter surface were determined at the outcrop of the St. Peter-Glenwood contact. These are represented on the map by triangles with surface elevation stated. Eleva- tions determined by well records are indicated by circles and elevations. Where the elevation of the St. Peter-Glenwood contact is known, the ele- vation of this surface is stated ; where the well was stopped in Platteville limestone, the elevation of the bottom of the well is given with a minus sign, showing that the St. Peter is some distance below ; where the well entered the St. Peter immediately beneath the till, the elevation of the present top of the sandstone is followed by a plus sign to indicate that the original sur- face of the St. Peter was higher than the present rock. In a few cases, approximate elevations were calculated from the surface exposure of some higher bed, and these are represented by the quarry symbol and the calcu- lated elevation of the St. Peter with a minus sign to indicate that the for- mation is at least as low as the elevation given, or a plus or minus sign is used to call attention to the fact that the figure is only approximately cor- rect. In addition to the points shown on the map, the following elevations also were taken into consideration. Table 6. — Elevations of top of St. Peter sandstone at points outside the Dixon quadrangle used in preparing structure map Location Elevation Feet City water well, Sterling, sec. 22, T. 21 N., R. 7 E., about 11 miles S. 70° W. from Dixon -33 Outcrops in NW. 14 sec. 33, T. 21 N., R. 11 E., about a mile east of Hart School 800 Outcrops in SW. % sec. 4, T. 22 N., R. 11 E., about three-fourths of a mile east of Prairie View School 745 Outcrop in roadside cut, NW. % sec. 2, T. 23 N., R. 10 E., about 1.2 miles northeast of point where Rock River enters quadrangle 745 Outcrop on hill, NW. % sec. 9, T. 23 N., R. 11 E., about a half mile northeast of NE. corner of quadrangle 790 Structure contours are broken where their positions are inferred or doubtful; the lines are solid when they are known to be practically accurate. STRUCTURAL GEOLOGY 109 Structure of the Quadrangle The crest of the La Salle anticline enters the Quadrangle annroximatelv the limestone along both the axis and the western limb of the anticline, leav- ing the eastern flank exposed as an eastward-dipping monocline. The west dips of the limestone are confined to the SW. Y\ sec. 26 and not unnaturally Op. cit., p. 11! 108 DIXON QUADKANGLE the St. Peter sandstone. These structure contours are similar to topo- gram* • ~~~ u 1 ™ ^occino- thrnnp-h ooints of equal elevation on top oft the: val to ' hal 40 det rej tio: W va lin sh th of fa ai hi la n u r< a Outcrop on hill, jnw. ■% sec 6, x. -«. _. northeast of NE. corner of quadrangle, Structure contours are broken where their positions are inferred or doubtful; the lines are solid when they are known to be practically accurate. STRUCTTTKAL GEOLOGY 109 Structure of the Quadrangle The crest of the La Salle anticline enters the quadrangle approximately on Rock River, extends southwestward almost to Ridge Road, turns south and runs past Grand Detour, and then turning southeastward, continues to the southeast corner of the quadrangle. From this axis the strata dip about 60 feet per mile in a generally S. 60° W. direction. From a shallow bi- furcated syncline near Pennsylvania Corners, one arm extends southeastward toward Grand Detour and the other northeastward past Columbian School to Devils Backbone. The southern branch is not marked by pronounced disturbance of the strata and is recognizable only by study of the elevations of the St. Peter. The northern syncline is more evident in the field. In sec. 16, T. 23 N., R. 9 E., southwest of Columbian School, local dips to the northwest as high as 14° are found on the south side of the syncline, and dips ranging up to 5° toward the south occur on the north limb. So far as outcrops show, none of these attitudes are persistent for more than 200 feet, and the structure map indicates that the steep slopes flatten out within a short distance. From the angle in the La Salle anticline crest, near Grand Detour, a broad plunging anticline or nose extends westward down the flank of the main structure. A minor syncline parallels this nose on the south, while the better developed Pennsylvania Corners-Grand Detour syn- cline lies to the north. The east flank of the La Salle anticline is less regular. The high area on the St. Peter surface in sees. 22 and 23, T. 22 N., R. 10 E., has been referred to in the discussion of the St. Peter-Glenwood contact. The sandy character of the overlying limestone and the thinning out of limestone beds against the sandstone indicate that this was an original irregu- larity of the St. Peter surface and is not a result of later disturbance. The depression west of Lighthouse Point in sees. 26 and 27, T. 23 N., R. 10 W., may also represent an original surface irregularity, but there is no evidence for this suggestion. It is possible that a good exposure in this basin would show a greater thickness of the Buff member of the Platteville, but the out- crop is so heavily covered by till slumping from above that this possibility cannot be checked. A narrow anticline with strong dips extends southeastward from Devils Backbone, the highest part of the structure. Cady G referred to this feature as a monoclinal fold, as it appears to be in sees. 22 and 23, T. 23 N., R. 10 E. Erosion by the unnamed creek immediately to the west has removed the limestone along both the axis and the western limb of the anticline, leav- ing the eastern flank exposed as an eastward-dipping monocline. The west dips of the limestone are confined to the SW. T /\ sec. 26 and not unnaturally 6 Op. cit., p. 118. 110 DIXON QUADRANGLE were overlooked in the general study of the anticline in its larger aspects. The east limb of this structure flattens out in the mesa in the center of sec. 23. Probably the structure rises northeastward toward the center of the Oregon Basin, but erosion by Kyte River has destroyed all evidence. The small anticline is prolonged southeastward by a broad, flat area east of Carthage. The most striking feature of the whole quadrangle structure is the compressed syncline in the southwestern part of T. 23 N., R. 11 E. and adjacent areas. This structure is so sharply divergent in strike and form from the other areas of the quadrangle that it was very carefully studied in the field. The north flank of the syncline makes the south side of the Oregon Basin, as already described. For a short distance, very steep dips occur, amounting in one case to 30°. The dip decreases even across a 50-foot quarry face and averages only 2° from the escarpment face to the valley on the south. The south limb of the syncline is not well exposed, but its position is fairly closely controlled by the Platteville outcrops in east- ern sec. 36, the St. Peter-Glenwood contacts north and east of Carthage, and the outcrops north of Prairie View School in sec. 5, T. 22 N., R. 11 E. Origin of the Structure The following experiment will show how the folds of the area were de- veloped. Place a moderately heavy, but not stiff, cloth on a table, warp it into a gentle anticline extending N. 20° W. to represent the La Salle anticline, and place weights, such as books, on the sides to hold the fold in place. Leave a few inches of the cloth free at the north end, and under this end insert a ruler on an east-west line to represent the Savanna-Sabula anticline. Now raise the ruler vertically two or three inches, being careful to keep it horizontal. If this is done carefully, a sharp syncline represent- ing the Prairie View syncline will form on the east, the axis of the anticline will be bent, so that south of the syncline it will trend farther west of north, and a high point will develop on the axis north of the syncline, correspond- ing to the Devils Backbone which is apparently the high point of the Oregon Basin. On the west side of the anticline one or more synclines will appear, analogous to the Pennsylvania Corners synclines and a plunging anticline which corresponds to the Grand Detour nose will extend westward from the point of maximum displacement of the anticline. The first event was the formation of the La Salle anticline, perhaps, as Cady 7 suggests, by movement along a deep-seated fault. This structure has repeatedly been a zone of movement, as was stated in Chapter IV. Cady has also pointed out that the La Salle and other areas show similar evidence of movement at several times. Second, there was an uplift along the Savanna-Sabula axis. This may easily have resulted from thrusting of Op. cit., p. 179. STRUCTURAL GEOLOGY 111 the Wisconsin land mass against the younger sediments of the Illinois basin. Such thrusting necessitated a crumpling of the earth's strata, if it was to be accommodated. When a crumpling uplift occurred along an east-west line, the beds between the old anticline and newly uplifted fold were compressed, and in yielding formed a syncline. They were already bowed down and continued yielding in the same way. In compe- tent rocks, folding may be localized along the line of earliest strain-relief, and so produce a compressed structure, like the Prairie View syncline. As the east-west syncline developed, it thrust against the side of the old La Salle anticline. The old anticline was already under linear compression; and with the application of the side thrust by the syncline, the earlier structure yielded by permitting displacement of its axis to the west. As the old anticline failed, it thrust against its western limb at the point of failure and formed a plunging anticline extending down the west limb from the newly formed angle in its crest line. This subsidiary western anticline is the plunging anticline or nose which runs northwestward from Grand Detour. Subordinate synclines formed between the plunging anti- cline and the east-west uplift on the north. There is no evidence which closely defines the time of cross-folding. It was post-Galena, since that formation is affected equally with the others, and was pre-Tertiary, for the Tertiary peneplain bevels all the struc- tures. Pre-St. Peter Structure At several small outcrops in a valley tributary to Franklin Creek (NW. % sec. 33, T. 22 N., R. 10 E.), Shakopee dolomite is exposed with very steep dips and widely varying strike. Another outcrop in sec. 9, T. 23 N., R. 10 E. shows a 12° dip to the northeast for about 120 feet. It is possible that these beds indicate pre-St. Peter folding, but the writer regards them as more probably a result of slumping incidental to the deep erosion that preceded the St. Peter. The very large Shakopee area in Franklin Creek shows no disturbance and several exposures near the steeply dipping beds in the Franklin Creek tributary are not much distorted (fig. 11). Other large outcrops one, three and four miles north, show no dip exceeding 5° and the principal outcrops west of Rock River are of flat- lying strata. If pre-St. Peter folding occurred, it failed to disturb most of the strata, and its character and amount are indeterminable. Faulting The only fault recognized in the area is located in the SE. % NE. T 4 sec. 16, T. 23 N., R. 9 E., on the south bank of Pine Creek. It strikes N. 25° W., dips 70° W., and has a normal downthrow on the west of 30 inches. CHAPTER VI— MINERAL RESOURCES General Statement The preceding chapters have discussed the origin and history of the rocks of the area. In this chapter, the value, distribution and amount of the mineral resources will be considered. Mineral resources are use- ful rocks or useful substances that can be obtained from the rocks. The demand for most substances determines their selling price, and the value of most mineral resources depends upon the difference between the price obtained and the cost of production. The value in place is small, for supplies are generally abundant, and the selling price is only a little greater than the cost of production. For instance, the cost of coal delivered in a bin consists chiefly of costs of mining, transportation and marketing, and of profits on these operations. In the ground, the coal commonly sells for less than 20 cents per ton. In one sense, soil is a mineral resource, for it is a mixture of fresh and decayed rock matter with plant substances, but because it is the basis of agriculture and is inextricably connected with plant and animal growth, it is not usually considered in discussions of mineral resources. In this area, the principal mineral resources are water, cement mate- rials, limestone and its products, glass sand, and building sand and gravel. Potential resources of potash occur and unimportant traces of natural gas, petroleum and some ore minerals have been reported. Of the resources now developed, ample and cheap supplies are available for all industries, ex- cept cement making. The area contains an abundance of cement materials, but most of this cannot be secured at a reasonable cost. Water SURFACE WATER Both surface and ground waters are important resources in most regions. Water is essential to all forms of life and a steady supply of pure water is one of man's fundamental needs in any place. None of the sur- face waters within the quadrangle is important for drinking or manu- facturing purposes. Most of the streams are dry, or nearly so in late summer. Rock River and five of its tributaries, Kyte River, and Clear, Franklin, Chamberlain and Pine creeks, are the only streams that can be depended upon for a flow exceeding 10 gallons per minute throughout the 112 WATEE 113 year. Their waters are so turbid, because of the overloaded condition resulting from the present rapid erosion, that they could not be used for domestic or boiler purposes without expensive treatment. The only water-power utilized is obtained from Rock River at Dixon, where Platteville limestone outcrops in the present bed of the stream and rapids have developed. Probably North Dixon stands on the filling of the old stream channel, for bed rock is over 60 feet lower within a mile, both up and down stream. The limestone affords an excellent foundation. Additional power could be developed by a dam near Grand Detour. Like the Dixon property, the head would be low, for Rock River falls an average of only 0.9 feet per mile and the dam in Oregon is not 20 feet above normal river level in Grand Detour. GROUND WATER In this area, ground water is of economic importance as the source of shallow and artesian wells, springs and water for plant growth. WATER FOR VEGETATION Growing plants receive an adequate quantity of water throughout the area, except in sand-dune areas and on the higher outcrops of the St. Peter sandstone. In these places, the ground- water table may be so deep that only very hardy or long-rooted plants can live. Because of its high porosity, the St. Peter contains abundant water near the level of the streams, but the ground-water table is nearly horizontal and lies far below the hill tops. Where glacial till still covers the surface, there is abundant ground water in the till and vegetation is normal ; but closer to Rock River and near the main creeks, the sandstone forms the hills and valleys. The aridity of St. Peter outcrops has produced a curious assemblage of desert plants in a re- stricted area between Franklin Creek and the southern line of Nashua Township. There, the clumps of dry rustling grass, cactus, and sand burrs scattered over the sandy, glistening plateaus, and the miniature, flat-topped, vertical-sided mesas combined with a desert fauna, are typical on a small scale of the sand wastes of Arizona. SPRINGS The contact of the Platteville with the underlying Glenwood is marked by springs, outlining the limestone boundary, even in places where it is thoroughly covered. Water traveling downward through the limestone can- not pass through the shale, and so moves along it laterally, later appearing in springs, such as those along the bluffs of Rock River near Lowell Park, and along Pine Creek, Sevenmile Branch and the southern tributaries of Kyte River. Some of these springs fail in late summer, but most of those which have much Platteville limestone above to draw from flow throughout 114 DIXON QUADRANGLE the year. Where the limestone-shale contact passes through a hill, springs are more common on the hill-side toward which the strata dip, and such springs are normally perennial. The largest Glenwood-Platteville spring in the quadrangle is located in sec. 12, T. 22 N., R. 10 E., and has a flow of about 400 gallons per minute in late August. In addition to the Glen- wood-shale springs, three springs issue from the Shakopee dolomite in Franklin Creek valley. One of the springs from the Shakopee carries much iron, which is deposited around the spring as limonite. With this exception, all the springs of the quadrangle supply palatable water. No spring in this quadrangle is known to be polluted, but the sink holes in the Platte ville are evidence of large underground channels, and some of the springs may be supplied from such a source. Spring water, therefore, is not necessarily pure and safe for drinking. SHALLOW WELLS IN TILL Wells less than 200 feet deep are common and easily drilled. They are of four general types, drawing water from till, alluvium, limestone, or the St. Peter sandstone. The wells drilled in the till are the most common, and range from 15 to 185 feet in depth. In the till, irregular patches and layers of sand occur, which are chiefly stream deposits and small out wash plains later overridden by a short advance of the ice. Irregular joint planes also traverse the till, and water follows these channels closely. On cliff expos- ures of the till, the joints may often be seen, marked by staining and leach- ing by the water. Their origin is uncertain, but may be due to the fact that the till is not an ordinary clay, but contains so much rock flour that it has a tendency to cement together and crack like other sediments. More commonly, the till water comes from sand lenses of uncertain extent. Their irregularity is attested by the number of dry holes which have been drilled on some farms before a good well was secured. The till is never dry in depth, but the water may seep out so slowly that the well is unsatisfactory and is called dry. Because of the uncertain size of the sand lenses, the driller always tests the well by pumping at a rate much greater than normal- ly will be required. If the test pump fails to lower the water, it is being supplied as fast as pumped, and the well is pronounced a success. Till wells bored or dug during the early settlement of the area have been deepened an average of 20 feet. The greater depth may have been necessary in part to secure a larger supply of water because of the increased demands of the modern stock farm, but drillers agree that it is necessary to go deeper than formerly to get a good well. Deeper drilling has been made necessary by the lowering of the ground- water table. Some drillers estimate that the lowering of the water surface amounts to 35 feet. Accu- rate data are not available. Well records do not furnish good evidence, for WATER WELLS 115 the demand on the wells may have reduced the water table locally ; but it seems probable that the general water level is at least 20 feet lower than it was 60 years ago. The great increase in run-off, which has accelerated the rate of erosion, has reduced the opportunity for run-in, decreased the amount of ground water, and accordingly lowered the ground-water table. The only artesian well reported in the glacial drift was drilled near the northwest corner of sec. 9, T. 23 N., R. 10 E. near Tealls Corners. This well was made by driving a two-inch pipe 18 feet into the ground. For several years water flowed with a head of about two feet above the surface. In deepening the well to increase the flow, the St. Peter sandtsone was en- countered, the water escaped downward into the sandstone, and the well was ruined. WELLS IN LIMESTONE Many wells in the limestone area pass through the overlying soil or till without encountering a satisfactory supply of water, and are then drilled into the limestone. Like the till, the limestone may be saturated and yet the water may not flow into a well fast enough to satisfy the demand. There are no sand layers in the limestone, but it is traversed by many joint planes. Water travels along these planes very freely, in many places dissolving the rock to form large channels, and the well that strikes one of them will have an abundance of water. Because water travels so readily through the lime- stone it is not filtered and may be highly dangerous. The sink holes de- scribed in Chapter IV are the result of water dissolving the limestone as it passes through. Rubbish thrown into these holes drains into the water channel and may supply highly dangerous water to wells a long distance away. Good water may sometimes be obtained from limestone, but it is always subject to suspicion, and should not be drunk if its taste, color or odor is peculiar. Freedom from peculiar smell or taste, however, does not prove that it is pure. The analysis cited below shows the fallacy of the popular idea that a deep well has pure water. This well is deeper than 95 per cent of the wells in the quadrangle, but the high chlorine and very high nitrate content prove that the well is contaminated with sewage. No uncurbed well is necessa- rily safe because it is deep and limestone wells are especially dangerous. This water is not drunk, but is used in the power plant. Water is retained in the limestone by the underlying Glenwood shale at many places where the contact between the formations lies well above the adjacent drainage lines. If the water were free to move downward into the St. Peter formation, it would soon be drained away. Several limestone wells have been ruined by drilling them too deep, penetrating the sandstone, and thus permitting the water to escape from the bottom of the well. 116 DIXOX QUADRANGLE The following analysis shows the character of the water in a 200-foot well at Dixon State Hospital. The analysis (Lab. No. 42,296) was made by the Illinois State Water Survey, Dec. 16, 1919. Table 7. — Analysis of water in 200-foot well at Dixon State Hospital Parts per million K and Na potassium and sodium 7.92 NH 4 ammonia 0.00 N0 2 nitrogen peroxide .008 Mg magnesium 31.36 Ca. . . . . calcium 68.44 Fe iron 0.00 N0 3 nitrate 35.43 CI "chlorine 11.00 S0 4 sulphate 13.36 HC0 3 carbonic acid 334.28 AL0 3 alumina 1.4 S*0 2 silica 15.6 Total 518.798 The analysis shows about 30 grains per gallon of dissolved matter, or 20 grains of residue on evaporation, as compared with 14 for Rock River and 18 for Croixan water. Although the bottom of this well is in St. Peter, it is not curbed or cased, and the water has come largely from Platteville and Galena limestones, and is essentially a limestone-well water. The high lime and magnesia with low soda and potash are typical of limestone waters, as are also high carbonate and low sulphate content. WELLS IN THE ST. PETER SANDSTONE For shallow wells, the most consistent water-yielding stratum is the St. Peter sandstone. Whenever a well penetrates the sandstone to the depth of Rock River, an abundance of excellent soft water is obtained. Even where the water comes from surface drainage, it is thoroughly filtered and purified in traveling through the sand. Away from Rock River, water may be obtained at higher elevations, and as a rule, except within three miles of the river, wells in the St. Peter need not be drilled more than ten feet below the level of the nearest valley, even though it does not contain a per- manent stream. No artesian wells obtain water from the St. Peter in this quadrangle, but farther from the outcrop, it yields abundant flows through- out an area in Illinois bounded by Chicago, Peoria, and St. Louis. Further discussion of St. Peter water supplies may be found in Leverett's 1 paper on Illinois water resources. 1 Leverett, Frank, Water resources of Illinois: U. S. Geol. Survey Seventeenth Ann. Rept., pp. 695-849, 1896. WATER WELLS 117 WELLS IN ALLUVIUM The success of a well in alluvium depends upon the ground-water level and the composition of the alluvium. Where the alluvium is a valley train, as in Rock and Kyte river valleys, it consists of sand and gravel and a well reaching below stream level will have abundant water. Valleys in the St. Peter usually have a sandy alluvium also, and wells there obtain plenty of water. In limestone areas, the alluvium may be largely clay, and it is then necessary to penetrate the underlying limestone to get water. Water from alluvium is usually good ; the principal drawback to these wells is the liabil- ity to damage and contamination through flooding of the lowland in which they are situated. ARTESIAN WELLS Wells in which water rose above the surface of the ground under nat- ural pressure were first drilled in the department of Artois, France, and so were called artesian. By an extension of the term, any well in which water rises a considerable distance above the original containing stratum is called artesian, whether the water flows at the surface or not. In this area, six artesian wells produce water from the Croixan series. The conditions necessary for a flowing artesian well are (1) a porous rock to carry the water, (2) an impervious overlying formation to retain the water under pressure, (3) an outcrop of the porous rock at a point higher than the top of the well, (4) an adequate supply of water at the outcrop or intake area, and (5) no outlet lower than the well sufficiently large to permit all the water from the intake area to escape. The Croixan series underlying the Dixon area meets all these require- ments (fig. 15). The sandstones furnish the porous rock outcrop in central and southern Wisconsin at a higher elevation than the surface of the Dixon area and numerous lakes on the outcrop supply all the water that can be absorbed ; the Prairie du Chien formation with its interbedded shales pro- vides an impervious cover, and there is no lower outlet. Artesian wells supply the water for the municipal system in Dixon and at the Dixon Epileptic Colony. The Dixon Water Company has drilled four artesian wells to depths of 1610, 1720, 1765 and 1860 feet, the deepest reaching 1250 feet below sea level. The water from these wells will rise eight feet above the surface, but the rate of flow is too slow to supply the city, and the wells are all pumped to secure a larger quantity. Two wells at the Dixon Epileptic Colony in sec. 21, Dixon Township are 1900 and 2217 feet deep, the deeper one extending 1415 feet below sea level. The water rises within four feet of the surface, or 128 feet higher than in the water company's wells, the Colony wells being located 110 feet higher than those in Dixon. No other deep artesian wells qre known in the quadrangle ; others nearby are those supplying the municipal systems in Sterling, Oregon, and 118 DIXON QUADRANGLE •fliP-ii, '■'.{■&> Dixon Green River L.::l If w n CO ^ Princeton if aisl 3 " ?m m Vvl : CEMENT MATERIALS 119 Amboy. A large supply of water from this source can be obtained at any point in this area, where the need is sufficient to justify the expense. The depth of an artesian well will vary from 1600 feet at the northern border of the quadrangle, to 1900 feet at the southern. The amount of water obtainable from any one well will depend upon the size of the hole and the rate at which it is pumped. Where more than one well becomes necessary, later wells should be placed along an east-west line so as not to interfere with one another, since the underground movement is from the north. They should be at least 600 feet apart, or should vary in depth by 200 feet or more. If the bottoms of the wells are too close together in the same horizon, they will be taking water from the same limited rock mass ; but by spacing them more widely, or by varying the depths, each well will be supplied from an independent body of rock, and the total flow will be much greater. Water from such wells is of necessity healthful, for all organic matter is filtered out in the years the water spends en route from the point where it enters the sand in south-central Wisconsin. The following analysis of Dixon city water, which is the Croixan artesian water, was made by the Dearborn Chemical Company of Chicago for the Northern Illinois Utilities Company and is published through the courtesy of the latter company. Table 8. — Analysis of Croixan water from Dixon, Illinois Parts per million Si0 2 silica 3.0 Fe.0 3 and A1 2 3 ferric oxide and alumina 1.6 CaCO :! calcium carbonate 149.4 CaS0 4 calcium sulphate Trace MgCO. magnesium carbonate 123.8 Na 2 S0 4 and K,S0 4 sulphates of soda and' potash 16.7 NaCl and KC1 chlorides of soda and potash 10.2 Loss, etc 2.9 Total 307.6 This analysis indicates that the Croixan water contains about 60 per cent as much dissolved matter as a water from limestone in this area. Cement Materials Portland cement is made from an artificial mixture of limestone, chalk, or marl, with impure limestone, clay, shale, slate or slag. Correct propor- tions of these materials are finely ground ; calcined until fusion begins ; the resulting clinker is cooled, ground to flour-like fineness; mixed with a re- tarder, and placed on the market. The raw materials may be any of the 120 DIXOH QUADRANGLE substances listed if they contain lime, silica, alumina and iron in the desired proportions and do not contain too much sulphur and magnesium. Mag- nesium is highly objectionable, and raw materials are carefully and contin- ually being analyzed to guard against its presence in excessive amount. The Dixon area contains satisfactory limestone in the fossiliferous Blue and most of the Glass Rock members of the Platteville. Loess is the only commercial source of alumina, silica and iron. The limestones and dolomites of the Prairie du Chien. Burt and Lowell Park members of the Platteville. and the Galena dolomite are all too magnesian to be used. The Glenwood shale is too thin to be a commercial source of clay for cement manufacture. Glacial till has a suitable composition, but the cost of remov- ing boulders and gravel, followed by extremely tine grinding of the hard quartz-sand grains is prohibitive. Loess, being largely the liner material Fig'. 16. Quarrying of the Blue limestone at the plant of the San- dusky Cement Company. of the till already sorted by streams and deposited by winds, is very satis- factory. The Sandusky Cement Company has located its plant beside Rock River two miles northeast of Dixon, where the full thickness of the non- magnesian Blue limestone is available without a capping of Lowell Park or higher dolomites which would have to be wasted because of their content of magnesia. Overlying the limestone is a mantle of till ranging up to 25 feet in thickness which is stripped away and dumped into a nearby ravine. Above the till lies the loess with a maximum thickness of 15 feet. The quarry is worked in three benches. Steam shovels standing on the Burt limestone remove the Blue member as it is blasted loose i fig. 16), and load CEMENT MATERLALS 121 it into cars for transportation to the mill. The limestone shovels often work at a face 50 feet high. On the next bench above, other shovels strip the till, which is from 4 to 25 feet thick, and load it into cars to be hauled away and dumped. The top of the till forms the third bench, and work- ing from this level, other shovels strip the loess and load it for transporta- tion to the mill. The loess varies in thickness from 1 to 15 feet, with an average of 4*/2 feet. Each bench is kept well in advance of the one beneath so that neither caving ground nor heavy rains can mix the materials (fig. 17) . Fig. 17. Quarry of the Sandusky Cement Company, showing the quarry face, the limestone below, the till behind the rigs which drill holes for blasting, and a thin bed of loess above the till. The limestone and loess are taken to the mill where the loess is sampled, dried, and stored until it is needed for making up the "mix" (fig. 18). The limestone passes through two jaw crushers, and then to hammer mills where it is broken into pieces less than one twenty-fifth of an inch in diam- eter. It is then dried and passed to storage bins from which it is drawn as operations require, mixed with the loess and ground until more than 92 per cent of the mixture will pass a screen having L0,000 meshes to the square inch. Sieves for testing this material are so fine that they will hold water. The "mix" is fed into the kilns, where it is calcined until it forms a clinker or slag-like mass. To this clinker about two per cent of gypsum is added. 122 DIXON QUADKANGLE Gypsum delays the setting of the cement and makes the handling and work- ing of concrete mixtures and cement mortars possible for nearly an hour after the water has been added. The clinker and gypsum are ground to- gether, sacked, and the resulting cement is ready for market. Additional supplies of cement materials may be found in sees. 15 and 16, Dixon Township, in the till-covered area northeast of Franklin Grove, and less probably along Ridge Road in sees. 24, 25, 35, and 36. In each case, unless the overlying till is more than 25 feet thick, the non-magnesian part of the Platteville has a thickness ranging up to 35 feet. There are no other extensive areas known where the Blue limestone forms the surface, and so can be quarried profitably. Where the Lowell Park member is present, it would be necessary to remove it first, and to discard it because of its high content of magnesia. The thickest loess deposits are east of Rock River, south of Grand Detour. \ Fig. 18. The mill at the plant of the Sandusky Cement Company. Limestone and Limestone Products The limestones of the quadrangle are valuable also for lime, building stone, road metal and crushed limestone for agricultural purposes. When the region was first settled, many quarries were opened, with lime kilns beside them for burning lime. As long as timber was available along the streams for fuel, and freights were high, this was profitable. Cement has largely replaced lime for building purposes, because of its greater strength and weather resistance, and there are now no active kilns in the quadrangle. Such lime as is still used is shipped in from large plants where cheap fuel is available. From the Platteville, a magnificent blue-gray limestone formerly was quarried and used for many of the public buildings in Dixon and Ashton. LIMESTONE AND LIMESTONE PRODUCTS 123 The stone makes an excellent appearance, although it is said to fade on long exposure to weather. The principal quarries were on Ravine Road, Dixon ; in sees. 3 and 27, Dixon Township; at Lighthouse, Nashua Township, (T. 23 N., R. 10 E.), and in sec. 23, China Township (T. 22 N., R. 10 E.). While the Platteville has furnished the favorite building stone, some Shako- pee has been quarried along Franklin Creek and used in Franklin Grove. Because of the shaly character of the limestone, there was too much waste in the quarrying to make it profitable. The Galena furnishes a very satisfactory dimension block, but is not favored for buildings, as it readily absorbs moisture, and renders the in- teriors damp. It has been quarried extensively, however, for bridge abut- ments and similar uses by the Illinois Central Railroad along its route, both north and south of Rock River. A large quarry in the Galena and Lowell Park member of the Platteville was opened along River Road, Dixon. Part of the quarrying was solely to make room for the road, and for buildings along the bluff line. How much of the rock was used for building purposes is not known. In spite of its high porosity, freezing and other types of weathering have only a slight effect upon the Galena dolo- mite and it retains a fresh appearance for years. Concrete and brick have largely displaced stone for building, and at the present time no quar- ries are working out building stone. Small amounts of limestone are crushed by farmers locally for use in their fields. Limestone neutralizes the acids produced by the decom- position of plant remains, which make soil sour, hindering or preventing the growth of vegetation, especially of such essential leguminous crops as clover and alfalfa. An unleached limestone soil is generally very fer- tile, provided it does not contain an excess of lime. Limestone is slightly soluble in water, and rain water soaking into the soil dissolves the limestone and carries it away. Accordingly, a limestone may be overlain by several feet of clay from which all the original lime has been removed and the resulting soil is sour and poor. One of the great benefits of glaciation was that practically all the worn-out and leached-out soils in this area were removed or ground up with crushed limestone and other rocks, produc- ing one of the most fertile soils in the United States. On much of the flat land, however, leaching has removed the limestone content of the sur- face till since the Illinoian glaciation, and a less favorable soil is found than in the areas covered by the last (Wisconsin) glaciation to the north and east. It is necessary to treat this land with limestone, and many car- loads of crushed limestone are hauled into this area yearly. The local limestone is as good as any that is imported and better than much of it. The value of limestone depends upon its content of cal- cium and magnesium. One unit of either metal neutralizes another unit 124 DIXON QUADRANGLE of certain acids. It follows then that all impurities in limestone are simply inert matter, and the money and time spent for freight, hauling and distribution of the impurities is dead loss. Ten per cent of clay in a limestone, which is not an unusual amount, means that one load in ten is wasted. The purity of limestone should therefore be carefully consid- ered in purchasing it. None of the Galena or Platteville limestone analyses available show 10 per cent impurity, and it is safe to regard any local lime- stone, except the Prairie du Chien, as pure enough to be valuable. The acid-neutralizing power of limestone varies with the amount of lime and magnesia. A pure dolomite has 108.8 per cent of the acid- neutralizing power of a pure limestone, and is therefore worth nearly nine per cent more. All of the Galena and most of the upper Platteville is dolomite: the Buff limestone of the Platteville is highly magnesian, but not actually a dolomite. It follows, then, that the limestone occurring im- mediately above the St. Peter, which is the Buff limestone, the upper Platte- ville and all of the Galena are the most desirable agricultural limestones. Whether the extra nine per cent of acid-neutralizing power is worth a special effort to secure it depends upon the individual case. There is abundant limestone for all agricultural needs : any limestone outcropping in the area of the Platteville or Galena formations will be satisfactory. The most important use now being made of limestone in the quadrangle is for road metal. Limestone is so widely distributed over the area that a new quarry can be opened in many cases more cheaply than the stone can be hauled from an existing pit. The amount of stripping is usually slight ; a place where rock is already exposed in a bluff is best, because of the ease of blasting and loading it. The value of limestone for macadam road depends upon its strength, toughness, resistance to wear and cementing power. Strength is the ability to stand up under loads without crushing; toughness in contrast to brittleness is the power to withstand repeated blows ; wear resistance denotes the amount of grinding and pounding the rock will stand, while cementing power depends upon the way the dust produced by crushing and wear of the lime particles is partially dissolved by rain and cemented or compacted onto and between the rock fragments. Ce- menting power is of extreme importance, for even though the other quali- ties are favorable, if the dust does not partially dissolve, then precipitate between the pieces and cement them together, the macadam will have little cohesion and will "ravel" along the edges, while the surface becomes rough and jagged due to absence of dust and fine material which are needed to make a surface cushion. Both the Platteville limestone and the Galena dolomite make satis- factory roads. The Platteville is stronger and harder, and has a higher wear resistance than the Galena, but the latter has much better cementing GLASS SAND 125 power, and is tougher. Either makes a better-than-average macadam road, but under the present high-speed traffic, the greater cementing power of the Galena makes it superior to the Platteville. Before automobile days, the Platteville made more desirable road metal, but the severe grinding and high-wind suction of modern cars remove so much of the rock dust that greater cementing power has become essential. Probably the more mas- sive beds of the Shakopee dolomite could make good roads, but the more satisfactory Platteville outcrops near all of the Shakopee exposures. In addition to quarries supplying merely the farm on which they are located, 30 quarries have been opened in this quadrangle to secure build- ing stone, commercial lime or road metal. Glass Sand Glass is made by fusing soda or lime and silica together and cooling the product so quickly that it does not have time to crystallize. Partial crystallization produces a cloudy, non-transparent material which is worth- less. Other substances than those listed are sometimes contained in the original materials, or added to the mixture in order to produce special color, brilliancy or strength. Pure white quartz sand, free from iron and clay, is the most desirable source of silica. Iron makes a green glass, such as is common in low-grade bottle or window glass. Alumina, which is an essential constituent of clay, has a very high fusion temperature and almost inevitably produces a "milky" glass. Removing the clay by washing greatly reduces or practically eliminates this most undesirable con- stituent. Much of the St. Peter sandstone answers all requirements. It was thoroughly cleaned by wind and water before it was deposited, and its total impurities are normally less than the permissible amount of iron alone in good window-glass sand. The St. Peter makes a glass of exceptionally high quality, and is the basis of the glass sand industry both in this area and near La Salle. In the ground, the sand has little value, for the available quantity is unlimited, and the selling price is limited to the cost of production, plus a moderate profit. Under these conditions, the success or failure of any glass-sand plant depends upon the ability of the management to finance the undertaking in its early stages and to market the sand efficiently and cheaply. The National Silica Company is operating a modern plant in sec. 8, Oregon Township (fig. 19). At the plant, the sand is quarried by drill- ing three-inch holes 20 feet from the face, filling them with explosive, and breaking off the face of the quarry (fig. 20). The sand is loaded into quarry cars by a steam shovel, crushed and shipped without special treat- ment if merely a good grade of sand is desired. If the more expensive, 126 DIXOX QUADRANGLE Fig. 19. Sand crushing and washing plant of the National Silica Company, SE. % NE. i/i sec. 8. T. 23 N.. R. 10 E. Much of the sandstone face has been covered by soil wash from above. (Photograph by National Silica Company.) ;~^feto& Fig. 20. View in sand pit of the National Silica Company, Oregon, SW. y± strata and its uniform texture; a churn drill in the right foreground mak- ing blasting holes in the rock; a steam shovel loading sand for the plant; in the distance, another shovel stripping soil and vegetation. (Photo- graph by National Silica Company.) GLASS SAND 127 high-grade sand is required, after being crushed to separate the grains, it is washed to remove clay and other impurities. The company is able to ship this sand with a guarantee of not over one-half per cent of impuri- ties. The following analyses made by Edward Orton, Jr. for the National Silica Company are published through the courtesy of the company, and show the reported character of the sand at the outcrop and of the product as shipped after washing. Table 9. — Analyses of surface and washed sand from quarry of the National Silica Company Surface sand Washed sand Per cent Per cent Si0 2 Silica 99.000 99.58 A1 2 3 Aluminum oxide .585 Trace Fe 2 3 Iron oxide .005 .23 Ti0 2 Titanium oxide Very faint Faint trace trace CaO Lime .235 None MgO Magnesia None None K 2 Potash .016 ..... Na 2 Soda .039 Loss on ignition .046 .05 Total 99.926 99.86 The elimination of the most undesirable impurity, alumina, is clearly shown by the analysis. The less harmful alkalies, soda and potash, and lime have also been eliminated. The iron has increased as a result of the grinding of machinery by the silica and possibly because the washed sand came from a lower, more ferruginous portion of the deposit. Experience has shown that iron increases with depth and has even reached 1.5 per cent. This is not because different strata are being worked, for the same bed contains more iron as it is followed back under cover. In most cases of weathering, the iron content increases at the surface. Here the iron de- creases. Probably the abundant vegetation at the surface helps to re- duce the iron, and the porosity of the rock favors quick transportation of the iron downward by percolating ground water. In most areas, also, iron is concentrated at the surface by removal of other substances. In the absence of alkaline carbonates, silica is very difficultly soluble. Since silica is practically the only constituent of the sand besides the iron, the ferrugi- nous content of the surface is not appreciably increased by weathering pro- cesses. The sand is used not only for ordinary and plate glass, bottles and cheap glassware, but also for the finest flint and cut glass. It is also 128 DIXON QUADRANGLE purchased in great quantity for use in the sand-blast process of cutting and frosting glass, and as an abrasive, for sawing stone, polishing and grinding. Much sand is also crushed here to pass a 140-mesh screen and sold under the trade name of "flint." True flint is not a crystalline quartz like this, but is a form of chert. However, powdered until no piece is more than 1/250 inch in diameter, this rock flour is said to serve all pur- poses of flint in making chinaware. In addition to these uses, because of its purity large sales are made for chemical purposes, particularly for making sodium silicate or, as it is commonly known, water glass. Supplies of this sand are practically unlimited since it is available at the surface over all the area indicated on the geologic map. The sand averages at least 50 feet in thickness, and in some places reaches over 150 feet. A thickness of 50 feet would produce 50,000,000 cubic yards or 110,000,000 tons per square mile. Sand and Gravel Building sand and gravel are found in this area primarily as a result of glaciation and do not bear any necessary relation to the formations un- derlying the valleys in which they occur. In regions of stream erosion, un- interrupted by glaciation, sand and gravel accumulate in valleys where erosion is rapid, or where certain materials successfully resist weathering. In this area, the commercial sources of these building materials are the Rock and Kyte River valley trains and the Grand Detour esker. Glacial erosion was rapid and deep, with no opportunity for chemical weather- ing. Accordingly, the glacial gravels consist of both hard and soft rock fragments. Limestone predominates because it is the common surface rock in the region and most glacial drift has had a local origin. Sandstone pebbles are not common in the gravel, for most of the neighboring sand- stones are poorly cemented. Rolling by the streams disintegrated much of the softer local material, and the swiftly running water washed away the finer constituents. Igneous and hard metamorphic rocks have there- fore contributed a much larger percentage to the gravel than to the till. The long transportation of the Late Wisconsin valley train material thoroughly washed the sand. Most of the gravel was not carried to this quadrangle. Estimates of the material over one-fourth of an inch in diameter commonly vary from less than 5 per cent to 10 or 12 per cent. Much of the sand is sharp and angular ; but readily recognizable, wind- blown sand grains from the St. Peter and Croixan formations are com- mon. All of the valley-train gravels are fairly well rounded. The supply of sand and gravel is practically inexhaustible. Pits may be opened at any point in the valley train and a good quality and abundant quantity of sand and fine gravel will be obtained except where bed rock SAND AND GRAVEL — POTASH 129 is close to the surface. Many terraces of this valley train stand over 30 feet above present water level in Rock River, and by dredging, sand and gravel could be taken from a depth of 60 feet beneath the present river level. The distribution of the upland gravels of the Grand Detour esker is indicated on Plate I. This material was water-transported for a very short distance and is not so well washed and sorted as the valley-train deposits. The esker deposits carry a much higher percentage of gravel, at least 40 per cent of the material being over a quarter of an inch in diameter. The beautifully rounded St. Peter and "New Richmond" sands are not satisfactory for building purposes. A sharp, angular sand is desired, first because sharp grains cannot be rotated or twisted in the mortar or cement so easily, and second, lime or cement must be added to fill all the spaces between the sand grains. Otherwise a porous, unsubstantial struc- ture results. There is more space between rounded grains of uniform size than between angular ones, and the cement or lime consumption is therefore higher. The sandstones would be more expensive to quarry than the valley-train and esker sands. Potash Three elements are required in great amounts annually for fertilizers in this country ; namely, phosphorus, nitrogen and potassium. The deposits of Florida are sufficient for the present demand for phosphorus in the form of phosphates, and tremendous reserves in Idaho, Utah and adjoining states promise a supply for at least 6,000 years for all the world at the present rate of consumption. Nitrates come in abundance only from Chile. The farmer can supply his own nitrates, aside from those contained in manure and other organic material, largely by growing various legumes, such as clover, alfalfa, peas, and beans, which take nitrogen from the air and store it in the ground in useful form. In case of urgent need, nitrates may be manufactured from the air by use of electricity, and the government built the great Muscle Shoals, Alabama, plant to provide nitrates for military purposes. In potash, the country is not self-sufficient. In spite of years of re- search and effort to secure an adequate supply from various sources, during the World War and under the stimulus of four-fold increase in price, only 40 per cent of the normal potash consumption was produced in this country. Germany had a virtual monopoly on the potash of the world before the war: with the return of Alsace to France, Germany lost the lesser of the two deposits which she had, and France is now a competitor for the world 130 DIXON QUADRANGLE trade. The two largest producing areas in the United States are the alka- line lakes of northwestern Nebraska and the salt marshes of southern Cali- fornia. Neither of these has a large reserve, and the 1919 rate of produc- tion would exhaust them both within 20 years. Other sources of potash are therefore of great importance, even though they may be reserves for the future, rather than commercial possibilities at present. The Glenwood shale carries an unusual amount of potash, apparently in the form of glauconite. The following analyses were made by J. M. Lindgren of the University of Illinois of samples taken by the writer in the Dixon quadrangle. Table 10. — Potash content of samples from deposits in the Dixon quadrangle Sample number Location Potash content Part of section Section Township 1 1000 feet E. and 500 feet N. of S W corner 15 23 34 11 Dixon Nashua China Grand Detour Per cent 5.83 4 2000 feet N. of SW. corner on west line 5.79 7 500 feet E. and 1600 feet N. of SW. corner 5.83 8 1000 feet E. and' 3200 feet N. of SW. corner .22 Sample No. 1 covers 5 feet of Glenwood shale outcropping in a ravine. This is practically the complete section of the Glenwood at this point. The top 6 to 10 inches was covered by slumping rock. The sample was taken by channeling three inches wide, one inch deep across outcrop, and quarter- ing down. Sample No. 4 represents 3^ feet of Glenwood shale exposed in the bottom of a small quarry. The sample was taken by channeling, as before. The total thickness of Glenwood here is about six feet, as esti- mated by leveling between exposures. Sample No. 7 is not from the Glen- wood, but from the green shale which underlies the St. Peter in Franklin Creek valley. It was taken by channeling and quartering across 30 inches of the shale. The top and bottom are both exposed. No. 8 is not a sample, but simply a piece of the exposure at Green Rock of the most prominent green glauconite bed in the upper St. Peter. As noted in the description of the St. Peter, the green clay is a coating and matrix embedding quartz grains identical with those making up the remainder of the sand. In the hand specimen, and under the microscope, it appears to be identical with the typical Glenwood shale which lies 10 feet higher. Under the micro- scope, this glauconite was estimated as 5 per cent of the rock: since the PETROLEUM 131 potash is confined to the clay, there must be about 4 per cent potash in the clay, indicating that it is essentially similar to the Glenwood above. The Glenwood was found wherever the top of the St. Peter was ex- posed, on the west flank of the La Salle anticline. It is difficult to meas- ure the shale thickness because the overlying Platteville slumps badly on this slippery "soapstone", as it is locally called. Thicknesses as little as 2 feet were measured, but where the relations seemed undisturbed by sliding, the amount varies from 3 to 7 feet. On the crest of the anticline and at the only good exposure on the Prairie View syncline, the Glenwood shale is commonly thin, and locally is missing entirely. In important quantities it can be looked for only west of the anticline in the Dixon area. The aver- age shale, as determined from 78 analyses by Clarke 2 , carries 3.25 per cent potash, so that the Glenwood contains nearly 180 per cent of the normal amount. Under present costs and methods, it is impracticable to work this material for its potash alone. It is probable that the potash may be re- covered as a cement plant by-product, and be of value in the future. It is hoped that the Glenwood may be studied to the north and west where it is thicker to see whether its potash content is as favorable there, where min- ing it would be much less expensive. Petroleum The bright, iridescent scum of limonite (iron oxide), commonly seen on the surface of swamps, has frequently been regarded as petroleum. Much time and energy have been wasted, as in the case of two or more wells drilled near Honey Creek, in search of the oil that was supposedly indicated by this iron oxide scum. Two very simple tests will quickly tell whether petroleum is present. Oil floating on water makes a film which will unite again if broken by gently stirring the surface, whereas limonite will break into sharp-edged fragments which do not merge, but may overlap if one is carried over the other by splashing water or a sudden wave. The other test is to see if the substance will burn. Petroleum may not flash to a match, like gasoline, but any petroleum found in the Mississippi valley will burn if a flame is touched to it. Petroleum may be collected by skimming, or if there is a very small amount, it may be skimmed into a bottle having a large body, but very small neck. The petroleum from a large quantity of water is thus collected in the small neck, and may be removed and tested. Limonite will not col- lect in this way. The smell and taste of the water are often appealed to, but few people know the taste of stagnant water, and think it tastes as pe- troleum probably does. Testing by breaking the surface and by flame will 2 Clarke, P. W., The data of geochemistry: U. S. Geol. Survey Bull. 695, p. 29, 1920. 132 DIXON QUADRANGLE save the expense, trouble, and disappointment of having an examination made in the great majority of supposed petroleum showings. There is little possibility that petroleum will be found in this area. In the first place, petroleum production is conditional upon the presence of favorable structures covered by impervious strata which will confine the oil and gas until man reaches them with the drill. The only desirably shaped structures in this area have St. Peter sandstone outcropping, and no oil has been produced from rocks as old as those underlying the St. Peter. Secondly, as already stated, oil accumulation demands a trap which will retain oil and gas. An impervious rock, usually a shale, must cover the reservoir, in order to make this trap. The only continuous shales of the area are those in the Prairie du Chien, and the upper part of the Croixan formations. If they were capable of confining petroleum, the original salt water of the ocean could not have escaped from beneath them. All the deep wells in the area penetrate these shales and find below them an adequate supply of fresh water for Dixon, Amboy and Oregon. Such quantities of fresh water indicate that water is moving through the underlying sands with ease, and if oil has ever existed here, it has long since escaped and disap- peared. Natural Gas Gas may be found in small quantities, as it is now produced from the Green River valley to the south and near Chana, adjacent to the northeast corner of the quadrangle. This gas is not associated with oil, but probably comes from the decay of plant matter buried by glacial deposits. Such gas also forms in wood-curbed wells which are tightly closed in, and in abandoned mine workings, where much timber is left in the ground. These gas pockets will have small production and probably a rather short life. They cannot be predicted from surface indications, except that the chances are better for them where a preglacial channel has been buried by drift. In such a place, much plant matter may have been covered, and may since have generated considerable gas. The chances for a large gas field are the same as those for oil, since oil and gas are formed and collected under identical conditions. There is no hope for gas development on a commercial scale in this area. Ore Minerals The term ore minerals is used, because there are no ores, nor prospect of any within this area. An ore is a rock or aggregation of minerals from which one or more metals may be extracted at a profit. An ore mineral is a mineral which in sufficient concentration constitutes an ore. NATURAL GAS ORE MINERALS 133 SULPHIDES In the Platteville limestone, three sulphides are found which in suffi- cient quantity would be valuable. They are the sulphides of lead, zinc and iron, known technically as galena, sphalerite (zinc blende, rosin or black jack, depending on its color) and pyrite (pyrites, fool's gold, sulphur or iron). In the northwestern part of the State, all of these are found in suffi- cient quantity to be ores, both in the Galena dolomite and to a slight extent in the Platteville limestone. In this quadrangle, they have been found only in the Platteville and Glenwood formations and only in very small amounts. Galena occurs in steel-gray cubes which are easily scratched w T ith a knife, and breaks conspicuously along planes parallel to the sides of the cube ; the mineral is very heavy ; its powder is black, and it will fuse to a lead globule on a red hot stove, with the typical smell of burning sulphur. The sphalerite looks like masses of yellow, red-brown or black rosin. Scratching with a knife produces a yellow or buff powder. Sphalerite will not melt nor burn at ordinary temperatures, and is only iy 2 times as heavy as limestone. Pyrite is best described by its popular name, fool's gold, for it is more often mistaken for gold than any other mineral. Pyrite is usually found in per- fect cubes, which do not break with plane surfaces. It is brassy to golden yellow when fresh, but turns brown and eventually becomes iron rust upon long weathering. It is too hard to scratch with a knife, has a green-black powder, is twice as heavy as limestone, ignites with difficulty, produces the typical odor of burning sulphur on ignition, and leaves a magnetic residue. As already stated, none of these has been found in such amount as to give any hope that valuable quantities occur in this quadrangle. The amounts are extremely minute and similar small quantities are often found in limestones. In 1922, the lead and zinc ores sold for about three cents per pound and pyrite was worth about $6.00 per ton. These prices are cited, not because people may find ore deposits, but to prevent the fre- quent arousing of false hopes based on exaggerated ideas of the value of these minerals. All of these sulphides appear in the casts of gastropod shells in the limestone, none of the specimens being found in veins. Their uniform occurrence, coating the interior casts, shows very clearly that iron, lead and zinc were present in the ocean of that time and were reduced and precipi- tated by organic matter. No reason is known for their appearance in gastropod shells alone, but possibly these animals contained much sulphur, and on decaying, the sulphur formed hydrogen sulphide and precipitated the metallic sulphides. If the sulphides were introduced at a hter time they would have been found in veins or fissures in the rock, rather than being confined to the one type of occurrence noted. 134 DIXON QUADRANGLE Pyrite is very common in shales and clays. The only mine ever opened in this quadrangle was a drift in the Glenwood shale where pyrite led the prospector to expect gold. This prospect was opened on the south side of sec. 22, Dixon Township, on the outcrop of the Glenwood shale. Small lumps of concretionary pyrite scattered through the shale attracted the attention of Simon Smith, who worked here for several months in 1907 . His equipment consisted of a small car and some trackage, a pick and a shovel. Xo blasting was done : since he was working up dip. the water drained out. The workings consisted of a straight drift extending- into the hillside from 150 to 300 feet, according to various reports. The caved ground at present proves only that he went at least 50 feet, and there is no way to determine how much farther he proceeded, since he never per- mitted any one to enter the prospect. The drift was four feet wide and live feet high, abundantly timbered with 3-inch posts, caps and sills. Va- rious reports of gold, silver, lead and platinum production are current, but no one interviewed ever saw these metals, except the lead. Mr. Smith finally abandoned the prospect and left the region. The dump shows a quantity of Glenwood shale, with concretionary masses of pyrite and an occasional cube of galena. No trace of the other metals was found, and there is no reason to think they ever were present. LIMONITB Iron ore is frequently reported by people engaged in drainage opera- tions. Masses of limonite I hydrated iron oxide, or iron rust I are com- monly found in the muck of old swamps and marshes. Limonite is an iron ore mineral and would be valuable if found in large quantity. Water circulating through the till may take iron into solution and carry it until it comes in contact with air. Then the iron is precipitated as an iri- descent, oily-appearing him which floats on the water, but eventually set- tles to the bottom and forms the lumps of limonite referred to above. Such limonite films, frequently mistaken for petroleum, are commonly found on stagnant water in the glaciated regions, and the small lumps are found in the deposits of plant matter that accumulate beneath these swamps. It is natural, therefore, that the}" should be found most commonly in drainage operations, where a poorly drained depression tilled with vege- tation is being prepared for cultivation. This iron ore is worth about $2.00 per ton, so that it is obvious large quantities would have to be available to make a commercial deposit. No iron mines in this country are work- ing in any such ore. and it is highly improbable that such ore exists here. It would have to be at least 1 () feet thick over a large area to have any value. There is no source for such a quantity of iron, nor is any present or ex- tinct marsh of sufficient size known. ORE MINERALS 135 COPPER AND GOLD Fragments of native copper transported from Canada, or doubtfully, northern Michigan, have been found in the drift. Several people report having seen gold panned from the gravel in Rock River. No one is said ever to have made a living in this way, nor is any one likely to. Gold is known in the drift, but always in infinitesimal quantities, and no com- mercial production from the drift is known. The gold in this quadrangle, like the glacial erratics, came from Canada and is so mixed with tremen- dous quantities of other material that it cannot be profitably obtained. INDEX A PAGE Acknowledgments 12 Alluvium, flood-plain 78 Amboy, city well of 34, 35 Amplexopora sp 64 Area of quadrangle 11 Artesian wells 117-119 Arthropora simplex Ulrich 59 B Backwater deposits 78 Batostoma cf. magnopera Ulrich 59 Batostoma winchelli Ulrich 59 Bellerophon troosti D'Orbigny. .. 64 Bibliography 11-12 Bucania nashvillensis Ulrich.... 64 Buff limestone, description of... 54 section of 56 C Cady, G. H., cooperation of 12 Cambrian period, deposition dur- ing 81 life in 80 Cambrian system, description of. 36-39 Garabocrinus cf. radiatus Bil- lings 59 Cement materials 119-122 Cenozoic group 66-79 Chamberlain Creek, preglacial course of 99 Clastics, formation of 24 Clathrospira subconica Hall.... 59 Clear Creek basin, dolomite in.. 83 Clear Creek, preglacial course of 99 Climate 13 Columnaria halli Nicholson 59,89 Communication 18 Consolidation of sediments 27 Constellaria varia Ulrich 59 Continental glacier, definition of 23 Copper in the drift 135 Correlation of formations 28-30 Crania trentonensis Hall 59 PAGE Cross-sections illustrating pene- plain surface 91 Croixan series 36-39 water wells in 117-119 Cuestas, formation of 92 Culture 18 Cyrtoceras sp 59 Gyrtodonta obliqua Meek and Worthen 59 Gryptozoon 83 Gryptozoon minnesotense Win- chell 47 Dalm.anella testudinaria Dalman 59 Daysville, sand dunes southwest of 102 Dekayella praenuntia Ulrich.... 59 Demangeon, Albert, explanation of unequal erosion in valley sides 103 Devils Backbone, anticline south- eastward from 109 elevation of 13 Dinorthis pectinella Emmons. ... 59 Dixon, deep well near 36 population of 18 Dixon Epileptic Colony, wells of 35, 36, 38,45,50, 117 Dixon State Hospital, analysis of water from well of 116 Dixon Water Company, wells of 34, 35, 36, 37, 50-51,117 Dolomite, formation of 24-25 Drainage 16 Drainage development, post-Illi- noian 98-101 Drift, definition of 27 Dunes, definition of 22 formation of 25 Economic geology 112-135 Elevation of area 13 137 13S INDEX— Continued PAGE Encrinurus vannulus Clarke.... 59 Endoceras proteiforme Hall 59 Eolian deposits 25 Eridotrypa aedilis Eichwald 59 Eridotrypa aedilis minor Ulrick 59 Erosion, definition of 22-23 Escharopora subrecta Ulricli.... 59 Eskers. definition of 27 Eurychilina reticulata Ulrick.... 59 F Faults in Dixon area Ill Field work 12 Flood-plains 16 Fossils, definition of 29 in Galena dolomite 64 in Platteville limestone 59-60 in Skakopee member 17 Franklin Creek section 45 Fusispira angusta Ulrick and Scofield 64 G Galena dolomite, areal distribu- tion of 64 correlation of 65 description of 61-62 dolomitization of 90 fossils from 64 paleontologic ckaracter of 64 quarries in 123 tkickness of 62-63 Galena in Platteville limestone.. 133 Galena stage, conditions during. .S9-90 Galloway, J. J., assistance of . . . . 12 Geologic column for quadrangle 32 Geologic formations, table of... 33 Geologic time table 28 Glacial deposits 26-27 Glacial invasions of the United States 94 Glass rock of the Blue limestone, description of 54 section of 57 Glass sand 125-128 Glenwood shale, conditions dur- ing formation of 86-87 correlation of 53 description of 52 extent of 53 PAGE source of potash 130-131 thickness of 52 Gold in tke drift 135 Gonioceras oceidentale Hall 59 Grand Detour, a summer resort.. 16 sand dunes nortk of 102 Grand Detour esker 73-74 age of 96 evidences of direction of ice movement in 95 source of commercial sand and gravel 128 Green Rock, exposure of St. Peter sandstone at 49, 51 Ground water, definition of 28 H Heoertella sp 64 Hemipliragma irrasum Ulrick... 59 Holopea excelsa Ulrich and Sco- field 64 Homotnjpa minnesotensis Ulrich 59 Homotrypa similis Foord 64 Homotrypa sp 64 Honey Creek, well near 36 Hormotoma gracilis Hall 64 Hormotoma major Hall 64 Hormotoma sp 47 Human activities, erosional effects of 104-106 Illaenus americanus Billings.... 64 Illaenus punctatus Raymond.... 59 Illinoian ice-skeet, kistory of.. ..95-96 Illinoian till 67-73 Industry 18 Iowan (?) till 67-73 Iron ore 134 Iscliaditcs ioicensis Owen 64 K Keweenawan sandstone 34-36 Kyte River, preglacial course of. 99 sand and gravel from valley train of 128 valley train in valley of 76 L Lake deposits 26 Land division, method of 20-21 INDEX — Continued 139 PAGE La Salle anticline 86, 87, 89, 107, 109, 110 Late Wisconsin valley train, sec- tion of 77 Leighton, M. M., cooperation of . . 12 Leperditia fabulites Conrad 59 Le'ptaena charlottae Winchell and Schuchert 59 Leverett, Frank, views on pre- glacial drainage 99-100 Lichenaria typa Winchell and Schuchert 59 Limestone, formation of 24 Limestone and limestone prod- ucts 122-125 Limonite, occurrence of 134 Lingula elderi Whitfield 59 Lingulepis acuminata Conrad.... 47 Liospira americanus Billings ... 64 Liospira obtusa Ulrich and Sco- field 59 Liospira progne Billings 59 Liospira sp 47 Liospira vitruvia Billings 59 Location of area 11 Loess, age of 75-76 definition of 25 description of 74-76 for use in manufacture of ce- ment 120 Lophospira augustina Billings... 64 Lophospira bicincta Hall 59,64 LopTiospira obliqua Ulrich 59, 64 Lowell Park member, descrip- tion of 54 section of 57, 58 M Maquoketa shales 65 Marine deposits, stratification of 23 Martin, Lawrence, views on origin of land forms in southern Wisconsin and northern Illinois 91-92 Middle Ordovician series 48-65 Moraines, definition of 26 Muck deposits 78-79 N National Silica Company, analy- ses of sand from quarry of.. 127 PAGE plant and sand pit of 125-126 Natural gas 132 "New Richmond" sandstone, areal distribution of 42 correlation of 42-43 description of 40-42 "New Richmond" stage, condi- tions during 81-82 Niagaran limestone 65 O Oneota dolomite, areal distribu- tion of 40 conditions under which de- posited 81 correlation of 40 description of 39-40 Ophileta sp 47 Ophiletina sublaxa Ulrich and Scofield 59 Ordovician system 36-65 Oregon Basin, structure of 110 Ore minerals 132-135 Orthis tricenaria Conrad 59 Orthoceras cf. sociale Hall 59 Orthoceras sp 59 P Pachydictya actua Hall 59 Paleozoic group, description of.. 36-66 Paleozoic record, later 90 Peat deposits 7S-79 Peneplains 90-93 Pennsylvania Corners, bifurcat- ed syncline near 109 Pennsylvania Corners-Grand De- tour syncline 109 Peorian interglacial epoch 96-97 Petroleum 131-132 Phragmolithcs fimbriatus Ulrich and Scofield 59 Physiographic cycle 30-31 Physiographic province 13 Pianodcma subaequata Conrad.. 59 Pine Creek, preglacial course of. 99 Platteville limestone, correlation of 60-61 description of 53-55 distribution of 5S division of 53-54 140 INDEX— Continued PAGE fossils from 59-60 ore minerals in 133 paleontologic character of 58 quarries in 122-123 sections of 56, 57, 58 thickness of 55 topographic expression of 55 uses of 124-125 Platteville stage, conditions dur- ing 87-89 Plectambonites sericeus Sowerby 59 Pleistocene period 94-101 Population 18 Potash 129-131 Prairie du Chien series 39-48 division of 39 Prairieside School, sand dunes north of 102 Prairie View syncline 110, 111 Pre-Cambrian eras, history of... 80 Pre-Cambrian rocks 34-36 Pre-Illinoian glaciation, evidenc- es of 94-95 Pre-Illinoian deposits 66 Pyrite, in Glenwood shale 134 in Platteville limestone 133 R Rafinesqui?ia alternata Emmons. 59, 64 Rafinesquina minnesozensis N. H. Winchell 59,64 Recent history 101-106 Recent sediments 78-79 Recevtaculites oiveni Hall 63,64 Relief of area 13 Rhinidictya exigua Ulrich 59 Rhinidictya grandis Ulrich 59 Rhinidictya mutabilis Ulrich.... 59 Rhinidictya sp 64 Rhinidictya trentonensis Ulrich. 60 Rhynchotrema increbescens Hall 64 Rocks, classification of 21-22 Rock River, dissection by 14 drainage changes in 99-101 dunes along 79, 102 effect of glacier on 97-98 gagings of 17 Platteville-Galena contact at dam of 62 PAGE sand and gravel from valley train of 128 valley of 16 Run-off, definition of 28 S St. Peter sandstone, areal distri- bution of 50 conditions of deposition of.... 85-86 correlation of 51-52 description of 48-50 elevations of top of 108 paleontologic character of 51 thickness of 50 topographic expression of 50 uses of 125 water wells in 116-117 Salpingostoma buelli Whitfield. . 60 Sand analyses 127 Sand and gravel 128-129 Sand dunes 79 Sandusky Cement Company, plant and quarry of 120,121 Sangamon interglacial epoch.... 96 Savanna-Sabula anticline 107, 110 Scenidium anthonense Sardeson. 60 Scolithus minnesotensis Hall.... 51 Scolithus sp 60 Sevenmile Branch, preglacial course of 99 Shakopee dolomite, areal distri- bution of 45 conditions under which formed 82-85 correlation of 47-48 description of 43-44 fossils from 47 paleontologic character of 46 sections of 45, 46 thickness of 45 Shakopee sediments, deformation of 83-84 Sink holes in quadrangle 102 Sinuitcs canccllatus Hall 64 Sinuitcs canccllatus trentonen- sis Ulrich and Scofield 64 Sphalerite in Platteville lime- stone 133 Springs 113-114 Spyroceras bilincatum Hall 60 INDEX— Concluded 141 PAGE Stream deposits 25-26 Streptelasma corniculum Hall... 60, 64 Streptelasma projundum Conrad (Owen) 60 Strophomena emaciata Winchell and Schuchert 60 Strophomena incurvata Shepard 60 Strophomena scofieldi Winchell and Schuchert 60 Strophomena trentonensis Win- chell and Schuchert 60 Strophomena trilobata Owen.... 64 Strophomena winchelli Hall and Clarke 60 Structure-contour map, explana- tion of 107-108 Structure of Dixon quadrangle 109-111 origin of 110-111 pre-St. Peter Ill Subulites regularis Ulrich and Scofield 60 T Tealls Corners, preglacial valley southeast of 99 Temperance Hill School, sand dunes near 79, 102 Terraces 16 Tertiary peneplanation 90-93 Thaleops ovatus Conrad 60 Till, age of 70-73 definition of 27 Topographic map, interpretation of 19-20 Topography 13-18 Transportation 18 Trochonema oeachi Whitfield.... 60 Trochonema umoilicatum Hall.. 60, 64 U Upland plain 13-14 V PAGE Valleys 14-16 "box" valleys 15, 50 limestone valleys 14 sandstone valleys 15 sandstone with overlying lime- stone 15 till valleys 14 Valley slopes, formation of asym- metrical 102-104 Valley train, Early Wisconsin... 76 gravel pit in Late Wisconsin. . 77 Late Wisconsin 77-78 Vanuxemia wortheni Ulrich 64 Vegetation 13 W Water analyses 116, 119 Water resources of the quad- rangle 112-119 ground 113-119 surface 112-113 Water wells 114-119 artesian 117-119 in limestone 115-116 in St. Peter sandstone 116-117 shallow, in till 114-115 Weathering, definition of 22 Wells in quadrangle, deep 70 West Dixon, section of gravel pit in 77 Wisconsin epoch 97-98 Y Younger Paleozoic formations. . .65-66 Z Zygospira, nicoletti Winchell and Schuchert 60 Zygospira recurvirostris Hall. ... 60 PLAiB I ■' '."~ :: jSSB - SksS H ism WBttBA ■ hMBB EH Hi HI Bfl BMP 1 " ■I I IS I 1 1 ■ ■ ■ ■ ■ I ■ I ■ I T -L^- 99 H9 33 I^HU anna