Illinois State: Geological Survey Mf7. Resistant bed of limestone on Cedar Creek between beds of soft shale 136 58. Diagrammatic illustration of the relation of falls to a hard stratum 137 59. Diagrammatic cross-sections of different parts of the valley of Clark's Run. . . . 139 60. Destruction of soil on a slope as a result of denudation in Fox Valley 140 61. Gravel sand spread over field on Cedar Creek 141 62. Gully formed in silt east of Marseilles 143 63. Graph showing chief commodities carried on the canal from 1849 to 1858 169 64. Graph showing tolls collected by Illinois and Michigan canal from 1848 to 1907 . . 171 65. Graph showing tons transported on Illinois and Michigan canal from 1849 to 1907 172 66. Canal boat above Morris, a relic of bygone days 179 67. Locks at Channahon 180 68. Map showing the Illinois Central system in Illinois 182 69. Graph showing grain produced in La Salle, Bureau, and Grundy counties 195 ( 10 ) GEOGRAPHY OF UPPER ILLINOIS VALLEY By Carl Ortwin Sauer CHAPTER I— INTRODUCTION Purpose of Report This bulletin has been written for the purpose of giving a non-technical account of the geology, physiography, and geography of the upper Illinois Valley. The region is rather typical of the great Prairie Plains to which the major part of Illinois belongs and is of interest therefore to many who do not have a first-hand acquaintance with the Middle West. The report is intended, however, primarily for those who live in this area, for the farmers of the prairie, for those engaged in the industries of Illinois Valley, and for the teachers and high-school students of the upper river counties who may wish to read the story that is written in the rocks and soils of their home. Acknowledgments The field work on which this report is based was done in the summer of 1910. Professor R. D. Salisbury and Professor H. H. Barrows furnished valuable criticisms and suggestions in the field. Acknowledgment is due to Professor R. D. Salisbury for a careful ^supervision of the entire work and for the critical reading of the manuscript. Professor H. H. Barrows also revised the last chapter of this bulletin, and made numerous helpful suggestions. To the many residents of the region, who freely aided me in many ways, I wish to extend in cordial remembrance my hearty thanks. ( 11 CHAPTER II— LOCATION AND TOPOGRAPHY Location of Area The region with which this report is concerned is the upper Illinois Valley, located in north-central Illinois, about four-fifths of the way from Ohio River to the Wisconsin State line, and midway between Mississippi River and the Indiana State line. Denned in terms of latitude and longitude, the area lies between meridians 88°10' and 89°25' and parallels 41°15' and 41°30'. The eastern limit of the area included in the bulletin extends somewhat beyond the head of Illinois River (at 88°15'30" west longitude, and 41°23'30" north latitude). Likewise the limit on the west overlaps slightly the "Great Bend" of the Illinois (fig. 1). The upper Illinois Valley is defined for the purposes of this report as that part above the great rectangular bend of the river at Hennepin. In this upper course the river flows almost due west, deviating by only about 6 degrees to the south of this direction. The northernmost point reached by the river is a few miles below its head, at 41°24' north latitude. A straight line drawn thence to the bend shows the maximum deviation of the river from a straight course to be at Seneca, and this is a departure of less than 5 miles. The east-west direction of the river and the linear nature of its valley have played an important part in the economic develop- ment of this region. Figure 1 shows the general location of the area and its relation to other regions on which similar reports have been issued by the State. In the area is included the greater part of La Salle and Grundy counties, smaller portions of Bureau and Putnam counties, and very minor parts of Kendall and Will counties. Of the cities and villages, Ottawa is most centrally located and is the county seat of La Salle County. The most important cities of the western region are La Salle and Peru, a single city in all but corporate limits. Farther west is Spring Valley, the largest town in Bureau County. Morris, the county seat of Grundy County, is the only important place in the eastern part of the region. The United States Geological Survey has divided the region into six rectangular divisions, known as quadrangles, 1 for purposes of uniformity *0f each of these quadrangles, a topographic map (i. e. a map showing features of relief, drainage, and culture) has been prepared by the U. S. Geological Survey, on the scale of one mile to the inch. These maps represent clearly and simply the character of the surface of the region. The U. S. Geological Survey distributes them at 10 cents each. ( 12 ) LOCATION AND TOPOGRAPHY 13 Fig. 1. — General location of Illinois basin and areas described in educational bulletins. 14 UPPER ILLINOIS VALLEY in mapping. Beginning at the east, the quadrangles concerned are: Wilmington, Morris, Marseilles, Ottawa, La Salle, and Hennepin. The shaded area in figure 1 shows the portions of these quadrangles which have been included in the report. The upper Illinois Valley is known to most residents of Illinois because of the Starved Rock State Park, and the busy industrial district centering about La Salle. It has also a distinguished place in State history because 11 Fig. 2 — Graph showing fall at La Salle. average monthly temperatures and average monthly rain- of its role in the early settlement of Illinois, and because the once famous Illinois and Michigan Canal terminates within it. Location as Determining Climate The climate of a region is an expression primarily of the various elements of its location. The most important of these are position in latitude, location with reference to large bodies of water and to mountain masses, and position with reference to prevailing and storm winds. Position in middle latitudes a thousand miles inland, a moderately low elevation, the absence of any nearby highlands, and the location in the LOCATION AND TOPOGRAPHY 15 track of cyclonic storms that come from the west are the leading factors in determining the character of the climate of the Central Prairies, to which the upper Illinois Valley belongs. The position in intermediate latitudes expresses itself in fairly long winter nights and equally long summer days, and in a sun nearly overhead in summer but shining very obliquely in winter. As a result, the seasons are sharply contrasted. The lengthening of the day in the summer months is a factor of some importance in accelerating the growth of vegetation. Interior location has given a continental climate with great temperature ranges, strong and shifting winds, and a moderate rainfall. The distinctness between the seasons has thus been emphasized — the winters are cold and characterized by high winds ; the summers are hot and have rain storms of irregular occurrence. The accompanying graph (fig. 2) shows average temperatures and rainfall at La Salle for each month in the year. On the whole, the advantages of such a climate are great. It is well suited to the production of most temperate-zone field products, particularly of grains. The frost-free season of more than five months, the long, hot summer days, the abundance of moisture during the summer months, and the rather sharp lowering of temperatures during the latter part of the fall provide excellent conditions for the growth of Indian corn, the first crop of the State. Droughts come occasionally, but do not constitute a serious agricultural problem. The health and vitality of the people are favored by the invigorating seasonal changes, the purifying strong winds, the wealth of sunshine, and the moderately low humidity of the air. Climatic extremes are not so great that the activity of man is seriously impaired at any time. The energizing influence of the change of seasons is also one of the causes of the thrift and prosperity of the people. As in all similar latitudes, the need of growing a sufficient surplus to tide over the non-productive winter season has stimulated progress. The big barns which liberally dot the prairie landscape and dominate the cluster of buildings around the farm houses speak not only of a sturdy race of farmers and of fertile soils, but as well of the long winters which have taught the farmer providence. Relation to Central Plains Illinois Valley is located almost in the heart of the great Central Plains or prairies. Eastward the prairies stretch to the plateaus on the western flanks of the Appalachians, westward to the high plains that lead up to the Rockies. Southward they merge gradually into the low Gulf Plain, and at the north the prairie joins the timbered uplands of the northern lakes. In all directions the surface features are very similar for hundreds of miles. All about, the region is one of moderate elevation 16 UPPER ILLINOIS VALLEY (below 1,000 feet) ; the relief is slight, and the surface rather monotonous The characteristic surface shows a uniformly gentle, billowy outline. Transportation lines cross at will, supplying rail facilities wherever there is sufficient traffic. The prairie region is belted particularly by east-west lines of railways that connect the Middle West with the Atlantic and also the Pacific seaboard. Similarity of conditions extends to more than surface features. The conditions of climate which have been traced for La Salle (fig. 2) hold with slight variations for the rest of Illinois and for Indiana and Iowa. The agricultural products are very similar for all the interior prairie States. All are poorly supplied with timber and have, as their only great mineral product, coal. Because of this uniformity of physical conditions, conditions of life have also been similarly uniform throughout the region. The sameness of surface, climate, and resources in the Central Plains has meant a rather even economic development in all parts. The history of the settlement and growth of the upper Illinois basin does not differ in any large measure from that of the adjacent districts. Provincialism has never been a prominent feature. There is a stereotyped quality in all its history, geologic and human, to the present; in a general survey it is essentially the same as that of the surrounding country. It is only in a detailed study that uniformity disappears, and that differences are brought to light which give a stamp of individuality to the region. General Features of Illinois Valley relation to other drainage lines Centrally located within the prairie region, Illinois River is, next to the Ohio, the most important eastern affluent of the Mississippi. It joins the Mississippi about midway between the source and mouth of that stream, and almost opposite the confluence of the Missouri with the Mississippi. Not far below the Ohio enters the Mississippi. This position of Illinois Valley within the greatest developed river basin of the world is most advantageous. It is located centrally to a long line of waterways, which stretch from the gates of the Yellowstone to the base of the Appalachians, and from St. Paul to the Gulf. The Illinois, however, derives an added importance, because of all parts of the Mississippi Basin it is most intimately associated with the Great Lakes. The outflow from Lake Michigan at times during the Ice Age was directed down Illinois Valley. Even now the headwaters of the Illinois crowd the watershed between the Mississippi Basin and the Great Lakes hard against Lake Michigan. Occasionally the abandoned glacial channel leading from lake to river becomes flooded, and water again flows through it to the Illinois. Here LOCATION .AND TOPOGRAPHY 17 then is an all but continuous natural waterway from Lakes to Gulf, which early attracted the attention of men to its completion. DRAINAGE BASIN OF ILLINOIS RIVER The dotted line in figure 1 encloses the drainage area of Illinois River. This river cutting across the State from northeast to southwest, gathers in the drainage from almost half the State, or about 25,000 square miles. Illinois River proper has its source and mouth within the limits of the State, but the Desplaines and the Kankakee, which form the Illinois, and the Fox River, which is the largest tributary entering its upper course, have their sources outside the State. The Desplaines rises in Wisconsin, and the Kankakee in Indiana ; both streams are marginal to Lake Michigan, and their courses are determined by a series of parallel morainic ridges bordering the lake. The size of the drainage basin has been estimated at 32,081 square miles, 1 supporting in 1910 a population of about one and a half millions. Two-thirds of this territory lies south and east of the river, the shorter slope of the drainage basin being formed by the watershed between the Illinois and the Mississippi rivers. DIVISIONS INTO UPPER AND LOWER VALLEY Sixty-three miles below its head, the Illinois changes its course from westward to southward. The turning point is known as the "Great Bend" of Illinois River. Below this point, the valley widens so markedly that even to a casual observer the change is striking. In this report the upper valley is considered as the part above the "Great Bend." In this upper stretch the average and rather uniform width of the valle}' is about one and one-half miles, except in the flat Morris basin, where there scarcely can be said to be distinct valley sides. Below the bend, as one passes Depue, the change is striking. The valley sides recede, the flood plain becomes two to five miles wide, and within it the river wanders about aimlessly. Here and there the floor of the valley narrows abruptly and re-expands below. In this lower part, the width of the valley is two to four times as great as in the upper part. Since valleys normally widen gradually downstream, the immediate inference drawn from the sudden change in width at the bend is that the lower valley is of much greater age than the upper valley. Other features which distinguish the two parts are: (1) the general absence of rocky bluffs in the lower valley and the prevalence of them in the upper valley; (2) the presence of great gravel terraces in the lower valley, and their absence as conspicuous features along the upper course; and (3) the gradient. 1 J. W. Hill, in Water Supply and Irrigation Paper 194, p. 315. L. E. Cooley places the estimate- at 27.914 miles (Lakes-Gulf Waterway). 18 UPPER ILLINOIS VALLEY GRADIENT The change in gradient occurs below the rapids at Starved Rock, and this place has been used in hydrographic surveys to mark a division of the channel into two parts. The gradient of the stream is shown in Table 1. Table 1. — Gradient of Illinois River for different parts along its course From head of Lake Joliet — to Treat 's Island to head of Illinois to Marseilles dam to foot Marseilles rapids to foot Starved Rock rapids to end of I. and M. Canal to end of Hennepin Canal (limit of area covered in this report) to Peoria to Grafton istance Total fall Fall per mile Miles Feet Feet 6.3 10.7 1.70 6.2 7.5 1.41 25.4 8.5 0.34 1.5 18.6 12.40 14.6 20.2 1.30 7.6 1.1 0.15 12.9 1.4 0.10 47.9 2.0 0.04 167.3 23.2 0.14 300 277 Head Marseilles / PEORIA Hennepin yS Bend Q /Starved Grafton . — o Rock 200 Miles 100 Fig. 3. — Profile showing gradient of Illinois River. 25 50 75 Figure 3 shows the gradient of the Illinois in profile. In the upper 63 miles of its course (that portion within the limits of the present report) the stream has a fall of 49.8 feet. In the lower 215 miles of its course the fall is only 25.2 feet. So low is the gradient of the lower course, that in the Mississippi at Grafton have been reported floods higher than low-water level at Utica, more than 240 miles up the valley. 2 The lower valley is eminently useful as a waterway, but entirely unsuited to the development of power ; the upper valley is capable of a very considerable development of power, but in its natural condition is unfit for navigation. Collected from Cooley, L. E., Lakes-Gulf Waterway LOCATION AND TOPOGRAPHY 19 VOLUME OF WATER In its unimproved condition the flow of Illinois River was so irregular that in former years it became a reeking slough in seasons of drought, and in flood-time discharged occasionally a volume of water forty times that of its normal flow. Extreme low water has been reported at Morris, as from 250 to 350 second-feet (cubic feet per second) ; and at La Salle 633 second-feet have been measured. The bank-full capacity at La Salle is about 20,000 second-feet. Periods of extreme drought formerly caused the river to dwindle to a mere ribbon of water within its banks. Once or twice within a decade a maximum of 60,000 to 67,000 feet is reached, or about three times the bank-full capacity of the stream, and about 120 times the average minimum. 3 The average flow of the stream in its various parts is given in Table 2. Table 2. — Average flow of Illinois JRiver in its various; parts given in second-feet Average low water Average high water Gauge station (for three Ordinary (for three driest months) wettest months) Head of Illinois 444 1,577 8,424 Mouth of Fox Kiver. 697 2,369 13,180 Mouth of Vermilion. . 796 2,820 15,066 Mouth of Illinois. . . . 1,904 6,747 36,045 a a Record kept from 1890-1899. Water from drainage canal is not Water Supply and Irrigation Paper 194, p. 159. included in these figures. These great fluctuations of volume are due to: (1) The character of the precipitation, which is irregularly distributed through the year and varies greatly from year to year. (2) Temperature conditions permit snow to accumulate through the winter far beyond the amount of precipitation from any single rain. The snow may melt rapidly in the spring, and the run-off, flowing over the still frozen ground, may flood the valleys suddenly. The spring "break-up" is directly responsible for many floods. Kankakee River, for instance, has a habit of thawing out before the ice moves at Morris, and ice- jams result which flood the lowlands about this city. (3) The character of the soil aids floods. The soil is largely clayey and quite impervious; hence much water runs off, and little sinks in. Observations made on the precipitation show that the greater part of the water reaches the streams by rapid run-off and not by gradual seepage. With the exception of the Morris basin, the slopes of the Illinois and the sides of the tributaries are steep and aid run-off. (4) In the cultivation 3 Report of Internal Improvement Commission of Illinois, p. 23. In Claypool's record at Morris, kept for 56 years, it is shown that during 20 years the river was not out of its banks; in the other 36 years there were 53 floods; the time out of banks averaged 9 days. The greatest recorded flood occurred in 1892; the flood was gauged at Morris as 73,730 feet; at La Salle-Peru as 93,600 feet. See also Cooley, L. E., Lakes-Gulf Waterway, pp. 49-51. 20 UPPER ILLINOIS VALLEY of the land, much of the timber and most of the original soil cover of grasses have been destroyed. Plowed fields with their well-spaced crops present no such check to the rapid run-off of water, as did the forest cover and the matted turf of the original prairie. The floods still come as they did formerly, but periods of low water are no longer seen on the Illinois in their former extremes. The Chicago Ship and Drainage Canal now constantly discharges water from Lake Michigan into the Illinois. In periods of normal or high water this volume is not very noticeable ; but at low-water stage it makes up a great part of the volume of water flowing down the upper valley. The river, which formerly at low water became fouled with the sewage of the upper river towns and was seriously impaired in the use of its water-power, "is now Fig. 4. — Lovers' Leap, looking up Illinois Valley from Starved Bock. a comparatively clear stream to which fish have returned," no longer a menace to public health, and much more valuable for power purposes than formerly. Surface Features of Upper Illinois Valley VALLEY SIDES Scenic effects for the most part are not diversified nor grandly massed in the prairies of Illinois. To this statement the upper Illinois Valley with its varied relief presents an agreeable exception. At its head, the Minooka ridge rises northward, whereas to the south and west a broad, low plain stretches halfway around the horizon. This is the Morris-Kankakee plain, a basin which includes most of Grundy County. The river here flows upon the prairie and has no well-marked banks. At Seneca the river begins to LOCATION AND TOPOGRAPHY 21 sink beneath the prairie, and valley walls become well-defined. The slopes at first are low and gentle, and are farmed or used for pasturage. Downstream, the valley sides steepen and become higher; pastures and fields give way to woods or brush-covered slopes. At Marseilles the valley sides are almost 200 feet high and have become well-defined bluffs. Below Ottawa they become sheer walls, with bare rock faces, most pronounced between Ottawa and the months of the two Vermilion rivers. Between La Salle and the "Great Bend" the slopes are again gentler, and narrow, discontinuous benches appear upon them here and there. In the valley are several large masses of rock which have become detached from the bluffs by erosion. The most conspicuous of these are Buffalo Rock, Starved Rock, and Lovers' Leap (fig. 4) between Ottawa and Utica. These are isolated bodies of sandstone, that rise like towering fortresses above the valley floor. VALLEY FLOOR The width of the valley is quite uniformly one and one-half miles, but the surface of the valley floor varies much from place to place. A true flood plain with broad alluvial bottoms and sloughs has been developed only below Utica. In the Morris basin is much low land which is often flooded. From 10 to 20 feet above the flood plain of the Morris region lie the second bottoms, which are low extensive terraces. The land marginal to the river about Mor- ris is not as desirable as the second bottoms and other lands more distant from the stream, partly because much of the riverward portion is subject to overflow, and partly because much of it is too sandy for the best growth of anything but truck crops. Below the Morris basin and above the mouths of the Vermilion rivers the valley floor consists in the main of a terrace about 40 feet above the narrow channel. About Seneca this terrace is level and covered with a deep soil, well suited to agriculture. Between this point and Utica the floor of the valley is in general irregular, and exposes at numerous places bare rock surfaces. Here is rarely more than a thin veneer of sand or silt above bed rock, and because of the scant soil, most of this part of the valley is not cultivated and is commonly used for pasturage. Occasionally small alluvial fields of high fertility lie beside stony pastures, where the meager soil can scarcely sustain even the grasses against the summer heat. Below Utica the typical alluvial river bottoms reappear. The soil is deep and fertile, but subject to floods and poorly drained, and in its unimproved condition unfit for agriculture in many places. Here we find most of the land given over to the wild growth of swampy bottoms — sycamores, willows, and reeds. 22 UPPER ILLINOIS VALLEY PRAIRIE Beyond the bluffs the prairie begins. As viewed from the valley, the bluff line lies smooth and straight against the sky except for occasional notches made by tributary streams. Viewed from the prairie the valley appears merely as a gash in the generally fiat surface. In reality the bluffs are joined to the upland behind them by a gentle, partially wooded slope which rises 20 to 40 feet in a quarter of a mile or less. The prairie at its riverward margin is 160 to 200 feet above the river level in most places ; away from the valley it rises gently another 50 to 200 feet. Figure 5 shows a profile across the valley at Ottawa and illustrates the general topographic relations for the region. 640 south Ottawa 600 .450 PERU 630 LASALLE 630 L440 Fig. 5. — Cross-sections of Illinois Valley at Ottawa, Peru, Morris, and La Salle showing relation of cities to physiographic features. Numerals represent elevations above sea level. The prairie of La Salle and Bureau counties is typical of northern Illinois. It has a slightly undulating surface, lacking in the ponds and swamps of the country farther north, yet almost unfurrowed by valleys. A few broad ridges interrupt the generally smooth surface: (1) at Princeton; (2) the Farm Ridge south of Utica; (3) most prominent of all, the Marseilles moraine, part of which is known as the Rutland Hills; and (4) the Minooka ridge. These ridges are roughly parallel to one another, and are at right angles to the river valley. Because of their gentle slopes, they are more conspicuous from a distance than nearby. This is part of the best agricultural section of the Middle West — the famed region of prairie farms — in which the upper Illinois country equals any section of this or adjacent states. Almost every foot of ground is cultivated here, and the prospect is one of highly developed farms, because LOCATION AND TOPOGRAPHY 23 of the broad fields, straight roads that run with the cardinal directions, and scattered farmhouses with well-appointed farm buildings grouped about them. These fill the body of the scene, and against the distant horizon may stand a thin line of trees that marks the course of some prairie stream. This is the home of the prairie farmer, one of the finest of American types. TRIBUTARY VALLEYS The character of the drainage of this region is in striking contrast with that of southern Illinois or the Wisconsin border. Compared with the former the drainage is less well developed; there are fewer streams, and these are shorter and have fewer tributaries. Northward, drainage lines are more poorly developed than in this region; streams follow depressions which they have not made, and undrained areas occupied by swamps or ponds become common. The local area is fairly well supplied with surface drainage. Below La Salle the tributary valleys are very wide for the size of the streams that flow through them, and rock outcrops are few. Spring Creek is an example of this type. Between La Salle and Ottawa the tributaries are cut in rock, and have developed canyons that are striking for their scenic beauty. Both the Vermilion river valleys are tortuous chasms in their lower courses and show picturesque rapids and overhanging rock walls. It is about the lower Vermilion Valley and about Starved Rock that scenic attractions are centered especially. Here each turn discloses new scenes of nature's beauty — canyons crowned by a fringe of cedar and pine with a glimpse of blue sky between, bold cliffs of rock that bathe their feet in shining pools, and companies of forest trees encamped around a noisy waterfall. The general effect is one of beauty, almost of grandeur, a sight unlooked for in its impressiveness in a prairie region. At Ottawa, the tributary valleys again grow wider and the slopes less steep. Fox River valley, which joins the Illinois at Ottawa, has rather gently sloping sides, but almost no flood plain. It is much interrupted by rapids and was once made to drive numerous mills. The upper tributaries of the Illinois show a great variety of conditions. Mazon Creek is in its lower course a widely meandering stream with alluvial banks ; its middle course is marked by rapids and rock w r alls ; and the headwaters again are sluggish and shallow prairie streams. The Au Sable is alternately rapid and sluggish, stretches of fairly rapid flow succeeding stagnant pools that are overgrown by water weeds and are the home of waterfowl. 24 upper illinois valley Relation of Topography to Occupations of Man The various surface features — the flat upland, the steep-sided valleys, the alluvial bottoms — have affected the development of the region by man. The surface has either invited or retarded the growth of population, according to the possibilities of its cultivation, the accessibility of its mineral resources, its transportation facilities, and the character of its drainage. CONCENTRATION OF POPULATION IN ILLINOIS VALLEY Comprising only a small part of the area under consideration, the immediate valley of the Illinois has attracted a greater population than has any similar area in this part of the State. Every important city of this region lies within the valley. The rural population, however, is less dense in the valley than upon the prairie, because of the low lying and ill-drained, or the uneven and infertile bottoms. Towns have sprung up in the valley (1) because Illinois River served as the first highroad by which settlers moved into this country, and upon which their early commerce was carried; (2) the first transportation line built, the Illinois and Michigan Canal, followed Illinois Valley because it is the lowest line leading west from Lake Michigan; (3) an early railroad chose the valley of the Illinois for its route, partly because a number of settlements had become established there, but principally to avoid bridging the numerous deep tributary valleys which dissect the upland. The valley still possesses transportation facilities which are superior to those of the adjacent prairie regions. Chiefly by these superior advantages of the valley for transportation, is the growth of its urban population to be explained. A secondary reason is found in the accessibility of its mineral resources. The principal exposures of bed rock and its mineral wealth are in the valley bluffs. It was along the valley that the first development of coal mining took place, and it is here that exploitation of mineral resources is most extensive today. INFLUENCE OF THE SURFACE ON DEVELOPMENT OF TRANSPORTATION LINES The number and direction of transportation lines have been determined chiefly by the character of the surface. Within the area under consideration Illinois Valley presents the easiest line to follow, and it is at the same time the most difficult belt to cross, since it is 150 to 200 feet below the general level of the upland and more than a mile in width. It is therefore an obstacle to communication between the uplands on opposite sides of the Illinois. Roads, both rail and wagon, enter the valley by following tributary valleys, thus reducing the grade of their descent. The North Western and Burlington railroads enter Illinois Valley by way of LOCATION AND TOPOGRAPHY 25 Spring Creek, and the Illinois Central and Burlington similarly cross by way of the two Vermilion rivers. The wagon roads that converge at .Marseilles lead down from prairie to river by following various tributary valleys about Marseilles. On the upland, movement becomes increasingly easier away from Illi- nois Valley, since the tributary valleys become shallower headward and cease to present serious obstructions to transportation. Upon the level prairie, movement is equally easy in all directions. The wagon trails of the pioneers ran in straight lines from settlement to settlement. One of these led from Ottawa diagonally across to Vermilionville ; another struck southeast toward Danville, and because of its straightness became known as the "Danville Air-Line." The best known of all of these pioneer roads, the ' ' Chicago Road, ' ' was worn deep in early days by the trains of ox wagons which sought a market in the distant lake port. This road runs northeastward from Ottawa through Danway. Of these old roads, remnants only are left. The surveyor came soon after the settler and laid out roads by the compass. The old diagonal roads were abandoned for the most part, and the new ones conformed largely to the network of squares laid out by the surveyor. Only those diagonal roads which had become most firmly established as short cuts between settlements have been suffered to remain. In the valleys the roads could not be laid out on section lines, and here they are controlled by the character of the relief. A map of the roads of this region gives some idea of the character of its topography — on the prairie, a rectangular system of roads; in the valleys, irregular roads controlled by the direction of the drainage. LOCATION OF TOWNS The exact location of villages within the valley of the Illinois was determined largely by terraces which furnished room enough for settlement, gave easy access to both river and prairie, and were out of harm's way during floods. Below the mouths of the Vermilion rivers the terraces of the Illinois are discontinuous, and small, so that favorable sites are not numerous in the western part of the valley. Depue, Peru, and La Salle are examples of settlements located on terrace remnants. Between Utica and Morris the broad, high terraces furnished abundant room for settlements. In the Morris basin the lowlands are extensive, and the city of Morris was located in consequence on a terrace which affords reasonable security from flood damages (fig. 5.) The growth as well as the location of the river towns has been influenced by the relations of river floor, terrace, valley side, and upland. Marseilles, dependent upon the rapids of the Illinois, and located north of the river because of canal and rail shipping facilities, had only a narrow 26 UPPER ILLINOIS VALLEY strip of land north of the river available for its expansion and accordingly grew to be two miles long and only two streets wide. Several ravines that come into the valley at Marseilles made it possible for roads to ascend to the upland, and here the newer part of the city has been built. At La Salle and Peru the river has left above its broad alluvial floor a prominent terrace remnant about 60 feet above the stream. At La Salle, the terrace is broader, and the back slope gentler than at Peru; and the past greater growth of La Salle has been due, in part, to the greater amount of available room. Both towns have long since outgrown the limited area of the river terrace, overspread the valley slope, and reached the prairie beyond. At present, with the expansion of both towns on the upland, the only advantage of surface left to La Salle is the gentler slope connecting the upper with the lower town. In both cases the broad, low, alluvial bottom precluded the growth to the southern side of the valley, as was the case at Ottawa, and as may be the case at Marseilles. In La Salle, First and Second streets occupy the terrace flat. Back of Second Street is a rise of 70 to 80 feet to Fifth Street. Beyond Fifth Street the city lies upon the prairie. A similar condition prevails at Peru. Thus it happened that as these cities outgrew their terraces the people living in the newer or prairie section found their dependence upon the valley section inconvenient. To the older, lower business district there was added a second business district on the hill which avoided the difficulties of the intervening slope. In Peru the upper business section has passed the lower in importance, because it serves the majority of the city's population. Figure 5 shows cross-sections for the cities of Peru, La Salle, Ottawa, and Morris, and represents graphically the conditions of surface which, in different ways, have influenced the conditions of growth of these places. In building on the slopes of Illinois Valley, artificial terracing has been resorted to extensively. The houses front chiefly on roads that parallel the strike of the slope. Because the lots above the road have a more commanding position than those below, the more expensive residences have been built on terraces above the road, and humbler houses on the unterraced side below the road. Because bared hillsides wash readily, the care of the roads early be- came a necessity, and excellent macadamized and paved streets are the rule in these river towns. RELATION OF TOPOGRAPHY TO UTILIZATION OF LAND A great advantage of the Prairie States for agriculture lies in their surface, the greater part of which is sufficiently flat for cultivation, and for the use of machinery in the production of crops. In the prairie townships of this region, almost every foot of ground may be cultivated. LOCATION AND TOPOGRAPHY 27 Farm Ridge and Miller townships in La Salle County, for example, are made up almost entirely of cultivated fields. Near Illinois Valley, the surface is not so favorable for agriculture. Because the river is depressed more than 150 feet below the level of the prairie, the riverward margin of the upland has become dissected by numerous tributaries. These tributary valleys, as well as the Illinois, have slopes in general too steep for cultivation, except in the Morris basin. On both sides of Illinois Valley, therefore, is a belt of timbered land or of pasture, varying from one-fourth to one-half mile or more in width. Correspondingly narrower belts flank the tributaries. Beyond the immediate valley slopes, however, the upland is nearly flat, so that cultiva- tion on the upland may extend to the margins of the valleys. In the region of Starved Rock with its box-like valleys, the prairie fields run in many places almost to the brinks of the canyons. As the valleys grow shallower headward, the amount of waste land decreases, so that the upper third of many valleys consists of cultivated fields or of meadows. During the growing season, some of these " draws" on the prairie are more readily discovered on the map than in the field, as even a stand of tall corn may obscure the shallow depression. The larger stream lines may be accurately followed by noting the lines of trees that almost invariably follow them. Practically the only timber left in the region is in the valleys, and they fill accordingly an important position in the agricultural economy of the prairie. 4 The larger tributaries furnish in some cases limited areas of farming land on the alluvial flats of their lower courses, but their chief uses are for pasturage and for timber supply. In the valley of the Illinois is considerable low-lying land which is either marshy or subject to flood and has not been cultivated. The most of these first (lowest) bottoms lie below Utica and about Morris. The quality of the land is excellent, and its only drawback is its lack of drainage. It may be expected confidently that its reclamation will take place within a brief period and will add an important class of lands to those already farmed. EFFECT OF TOPOGRAPHY ON ECONOMIC AND SOCIAL CONDITIONS The character of the surface affects the culture and prosperity of the region in many ways, chiefly through the conditions of communication and of agriculture. The prairie farmer (1) can put practically his entire farm under the plow and make all his land productive, and (2) has had the drudgery of farming reduced to a minimum because he is able to use machinery extensively. The hill farmer, on the other hand, (1) can clear 4 The timbered slopes of the valleys are of course less valuable than the flat surface of the prairie. Agriculturally, therefore, those townships are most desirable which lie far enough from the river to have a minimum of dissected surface. In Farm Ridge township, the farms were said in 1910 to be worth, on an average, $200 per acre, whereas the more "broken" land marginal to the valley of the Illinois sold for $125 to $150, and the land which was all in timber for $50 to $75. 28 UPPER ILLINOIS VALLEY only part of his land; (2) fields are small and uneven, so that much hand labor is required in the production of his crops; and (3) the soil is poorer than on the prairie and needs more care in cultivation, so that slope wash may not remove the rich surface materials. The hill farmer must work harder than his neighbor of the prairie for smaller returns. Prosperity thus avoids the timbered fringe of the valleys and keeps to the open prairie. The uneven surface imposes a handicap upon the hill farmer in the marketing of his products as well as in their production; he begins with a harder row to hoe, and ends with a harder road to travel to market. Socially an equally great advantage lies with the prairie farmer. It requires less time for him to do an equal amount of work than it does the hill farmer. Consequently he has more leisure than the latter for social purposes. His neighbors are also nearer and easier to reach because of better roads. As a result the prairie farmer develops by social contact, whereas the other too frequently retrogrades in his isolation. The lot of the average farmer in this region is excellent both as regards his farm labors and his social opportunities. But even in this area, examples can be found of this difference in condition, illustrated most strikingly by the highly developed prairie farms of Vermilion Township, contrasted with the isolated backward farms which are tucked away in the be*nds of the chasm-like Big Vermilion River. CHAPTER III— DESCRIPTION AND HISTORY OF THE HARD ROCKS Classes of Sedimentary Rocks and Their Origin general processes Bedded roeks or "rock ledges" may be seen along almost every valley in this region and offer abundant opportunities for studying geologic history. Well records and mine shafts furnish additional information concerning the materials underground. The local geologic record is of particular interest, both because it shows a diversity of geologic history which cannot be duplicated in the State, and because the life of the people of this section is bound up most intimately with its mineral resources. The geologic history is such that, with a little help, anyone who will may read it in the characteristics of the formations of the bed rock and their relations to each other. The simple fundamental idea is, that these bedded rocks are deposits of sediment, such as mud or sand, which formed a very long time ago, on land or under water became buried by other deposits and were slowly hardened into rock. The change of many of these rocks from their original condition has not been great, and the origin of the formations may still be seen clearly. There is no reason to believe that the processes of the geologic past differed greatly from those now in operation at the surface of the earth. Streams, waves, and winds were then at work as at present. It is necessary merely to remember that the scene of activity of the various geologic processes has been shifted from time to time. Where now there are farming lands there once may have been a shallow sea, and waves shifted about the sand which now appears in the sandstone of the valley sides. Wind, water, and ice have acted at various times as agents of deposition in this region, but of these water has been by far the most important in the geologic record. The work of water has consisted partly in dissolving and redepositing rock matter, but more largely in the mechanical transportation of sand and mud. By depositing these materials in large quantities water has been responsible chiefly in the formation of clastic sediments, the most common class of sedimentary rock. MECHANICAL OR CLASTIC SEDIMENTARY ROCKS Most of the rocks composed of mechanical or clastic sediments were formed by shore or stream deposition. The size of the materials which water may handle depends upon the vigor of its movement. The upper ( 29 ) 30 UPPER ILLINOIS VALLEY part of a stream has, as a rule, the most rapid flow. Here the transporting power is generalty great, sand and silt are carried easily by the swift current, and gravel only is lodged in the stream bed. Even large stones at the bottom are subjected to vigorous wear by the incessant pounding of rock fragments upon them, and in time may be so reduced in size that they may be rolled along by the current. Downstream the velocity gradually lessens, and the stream's ability to carry coarse material is decreased correspondingly. It is thus forced to drop successively finer and finer sediments, first gravel, then sand, and lastly silt. In its lower course it may be able to handle only fine sand and mud, alternately depositing and removing them as the current varies in strength or amount of load. Some of the mud may be carried out to sea and built into deltas. Similarly slwre deposits vary according to the strength of the waves which formed them. Where the waves break in shallow water, and especially where they dash against the shore, gravel and sand may be the most abundant materials. With increasing depth of water the waves agitate the bottom of the water less and less, and finer sediment is shifted about. From the shore outward the sediments commonly grade from gravel along the beach, to fine mud in the deep water. These sediments have formed three general sorts of sedimentary rock, which are based on contrasts of texture. 1. The mud deposited by ancient streams or seas may have changed only slightly and is called clay. If it has been compressed and cemented it becomes shale. Under great pressure, shale may be converted into slate, which cleaves into thin sheets like roofing slate. 2. Sandstone is cemented sand. If the water which circulates through the pores in the sand carries dissolved mineral matter and deposits it between the grains, the individual grains become cemented and sandstone is formed. If silica (the substance of sand itself) forms the cement, a hard and durable sandstone or quartzite is the result. A cement of a lime or other carbonate, on the other hand, is easily redissolved, and a sandstone with such a cement weathers rapidly on exposure. 3. Conglomerate is the rock equivalent of gravel. It too may vary greatly in compactness and in its resistance to weathering, according to the kind of gravel from which it was formed and the degree of pressure and the kind of cementation to which it has been subjected. The distribution of conglomerate is generally much more limited than that of the other clastic sediments because gravel is deposited less generally than either sand or mud. Shales and sandstones abound in this region ; conglomerate on the other hand is rare. HARD ROCKS 3I ORGANIC SEDIMENTARY ROCKS In rocks of organic origin, the agency of plants or animals or both is essential. LIMESTONE In the sea water is a vast number of animals, largely shell fish, which secrete lime carbonate. When they die their remains, consisting largely of lime carbonate, may sink to the bottom and there accumulate in large beds. These beds may be hardened into limestone. There are other ways in which limestone is formed, but this is the most common. A pure limestone signifies ordinarily a clear sea as the place of its origin. If the limestone is clayey, the floor of the sea in which it accumulated was muddied by the inflow of streams or by the drag of waves. COAL That coal is derived from plant remains is evident to anyone who has observed the imprints of leaves, the portions of stems, the woody fiber, and even the roots common in soft coal. The beginning of the story of coal may be read from almost any swamp or bog. A body of quiet water is required, into which little or no sand or mud is washed, and which is shallow enough for the growth of plants. The seeds and dead leaves and stems drop into the water which soon acquires preservative qualities that arrest decay. By the continued accumulation and partial preservation under water of plant matter, peat is formed, the first step in the formation of coal. The next step takes place by the burial of the peat beneath sediments. But coal is more than compressed vegetable matter, for the vegetable tissues have suffered chemical changes. The overlying sediments exert pressure and shut off the free access of air and water. As a result, chemical changes take place which cause the buried vegetation to give off gases that are combinations of oxygen, hydrogen, and carbon, the principal constituents of organic matter. More oxygen and hydrogen are given off than is carbon, so that the percentage of remaining carbon increases with time. This concentration of carbon gives coal its high fuel value. 1 Com- pression and loss by chemical change are so great in the formation of coal that the vegetable growth of at least 3,000 to 4,000 years is estimated as required to afford material for one foot of coal. 2 Both coal and limestone are of wide distribution in this region. a The process of burning consists in the combination of oxygen from the air with the carbon of the fuel. -Ashley, Geo. H., Economic Geology, vol. 2, p. 47. 32 UPPER ILLINOIS VALLEY SPECIAL FEATURES OF ROCKS Certain special features may be mentioned which are not peculiar to any one kind of local rock and which occur prominently in several formations of the region. VEINS Some of the limestones along the Vermilion River, particularly at Oglesby, show irregular bands or veins of white crystals (calcite) that run at various angles to the beds or bedding planes of the rock. These veins are generally short, and many of them are not connected. The St. Peter sandstone at Dayton affords a striking example of vein fillings : The river floor at that place has a peculiarly honeycombed appearance, caused by Fig. 6.— Honeycombed bed of Fox River at Dayton. The knife-like ridges are resistant veins in the St. Peter sandstone. knife-like ridges in the rock which intersect each other variously (fig. 6). These ridges are caused by veins of harder material in the softer sandstone, exposed through stream erosion. Veins are the filling of cracks in rock. Due to some strain, a rock develops cracks in which the circulating underground water deposits some of its dissolved mineral material. Ordinarily, ground water circulating in a limestone formation fills these crevices with calcium carbonate, which HARD ROCKS 33 crystallizes into calcite; in sandstone the veins are commonly of silica and very resistant to weathering, as shown by the small ridges in the St. Peter sandstone. CONCRETIONS Concretions are to be seen in widely varying forms in practically every formation, from the oldest bed rock, the limestone of the npper Prairie dn Chien group (Lower Magnesian) at Utica to the post-glacial clays found along the Illinois Valley. Figure 7 shows concretions which have weathered out in the bed of the Au Sable Creek, immediately above the aqueduct. The rock in which they occur is a sandstone containing many shining flakes of mica; the concretions consist of a groundmass of plates of calcite in which are set grains of sand and plates of mica. In these concretions the materials of the sandstone have been replaced largely by calcium carbonate (calcite). They range from a spherical to a flattened, disc-shaped form, and occasionally are twin groivths. Some of the most famous concretions of the country are from Mazon Creek. They are flattened, elliptical bodies of a hard iron-bearing shale, imbedded in a soft clay shale. The nucleus about which these, concretions formed, consists of parts of plants or animals that chanced to be buried in the Carboniferous mud. Fern leaves or bits of bark are the most common nuclei, but occasionally insects, small fishes, and other material have had these concretionary forms cased about them. The perfection of their preservation, to the minutest and most delicate detail, is marvelous. The shape of the nodules corresponds somewhat to the form of the enclosed leaf or animal. In chemical composition, concretions are commonly unlike the formation in which they are found : The Prairie du Chien limestone carries concretions of silica (chert) ; in the St. Peter sandstone, the concretions are composed of iron compounds (largely pyrite or iron oxides) ; "ironstones" are most common in the "Coal Measures" clays, and pyrite in coal ; the concretions in the Carboniferous sandstones are mostly calcium carbonate. Concretions are formed after the deposition of the material in which they are imbedded, chiefly by the action of ground water, which by selective solution and deposition of the minor constituents of a formation tends to segregate these ' ' impurities. ' ' About a convenient nucleus the circulating waters deposit a film of some mineral, continuing the process and building ever larger concentric layers about the older films, until a concretion is developed. This process may go on until the most of a minor mineral of a formation is extracted from the main mass, and assembled in these concretions. 34 UPPER ILLINOIS VALLEY Fig. 7. — Views on An Sable Creek above the aqueduct of the Illinois and Michigan Canal. The bed of the creek is here covered with disc-shaped and spherical concretions originally contained in the soft Carboniferous sandstone and left behind when stream erosion removed the sandstone hard rocks 35 Hard Rocks of Illinois Valley unexposed rocks By erosion and deposition continued through many millions of years the earth has beeome mantled generally with sediments of great variety, which are disposed in rather orderly succession, and record within themselves the past history of the region. The oldest formation definitely known to exist beneath this region is not exposed at the surface within the limits of the State. Its presence is known through deep-well drillings only. By this method it has been located in the eastern portion of the upper Illinois Valley where the rock formations lie at a higher elevation than farther west. Hence the older and deeper-lying formations are more easily reached by borings at the east than at the west. This completely buried formation is the Potsdam sandstone. It has been located at Ottawa, at a depth of about 1,100 feet. The rock is porous and carries a great amount of water. It comes to the surface in central Wisconsin and is a prolific source of water for many deep wells in the southeastern part of that State, as well as in northeastern Illinois. EXPOSED ROCKS PRAIRIE DU CHIEN GROUP The Prairie du Chien group (formerly known as the "Lower Magnesian" limestone) comprises the oldest formation which appears at the surface in Illinois. Its largest area of outcrop is in this region, distributed in three principal localities (PL II). The thickness of this formation is several hundred feet. The most extensive of these is a belt about two and a half miles wide between Utica and Split Rock. Its eastern limit coincides with the eastern limit of the village of Utica. Westward it rises to the prairie beyond the northern bluff of the valley. Southward it crosses the river a short distance below the Utica bridge. Along the lower Pecumsaugan Creek it outcrops rather extensively on the upland. A few hundred feet east of Split Rock, the formation dips beneath the St. Peter sandstone, and disappears under the floor of the valley. The second outcrop is on Tomahawk Creek, a tributary of the Little Vermilion River. This outcrop is intersected by the road which crosses the creek half a mile north of Mitchel School. It is confined almost entirely to the floor and sides of the valley and is exposed for a distance of slightly more than half a mile. A similar outcrop occurs on Little Vermilion River northwest of the one mentioned above. These three outcrops form a straight line running somewhat west of north to east of south. The Prairie du Chien is one of the great limestone formations of the Middle West. It is well known locally because of its beds of hydraulic- 36 UPPER ILLINOIS VALLEY cement rock. It is not common limestone (calcium carbonate), but a magnesian limestone (calcium magnesium carbonate) called dolomite. Dolomites are harder and more resistant to weathering than true limestones. A little clay is present very generally in the Prairie du Chien limestone as an impurity, and here and there is also some sand. In its upper part the formation contains thin beds of quite pure sand, alternating with beds of limestone which have little or no sand. In its lower exposed parts sand is almost absent, and the formation consists of massive dolomite, either clayey or pure. The color of the limestone in fresh exposures is a dull Fig. 8. — Quarry in St. Peter sandstone near Twin Bluffs. drab. Weathered surfaces have a slightly buff color on account of the iron which has been oxidized where the stone has been exposed to the air. The presence of iron in any formation is, as a rule, readily betrayed by weathering, as iron compounds soon oxidize (rust) and become brownish yellow. The Prairie du Chien limestone is characterized by a lack of persistent qualities. Variations of mineral composition are shown by the very irregular surface which the dolomite develops on weathering, due to the unequal solubility of its various parts. It is variously thick and thin bedded ; some beds may be a dozen feet thick, others are mere laminae a dozen of which may not exceed a foot in thickness. Where rather pure, 4 3 2 1 O Scale K>Mil^« I 8kC0 BALTIMORE, Ml Prairie du Chien group (Lower Magnesian formation) (Massiw magnesian limestone carry- ing a little clay and sand as impurities; in the upper part distinct thin beds of sand are present) HARD ROCKS 37 it is finely crystalline in texture, but this quality disappears with an increase of clay. One of its most striking features is its beds of concretionary chert, which is siliceous matter, akin to flint. The formation is highly concretionary, and most of the concretions are segregated into rather distinct layers. Many of the cherts are 8 to 12 inches in diameter and if broken through, show beautiful banding due to concentric deposition. Fig. 9. — Tributary canyon in Deer Park Glen. This small canyon, like the larger ones about it, is cut in St. Peter sandstone. It shows the unequal erosion of the sand- stone, and in its caldron the light streaks indicate the deposition of soluble salts, chiefly magnesium and calcium carbonate, leached from the cement of the sandstone. ST. PETER SANDSTONE Next in the geological series is the nonfossiliferous St. Peter sandstone, which lies above the Prairie du Chien limestone and ranges from 140 to 200 feet in thickness. Its area of surface exposures within the State is almost as small as that of the preceding formation, and most of its outcrops are disposed marginally about the outcrops of the older formation. In this region the western limit is Split Rock, two and a half miles east of 38 UPPER ILLINOIS VALLEY La Salle, and thence it extends across to a similar point on the south side of the valley. South of the river the outcrop does not rise above the bluff line. Northward it occurs as the surface formation beneath the upland prairie from Little Vermilion River eastward to Clark's Run. East of Utica its surface declines rapidly. In Ottawa it is slightly above the level of the canal, and a mile and a half to the east it dips beneath the floor of the valley. In the Fox Valley the formation is exposed at the surface, with slight interruptions, far beyond the area covered by this report. A small isolated outcrop has been exposed in Deer Park Glen. The St. Peter sandstone is characterized by a striking uniformity of qualities. It is throughout a sandstone of unusual purity. The well- rounded sand grains are fine, generally of dazzling whiteness in fresh exposures, and almost without admixture of clay, the absence of which makes it valuable for the manufacture of glass. Cementation has commonly been slight so that the freshly exposed sandstone may be freely worked with pick and shovel. Figure 8 shows a characteristic exposure of the sandstone with accumulations of loose sand at the base of the pit. The cementing material is most commonly silica; but in places a little iron oxide, and more rarely calcium-magnesium carbonate, is present. Figure 9 shows white stains in the caldron at the base of the falls. This is mostly silica and calcium magnesium carbonate which has been leached out of the cement of the sandstone. Locally there are distinct veins of the sandstone, and in veins a little sand is included. The veins are of quartz, and where the sand is included in them the vein looks something like quartzite. On Lower Buck Creek above Wedron an unusual and beautiful form of cementation may be seen. The sand is here cemented by iron sulphide (pyrite), and the blue-gray quartz grains set in the glinting gold-colored pyrite flash like precious gems. The water which comes from the St. Peter sandstone is heavily charged with sulphureted hydrogen from the decomposition of the pyrite. A well penetrating to this sandstone may be recognized almost unmistakably by the sulphurous taste of its water. Concretions, although rather common, are inconspicuous. They generally contain more iron oxide than the body of the rock, and by reason of their superior resistance and darker color, they are conspicuous on weathered surfaces as irregular reddish-brown knobs. They may be seen on Starved Rock. On Tomahawk Creek the contact between the Prairie du Chien limestone and the overlying St. Peter sandstone (fig. 10) shows: (1) that with uniformly dipping beds the line of contact between the two formations varies considerably and irregularly in elevation. (2) On close examination of the surface the contact shows an irregular line separating the two formations ; at one place this line departs from the dip 9 inches vertically within a horizontal distance of two feet. (3) At the contact may be HARD ROCKS 39 observed in places loose cherts and blue, noncalcareous clay, the products of long weathering. (4) The two formations are quite distinct at the plane of contact; the Prairie du Chien below is typical dolomite; the St. Peter above as typical a sandstone. There is no gradation. The phenomena mentioned under (1), (2), and (3) record an interruption in the process of sedimentation, known as unconformity, and the fourth point is consistent with the other three. The general relation is shown by figure 10 ; a conformable relation is shown in figure 12. In the conformable relationship, the change from the deposition of one kind of sediment to OT C_n Uower Magnesian limestone 7K~ Fig. 10. — Diagrammatic illustration of the unconformable relations of the Prairie du Chien ("Lower Magnesian") limestone and the St. Peter sandstone. another was gradual, and there was no break in sedimentation. The unconformable relationship indicates that after deposition of the older formation sedimentation was stopped, the surface was eroded, and the products of weathering accumulated before later beds were deposited upon the older formation. Similar evidence of an unconformity between the Prairie du Chien and the St. Peter may be secured in abundance along Illinois Valley, particularly in the second ravine east of Split Rock. PLATTEVILLE-GALENA LIMESTONE The Platteville-Galena limestone (formerly called the "Trenton- Galena" limestone) is a general name used for the Middle Ordovician limestone which includes both the Galena (now correlated exactly with the Trenton proper) and the somewhat older Platteville limestone. 40 UPPER ILLINOIS VALLEY The distribution of this formation, for causes to be noted later, is irregular. Many of the outcrops are too small to be shown on the sketch map (PL II). The three principal districts where this formation outcrops in this region are (1) along the line of the Vermilion rivers, notably at Deer Park and on the Little Vermilion about Troy Grove; (2) in the vicinity of Ottawa, including a broad outcrop on the valley floor west of Ottawa, and a narrow area occupying the valley of lower Covel Creek ; and (3) an obscurely defined area east of Morris on Au Sable Creek. The thickness of the formation is more variable than that of any other formation exposed in this region. Over a considerable part of the area Platteville-Galena limestone • St. Peter sandstone Fig. 11. — Diagrammatic illustration of the unconformable relation between the St. Peter sandstone and Platteville-Galena (''Trenton") limestone. underlain by formations younger than St. Peter sandstone, it is wanting entirely, whereas the two older formations are present everywhere beneath the beds of later age. Especially to the west, it is in irregular remnants of slight thickness between the St. Peter sandstone and the Pennsylvanian series ("Coal Measures"), or else is missing. In most of these patches it is not more than 20 to 50 feet thick. Westward and southward it thickens considerably and it is also more persistent. At Lowell it has been reported about 200 feet thick, at Marseilles it is 56 feet thick, and the Chicago, Rock Island, and Pacific Railway well at Morris records a thickness of 200 feet. The Platteville in this region is a limestone formation. The color is commonly a light drab, which changes to buff on weathering. Its texture is finely to moderately crystalline. On Au Sable Creek the lower part of the formation is gray, crystalline limestone containing large plates of calcite and disseminated particles of zinc blende and pyrite, appearing as shining metallic spots in exposed faces. Perhaps the most distinctive feature of the limestone is its unusual hardness, which has given to it unfavorable notoriety, particularly among well drillers. As it is of quite HARD ROCKS 41 uniform composition, it weathers very evenly. Thin films of clay between beds of purer limestone cause it to weather into thin slab-like layers, whereas in fresh cuts it appears massive. The formation carries abundant fossils, but these are confined mostly to certain beds. Among the fossils the shells of brachiopods and the cylindrical stems of crinoids are most abundant. The Platteville rests unconformably upon the St. Peter sandstone. In Deer Park Glen, immediately above the falls, a good exposure may be seen, which shows the contact as an irregular wavy line. Again at the Federal Plate Glass Company's plant west of Ottawa and on Covel Creek, the unconformity is well shown. In this section the Platteville-Galena lies in depressions in the St. Peter sandstone. A cross-section on Covel Creek is represented diagrammatically in figure 11, which shows outcrops of St. Peter sandstone rising well above the strip of Platteville-Galena limestone whieh they enclose. The uneven base of the Platteville-Galena may also be seen along the bank of the Illinois opposite the mouth of Covel Creek. RICHMOND LIMESTONE The next younger formation of this region, the Kiehmond limestone, is the surface formation in the extreme eastern part of the area. It is wanting between Morris and La Salle, but at La Salle, it is again found far beneath the younger "Coal Measures." East of Morris, between Morris and Au Sable Creek, it is covered by a slight thickness of the ' ' Coal Measures." The westernmost outcrop is on Au Sable Creek, almost directly above Sand Ridge. Thence outcrops continue up the Au Sable far into Kendall County. Southward it outcrops widely on the valley floor between the canal and the river and again south of the river. At Goose Lake the line of outcrop bends sharply to the east and thence loops back to a point about a mile below the head of the Illinois. The country here is flat and outcrops are few and indistinct. As exposed within this area, the Richmond limestone consists of the eroded remnants of a formation which was later buried by "Coal Measures." Its thickness is therefore very irregular and slight, compared with its development east of this region. The Richmond limestone is the only member of the Cincinnatian series, recognized in outcrops in this region. (Cincinnatian is a group name for the upper part of the Ordovician system.) The formation is rather uniform in its characteristics — massive, coarsely crystalline, and high in iron content, "When fresh the stone is hard, but it weathers rapidly into thin beds and assumes a granular appearance because of its coarse texture. This formation is by far the richest in fossils of any in the region ; fossils in great number, variety, and range of size, crowd its beds. 42 UPPER ILLINOIS VALLEY The Richmond beds doubtlessly rests unconformable on the underlying Platteville, though no contact has been seen in the region. It is known, however, that several formations of intermediate age are missing, and this absence records a break in deposition. NIAGARAN LIMESTONE The Niagaran limestone does not outcrop in this area, but is prominent to the east. West of La Salle the limestone is reported in well drillings. PENNSYLVANIAN SERIES In each of the formations described above, a distribution marginal to that of the next older formation is to be noted (PL IV). As one goes toward the vicinity of Utica from either east or west the younger forma- tions disappear successively, and older ones come to the surface in their places. Lying over parts of all the formations mentioned heretofore, are the Pennsylvanian series of strata belonging to the Carboniferous period. This series is commonly termed the ' ' Coal Measures. ' ' With the exception of narrow belts restricted almost wholly to valleys, the "Coal Measures" underlie the entire area west of Au Sable Creek. The relations to the older formation are shown by the cross-section on Plate II. East of this region the "Coal Measures" outcrop in a narrow, irregular belt in the valley of Dupage River. The northern limit does not extend beyond La Salle County, but the formation has a great extent west and south. ; |jj ~, The "Coal Measures" of this region were deposited in an extensive, shallow basin, the long axis of which stretches southeastward from La Salle to the mouth of the Wabash. The beds dip toward this axis except where they have been deformed, and toward this axis the formation thickens. At Morris (on the rim of the basin) it is only 64 feet thick; in La Salle County (near the axis) the maximum thickness is reported at 570 feet; in Bureau County, west of the axis, the formation varies from 250 to 400 feet in thickness. The great variations in thickness are the result (1) partly of the unequal deposition in a great basin made up of several minor basins, so that the original thickness was variable; (2) partly of unequal erosion or removal of unequal amounts from different parts of the area during the long period of exposure at the surface. One of the most striking characteristics of the ' ' Coal Measures ' ' is that almost all sediments are repeated again and again in any considerable vertical section. Figure 12 shows the following succession of beds on Cedar Creek: (1) shale, (2) coal, (3) shale, (4) limestone, (5) shale, (6) coal, (7) shale (overlain by drift.) Not only do vertical sections of the ' ' Coal Measures ' ' vary greatly, but most beds vary horizontally within HARD ROCKS 43 short distances, both as to thickness and kinds of rock. The basal beds in some places are sandstone, and in others clay as at Lowell, where potter's clay is found below coal No. 2. Above the ox-bow of Mazon Creek these horizontal changes are well shown: (1) At the ox-bow, at the base of the Fig. 12. — Exposure of "Coal Measures" on Cedar Creek. This section shows well the variety of strata in the ' ' Coal Measures " : a, shale ; b, coal ; c, shale ; d, limestone ; e, shale; /, coal; g, shale; and h, cover of glacial drift. bluff, is a shaly limestone, upon which lies soft, blue clay, and upon this, a thin sandy shale. (2) Upstream the blue clay becomes shaly, and the shales grade into sandstones. (3) Farther on all give way to sandstone. (4) Upstream again this sandstone becomes a sandy shale, and (5) finally in the shale thick beds of blue clay appear over thin beds of impure limestone. Coal beds vary greatly in thickness within short distances; above the Farm Ridge crossing of Big Vermilion River, may be seen a sandy 44 UPPER ILLINOIS VALLEY bituminous shale which stains the water with oil. Southward this bed develops into a workable coal seam. From these widespread and irregular variations it is judged that the beds were deposited under very unstable and variable conditions. In spite of these variations, sediments in the "Coal Measures" show characteristics which set them off from other formations. The Carboniferous sandstones are distinguished from all other sandstones of the region by the flakes of shining mica and the small crystals of calcite which they carry. The sandstone varies from very shaly phases, as found on Mazon Creek, to a freestone used for building purposes, as on lower Au Sable Creek. The sandstone is cross-bedded (thin beds at an angle with the main bedding planes,) in places so sharply as to appear deformed. The best development of the sandstone is along the margins of the old basin, particularly about Morris, where it appears that sand was accumulating at the same time that clay was being deposited in deeper water to the south and west. The clays and shales likewise bear a strong resemblance to each other throughout the series. At certain horizons they are the most persistent members of the series. The texture is in many places marvelously smooth, particularly that of the fire clays which are almost without grit, and become plastic when wet. Good exposures of thinly cleaving (slaty) shales may been seen on the Big Vermilion below Lowell. Other phases are represented in various places. Pyrite is found in the fire clays. As a result the fire-brick companies prefer to use the clays near the surface, from which the pyrite has been removed by oxidation and leaching. Concretions are very prominent in the "Coal Measures." Near the mouth of Tomahawk Creek, the creek bottom is covered by large, generally flattened, concretions. Most of them are at least three times as long as wide, and many reach 6 feet in diameter. The concretions are peculiar in that they are crossed by several sets of radial cracks, which break up their surfaces into rude geometrical forms (trapezoids). The cracks are filled with colored crystals, composed largely of calcium carbonate, or with shaly matter harder than the body of the concretion. These concretions are known as septaria, and were formed by the cracking of the concretions subsequent to their formation, and the filling of the cracks with mineral matter deposited from solution, forming veins. Septaria may be seen at Lowell and on some of the eastern affluents of the Fox. The widely distributed ironstone concretions, often of fantastic shape, and the fern- concretions of Mazon, also belong here. Limestones are developed extensively in the upper part of the "Coal Measures" only. They arc best represented west and south of the area of older rocks about Utica. The uppermost limestone has been named the La Salle limestone, which outcrops particularly along the two Vermilion HARD ROCKS 45 rivers. Bailey's Falls arc over it, and along the line of its outcrop arc located the Portland cement plants of La Salle and Portland. It has an upper and a lower phase, is finely crystalline, of a blue-gray color, compact, thick bedded, moderately fossiliferous, and in places mottled with vein ealcite. The limestone contains a varying amount of clay and a little iron oxide. Coal is the least of the formations in quantity, but its economic value is greater than that of all other local mineral resources. Of the half dozen •••••• • • • - .' • . ' . . •' • • • • • *. *. . • • • • • • • ••'•••'.•••.•• •••••.•••;•••■,•-•••'.••■•. • \ ' ~ i ~ r ^r 1 - .* • • *••'•"". ', '••'•'••'.'• : '. . ■ ' ' ' ' , . ures ''.'.'• ■' ' -' :'• .'•'. *•. ■-'.' • . - . Coal Meas ', '")..•.•'.'••.• . ■ . '• ■ .. \ .•*.•.'.' \ '••'«'■•.'•. • • • '.•..'*'.• ' ''...'•''''.:.'. '.\.'~ ' •' ' '.' ' ' ■ '• '. ' • 7T~S • • • '• • '. .'•"••' <"^v ' '. '•'••.•'.. >•'( lW0^M^f :\'\:i\-y^\:'':} ■v^@v/;:^\::«;': SllSBHil ))[\\'}':\[ \ }\'':-\-\ :'/'■[ W0:^':^:l:' : ^M:^:^--v'-'-'-:- :: yf--. V~^ / ;i;^:V;::V/'st, P '.V:.'\:--.'«'. ';'.•'•'• '.'■:■'■'.'■' '•']y^\$A-}lY$'X ■ '•'■:!:}^\-''.'y-\; : '::::: : Bter)sandstone .*;.••; : . - : ■ : :'• • .' •' -. : •/.*,' : *. ; - ; .• ■ '• : ; Fig. 13. — Diagrammatic illustration of the unconformable relation of the St. Peter sandstone and the Pennsylvanian ("Coal Measures") series. coal beds in a single section, not more than two or three are of economic importance. The most valuable coal bed is about 12 feet above the base of the series. This is coal No. 2, familiarly known as the "Third Vein" coal. The bed averages about three feet in thickness, and furnishes coal of unusually good quality. Two thicker beds of poorer coal lie above it. The coal varies considerably in hardness and composition. Particularly undesirable are concretions of pyrite, which form platy clusters or have replaced portions of stalks or of bark. The unconformable relations of the "Coal Measures" with the underlying formations may be established in almost any section that shows a contact with one of the older formations. At the crossing of the 46 UPPER ILLINOIS VALLEY Little Vermilion along the La Salle-Dimmick town-line road, the relations of figure 13 are shown. Here the basal Carboniferous or "Coal Measures" sandstone lies on the St. Peter sandstone. The latter was fissured, the fissures were filled, and then erosion wore down the sandstone so that the vein fillings stood out in relief before the first "Coal Measures" sands were deposited upon it. In many places the "Coal Measures" may be seen lying in a depression in the older rock, similar to the Platteville noted above. The position of the "Coal Measures" upon each and all of the formations of the region serves to establish its unconformity with all but the youngest underlying formation. Field evidence of a break in the history of sedimentation between the Richmond limestone and "Coal Measures" is lacking. It is known however that all the formations of two great intervening periods, the Silurian and Devonian, are wanting. Within the "Coal Measures" are numerous minor and local unconformities. These represent only short intervals of erosion in limited areas, and are not comparable to the breaks between larger divisions of the geologic series. An unconformity of this sort may be seen in the eastern part of the city of Marseilles, where "Coal Measures" sandstone may be seen overlying "Coal Measures" shale with an erosion contact. On Covel Creek, half a mile south of Hitt's, eroded shale is overlain by sandstone. Structure of the Rocks general southward dip In most sedimentation, the beds are laid down in layers that are almost horizontal. Sediments are deposited normally on gently sloping surfaces, either on the floor of a sea or on low-lying land. The beds thus formed have a slope which may be too slight to be noted by the eye. The sea encroached upon this region from the south, and the sediments on its sloping floor had a slight southward dip. This may be shown by the fact that to the southward successively younger formations are encountered, and that the elevation of the various formations above sea level decreases constantly, although slowly, southward. The Potsdam sandstone outcrops at the surface several hundred miles to the north of this region ; here it is about 600 feet beneath the level of the sea. This obscure depositional dip, however, is locally masked by deformation which has warped every formation of the region out of its original position. LA SALLE ANTICLINE The older beds in the west-central part of the region about Utica have been brought to the surface by their upfolding into a great arch, or anticline. This anticline crosses Illinois Valley between Utica and the two HARD ROCKS 47 Vermilion rivers, crosses the Little Vermilion on Tomahawk Creek and again below Dimmick, and is again seen to the northwest at Dixon on Rock River, where it brings the St. Peter sandstone to the surface. Southward it passes out of this area at Lowell. The axis of the fold is about N.20°W. (in the shaft of Black Hollow mine recorded as N.12° W.) Its general course is shown by the outcrops of the inclined La Salle limestone which run northwestward from the bluffs of the Illinois at a point half a mile west of Split Rock to the Little Vermilion. The line of greatest uplift, or the axis of the fold crosses the north bluff of the Illinois about at the mouth of the Pecumsaugan canyon, for here the Prairie du Chien, the oldest formation, is at its maximum elevation. On both sides the beds dip away, more gradually on the east than on the west. The western flank of the fold shows dips exceeding 30 degrees, which carry the Prairie du Chien limestone beneath the valley floor within half a mile of the crest of the fold, where the limestone outcrops more than 160 feet above the river. The dip on the western flank is so steep that the outcrops of the various formations are tilted nearly on end the entire St. Peter sandstone, for instance, having a surface outcrop only about 120 yards wide. On the easteim flank of the fold the dip is very gentle, not over 5 degrees, and the beds are in many places apparently horizontal. As a result the outcrops are wide; the eastern limit of the outcrop of the Prairie du Chien limestone is two miles east of the crest of the fold. The width of outcrop of the St. Peter sandstone on the eastern flank of the anticline, with the same thickness as on the west, is about 11 miles. These comparisons show strikingly the unsymmetrical character of the fold (see Plate II). In the bowing up of the strata a great many beds of greatly varying resistance were involved. The weakest beds yielded most readily. On the western flank of the fold where the deformation was most severe, the shaly beds in the Prairie du Chien formation have been crumpled into sharp folds, whereas the beds of limestone and sandstone between are tilted, or perhaps broken, but not crumpled. Where the more resistant beds were broken in the deformation, the clay was forced in around the fragments, filling the spaces between and giving the rock a brecciated character. The La Salle anticline was not developed all at one time. The folding consisted of several movements extending through long periods of time, which did not cease finally, until long after the first beds were deformed. The Prairie du Chien shows beds which have been deformed more than any later ones. These beds and other rather indefinite data suggest the possibility of a first deformation after the deposition of the Prairie du Chien limestone. There is positive evidence of deformation between the Platteville-Galena epoch and the beginning of the "Coal Measures" period. During this interval, the first great bowing up of the strata occurred. At 48 UPPER ILLINOIS VALLEY the close of the "Coal Measures" period, the beds were again deformed. These two great periods of deformation find confirmation in many places along the western flank of the fold. In many places the Platteville and older formations show dips of 30, 32, and 40 degrees, and directly overlying them are the Carboniferous beds having a dip of less than 20 degrees and commonly less than 15 degrees. Good exposures of this disparity of dip between the "Coal Measures" and the older formations may be seen at Split Rock, and on the Big Vermilion just above the mouth of Deer Park Glen. MINOR DEFORMATIONS Upon this large fold, minor folds were developed locally. The most notable are listed as follows: (1) Opposite La Salle at the suspension Fig. 14. — Small syncline in the ' ' Coal Measures ' ; along Big Vermilion Biver below Lowell. bridge on the Little Vermilion a trough, less than twenty feet deep, has been formed in the La Salle limestone. (2) Another minor syncline (structural trough) on the Big Vermilion is illustrated in figure 14. (3) The St. Peter sandstone shows several deformations, notably at Wedron on Fox Eiver. On the eastern flank of the anticline, the gentle eastward dip carries the St. Peter sandstone beneath the surface a short distance above Ottawa (both in the Illinois and Fox valleys). Farther up Fox Valley the formation reappears in three prominent outcrops about Dayton, about Wedron, and about Sheridan. At Wedron the sandstone rises more than 120 feet above the normal elevation of its surface. At this place the sandstone is domed up, and dips to the south. A lesser bowing has probably exposed the formation in the river bed at Dayton. (4) Sags and HARD ROCKS 49 swells abound in the "Coal Measures," although they arc of very slight extent, both vertically and horizontally. In the Morris basin almost every creek shows such minor warpings of the beds. They appear through a considerable part of the ' * Coal Measures ' ' series, indicating the frequency of crustal warping in the course of the Carboniferous period. Occasionally the beds have been faulted. In shaft No. 3 of the Spring Valley Coal Company a fault has been encountered in which the beds have suffered a vertical displacement of 11 feet. These minor deformations are rather more common here than in most similar areas. The may be connected causally with the development of the larger anticline. History of Formation of Hard Eocks Geologic time has been divided into five principal eras. The history of the bedded rocks of this region falls entirely within the third of these, the Paleozoic, which includes the oldest sedimentary rocks with abundant remains of life. Of the oldest Paleozoic period, the Cambrian, there is no surface record here, but the buried Potsdam sandstone indicates that at the time of its deposition this region was covered by a shallow sea, which shifted sand widely over the area of the present interior plains. The second Paleozoic period, the Ordovician, was probably begun by a change to a clearer sea, in which marine life was abundant and formed in large part the beds of the Prairie du Chien limestone. Occasionally, the waters were disturbed by waves which carried in thin deposits of sand or mixed silt with the organic remains on the sea floor. The variable nature of the beds may point as well to rather frequent slight changes in the depth of the sea, putting its floor at times within reach of wave drag, and at times of deeper water protecting it from such agitation. The early Ordovician sea spread widely over the central states, the nearest land being in northern Wisconsin. It is known to have persisted for a very considerable time, allowing the deposition of a considerable thickness of limestone on the floor of a slowly sinking sea bottom. Later the sea withdrew from the region, and the newly formed land was exposed to weathering and erosion by streams. The land surface became gullied and generally uneven. This erosion interval is expressed by the unconformable contact between the Prairie du Chien and the St. Peter formations. Observations made in other regions indicate that the withdrawal of the sea was widespread and affected an area much greater than northern Illinois. The third scene in Ordovician history was introduced by another depression of the land. It is possible that the sea encroached again over the region, but if so the water was somewhat less extensive and shallower than before, for in it sands only were laid down. These later hardened 50 UPPER ILLIKOIS VALLEY into the St. Peter sandstone. It is possible that from some rather nearby land area, probably northern Wisconsin, rivers may have brought down great masses of sand to the sea coast, there to be shifted about by the wind and the waves. The conditions during the deposition of the St. Peter sand- stone were wonderfully uniform, as the sandstone shows almost no variation from top to bottom. This may be explained by a slowly and uniformly sinking land surface, by which the conditions for the deposition of sand were maintained constantly. This period of depression was of relatively short duration; the deposition of 200 feet of sandstone required probably but a fraction of the time which was needed for the formation of the Prairie du Chien limestone. Again the land was elevated and the surface of the St. Peter formation eroded. This erosion interval is not established over as wide an area as the preceding one. The Platteville sea which followed was perhaps more extensive than any since the Potsdam. The limestone is very uniform in character, and indicates deposition in water sufficiently deep to prevent the washing in of mud. Later the sea became more shallow, and mud was again swept in to form, after a time, the Cincinnatian shales. Another deepening of the sea brought with it abundant shell-bearing life, which accumulated in the beds of the Richmond limestone. After the deposition of the Richmond limestone, the sea withdrew again, and throughout the middle west a long interval of erosion followed, terminated by the invasion of the Niagaran sea, in which accumulated one of the most notable limestone formations of the interior. Another oscillation caused the region to emerge from the sea and brought it into a position to be eroded. After this time submergence of the land is not known to have taken place until the Pennsylvanian ("Coal Measures") period. If there was submergence in the meantime, as in the Devonian or Mississippian periods, that fact is not known. If formations of these periods were ever deposited here, they were completely removed by erosion before the period of the "Coal Measures." The first great recorded growth of the La Salle anticline occurred after the deposition of the Platteville and before the formation of the "Coal Measures," as shown by the contrasted dips of these formations. Deformation may have begun in the Middle Ordovician period, even during the Platteville epoch. The bowing up of the anticline was doubtlessly very slow. The arching may have elevated the older beds above sea level, and exposed them to erosion, at the same time that sediments were accumulating around the deformed area. This would account for the absence in the anticline of beds intermediate between the Platteville-Galena formation and the ' ' Coal Measures. ' ' HARD ROCKS 5l In the "Coal Measures" period sediments were again deposited over all the area. Deposition may have been due in part to the gradual wearing down of the land surface by stream erosion, which reduced it to a low, marshy condition, with sluggish streams; but the region was also at times beneath the sea. In contrast to the previous uniformity over large areas there were in the "Coal Measures" stage many small and variable basins in which deposition took place. Muddy water, clear water, and exposed land surfaces were within short distances of one another, and deposits of mud, sand, and limy material took place contemporaneously, while adjacent areas perhaps received no deposits at all. The greatest uniformity is found in the limestone members of the series, formed during epochs of depression while the sea level stood safely above the entire surface of the region. The greatest variety of conditions was recorded while the region lay about at the sea level, and very slight oscillations furnished the conditions for erosion, or for the deposition of coal, shale, sandstone, and sometimes of limestone. At the beginning of the "Coal Measures" period, beach conditions prevailed, under which much sand was shifted about. Marshes formed at the edges of the sea, and in them accumulated vegetable material which later formed coal. Oscillations of the sea level were very numerous, but a general tendency toward greater submergence became marked as the period progressed. Slightly submerged marshes gave way more and more to deeper waters, and these in turn to the open sea in which the formation of sand and shale was succeeded by that of limestone. The upper horizons of the "Coal Measures" are largely limestone, and indicate marine conditions for rather a long time toward the close of the period. Most of the Pennsylvanian limestone is in the upper horizons of the series, and most of the coal is in the lower. Shales are most pronounced in the upper part of the series. After the close of the period, deformation affected the central area again. Along practically the same axis as before, the beds were again folded, but not so severely as in post-Platteville-Galena time. Perhaps this folding Avas a minor expression of the great movements that were then taking place in many parts of the earth, as in the Appalachian region, and which brought the Paleozoic with its ancient forms of life to a close. With this uplift the history of marine deposition in this region closes. The great interior sea withdrew permanently, and the later history deals with processes that shape land surfaces and not with the beat of ocean waves. Pre-Glacial Topography and Its History character of bed-rock surface Outcrops of bed rock are widely distributed, but occupy only a very small part of the surface of the region. In most of the area the bed rock 52 UPPER ILLINOIS VALLEY is concealed by a thick cover of clay, sand, and gravel, of very unequal and irregular thickness. The present surface of the land shows only a slight similarity to the surface of the bed rock. A reconstruction of the buried bed-rock surface could be attempted only after an exhaustive study of the region, particularly after a close notation of elevations of outcrop, and an elaborate cataloguing of well records to show the distance of bed rock beneath the surface. The material for such a reconstruction is not now at hand, so that only certain large features can be stated definitely, and suggestions given which point to other conditions. On the whole, the surface of the bed rock is much more irregular than the present land surface, the thick drift cover hiding entirely in several places, ridges and depressions in the bed rock, having a vertical extent of several hundred feet. If these depressions and elevations could be traced in their entirety, they would be found to form buried valleys and ridges. Were the drift cover stripped from the region, the place of the present flat prairies would be taken by a region of rather sharp valleys and narrow, ridged uplands. These valleys were more numerous and deeper than those of the streams which now drain the region. The major buried depressions known are as follows: 1. In the western part of the region the surface of the bed rock declines into a great linear depression which runs southward from Rock River to Princeton, and thence follows the line of the Illinois. The present surface of the land lies quite generally well above 600 feet above sea level. At Spring Valley the surface of the bed rock is about at 600 feet. North of Marquette it declines to 500 feet. Bed rock has not been found at an elevation greater than 400 feet at Depue or in Hennepin Township, Putnam County. Four miles farther west, at Bureau Junction, the rock surface is only 340 feet above sea level. West of Princeton the rock surface again rises rather sharply. These records furnish a section across a buried valley, parts of which are covered by at least 350 feet of loose materials, and of which the present surface of the land shows no trace. Leverett 2 has reconstructed this old valley southward from Rock River to its junction with the present line of the Illinois at the "Great Bend." Where it joins the valley of the Illinois its floor is a hundred feet beneath the present channel. It has been suggested by Leverett that this buried valley may be in part the pre-glacial valley of Mississippi River. Certain it is that here was a pre-glacial valley, greater than the present Illinois Valley, both in depth and width, and it probably held a stream larger than the Illinois of today. Figure 15 is an attempted reconstruction of the old drainage system. Well records indicate several affluents to this buried valley above Hennepin. 3 Leverett, Frank, U. S. Geological Survey Mon. 38, Chap. 12, PI. XII. HARD ROCKS 53 One of these probably has been occupied by the Illinois below La Salle. That part of Illinois Valley with an alluvial floor below La Salle is considerably older than the rock-floored valley above. Very clear indications of this older valley are given above the bend. At Allforks Creek and at Marquette, the old valley bottom is at the very least 50 feet below the present channel. Another tributary appears to have come in from the northeast through Hollowayville and Ladd. 2. From La Salle to Marseilles and beyond, the elevation of the bed-rock surface is rather uniformly at about 600 feet above sea level. Fig. 15. — Reconstruction of the Rock-Illinois Valley. The dashed line is the course of the present Rock River (after Leverett). Buried depressions have been observed in but few places, and these record small steep-sided valleys cut in the general pre-glacial upland which occupied central La Salle County. On Buck and Indian creeks above Wedron, such old valleys are exposed in cross-section along the sides of the creeks. North of Marseilles, however, particularly through central Miller Township, well drillers have encountered repeatedly a large depression which appears to follow a northeast-southwest line, and which in at least one case reported descends to about 475 feet above tide. 3. South of Illinois Valley, another large buried drainage line can be traced for a distance of about 10 miles. This line has been followed from the Farm Ridge crossing on the Big Vermilion River, eastward to Grand 54 UPPER ILLINOIS VALLEY Ridge. Beyond, a number of wells in Grand Rapids and Brookfield townships record apparently a continuation of this valley to Illinois Valley above Seneca. In a series of wells west of Grand Ridge, the bed-rock surface falls to at least 430 feet above sea level, and a drilling near Vermilion River passed through sand and gravel to a depth of 70 feet below the level of that stream. 3 For more than two miles along the Vermilion south of Lowell, no rock is exposed either on the floor or sides of the valley. About three-fourths of a mile above the Farm Ridge crossing the surface materials may be seen resting against a sloping surface of rock, which marks one valley side of the buried river course. This valley, of which this fragmentary record was discovered, appears to have been comparable to the present Illinois both in depth and width. 4. Above Marseilles the elevation at which bed rock is found decreases rapidly. At Marseilles it is still 600 feet, or almost 150 feet above the river, but south of Seneca it drops below the level of the river. North of the river the surface of the bed rock does not descend beneath the valley floor except for a short distance in Erienna Township. In all the region east of Seneca, bedded rock is inconspicuous, and in numerous places its surface is at a considerable distance beneath the Morris Basin, which appears to be another broad, low-lying, pre-glacial valley. The general character of the bed-rock surface is that of a broad, elevated central region from Spring Valley to Seneca, flanked on each side by an extensive depression — on the west by the old valley at Princeton, on the east by the low Morris Basin. In the central elevated section the maximum elevation of bed rock is quite uniformly in the neighborhood of 600 feet above sea level. The surface is here and there depressed beneath this level, but a line extending across the summit elevations would coincide almost exactly with the 600-foot contour. The even surface of the bed rock is expressed by the level-topped valley bluffs which have but a slight covering of drift. In the frontispiece, the panoramic view from Starved Rock shows plainly the even sky line of the opposite side of the valley, which is rock almost to the top. This view reaches from La Salle to Buffalo Rock and shows the level surface stretching uninterruptedly across the whole anticline and including some of the horizontally bedded rocks on each side. This plane surface cuts straight across a great variety of forma- tions very unequally resistant to erosion, which range from the hard Prairie du Chien limestone about Utica, to the readily eroded Carboniferous clays and shales of Ottawa, and the intermediate St. Peter sandstone. Similarly, away from the valley, wells within this central zone commonly penetrate bed rock at an elevation of about 600 feet. Such a plane rock surface developed upon rocks of unequal hardness, is called a peneplain. information by Mr. Williams of Grand Ridge. HARD ROCKS 55 HISTORY OF PRE-GLACIAL EROSION PERIOD Between the deposition of the youngest member of the "Coal Measures" and the formation of the drift which covers the bed rock, many geologic periods passed involving great changes in the history of the earth through many millions of years. The youngest bed-rock formation of the region belongs to the ancient history of the earth ; the drift cover to modern geologic time. The character of the eroded surface of the bed rock is almost the only local record of what transpired in the time that intervened. While in many other regions great deposits accumulated in the intervening periods, in most of the eastern half of the North American continent, geologic activity was confined to the wearing down of the land by weathering and stream erosion. During this great interval, erosion was the dominant geologic process within this area, as deposition had been previously. Of the varying fortunes of the region during this long time, probably only the last chapters have been preserved in the character of the eroded surface. This shows particularly two distinctive features: (1) the central, elevated plain, and (2) a well-drained region, considerably dissected, with several broad, low valleys. The development of a peneplain is a late stage in the long process of erosion. The surface run-off erodes most readily where the material is least resistant and soon develops valleys on the weaker rock. The more resistant rocks thus gradually come to stand out as ridges above the more rapidly eroded softer materials. This differential rate of erosion causes a constantly increasing difference in elevation between the ridges of harder rock and the valleys of weaker rock, until the valleys have been brought as low as running w T ater can erode. When the gradient of the main streams has become too slight for further erosion, slope wash and minor streams flowing down the slopes of the harder ridges still continue actively to remove material. By the wearing down of the harder ridges, while the depressions remain at a constant elevation, relief gradually is lessened ; the valleys wait, as it were, for the ridges to be brought down to the level which they long since reached. Finally, when the whole region is brought as low as running water can erode its surface, the ridges disappear and a generally flat surface is the result. A flat surface produced in this way is a base level. When the surface has been reduced nearly to flatness it is a peneplain. Peneplanation is the only satisfactory explanation for the formation of an extensive flat surface across a region of folded rocks so unequal in hardness as in the La Salle anticline. How often the processes of erosion leveled the land to a monotonous plain, and how often the dying streams were quickened into new activity by uplift of the land, we have no means of knowing. One such cycle of erosion is, however, preserved in the flat summits of the central area. 56 UPPER ILLINOIS VALLEY Peneplanation was followed by an uplift and a reestablishment of vigorous drainage. Valleys were cut into the general flat, and again some of the larger streams lowered their floors to base level, and formed broad flood plains as indicated by the broad valley at Princeton. Central and eastern La Salle County were dissected by smaller streams which had not destroyed the older flat surface. The relief, therefore, was greater then than now. At this point the erosion history was interrupted by the mantling of the old surface by glacial drift. Although the general history of the interval is concerned with degradational processes, the conditions were afforded locally for occasional sedimentation, as in river flood plains. Lying upon the bed rock and apparently older than the drift are occasional thin beds of gravel. These are known particularly in the western region, and are shown in sections on lower Spring Creek, and on lower Negro Creek. The gravels are of local materials, mostly cherts and quartz, considerably weathered. Similar old stream gravels of probable pre-glacial age may be seen on the east side of Fox River, in the sand pits just above Wedron. These few fragments comprise the local record of a period of time comparable to that involved in the deposition of all the rocks of this region. CHAPTER IV— ICE AGE Relation of Drift Cover to Bed Rock In the weathering of solid rock there is formed gradually a mantle of rock waste on its surface (fig. 16). Such a rock cover has the following characteristics: (1) It grades downward from soil through subsoil into partially decomposed rock, and finally into firm bed rock. (2) Since it is merely the weathered outer portion of the bed rock, its surface conforms in outline rather closely to the surface of the bed rock. (3) Its thickness depends largely on the slope of the surface, being thickest on flats and thinnest on steep slopes. Slope wash keeps pace with or exceeds rock decay Fig. 16. — Diagrammatic illustration of the relation of mantle rock to the under- lying rock from which it was derived (courtesy of U. S. Geological Survey). on many slopes, so that on hillsides the rock cover is kept at a slight thickness. (4) Since the soil is residual from the decay of the underlying rock, its chemical composition is limited to the range of mineral elements found in the bed rock, and of these it contains for the most part only the relatively insoluble constituents. These characteristics are common to the greater part of the surface of the earth. The upland soils of the region south of Ohio River are of this nature, as are those of southwestern Wis- consin and part of northwestern Illinois. The rock cover in this region differs from that noted above in several particulars : 1. Contact between drift and bed rock is commonly clearly defined. Figure 16 shows contact by weathering; figures 29 and 30 show two types of contact of the local mantle (drift) with fresh bed rock. Figure 29 shows a clear-cut contact between drift and a coal bed. Figure 30 shows till (material worn and deposited by glacial ice) at the top of the section, and below is shown till mixed with fire clay and shale. Still farther down the material becomes a mass of disrupted fragments of ''Coal Measures" with occasional masses of drift. At the base of the section the fire clay of the "Coal Measures" may be seen in position passing from the upper crumpled beds to the lower undisturbed horizontal beds. This second type of contact, however, is evidently not a gradation due to weathering, but is the result of the forcible mixing of the different materials. ( 57 ) 58 UPPER ILLINOIS VALLEY On the whole the sharpest contacts between drift and bed rock are found where the drift overlies a resistant formation, such as one of the various limestones of the region. This is well shown along the Big and Little Ver- milion rivers (La Salle and Platteville-Galena limestones), and on Au Sable Creek (Platteville-Galena and Richmond limestones). These harder rocks show planed, smooth, and striated surfaces in many places. On the other hand, shales and clays rarely have a definite contact with the drift. Figure 17 shows a relation of the latter sort from the pit of the Utica Fire Fig. 17. — Diagrammatic illustration of the indefinite relations of soft bed rock and drift as seen in the pit of the Utica Fire Brick Company near Utica. Brick Company, half a mile south of the river at Utica. The top of the coal bed is very much crumpled; above it are several inches of residual material; and above this is fire clay mixed with till and grading upward into till. Clay pits between La Salle and Peru on Sixth Street show similar relations between drift and blue clay shales. The nature of the contact varies therefore with the hardness and texture of the bed rock, being sharp where the material is resistant, and indistinct where it is soft. The contact also depends upon the character of the surface of the bed rock. Where the rock forms an elevation, the contact is commonly definite, and in the rock depressions considerable weathered material may be left beneath the drift. 2. The surface of the drift does not correspond in most places to the surface of the bed rock. Although the surface of the bed rock falls off sharply west of Spring Valley, the land rises in this direction. Most of the pre-glacial valleys in the rock (see Chap. Ill) are buried so effectively as ICE AGE 59 not to leave the slightest surface evidence of their existence. The topography of the upland is entirely the topography of the drift, except along Illinois River, where most of the drift has been removed, and to some extent along Farm Ridge, which appears to be a moraine (ridge of drift) emphasized by an unusual elevation of bed rock. 3. The thickness of the drift varies greatly, being thicker in the pre-glacial valleys and thinner above the pre-glacial ridges of rock. The average thickness of the drift is much greater than is common for a residual soil, being well over 50 feet on an average for this region. The thinnest drift is immediately adjacent to Illinois Valley, particularly in the Morris basin. The greatest accumulations of drift known in the region are (1) in the old valley at Princeton, where it reaches a thickness of at least 350 feet, (2) north of Marseilles in Miller Township (250 feet), and (3) east of Grand Ridge. 4. Tlie composition of the drift is not limited to the materials of the underlying rock. The drift in one part of the area does not vary markedly from the drift in any other part, although it overlies different formations in different places. Limestone is the most important stony constituent of the drift, whether the drift covers a limestone formation, or rests upon shale or sandstone. In any single section of drift are found not only fragments of almost all kinds of sedimentary rock, but many kinds of rock entirely foreign to this region. They include crystalline rocks, igneous and metamorphic, whose nearest possible source was the Lake Superior region. Averages from the eastern part of the region give fully 50 per cent of the smaller stony material (about 1 inch in diameter) as limestone, about 25 per cent shale, 5 per cent chert, 5 per cent sandstone, and the remainder igneous rock. Dark-colored crystalline rocks (basic igneous rocks chiefly dark schists, trap rock, and gabbros) outnumber the light crystalline rocks of acid composition, such as granite, by a ratio of at least three to one. Among the larger bowlders, the percentage of igneous rock increases materially. Occasionally a glittering bit of hematite tells of its source in the iron country of Lake Superior. 5. The drift is made up of fresh, not weathered, rock materials. These characteristics of the drift indicate (1) that the agent which deposited it stripped the loose weathered material from the surface of the bed rock. (2) Some of the bed rock it smoothed or scratched. (3) In places it mixed the drift with the bed rock. This agent also was competent (4) to deposit great thicknesses of drift over wide areas, independently of the character of the underlying surface. (5) The agent which formed the drift collected rocks of many kinds without discrimination as to size, (6) ground them to various sizes and shapes, and (7) transported them great distances. Glacier ice can do these things, and is the only agent which can. 60 UPPER ILLINOIS VALLEY Materials of Drift and Their Origin till The till, bowlder clay or "hard pan" as it is commonly called in this region, forms by far the largest part of the drift. Outside Illinois Valley, it probably makes up more than nine-tenths of the mass of the drift. Several characteristics of the till show that it was formed and deposited directly by glacial ice: 1. Most striking is the absolute lack of assortment of its materials. A typical section of till shows clay, sand, gravel, and bowlders of all sizes mixed together indiscriminately. The main body of the till consists generally of clay (fig. 18). Fig. 18. — Glacial till along Indian Creek. The section here is about 60 feet. dark streak slightly below the top of the section is a bed of silt. The 2. Another characteristic is the large size of some of the material found in the till. Most of it is fine enough to have been carried by vigorously flowing water; but bowlders larger than a man's head are exceedingly common. They are strewn over valley bottoms, left by streams which in excavating their valleys have carried away the fine material of the till but left the large rocks. Here and there fence rows are piled high with rocks taken from adjacent fields. Bowlders weighing several tons are not uncommon, and a few of those in the region weigh ten to fifteen tons. Most of these are of distant origin, consisting of blocks of resistant igneous rock. ICE AGE 61 Figure L9 shows one on South Kickapoo Creek, weighing about ten tons. The carrying of 10-ton bowlders for five hundred miles or more, as in the case of the one mentioned, demands a transporting agent for which present conditions offer no parallel in this region. JHXmKssI ^M^HflHHB^«n\fii^! 1 ^■Sit^S^- ^^ EEs^^Pl Fig. 19. — Large igneous bowlder on South Kickapoo Creek. Compare the size with that of the hammer on top of the bowlder. Fig. 20. — Sketches illustrating the characteristics of glaciated bowlders. 3. Shapes of tJie bowlders. Many of the stones of the drift have distinctive shapes, especially flattened sides or faces, which meet at vary- 62 UPPER ILLINOIS VALLEY mg angles and give to the bowlders a subangular form (fig. 20). A well- glaciated bowlder has neither the rounded outline of a water-worn stone, nor the irregular surface and jagged edges of a newly broken fragment of rock. The flat faces have no particular relation to each other, unless the rock has a tendency to break along certain planes. In such cases the rock has been flattened along its planes of cleavage. In addition to planed faces, glaciated stones commonly show linear scratches known as striae. Many stones have been well polished on their smooth faces. The degree to which these features are developed depends largely on the hardness and texture of the rock ; moderately hard limestones are more apt to show them than friable sandstones or excessively hard igneous rock. The ma- jority of the stones in the till do not show these characteristics to any great extent. Many of them are not very different from water- worn peb- bles or broken rock fragments. But so many bowlders are well planed and striated, and so many others show these characteristics to some extent, that they become significant. The subangular forms, particularly, suggest that the bowlders were gripped in a vise and planed. This vise was the ice in which the stones were imbedded and then ground by friction against its bed, and against other bowlders. The stones were polished by the fine material carried in the ice, and scratched by fragments of hard rock against which they scraped. 4. In chemical composition, the clay of the till differs from the clays produced by weathering. The latter are known as residual clays. In their making, the soluble compounds of the rock are leached out largely, leaving the insoluble remainder as earthy matter, which if made up of very small particles, is commonly called clay. Glacial clay, on the other hand, contains all the constituents originally present in the rocks which were ground up in its formation. It may properly be called rock flour, as it is the fine product of the grinding of rocks in the glacier. It is made mostly from the weaker rocks, such as shales which crumbled readily under the pressure of the ice. Fragments of shale are not easily recognizable in the local till, although it is the most common rock constituent. This is due to the ease with which shale was ground up into the clay which forms the body of the till. 5. The till occasionally shows a kind of cleavage in wavy lines. Such cleavage (foliation) may be seen in the gravel-pits on the lower road to the west of North Kickapoo Creek. These bands are not a feature of deposition, but are due to shearing under the pressure of the moving ice. STRATIFIED DRIFT Unlike the till, the stratified drift is of limited distribution and generally of slight thickness. It was deposited by waters from the melting ice, and its materials were assorted into beds composed of sediments of similar sizes, according to the transporting power of the water which ICE AGE 63 deposited them. Most of the stratified drift shows distinct planes of bedding, but even where these are wanting the material has been assorted by stream action. Much of it is but slightly water worn, as the time during which it was in transport was too short for the development of water-rounded surfaces. The stratified drift may be divided into two classes according to its distribution: (1) that associated with present drainage lines, and (2) that of irregular distribution, not in valleys. Each of these classes may be divided into (a) surface deposits, and (b) deposits covered by till. 1. The most conspicuous beds of stratified drift are those in Illinois Valley and in the valleys of its larger tributaries. The surface deposits belong to two groups. The first group is composed of a series of gravel beds that extend westward beyond the area covered by this report; they lie at high levels, generally near the top of the valley slopes, extend up the Fig. 21. — Gravel bed of coarsely bedded "high-level" gravels on Cedar Creek. larger tributaries, and occur in beds as much as 60 feet thick. They will be described more fully in the succeeding chapter, as the "high-level" gravels (fig. 21). The second group extends into this area from the valleys of the Dupage, Desplaines, and Kankakee rivers, as part of the Late Wisconsin valley train. These beds have no definite limit downstream, 64 UPPER ILLINOIS VALLEY but disappear gradually below Marseilles. They are at lower levels than the former group and are confined more nearly to the main valley. They are finer, being, on the whole, sandy rather than gravelly. Some of the stratified drift of the valleys is covered by till. The two most important groups of this kind are a series of gravel beds between Seneca and Marseilles, best developed on the northern side of the valley in beds up to 70 feet thick to be described later as the "Kickapoo beds," and buried gravels and sands west of Peru extending beyond Spring Valley. The latter are mostly on the south side of the valley, and will be discussed later as the "Peru beds." 2. The stratified drift outside the valleys is of sparse and irregular distribution. Here and there patches of sandy or gravelly material may be seen on the prairie. These are of limited extent and slight thickness. An occasional knoll of gravel, known as a kame, may be seen along the front (west side) of the Farm Ridge moraine and similarly on the head- waters of Covel Creek; but most of the stratified drift is not so disposed as to make pronounced topographic features. The materials found in these deposits in general are not so coarse as those in the beds marginal to the valleys. The sands and gravels on the prairie were deposited in large part by water which flowed over a rather flat surface and without great velocity. Bedding is generally obscure and the gravels in many places have some clay mixed with them and are then known locally as loamy gravels. In large part, these deposits of stratified drift were left by the water flowing from the receding ice front. Buried in and beneath till sheets is much stratified drift which has no relation to existing valleys. This drift consists in many places of thin sheets of sand and gravel interleaved with till. Such material is found in almost all the wells of the region. (Sections will be given later.) Here and there thicker bodies of stratified drift lie beneath the surface till. Beds of this sort may record old valleys which were filled in part by glacial waters, and then obliterated by the deposition of till above them. LOESS Loess is typically a loam, intermediate in texture between clay and sand. On microscopic examination it shows many flattened particles. Though altogether uncemented, a vertical face of it once developed will stand for years (fig. 22). The prevailing color is buff, but drab may be seen in places. Shells of land snails (gastropods) are common in loess. Most of its material locally is fresh and quite calcareous, concretions of lime carbonate being abundant in many places. Most loess is now commonly thought to have been dust deposited by the wind. Much loess was formed at various times during the Ice Age. After the retreat of an ice sheet, the sweep of the winds over the bare surface was particularly effective in ICE AGE 65 blowing about the fine material left by the ice. As soon as the climate became mild enough to allow the growth of plants, the cover of vegetation arrested the accumulation of loess. An important source of loess was the silt-laden water that issued from the melting glacier, and built up great mud flats which, in drying, supplied the prairie winds abundantly with dust. There is much silt very similar to loess, associated with the drift of this region, but it contains occasional bowlders which loess lacks. They imply deposition by water, in which floated ice containing imbedded bowlders that were dropped on melting. East of Spring Valley there is little true loess at the surface. West of Spring Valley it increases in amount, and is well developed on the prairies W^^ . . :•%■• 2'; !isl . £]T "ViB*; ''■■■** *rj/f* ' 'A* •; 4- f ^^sS**t ; :;■ i — V '*'■ - ' .' ^*- — ^-* — ^— ~—L— — -J-"^f-, - A , Fig. 22. — Loess under gravel on Spring Creek, and the initials carved in it. Note the vertical sides of the loess about Bureau Creek. Here it attains a thickness of 30 feet in places, and rests either directly on till, or on gravel which overlies till. The prairie east and south of the Hennepin gravel flats is covered abundantly by buff loess. In very restricted patches it is found on top of the drift east of Spring Creek, as on Little Vermilion River immediately above the Matthiessen and Hegeler zinc works, and again east of Ottawa at the crossing of the first tributary above the mouth of Fox River by the Marseilles road. In this region loess is much more abundantly developed between sheets of till than it is as a surface formation. In this older loess, remains of life 66 UPPER ILLINOIS VALLEY are scarce. Good sections of buried loess are found on Indian Creek, and in almost every bank between Morris and Seneca. UPLAND CLAY Similar in many respects to the loess is a heavy clay which covers most of the upland. Although of almost universal distribution in this region, this upland clay is more extensively developed west of the Marseilles till ridge than east of it. The clay in the western region averages about three feet in thickness, and at Spring Valley it is as much as eight feet thick. In the eastern region such thicknesses are unknown. This clay is always the surface formation, and commonly is clearly differentiated from the base upon which it rests. It apparently has no limit in elevation but generally becomes thinner on the higher ridges. Its physical characteristics are very uniform. When cut, the clay is smooth, and shows a well-polished surface. Occasional bits of chert are found in the clay. Its color is brown of varying shades, but in thick sections may be greenish. It has a closer texture than loess and consequently it makes a heavier soil. The clay lies over all kinds of formations, from gravel and till to bedded rock, without showing appreciable variations of character. Its probable manner of origin is discussed in Chapter V. Surface of Drift The surfaces formed in the deposition of stratified drift by water are nearly plane, except as modified by subsequent erosion. The surface of the till, however, is undulatory. From a distance it may appear level; but viewed at close range, it is slightly billowy, due to numberless gentle swells and shallow sags. In this region the prairie is commonly so flat that these undulations readily escape notice. Cultivation has helped to destroy them by plowing down the swells and draining the depressions which formerly were emphasized by small marshes. Unlike water, the ice which deposited the drift was able to get rid of its load without particular reference to slope, and to deposit it with an uneven surface. In some places the till is heaped into long ridges which rise a hundred feet or more above the surrounding prairie. These are known as moraines. The principal ones within this region have been previously noted and are shown in figure 23. History of an Ice Sheet manner of development The drift was deposited by great ice sheets (glaciers grown to continental proportions) which once overspread the region. Due to atmospheric and climatic changes, it is believed that huge snow fields developed in the northern part of the continent. The snow fields gradually ICE AGE 67 bfi *h r- to c3 • _. u £~ fib. s s ° * > C ? J5 i o s +» o b " o w S ,„ o c w £ | i g££.s" , ^ m § j ^