K-o '^:wS!Mr^^ y ^mm ^Mfii ^'^^«^ly^«,^,^ ^vii lit-* p BULLETIN OF THE GEOGRAPHIC SOCIETY OF CHICAGO. NO. 1. THE GEOGRAPHY OF CHICAGO AND ITS ENVIRONS BY ROLLIN D. SALISBURY AND WILLIAM C. ALDEN CHICAGO Published by the Geographic Society of Chicago 1899 PREFATORY NOTE. It is the purpose of this essay to present an outline of the geography of Chicago and its immediate surroundings, and espe- cially to sketch in as simple a manner as possible the course of events by which that geography was developed. The essay is not intended to take the place of the detailed descriptions of special localities heretofore published or yet to be published. Rather is it meant to give such an account of the region about the city that the interpretation of local phenomena may be more easily and more generally understood. The detailed field work on which the essay is based was done by the junior author, who furnished the data for the maps, and made the first draft of the essay. Acknowledgment is also made of indebtedness to the work of earlier students, whose names are mentioned in the course of the essay. In the preparation of the illustrations, valuable assistance has been given by Mr. Wallace W. Atwood, Mr. Frank H. Harms, Dr. Henry C. Cowles and Miss Evelyn Matz. The Frontispiece is from a model made by Mr. C. E. Siebenthal. SI So3 Cop '^ OUTLINE. ^' THE CHICAGO PLAIN. Topography, yQ General Topographic Relations. _^ Topography of the Plain. 1^ Structure of the Plain. L* Relations of Rock and Drift. ^ The Rock. The Drift. "^ Unstratified Drift. Stratified Drift. Surface of the Rock Beneath the Drift. The Drift is of Glacial Origin. DEVELOPMENT OF THE PRESENT GEOGRAPHY. The Geography of the Rock Surface. The Glacial Period. Development of the Ice Sheet. ^ The Erosion Work of the Ice. * Deposits Made by the Ice. Lake Chicago. Origin. The Chicago Outlet. Stages. The Beaches of Lake Chicago. The Upper or Glenwood Beach. The Oak Park Spit. Cliff and Wave-Cut Terrace. Dunes on the Glenwood Beach. The Glenwood Spit. Duration of the Glenwood Stage. Changes of Water Level. Life. Blue Island. Interval of Emergence. The Calumet Beach. Rose Hill Bar. Evidence of Life in the Lacustrine Deposits of the Calumet Stage The Third or Tolleston Beach. Stony Island. Evidences of Life at the Tolleston Stage. Changes in Topography Effected by Lake Chicago. Recent Changes. Lake Michigan Beach. The Ddnes. Stream Erosion. Weathering. The Formation of the Soil. I. THE CHICAGO PLAIN. TOPOGRAPHY. General topographic relations. — The topography of a region is always significant of its history. The City of Chicago is situated on a low, strikingly flat plain, bordering the west side of the head of Lake Michigan. The limits of the plain for a tract about the city are shown in Fig. i (see also Frontispiece), from which it will be seen that the plain is roughly crescentic. Its inner border is formed by the shore of Lake Michigan, while its outer margin, marked by higher land (shaded in Fig. i), extends from Wyinetka on the north, through Galewood and La Grange on the west, to Glenwood and Dyer (Indiana) on the southwest and south. Its greatest width is about 15 miles in a direction southwest from the city. From the shore of the lake, the level of which is about 581 feet above mean tide level in New York Harbor, the Chicago plain rises very gradually to a nearly uniform height about 60 feet above the lake. At this level, the flatness of the plain is inter- rupted, and to the west and south the surface rises promptly, and its topography is rolling. The rise is continued until the rolling surface reaches an extreme altitude of 200 feet above the lake. From this considerable elevation there is a decline toward the west, southwest and south. In other words, the Chicago plain is shut in by a broad, ridge-like belt of gently rolling topography. Observations beyond the immediate vicinity of Chicago show that this ridge-belt comes down from the north and swings about the head of the lake basin. It is in reality a glacial moraine, and has been called the Valparaiso moraine from the city of Valparaiso (Indiana), which is situated upon it. Where crossed by the Wabash Railroad southwest of Palos Springs (Plate II), this moraine has a width of 15 miles, its outer edge being at New Lenox. These relations are shown on the accompanying maps 5 6 THE GEOGRAPHY OE (especially PI. II), to which constant reference should be made. North of the line of the Chicago, Burlington & Quincy Railroad the moraine is ill-defined, and the location of its eastern border is somewhat arbitrar\'. Fig. I.— The Chicago plain and its surroundings. Cutting directly across this low broad ridge in a southwesterly direction, from Summit to Lemont, is the valley now traversed by the Des Plaines river, the Illinois-Michigan canal, and the new Drainage Canal. This valley has abrupt slopes, varies in width from CHIC A GO AND ITS EN VI R ONS. 7 one-half mile to one and one-fourth miles, and is 30 to 100 feet deep. From side to side, the floor of this valley is nearly flat. At its lake- ward end, the bottom is continuous with the Chicago plain, and is less than 15 feet above the level of the water in the lake. These relations are shown in the Frontispiece, PI. II and in Fig. i. From Summit to Lemont, the fall is so slight as to be spoken of as "the twelve-mile level." Tributary to this valley at Sag Station, about three and one- half miles above Lemont, is a second valley of like dimensions known as "the Sag." This valley runs nearly due west from the village of Worth on the Wabash Railway, to Sag Station on the Chicago & Alton Railway. It is traversed by a small creek known as the Canal Feeder. These valleys converge and unite at Sag Station, including between them a triangular tract of elevated land of undulating topography. This isolated area, known as Mount Forest, has a length of six miles and a width of four. The floor of the Sag, as well as that of the Des Plaines valley, is continuous with the Chicago plain. These two valleys, there- fore, give ample outlet for drainage from the Chicago plain «outh- westward across the moraine belt, and thence by way of the Illinois and Mississippi rivers to the Gulf of Mexico. Following the line of the canal, there is a rise of less than 15 feet from the present level of the lake to the divide which separates it from the Des Plaines river. The lake, therefore, barely escapes drainage into the Mississippi river system, even without the new canal. Topography of the plain. — Apart from the Mount Forest island already mentioned, the most prominent topographic feature of the plain is the Blue Island ridge (PI. II), seven miles west of the lake at South Chicago, This ridge runs nearly due north and south, having a length of six miles, a width of about one mile, and an elevation of 25 to 50 feet above the surrounding flat. Just west of South Chicago, between the Blue Island ridge and the lake, is a minor elevation of rock known as Stony Island, (S. I. PI. II). Its longer axis has an east-west direction. The length of the "island" is one and one-fourth miles, its width about half a mile, and its height about 20 feet above its marshy surroundings. Traversing the plain, and converging to the two southwestward valley-extensions of the plain on either side of Mount Forest, are THE GF.OGRAJ'IIY OF 77«; YK 7/ioevmi a series of low ridges of sand and gravel so related to the lake, to the valleys on either side of Mount Forest, and to one another, both in elevation and arrangement, as to be most significant in working out the geographic his- tory of the region. In many parts of the city where the natural surface has not been destroyed by grading, as well as at many points outside the city, these low ridges are brought into prominence by the trees which grow upon them, while their surroundings are treeless. Some of these inconspicuous ridges are shown on PI. II. Apart from these features, some of which are not pro- nounced, the notable characteristic of the topography of the plain is its flatness. STRUCTURE OF THE PLAIN. Relations of rock and drift. — The sub-structure of the Chicago plain is solid rock. This may be seen in the several quarries about the city, arfd is made known by deep borings and excavations of other sorts at many points where the rock is not exposed at the surface. Overlying the bed-rock is a mantle of unconsolidated material composed of bowlders, clay and sand, and known as drift. Borings for wells, excavations for the foundations of buildings, and the exposures of rock in the quarries, show that the thickness of the drift mantle is extremely variable, and since the surface of the plain is nearly flat, it follows that the surface of the rock on which the drift rests must be very uneven (Fig. 2). If the drift mantle were to be stripped off, there would remain, instead of the flat plain on which the city now stands, a markedly uneven surface. The present rock outcrops, where the drift is thin or absent, would be the tops of hills rising above their surroundings, that is, above the plain where the drift is now thick. The slopes from the hilltops to the valleys about them would be sometimes steep and sometimes gentle. While city ^rSm^o^c^knon engineer, Mr. Samuel G. Artingstall prepared a map of caj)"""" *"'''" the city giving the elevation of the rock surface at CHICAGO AND ITS ENVIRONS. 9 various points, as shown by borings. While the data for this map were insufficient to determine the details of the topog- raphy of the rock, the map showed clearly the undulatory character of its surface, and the consequent varying thickness of the drift, not only in the plain, but also over the area beyond the plain, where the drift surface is undulatory. The lowest level of the rock surface determined is near the North Branch of the Chicago river, about one-half mile north of its junction with the South Branch. The surface of the rock is here 124 feet below the level of Lake Michigan. Passing out radially from this point the rock surface rises, with many undula- tions, and numerous exposures at the surface. This rise in the surface of the rock is continued under the moraine surrounding the Chicago plain, until it reaches an elevation of 100 to no feet above the lake level. While the rock surface is higher outside the Chicago plain than under it, thd greater elevation of the surrounding country is not due entirely to this cause. From the study of one hun- dred borings distributed over the plain, it has been esti|?iated that the average elevation of the rock surface under the plain is 45 to 50 feet below the level of the lake. From about sixty bor- ings west and southwest of the plain in the area of higher land with rolling topography, the average elevation of the rock under the moraine-covered territory, has been estimated to be 25 to 30 feet above the level of the lake. This gives a difference of 70 to 80 feet. In the plain the drift varies from o to 130 feet in thick- ness, with an estimated average of 50 feet. In the moraine belt, the average thickness of the drift has been estimated at 150 feet. It is thus evident that the belt of higher land above the Chicago plain is due partly to a rise in the surface of the rock beneath it, and partly to the greater thickness of the drift (Fig. 2). The rock. — The rock which underlies the plain about Chicago is limestone. At the various quarries, and wherever the rock is exposed, it may be seen to contain bits, or sometimes even large masses of coral, fragments of crinoid stems, and fragmentary or perfect shells of various forms of shell-bearing life. Locally, the limestone may almost be said to be made of such fragments. These fossils give positive evidence of the origin of the limestone, for all of them are the relics of life which lived in sea water. In 10 THE GEOGRAPHY OE the ocean to-day similar accumulations of coral and shells are making, where the conditions are favorable. Geologists are therefore confident that the limestone of this region was accumu- lated beneath the sea, and this means that the ocean covered the site of our city when the limestone was formed. By means of its fossils, and by other means less readily explained, the age of the limestone, in terms of geological chro- nology, is known. It belongs to the later part of the Silurian period, and the Silurian is the third of the six or seven long per- iods which make up the Paleozoic era, the first era when, so far as now known, there was abundant marine life. The local rock is known as the Niagara /irnesio?ie, because it is believed to be of the same age as the limestone at Niagara Falls, and the limestone at that point was long since named Niagara. The limestone may be seen at all the quarries about the city. The best exposures are at or near Stony Island, Hawthorne, Bridge- port, Elmhurst and Lyons. Until recently no formation of rock (barring the drift) younger than the Niagara was known in the immediate vicinity of Chicago. But recently Mr. Stuart Weller has found remnants of a forma- tion of the Devonian period (the period next following the Silu- rian) at the quarry a mile west of Elmhurst. Meagre as these remnants, are they show that beds of Devonian age once overlay the Niagara limestone. Like the Niagara formation, these Devon- ian remnants are of marine origin, and prove the existence of the sea in this region in the Devonian, as well as in the Silurian period. The waters of the- Silurian and Devonian seas in this region were probably not deep. Their shallowness is suggested by the character of the fossils. For example, corals do not flourish in deep waters, and corals are abundant in the Niagara limestone. It is possible that still younger beds (the Carbonifer- ous) once overlay the Devonian, but if so, there is now no con- clusive evidence of the fact. THE DRIFT. Good exposures of the drift may be seen along the lake bluff from Evanston northward, along the line of the new drainage canal, from Bridgeport southwestward to Lemont, and at the clay CHICAGO AND ITS ENVIRONS. ii pits of the various brick-yards of the city and its surroundings. Some of these pits, accessible from various parts of the city, are the following: Near the North Branch of the Chicago river, west of Lincoln Park; in the vicinity of South Robey and Forty-third streets; west of the Union Stock Yards; at Purington Station on the Chicago, Rock Island and Pacific railway. Fig. 3. Glacial drift or till. A typical section. (Atwood.) Apart from these exposures which are more or less perma- nent, temporary exposures are frequently to be seen at various points in the city where excavations are being made for the foun- dations of buildings, for water-pipes, gas-pipes, etc. Unstratified drift. The drift at various points presents various characteristics. In most of the localities where the more perma- 12 THE GEOGRAPHY OF ncnt exposures occur, the drift consists of a matrix of dense blue (in places buffish) clay, in which are imbedded many stones (Figs. 3 and 4). In size the stones range from pebbles to bowlders sev- eral feet in diameter. The material is in general without arrange- ment; that is, the fine and the coarse are intimately mingled. To put the matter in another way, the drift does not show the assort- ment and stratification characteristic of deposits made by water. Much of the stony material is too coarse to have been handled by waves or currents of any ordinary strength. Flc. 4. Till in the lake blurt" south of Winnetka. Beach gravel in the forcRrouud. (Harms.) The greater part of the stony material of the drift was derived from the Niagara limestone which underlies the drift, not only about Chicago, but throughout northeastern Illinois and east- ern Wisconsin as well. Another, but smaller portion of the stones of the drift are fragments of sedimentary rock from other forma- tions, while still another part are fragments of metamorphic and igneous rock. More commonly than otherwise the larger bowl- ders belong to this last class, and the formations from which they came are found about Lakes Superior and Huron, and other points to the north and northeast. If the stones imbedded in the clay be examined, they are CHICAGO AND ITS ENVIRONS. found to be partly angular and partly rounded, but largely sub-angular with num- erous flat surfaces or facets (Fig. 5). They show neither the rounding of shore pebbles nor the an- gularity of freshly broken rock. The facets often show polishing, parallel grooving and scratching, as though smoothed and striated while being held in a firm position and moved over a hard surface beneath. The fine material of the un- stratified drift, that is, the blue clay (which is sometimes yellowish at the surface), is found on exam- ination to be made up of minute particles of rock. It is, in fact, nothing more than finely pulver- ized rock. Particles from many sorts of rock enter into its compo- sition, though some are abundant and some rare. About Chicago, particles of limestone are by far the most abundant. Their presence in abundance is easily shown by putting a few drops of hydro- chloric acid on the blue clay. It will, as a rule, effervesce 14 THE GEOGRAPHY OF briskly. The effervescence is the result of the decomposition of the lime carbonate by the acid, the carbonic acid gas of the former escaping, and causing the bubbles. The surface portion of the clayey drift to the depth of two or three feet, is often buflfish or yellowish in color. This portion does not usually effervesce when acid is applied, showing that it does not contain much lime carbonate. The reason of the buff, non-calcareous surface portion, will appear later. As will be seen from the list of localities enumerated above, the unstratified drift occurs sometimes on the low, flat plain, and sometimes on the high, rolling land which borders it. In many places it is known to run down far below the level of the lake, lying on the rock with no stratified drift beneath. In someplaces about the city, and at numerous points through- out the drift-covered area, bits of timber, and even large logs are found in the drift. Vegetable mould and beds of peat, which represent buried swamps, are also found both about Chicago, and throughout the broader area of which this forms a part. These logs, beds of peat, etc., record the fact that as the glacier ice advanced over the region it found forests, soils and swamps. Trees from the forests were buried where they grew, or more commonly detached and carried forward by the ice and incor- porated with its stony and earthy debris. The soils and the peat of the swamps sometimes suffered a similar fate, but since they offered no resistance to the ice, they were overridden and buried without being carried forward, more commonly than trees. It is manifest that if the species of the plants could^be determined, they would give some clue as to the climate preceding the advent of the ice. Stratified drift. — In many parts of the City of Chicago, and at many points outside the city on the Chicago plain, the shallow excavations which are frequently to be seen show the upper part of the drift to be stratified, and to consist of sand and gravel, instead of clay and stones. If the excavations be deep, the blue clay with its content of stones is often exposed beneath the sand and gravel. The stones of the stratified drift are usually rounded, and almost never striated. This superficial mantle of stratified drift is wanting in many parts of the plain. The strati- fied drift is, however, not strictly confined to the plain. At the CHICAGO AND ITS ENVIRONS. 15 south end of the Blue Island ridge, for example, there is a consid- erable body of stratified drift running well up to the summit of the Unsubmergeo Till Areas. I I Till Area of the Chicago Pi \ ■'■■ ■ \ Sand and Gravel Deposits o Lake Chicago. Fig. 6. Map showing the general distribution of stratified sand and gravel and unstratified drift till on the Chicago plain. i6 THE GEOGRAPHY OF elevation. Nor is the stratified drift all at the surface, though this is where it is most commonly seen. Deep excavations sometimes show thin beds of stratified drift below thick or thin bodies of unstratified. A complete explanation of the drift must of course take account not only of the unstratified drift, but of the stratified drift in all its positions and lelations. The general distribution of stratified drift on the surface of the plain is shown in Fig. 6. Surface of the rock bencatJi the drift. — These various charac- teristics of the drift, stratified and unstratified, are hardly less sig- nificant, in the explanation of the phenomena about Chicago, than the surface of the limestone beneath. In general it may be said that the surface of the limestone where it is accessible is relatively Fig. 7. Diagrammatic section, showing the relation of drift to bed-rock. smooth. This statement is not to be confused with tlie idea already distinctly stated, that the surface of the limestone is very uneven. What is here meant is that the surface of the limestone over an elevation or in a depression is, for any small area, essentially smooth (Fig. 7). When the limestone is uncovered, its surface frequently looks as if it had just been smoothed or polished. It i^a-iiiiS Diagrammatic section, showing the relation of residuar\ lies, and from which it has arisen hy de iirths to tlie rock on which i has not the numerous little irregularities which characterize the surface of limestone which has decayed under the influence of atmospheric changes (Fig. 8). In such cases the surface of the limestone is irregularly etched, and often so soft and crumbling that an exact line marking the distinction between the earthy mat- ter above and the rock below cannot be drawn; but here beneath CHICAGO AND ITS ENVIRONS. 17 the drift, the surface of the limestone is in general hard as well as smooth, and the demarkation between it and the drift is per- fectly definite. Figs. 7 and 8 put the two types of rock surface in contrast. Not only is the surface of the rock beneath the drift hard and in general smooth, but it is also marked by numerous lines and grooves comparable to the lines and grooves on the surfaces of the stones of the drift. So striking is the correspondence between these marks on the bed-rock and those on the stones of the drift, that there can be no doubt that they owe their origin to a common cause. Furthermore, the striae which are to be seen on the surface of the limestone beneath the drift are, in any locality, essentially parallel to one another. ■M 1 « ■ 1 K g 5^ j^4^. ^^-# ■Hi ^ _ -,..^ ^^aas ^ Wtr^^Bjk SKj ^ »3B» ' -J^ ^ ^^* fy^;"^^'* ^^ tSffSl ^■fe'^^tai ^^BmiMHH^^^^H ^^^^^^H I 1 SH 1 1 1 B ■ Fig. q. Abandoned quarry at west end of Stony Island. The figure shows the dip of the rock, and the general smoothness of its surface. The stris on the surface do not show. (Harms.) The characteristics which have been mentioned as affecting the surface of the limestone, as well as many other phenomena which need not be here detailed, indicate that the limestone was worn in such a way as to smooth and striate its surface at the time the drip was deposited. The arrows on the map, Plate II, show the location of the striae which have been observed about Chicago, and their direction. The best exhibition of striatj is on the surface of the rock at the 1 8 THE GEOGRAPHY OF east side of the abandoned quarry near the west end of Stony Island. The stria; here affect the upturned edges of strata which have been planed down, as well as polished and scratched. Here as elsewhere, striye are to be seen on the surface of tlie rock only where the drift has been recently removed. Surfaces of limestone which have long been exposed do not show stria-. At the old quarry on the south side of Stony Island, striae may be seen on vertical and even on slightly overhanging surfaces. Fk;. lo. 01(1 quarry on the south side of Stony Island. The surface shown in the tiyiure does not show the stria, though it shows the general smoothing which characterizes glaciated surfaces. Near the right hand side of the Fig. near the bottom, the rock overhangs. On the overhanging surface, grooving is seen, and stri;c may be seen in the field. (Harms.) Other easily accessible localities clearly showing glacial stria- are the following: (i) In the vicinity of the intersection of Chicago avenue and Western avenue; (2) at Robey and Nineteenth streets; (3) at the quarry of Dolese & Shepard at Hawthorne Station; (4) at the Lyons quarries, south of Riverside; (5) in the bed of the Des Plaines river at low water, between Riverside and Summit; (6) at CHICAGO AND ITS ENVIRONS. 19 the quarries at Summit; (7) at the quarry one mile west of Elm- hurst; (8) in the creek south of the bridge at Thornton. Some other localities are shown on the map, but either the striae are not good, or they are not now easily accessible. The features of the drift and of the rock surface beneath which have been mentioned as characterizing the region about Chicago, hold, in a general way, over all the millions of square miles of ter- ritory which the drift affects; and the conclusions which follow are based, not on the phenomena about Chicago alone, but on the phenomena of this greater area, much of which has been studied with great care. The drift is of glacial origin. — The characteristics of the un- stratified drift, together with the characteristics of the surface of the rock on which it lies, point in no uncertain way to the origin of the drift and its accompanying phenomena. The drift is identi- cal in kind with the deposits now being made by glaciers in vari- ous parts of the world, and the characteristics of the surface of the rock beneath the drift are identical with those of the surface of rock over which glacier ice is known to have recently passed. These points are easily demonstrable. * In many regions existing glaciers are diminishing in size, and are therefore bordered by areas which they recently occupied, but from which the ice has now melted. In such situations both the debris deposited by the ice (the drift) and the surface of the rock on which it rests, are accessible. Here the surface of the rock is found to be smoothed and polished, and marked by lines or grooves essentiall}' parallel to one another, while on it rests a mantle of debris of variable thickness, made up of bowlders or smaller pieces of rock, imbedded in a clayey matrix composed of pulverized rock, the mixture being without stratirication. The stones are more or less faceted and striated. With this unstrati- fied debris there is often associated some which is stratified. Furthermore, the lower portion of existing glaciers may some- times be seen, and the lower part of the ice is thickly set with a quantity of earthy, sandy and stony material of all grades of coaise- ness and fineness. With these materials imbedded in its lower portion, the ice moves slowly forward, resting down upon the sur- face over which it passes with the whole weight of its mass. The grinding action between the stony matter in the bottom of the ice 20 THE GEOGRAPHY OF and the rock bed over which it moves, is powerful. The coarse material in the bottom of the ice grooves and scratches the bed over which it is borne, while the fine material, like clay, polishes the surface over which it is moved. At the same time that the material is carried forward in the bottom of the ice and used to grave the surface of the rock beneath, the stones in transit are themselves worn, for the bed-rock reacts on them. Like the bed-rock, they are striated. One surface of a stone in the ice is at one time held against the bed-rock, and worn flat and polished or striated. The stone may then be turned, and in the new position a new flat surface may be developed. The stones in the ice may be worn, not only by the bed-rock, but by one another. Thus they may be striated on several or all sides, and because the stone may change its position from time to time, the striae may run in any direction (Fig. 5). The bed-rock, on the other hand, not being free to move, is striated in one direction only. Its stria,", therefore, and not those of the stones of the drift, show the direction of ice movement after the ice has melted. So unique and so distinctive are the results of the work of glacier ice that they cannot be mistaken for the work of any other agency; and so many and so striking are the points of correspond- ence between the work of existing glaciers and the work of the agencies which produced the drift about Chicago (and the large drift-covered area about it), that there is no escape from the con- clusion that the latter, with all its accompanying phenomena, is the work of glacier ice. In the valleys and on the plains beyond the existing glaciers there are frequently deposits of stratified sand and gravel, borne out beyond the ice by waters which came from its melting. Water action necessarily accompanies glacier action, and the deposits made by water are stratified. Every glacier, therefore, gives rise to water, which is sure to stratify more or less of the material which the ice had deposited, or which it was carrying. It is through the agency of water, therefore, that the stratified drift accompanying the unstratified is to be explained. It will be seen in the sequel that the water which stratifies the drift may be lake or sea water, as well as that of streams. CHICAGO AND ITS ENVIRONS. II. DEVELOPMENT OF THE PRESENT GEOGRAPHY. The preceding pages should have made it clear that two for- mations determine the geography of Chicago. These are the rock, and the drift which overlies it. THE GEOGRAPHY OF THE ROCK SURFACE. After the Niagara limestone was deposited, and after such younger beds as once covered it had been laid down on it, the sea retired from this region, either because its waters were drawn off by the sinking of the deeper parts of the ocean bottom elsewhere, or because this section of the earth's crust was warped upward suf- ficiently to bring it above the level of the sea. So soon *s it became land, its surface was exposed to the action of heat and cold, of rain and wind, of plants and animals. Of primary importance was the rain, and the streams to which the rain gave rise. These streams, working as streams have always worked, began to cut valleys in the surface of the land, and ultimately wore away much of the rock, carrying the eroded material back to the sea. During the long period which followed the deposition of the youngest marine beds, almost all of the formations down to the Niagara were carried away by erosion. Not only were the formations above the Niagara destroyed, but the surface of the Niagara limestone itself was deeply eroded by the same processes which had carried away the overlying beds. The cutting of val- leys in the surface of the limestone left ridges and hills between them, and the surface, at the close of the long period of erosion, was even rougher than that which now affects the limestone beneath the drift. In northwestern Indiana, the Niagara limestone is overlain by Devonian formations. At the junction of the Des Plaines and Kankakee rivers is found the northeast margin of the formations of the Carboniferous system (next younger than the Devonian), 22 THE GEOGRAI'HY OF which covers most of tlie State, while farther west, in Iowa and beyond, the systems of the Mesozoic and early Cenozoic eras overlie the Carboniferous. The mantle of drift which covers the Niagara limestone of Chicago, covers all these systems of strata. It is therefore evident that all the vast geologic periods represented by these several systems of rock must have intervened between the deposition of the Niagara limestone, and that of the mantle of bowlder clay which rests on its surface. These relations show that the period of erosion following the deposition of the Devonian beds and preceding the deposition of the drift, was very long.' THE GLACIAL PERIOD. The long period during which the rock beds of this region were exposed to the ordinary agencies of rock disintegration and erosion was brought to a close by climatic changes the like of which had never occurred in this latitude, so far as now known, in all the earth's history. This change consisted in the development of arctic conditions, not only about Chicago, but over a wide area in the northern and northeastern parts of the United States, as well as over a still larger area farther north. Under the influence of these conditions, a vast continental ice-sheet, comparable to that which now covers Greenland, though many times larger, came into existence. Its area, when at its maximum, is represented in Fig. u. The cause of the climatic change which brought about the glacial conditions it not here discussed. Conjecture has attributed it now to great changes in the orbit or axis of the earth, now to changes in the elevation or distribution of the land, and now to changes in the constitution of the atmosphere, as well as to many other changes, real or speculative. Sufifice it to say, that scientists are by no means agreed as to the hypothesis which best explains the facts. Whatever the cause, the fact that a great ice- sheet, about 4,000,000 square miles in area, came into existence in the northern part of the continent, is no longer open to ques- tion. As already pointed out, the proof is found in the character 'Tliis period of erosion is here spoken of as if it wore continuous, tfiouRh tliis may not have been the case; but for our present purpose, it is not important to recite the many eleva- tions and depressions, and perhaps the submergences and emergences which this region is icnown to or thoufjht to have suffered since tlie deposition of the Devonian beds. CHICAGO AND ITS ENVIRONS. 23 of the drift, and in the peculiar and distinctive features of the rock surface beneath it. Fil; 11 Sliows the area covertd by ice during that epoch of the Glacial Period when the ice was most extensively developed. The main centers of accumulation are also shown. There were many small centers of glaciation in the mountains south of the main ice- sheet. (After Ch'ambcrlin.i The results of careful and extensive study of the drift in North America have led those geologists who have concerned themselves especially with the drift, to the confident conclusion 24 THE GEOGRAPHY OF tliat the glacial period consisted of several more or less distinct glacial epochs, separated b\' epochs which have been called inter- glacial. During the glacial epochs, the climate was severe, and the ice-sheets were being enlarged ; during the interglacial epochs the climate was less severe, and the ice-sheets diminished in area and thickness, if indeed they did not altogether disappear. During these mild intervals, plants and animals returned to latitudes from which they had been driven by the cold and ice, only to be driven southward again with the advent of the next epoch of rigorous climate. The most extensive invasion of the ice reached the Ohio river in Ohio and Indiana, and farther west reached northeast- ern Kansas. West of that point the margin of the ice was not far from the Missouri river. The ice-sheets of several of the glacial epochs passed over northeastern Illinois, and each contributed to the aggregate effects of glaciation. In the paragraphs which follow, it is the effects of glaciation, rather than the effects of the ice of any one glacial epoch, which are referred to; yet the effects of the last glacial epoch on the geography of Chicago are of so much more importance than those of the others, that the chief emphasis is laid on its results. It should be noted, however, that some of the great geographic features of the region, such as the basin of Lake Michigan, may have been formed before the latest advance of the ice and, perhaps, much before.' Development of the ice-sheet. — The especial feature of the gla- cial period was an ice-sheet of continental dimensions. The climate which preceded and caused the development of this ice- sheet, probably came on gradually, and the growth of the ice-sheet was probably slow. To gain a conception of the origin of the ice-sheet a few familiar principles may be recalled. The temperature and the snowfall of a region may stand in such a relationship to each other, that the summer's heat may barely suffice to melt the winter's snow. If under these circum- stances the annual temperature were to be reduced, or the fall of snow increased, the summer's heat would fail to melt the winter's snow, and some portion of the snow would endure through the 'A summary of opinions as to the time and mode of origin of tiu' Great Lake basins is given l>y Alexander Winchell in the American Geologist, Vol. XIX, 1897. CHICAGO AND ITS ENVIRONS. 25 summer. Were this condition once inaugurated, the depth of the snow would increase from year to year, and at the same time the area of the snow-field would be enlarged, since the presence of the snow would so far reduce the temperature of the surround- ing territory as to increase the proportion of the precipitation which would there fall as snow. In the course of time, and under favorable conditions, the area of the snow-field and the depth of the snow would become great. If at the same time the climatic changes which occasioned the snow-field continued to act with increasing effect, the total result would be still greater. As in the case of snow-fields to-day, the greater part of the snow-mass would eventually be converted into ice. Several fac- tors would be effective in accomplishing this result. The pres- sure of the overlying snow would tend to compact the lower portions of the snow into ice, and water arising from the melting of the surface snow by the sun's heat and percolating through the superficial layers of the snow, might freeze below, taking the form of ice. By these and other changes the snow-field becomes an ice-field, the snow being restricted to its superficial parts. • Eventually the increase in the depth of the snow will give rise to other phenomena. When the thickness of the ice has become considerable, the pressure upon its lower parts will be great. We are wont to think of ice as a brittle solid. If in its place we had some slightly plastic body, which would yield to pressure, it is evident that the weight of the overlying portions would press out the lower parts of the mass, and that these would spread in all directions by a sort of iiowing motion. Under great pressure many substances which otherwise appear to be solid exhibit the characteristics of plastic bodies. Among the substances exhibiting this property, ice is, perhaps, best known. Brittle and resistant as it seems, it may be moulded into almost any desired form if subjected to sufficient pressure, steadily applied through long intervals of time. The changes of form which may thus be produced in ice are brought about without visible fracture in its mass. Concerning the exact nature of the movement which takes place between the particles, there may be some question, but the result appears to be such as would be brought about if the ice were capable of flowing, with extreme slowness, under great pressure continuously applied. 26 THE GEOGRAPHY OF In the ice-field supposed, we have the conditions for f,'reat pressure and for its continuous application. If the ice be caj>a- ble of motion as a plastic body, the result would be that the great weight of ice, pressing down upon the lower parts of the ice field, would induce a gradual movement of the ice outward from the deepest part of the field, so that areas surrounding the region of snow accumulation, would gradually be encroached upon by the ice. Observation shows that this is what takes place in every snow-field of sufficiently great extent and depth. Motion thus brought about is glacier motion, and ice thus moving is glacier ice. Greenland affords an example of the conditions here described. A large part of the half million square miles which this body of land is estimated to contain, is covered by a vast sheet of snow and ice, hundreds and thousands of feet in thickness. In this field of ice and snow there is continuous, though very slow move- ment. The ice creeps slowly out from the heart of the icy continent by a sort of flowing motion, and advances to the south and east and north and west until it reaches territory where the climate is such as to waste (melt and evaporate) the ice as rapidly as it advances. The edge of the ice does not remain fixed in position. There is reason to believe that it alternately advances and retreats, according as the ratio between movement and waste increases or decreases. These oscillations in position are doubtless connected with climatic changes. The ice-sheets of northeastern North America appear to have had more than one center of growth. One main center lay east of Hudson Bay, and another west of it (Fig. 1 1). There were perhaps other minor centers, but ultimately the snow-fields, extending themselves from their several centers, united, and the resulting ice-sheet is commonly spoken of as a unit. In addition to the ice-cap of the northeastern part of the continent, there was an ice-sheet in the northwestern part. This extended eastward from the mountains and joined the one origi- nating farther east, near our national boundary, in the longitude of Montana. The centers of ice accumulation are, on the whole, higher than their surroundings, so that if the relative elevations of the CHICAGO AND ITS ENVIRONS. 27 different parts of the continent were the same as now, the ice moved from higher to lower lands. The surface of the land is, however, so uneven, and its inclination so slight, that some cause of movement other than its inclination must be involved. Furthermore, since the ice passed over valleys, hills, and even mountains, without having its general direction of movement notably affected by them, it is clear that its motion was not con- trolled primarily by the slope of the surface on which it rested. The direction of flow' of any liquid substance depends, not immediately on the slope of its bed, but on the slope of its upper surface. For the surface of the ice to have had sufficient slope to cause the movement, its thickness must have been great. The limit of the ice in southern Illinois is something like 1,000 miles from the center of the ice-sheet. How much slope of the upper surface would suffice to cause the movement which actually took place? Various estimates of the slope of the surface of the ice have been made. If the slope were no more than 10 feet per mile, and this seems like a very moderate estimate, the thickness of^the ice would have been 10,000 feet at the center of accumulation, if this slope held fro7n margin to center. It is possible that even this low angle of slope is excessive as an average. Near the margin of the ice-sheet, and this is the only place where its former surface slope can now be determined, the slope was certainly much greater than this, but with increasing distance from the margin the slope became less and less. Whatever the slope, the ice in the northeastern part of the continent was thick enough, except at its very margin, to fill all valleys and basins, and to cover all hills and ridges within its area, and some of the mountains covered rise 3,000 or 4,000 feet above their surroundings. When the ice covered the region about Chicago, its surface was probably essentially smooth, and not notably affected by the topography of the rock beneath. Its surface must have been many hundred feet above the surface of the highest rock hills of the region. Though the irregularities of the rock surface probably caused deflections of movement in the lower part of the ice, its 'It 15 not meant to assert that the ice actually flows. This is open to question; but the result of its movement is very much the same as it would be if it actually flowed. 28 THE GEOGRAPHY OF movement as a whole seems not to have been much affected by any topographic feature immediately about Chicago, unless by the lake basin itself. The erosion ivork of the ice. — When the ice invaded this region, the surface was probably covered with a mantle of soil and decayed rock, and vegetation was probably growing upon it. In its move- ment, the ice soon incorporated in its lower part much of the veg- etation, soil and decayed rock. So soon as these loose materials were removed, the surface of the rock beneath was exposed to wear, and the advancing ice polished, scratched and grooved it by means of the earthy matter and rock fragments which it slowly but steadily carried forward. The rock fragments in the ice were themselves ground, striated and polished at the same time, and perhaps crowded farther up into the ice and borne onward with the load of debris. Fu;. 13. Diagrammaiic riguru, showing the effect of ice wear on a hill of rock, such as tliat shown in Fig. 12, after it has been overridden l)y the ice. The wear effected by the rock-shod ice was not confined to a mere marking of the surface over which it passed. Where promi- nences of rock obstructed its progress, they were acted upon with a force proportional to their great resistance, and suffered a corre- sponding measure of abrasion. They were worn most on the sides which faced the movement — that is, on their stoss sides. All roughnesses of surface and all projecting angles of rock, being pressed upon with especial force, would in the course of time be reduced if not obliterated. Such as escaped destruction would come to have not merely polished and striated surfaces, but rounded forms, with the greatest wear and gentlest slopes on their CHICAGO AND ITS ENVIRONS. 29 stoss sides (Fig. 13). Even the minor rugosities of a glacier's bed will suffer wear in a similar manner, and, until entirely effaced, will present similar forms. The erosive effect of the ice was therefore to grind down the elevations and to make rough surfaces smooth. The rock surface beneath Chicago and its environs still remains about as it was left by the ice. Could it be seen, it would be found to be wanting in the many little rugosities which affect surfaces eroded subaerially. At the same time that the hills of rock were worn down by the ice, depressions in the rock were in some cases made deeper. This is especially true where the ice moved through a valley lengthwise. Where it crossed a valley, its effect was to wear down its borders, rather than its bottom. The moving ice must have covered the site of Chicago for long periods of time. During that glacial epoch when the advance of the ice was greatest, its stay in this region began when, coming down from the north, it reached this latitude. Glacier ice remained over this locality while the edge of the ice was advancing some 150 miles farther south, during such time as the edge remained sta- tionary in this advanced position, and during the time occupied in melting its edge back again to this region. If the edge of the ice advanced and retreated at the rate of but a few feet per day, it will be seen that a very long period of time, several thousands of years at least, would be needed. During other ice epochs, when the ice advanced less far to the south, its stay may not have been so long. Deposits made by the ice. — On melting, glacier ice leaves its former bed covered with the debris which it carried, chiefly in its lower part. Were this material equally distributed in the ice dur- ing its motion, and were the conditions of its deposition every- where the same, the drift would constitute a mantle of uniform thickness over the underlying rock. Such a mantle of drift would not greatly alter the topography. It would simply raise the sur- face by an amount equal to the thickness of the drift, leaving elevations and depressions of the same magnitude as before, and sustaining the same relations to one another. But the drift carried by the ice, in whatever position, was not equally distributed dur- ing the process of transportation, and the conditions under which it was deposited were not constant in the same area, much less in 30 THE GEOGRArilV OF different ones. Because of tfie unequal amounts of material carried by different parts of the ice, and because of the unequal and incon- stant conditions of deposition under the body of the ice and its edge, the mantle of drift has a very variable thickness; and a mantle of drift of variable thickness cannot fail to modify the topog- raphy of the region it covers (see Fig. 2). The extent of the modification will depend on the extent of the variation in thick- ness. This amounts, in our region, to 150 feet or more, and on our continent to upwards of 500 feet. The continental ice-sheet therefore modified the topography of the region it covered, not only by the wear it effected, but also by the deposits it made. About Chicago the average thickness of the drift on the high- lands is greater than on the low. From this it might be inferred that the relief of the present surface about Chicago is greater than it would have been without the drift. But this is probably not the fact, for there are somewhat deep valleys in the surface of the rock beneath the Chicago plain, and they increase the relief of the rock surface notably. At any rate, the angles of slopes of the pres- ent surface are probably notably less than some of the angles of slope of the rock surface beneath the drift. Reference has already been made to the belt of thick drift which skirts the Chicago plain. The greater thickness of drift along this belt seems to have resulted from the halting of the ice edge in this position, during its final retreat. If the edge of the ice had melted back at a constant rate, its position at one stage would not be marked by notably more drift than its position at another ; but if its edge remained in a given position for a time, drift was being continually brought to that position by the forward motion of the ice, and not carried beyond. Under the stationary edge, therefore, a belt of drift, thicker than that on either side, might be accumulated. This is the explanation of the Valparaiso moraine (PI. II) and of submarginal moraines in general. In its greater thickness only does it differ from the ground moraine which the great body of the drift constitutes. Not only did the deposition of the drift affect the topography about the city by diminishing (probably) the relief and by obliter- ating the more striking depressions in the surface of the rock, but its surface had a topography of its own. Like glacial deposits in general, its surface, as left by the ice, was undulatory, being CHICAGO AND ITS ENVIRONS. - 31 marked by many minor and gentle elevations and depressions, the latter often without outlets. In our own region this rolling topog- raphy, marked by low swells and basin-like or saucer-like depres- sions, is common outside the Chicago plain. The same topography is wide-spread throughout the whole area affected by drift. In the depressions lie many of the ponds and lakes which abound in the glaciated part of our country. The topography of the region as left by the ice was then the result of the superposition of an unequally thick mantle of drift,on an uneven surface of rock. LAKE CHICAGO. Origin. — Every ice-sheet has a period of advance followed by a period of decline. In the former, the growth of the ice-field exceeds its waste, and in the latter the waste exceeds the growth. The duration of the last ice-sheet in the region is unknown, but it is probably to be reckoned in thousands of years. When the con- ditions became such that the ice front was melted back faster than it advanced, the final retreat of the ice began. While the edge of the ice was being melted back to the Valparaiso moraine, and while it stood in that position, the water which arose from its melting flowed off to the south. That from Northern Illinois found, its way by various valleys to the Mississippi, and thence to the sea. One line of drainage was down the Des Plaines valley to the Illi- nois. When the ice retreated northeast of the Valparaiso moraine, the depression between the ice front on the one side, and the moraine ridge on the other, was flooded with glacial water, and a lake, marginal to the ice, came into existence. As the edge of the ice which formed one shore of the lake retreated northward, the lake enlarged. Its water rose until it reached a level about 60 feet above the present surface of Lake Michigan, when it over- flowed to the west along the line of the present Des Plaines river valley and through the Sag (see Fig. 14, also Frontispiece and PI. II, page 8). The accumulation of water between the moraine and the ice was the beginning of what has been called Lake Chicago,^ in some ' Leverett.— The Pleistocene Features and Deposits of the Chicago Area. Bull. II.. Geol. and Nat. Hist. Surv., Chicago Academy of Sciences, p. 57, iSgy. 32 THE GEOGKA/'HY OF sense the ancestor of the present Lake Michigan. This lake is the third great factor to be considered in studying the geography of this region. The lines of drainage which developed into the present Des Plaines valley and the Sag tributary to it, have long been known as the Chicago outlet. The Chicago outlet. — The general features of. the outlet have already been given. Near Lemont, the valley is largely cut in rock, the limestone beds rising 40 to 60 feet above the valley bot- tom on either side. This valley is probably not preglacial, though it may have antedated the last glacial epoch. If so, it was largely filled with drift during that epoch. The top of the rock in the bluffs has about the same elevation as that of the waters of Lake Chicago at its highest stage. At its maximum, the discharge of water through this outlet must have been comparable to that now discharged through the Niagara river. Below Lemont, the bed of the outlet declines 90 feet in 25 miles. Of this fall 76 feet is made in less than 10 miles, between Romeo and the Joliet pool. The nature of the rock is such that it is not probable that a waterfall was established, but the high gradient must have caused strong rapids. Stages. — There were several more or less distinct stages in the history of Lake Chicago. During the first stage, which has been recognized (the Glenwood stage), its water seems to have stood about 60 feet above the level of Lake Michigan. This stage lasted for a considerable period of time, during which the waves and currents did their appropriate work. Where they cut into the shores they developed cliffs; where they were depositing instead of eroding, they made beaches and spits of sand and gravel. All this time the ice may have been melting back, so that the ice-shore of the lake was receding to the northeastward, and the area of the lake increasing. Following this maximum stand of Lake Chicago, when its waters were 60 feet higher than those of Lake Michigan, there was a stage during which the waters are thought to have been too low to discharge through the outlet to the west, or even to cover all of the Chicago plain. On this plain, so far as not covered with water, vegetation gained a foothold, and where marshy con- ditions prevailed, distinct deposits of peat were formed. This was the second stage of the lake. The reason for the lowering of CHICAGO AND ITS ENVIRONS. 33 the lake level at this stage is not known. Probably the ice had retreated so far to the north as to open an outlet in that direc- tion, lower than that via the Des Plaines and Illinois. Later, the water of the lake rose again, though not so high as before, covering the plain and burying the peat and other vegetal deposits under accumulations of sand and gravel. This rise of the lake was the beginning of its third stage (the Calumet stage). The cause of the rise of the water may have been an advance of the ice from the north, blocking the outlet of the pre- ceding stage, or a rise of the land to the north, raising the outlet in that direction, and with it the level of the lake. As. the waters rose the discharge through the southwestern outl™ was again resumed. The third stage of the lake has left a record in a sec- ond line of beaches about 40 feet above the level of Lake Michi- gan, and about 20 feet below that of the first recorded stage. The outflow lowered the outlet, and with this lowering the level of the lake was gradually drawn down. About 20 feet above the present lake its level remained nearly constant long enough to allow a third series of beaches to be developed. This may be called the fourth stage (the Tolleston stage) of the lake. • Still later, an outlet was opened to the north, probably as the result of the recession of the ice. This outlet was lower than that via Lemont, and the level of the lake was drawn down suffi- ciently 10 cut off the flow via the outlet. When this was done, the present conditions were inaugurated, and the history of Lake Chicago was at an end. THE BEACHES OF LAKE CHICAGO. The Upper or Glenwood beach. — The different levels at which the waters of Lake Chicago stood for any considerable length of time are marked by a series of well-defined shore-lines, whose ridges of beach sand and gravel have been mentioned (p. 7) as significant features of the Chicago plain. The positions of the various shore-lines are indicated on Plate II (p. 8), and in Figs. 14, 17 and 18. As before stated, the water at first rose to a level about 60 feet above that of Lake Michigan, or 640 feet above sea level, before it found an outlet, and at this level was formed the first and highest beach. To this Mr. Leverett has given the name 34 THE GEOGRAPHY OF Glefnc'ooii, from the village of Glenwood, on the Chicago and East- ern Illinois railroad, four miles south of the Calumet river. At or THt VICINIT ot CHICAGO AT THE GLENWOOD STAGE \ LAKE CHICAGO. SHADED AREA LAND SCALL or MILES k^^ Fig. 14. The vicinity of Chicago during the Glenwood stage of Lake Chicago. The area between the present shore and the shaded area was covered by water. CHICAGO AND ITS ENVIRONS. 35 Glenwood the beach is especially well developed. The relations of land and water while the Glenwood beach was forming are shown on the map (Fig. 14). The shore-line corresponding to the Glenwood beach extends an undetermined distance northward into Wisconsin, but is want- ing between Winnetka and Waukegan, where the present lake shore is west of the position occupied by the shore of Lake Chicago when the Glenwood beach was formed. The northern end of the Glenwood beach, so far as the environs of Chicago are concerned, lies on the crest of the pres- ent bluff near Winnetka. From the bluff at Winnetka this beach swings southwestward for several miles to Norwood Park (PI. II, and Fig. 14) on the Wisconsin division of the Chicago and North- western railway. Thence its course is southerly through Dun- ning County Farm and Galewood, to a point about one mile south of the Chicago, Milwaukee and St. Paul railway. Extend- ing northward between Galewood and Maywood there was a shal- low bay, two or three miles in width. From Maywood the beach swings southwestward and southward through La Grange to the line of the present Des Plaines valley, near McCook Station, whence it makes for the outlet which, at this stage, was about three miles southwest of McCook. The outlet here was about one mile in width. The head of the Sag outlet at the Glenwood stage was about two miles west of the village of Worth, and about one-half mile in width. From the Sag, the Glenwood shore-line passes southeast- ward along the inner slope of the moraine, which rises now gently and now abruptly, from the plain. Passing about one-half mile north of Homewood, on the Illinois Central railroad, the shore extended southeastward through Glenwood, on the Chicago and Eastern Illinois railroad. Just southeast of Glenwood the beach deposits have been almost entirely removed by the erosion of Deer creek, but one- half mile farther on the beach is again seen, flanking the lake- ward side of a sharp, narrow ridge of drift. Thence it runs east- ward to Dyer, Indiana. The Glenwood shore-line has certain features which deserve special mention. Tlie Oak Park spit. — From Norwood Park south to the Chi- 36 THE GEOGRArilY OF cago, Milwaukee and St. Paul railroad and beyond, the old shore- line lay along the east margin of a moderately high and slightly rolling tract, to the west of which there is a depressed area which was a shallow bay (Fig. 14), two or three miles in width, at the time the Glenwood beach was formed. In the western part of this low area the Des Plaines river has now cut its channel. As there was little wave action in the bay its shore lines are not clearly marked. Across the debouchure of this bay the shore currents, mov- ing southward toward the outlet under the influence of strong northeast winds, as along the west shore of Lake Michigan to-day, gradually built out the shore drift into a long, narrow spit (PI. II and Fig. 15) diag- onally across the mouth of the bay. This spit (or possibly a barrier beach) passes through Oak Park, terminating at For- est Home Cemetery, near the Des Plaines river. Its double curve is probably due to the combined action of the northeast winds and the current of outward i^ow from the bay. The former turned the spit southwestward, until the outlet of the bay was somewhat constricted, when the outward flow of water from the north became sufficiently strong to deflect the spit-building cur- rent again to the south. The method of building a spit is readily explained. A current l-ig. moved along the shore in the direction indicated by the arrows. While flowing on the shallow bottom near shore, the current carried along more or less sand and gravel. As it reached the point of land below Galewood, it continued across the bay in the general direction already assumed, instead of following CHICAGO AND ITS ENVIRONS. 37 the re-entrant of the shore. As it reached the deeper and more quiet waters opposite the mouth of the bay, its velocity was checked, the carrying power reduced, and the load dropped. More material was constantly brought forward, being carried out each time a little farther over the deposit already made. Thus the narrow submerged ridge of sand and gravel was extended out from the headland in the direction of the shore currents. As a current flows across the mouth of the bay, the sweep of the winds across the open water of the lake is likely to deflect it into the bay, and the spit receives a like deflection. If turned in sharply, it forms a hook. If the process of spit-building con- tinues until the opposite shore of the bay is reached, the bay is completely cut off, and the embankment forms a bar. When it is built up to the level of water of quiet weather, the waves of storms may throw up the material still higher, and the bar becomes the shore-line, with a lagoon shut in behind it. The tendency to cut off shallow bays by means of spits and bars across their openings, and so to straighten and simplify the shore-line, is one common to all bodies of standing water. Cliff and zvave-cut terrace. — North of McCook the shore line is marked by a wave-cut terrace., and low cliff, cut in the glacial drift, rather than by a sandy beach. Fig. 16. Diagrammatic section, illustrating the formation of a cliff and wave-cut terrace. The method of formation of a cliff and wave-cut terrace is as follows: D A (Fig. 16) is a land surface sloping gently to a lake, the level of which is D' A, A being the original position of the shore- As the waves dash against the shore, the bank is more or less eroded, and the debris is either washed backward by the undertow and spread on the bottom, or carried along the shore by littoral currents to be 38 THE GEOGRAPHY OF deposited wherever motion sufficient for its transportation fails. As the zone of greatest erosion is at the water's edge, extending a little above and below the level of quiet water, the shore is grad- ually cut backward as with a horizontal saw, the material above sliding and falling down when undercut, and being worked over and carried away by the waves and currents. Thus with a surface rising back from the shore, the shore grows higher as the water advances on the land, and becomes a cliff, while the bottom of the lake near shore slopes gently up to the water's edge, forming a wave-cut terrace. The horizontality of its landward margin, which also marks its junction with the cliff, is the especial characteristic of the wave-cut terrace. It should be noted, however, that in the case of the ancient terraces from which the lake has withdrawn, their landward margins are locally rendered uneven by alluvial fans formed at the debouchures of ravines and gullies in the old lake cliff. Dunes of the Glenwood beach. — One mile east of Homewood, the beach is covered by dunes, or wind-blown sand, which origi- nated later than the beach itself. In the field, the shore gravels are seen to come out from under the dune sand, and to extend on toward Glenwood. The Glenwood spit. — Southeast of Glenwood, and near the Illi- nois-Indiana line, there was a shallow bay (Fig. 14). The shore cur- rents did not follow the shore of the bay, but as in the case of the bay farther north near Galewood, they swept onward across the inlet, bearing the shore drift of sand and gravel with them. As the currents came into deeper water across the opening of the bay, they dropped their burdens of detritus, and gradually built it out into a great spit nearly two and one-quarter miles in length, almost completely shutting off the bay (see PI. II, p. 8). The land- ward deflection of the detritus-bearing currents by easterly winds is well illustrated by the curved form of the spit. In quieter weather, the current flow, and the consequent spit-building, was southeastward in the general direction of the shore-line, but during periods of heavy storms from the easterly quarter, the currents were deflected into the bay, and the spit suffered a like deflection. During storms the distal part of the spit was probably washed away, and the material swept back into the bay in the form of a hook, and only with the return of more quiet weather was the CHICAGO AND ITS ENVIRONS. ' 39 extension of the spit in the original direction resumed. The structure as a whole is that of a great curved bar formed by a series of hooks extended, one from the other, with the same general front (PL II). As this bar increased in height until it stood at or near the water level, it became the real shore-line, and was further heightened by accumulations of dune sand. This ridge has now a height of 15 feet above the plain to the north and east. Duration of Glenwood stage. — How long the waters stood at this upper level cannot be told, but it was long enough to accom- plish considerable erosion of the outlet, and of the inner margin of the moraine. Most of the debris resulting from this shore erosion seems to have been swept through the outlet, instead of being deposited on the lake bottom. Changes of water level. — The level of the lake was probably not constaat during the Glenwood stage. The outlet was probably being cut down continuously, and the level of the water in the lake correspondingly lowered. How much it had been lowered before the next succeeding stage of the lake was inaugurated is unknown, but there is some reason for thinking that it may have lowered something like 20 feet. As the water level became lower, the level of wave-cutting was correspondingly lowered, and the cutting edge of the waves was felt at all levels successively from the highest stand of the lake (640 feet) to the lowest. Life. — No satisfactory evidence of life has been found in the waters of the lake at the Glenwood stage. This is as would be expected in waters mostly derived from the melting of the great ice-sheet. Blue Island. — Within the area of the Chicago plain shown on Plate II, and within the area of that part of Lake Chicago shown in Figs. I and 14, the only emerging land was Blue Island, the ridge of drift already mentioned (p. 7). That this is a ridge of drift and not of rock covered with a mantle of drift, is shown by well bor- ings. The well at Morgan Park Water Works shows 76 feet of drift overlying the rock, while the well at the Blue Island smelter, on the flat about one and one-quarter miles to the south, shows 40 feet of drift over the rock. The difference in elevation between the two points is such as to show that the surface of the rock has almost the same level under the ridge as under the flat. 40 THE GEOGRAPHY OF There seems to be no assignable reason why excessive depo- sition should have occurred at this point. It is probable that as left by the ice, this elevation of drift spread out to the east, north and south somewhat more than now, with more gentle slopes, such as now occur on the west. If this be true, it originall\' formed a broader and less abrupt swell than now. At the Glenwood stage of the lake, this drift ridge was an island rising lo to 35 feet above the waters of Lake Chicago. On its eastern side the waters developed a cliff, and the debris result- ing from the erosion was carried out some slight distance from the shore. The currents toward the outlet from the east and south- east appear to have been divided by the ridge, one part of the water sweeping about the north end, and the other part about the south. These currents gathered up the debris which the waves developed, and swept it out to the leeward of the island in a pair of spits, one at the north end and one at the south (Fig. 6). That at the north is best seen at the Catholic cemetery at Ste. Maria on the Chicago & Grand Trunk Railway. It may be that the accumulations of bowlders at the north end of the Blue Island ridge are a remnant from this erosion, being the coarse materials which the waves and currents were not able to carry away. The deposits of sand and gravel seen just east of the till ridge at Morgan Park, Washington Heights and elsewhere, were probably built in the form of barrier ridges by the action " of the waves and currents. The beach gravels along the west side of the island are buried beneath an accumulation of dune sand which was blown up later, when the sandy flat to the west emerged from the waters. This well-defined dune sand deposit gives evidence of prevailing west and southwest winds, as at present. Interval of emergence. — After the Glenwood beach was formed, a northern outlet for the lake seems to have been opened, and its level was lowered until the waters of the lake receded to or beyond the present shore-line of Lake Michigan. The opening of the north- ern outlet was probably due to a recession of the ice-sheet beyond some valley lower than the Chicago outlet. As before stated, the evi- dence of the emergence of the Chicago plain at this time is found in a bed of peat beneath the deposits of the succeeding stage, showing that the waters must have withdrawn from the plain for a time suf- ficiently long to allow vegetation to grow and accumulate before CHICAGO AND ITS ENVIRONS. 41 the area was re-submerged, and the later deposits formed. These deposits were best seen some years ago in a section on the campus of Northwestern University in Evanston. Further evidence con- cerning this interval of emergence is desired. Recent investiga- tion has not discovered new data bearing on the subject, which remains as set forth by Dr. Andrews years ago, and more recently by Mr. Leverett. The Calinnet beach.- — Following the period of emergence, the waters of Lake Chicago again rose and flooded the Chicago plain. This re-submergence may have been due to a return of the glacier ice to the northern end of the basin, blocking the outlet which had been opened to the north, or to a rise of the land in that direction, lifting the outlet and causing the water in the lake to rise. The height to which the water rose in the second submergence of the Chicago plain is marked by the second or Calumet beach, about 35 to 40 feet above the present lake, and about 20 feet below the beach of the Glenwood stage. It is the rise to this level, and not higher, which suggests that the water of the Glenwood stage had been drawn down about 20 feet by the lowering of the outlet (p. iq). Like the older beach, this lower and younger one has its correlative in Wisconsin. It h3.s been recognized 25 miles north of Milwaukee, but from this latitude southward to a point between Racine and Kenosha, a distance of more than 50 miles, it has been cut away by the advance of the lake on the land in later times. From the Wisconsin line southward to the Chicago river, the sec- ond beach is closely associated with the first, wherever the first remains. South of the North Branch of the Chicago river, the Calumet beach is seen in good development at Jefferson Park (Fig. 17). Thence it runs through Cragin, to Austin and Riverside. Through this distance of 12 miles there is a continuous well-developed beach ridge of sand and gravel. At this stage of the lake, the drainage of the region northwest of its border, along the line of the Des Plaines river, and probably along the line of Salt creek, entered the lake between Riverside and the rock elevation at Lyons. From Riverside to the outlet, the head of which at this stage was at Summit, the Calumet shore-line is not well defined. At the Calumet stage of the lake, the Mount Forest and Blue 42 THE GEOGRAPHY OF Island islands of the Glcnwood stage were no longer separate, for the plain between them was above water. These islands of the Fig. 1". The Calumet stage of Lake Chicago. The unshaded area west of the present lake \vas covered by water. The figure also shows the position of the Glcnwood beach. CHICAGO AND ITS ENVIRONS. 43 Glenwood stage, and the area between them now formed one large island between \he two outlets. Sag Station marks its western extremity, Summit the northern, and Blue Island village the south- eastern. From Summit, the shore-line of the Calumet stage swung in a broad curve southeastward about the north end of the Blue Island ridge, through Washington Heights. Throughout this distance of eleven and one-half miles, the Calumet beach is marked by a con- tinuous, well-developed ridge of sand and gravel, five to ten feet high, and 50 to 100 yards wide. From Washington Heights to the town of Blue Island, the outer of the barrier ridges mentioned (p. 40) marks the shore-line of this stage. The head of the Sag outlet at this stage may be considered as lying between the south end of the Blue Island ridge and the inner margin of the moraine three miles farther south-west. If this be considered the head of the outlet, the water passing through it was divided into two currents b}' a low body of land known as Lane's Island. The area of this island was submerged in the Glenwood stage, but the lower water of the Calumet staare left the crest of the low ridge exposed. The village of Worth, on the Wabash Railway, stands at its western end. The present Stony creek canal feeder now follows the course of the channel which lay to the north of this island. From the south side of the Sag outlet to the rock elevation at Thornton, the Calumet shore-line was almost parallel to that of the preceding stage, and but about one-half mile inside it. The line is here marked by a continuous ridge of beach sand and gravel. After swinging to the northward about the Thornton ele- vation, this ridge continues eastward into Indiana. Between Homewood and Thornton there is a considerable deposit of dune sand, in hillocks 20 to 30 feet in height, now well covered by vegetation. These dunes overlie part of the beach deposits of the Glenwood stage. The eolian sand which overlies the Glenwood beach may have originated at any time subsequent to the formation of that beach; but that which overlies the Calumet beach must belong to the Calumet or to some later stage. There is also much dune sand associated with the Calumet beach east of Thornton. Rose Hill bar. — From the Calumet beach, north of Chicago, 44 THE GEOCRA Pit Y OF there extended into the Chicago embayment at this stage a con- spicuous bar (PI. II). Its northern end is found at the present lake shore between Wilmette and Evanston. Its connection with the old shore-line has been cut away by the advance of the lake on the land. It runs southward through the western part of Evanston, and on it, near its southern extremity, is Rose Hill Cemetery. It is beneath this bar that the peat deposits which give the evidence of an interval of emergence between this and the Glenwood stage of the lake, were found. Evidence of life in the lacustrine deposits of the Cahimct stage. — In connection with the evidence of a withdrawal of the water from the Chicago plain at the close of the Glenwood stage, and its conse- quent submergence by the waters of the Calumet stage, the find- ing of evidences of life in these lake deposits is of especial interest. The occurrence of shells in the Calumet beach deposits at Summit and near New Buffalo, Michigan, has been reported, but no definite information has been secured concerning them. The only place where definite evidence of life has been found about Chicago is at the farm of Mr. J. H. Welch, about one and one-half miles southwest of Chicago Lawn. The Calumet shore-line was spoken of as being marked by a well-developed ridge of sand and gravel swinging in a broad curve from Summit southeastward about the north end of the Blue Island ridge. In Mr. Welch's field, just northwest of the point where this ridge is cut by the Belt railway, there have been found numerous molluscan shells, and one specimen of coral. An examination of these specimens showed them, without excep- tion, to be of marine species, whose present range is between Prince Edward Island and the West Indies.' With the specimens which could be identified there were many fragments so well worn and so thoroughly perforated as not to permit of identification. The character of the evidence which these shells seem to afford is 'Tlie species as identified by Mr. Frank C. liaker of the Cliicagfo Academy of Sciences, are as follows: Pelecypods: Ostrea virginica Gmelin, ranging at present from Prince Edward Island to the West Indies. These specimens are very largely perforated by boring sponges. Area transversa Linnc. ranging from Cape Cod to Key West. Venus cancellata Linne, ranging from Cape Hatteras to Trinidad. Venus mercinariiis {}) Linne. Pecten (Sp. ?) possibly CItlamys irradians Linne, a fragment. Gnathodon cuneatu^ Gray, Gulf of Mexico. Gastropods: Fulgar pcrversus Linne, ranging from Cape Hatteras to Cuba. Cerith- iutn (Sp. ?), apical whorls only found. Cerithiopsis (Sp. ?) apical whorls only found. Coral: Oculina robusta Pourtales, West Indies. CHICAGO AND ITS EiWIRONS. 45 of such a radical nature as to excite great interest, and conclusions must be drawn with extreme caution. The question is, were the shells left in their present position by natural agencies? To say that they reached their present posi- tion by natural means, is to say that the waters of Lake Chicago at the Calumet stage were salt. This would seemingly require the subsidence of this and surrounding areas to such a level as would allow the incursion of the sea over this part of the interior of the continent, and their subsequent elevation to the present altitude of 620 feet above tide, within very recent geological time. It is true that this is not the first or only suggestion of such a subsidence and marine incursion. Dr. R. W. Ells' of the Geo- logical Survey of Canada, has recently brought forward evidence to show that the ocean extended westward throughout the upper Ottawa basin in post-glacial time, leaving marine deposits which are now 1,000 feet above the sea level. Dr. Bell'^also records the presence of marine deposits north of Lake Superior, along the Kenogami river, at an elevation of 450 feet above sea level. It is not unreasonable that the subsidence of the area about Chicago should have occurred as a part of the more general subsidence of which these marine deposits to the north and northeast seem to be evidence. There also occur along the lake shore certain plants,^ long regarded as seashore plants, and these, together with the existence of a Mysis, a species of marine crustacean, in the lake, have been taken as evidence that salt water has at some time existed where Lake Michigan now is."* iDr. R. \V. Ells. Sands and Clays of the Ottawa Basin. Bull. Geol Soc. Am., Vol. IX, pp. 211-222, February, iSgS. -Report of Progress, Canadian Geol. Surv., 1895, Vol. VI, p. 340. ^These plants are Triglochin mari/ima (arrow grass) ; Sa/sola i'ali (Kussia.T\ thistle); Cakile americana (sea rocket); Prunus mantima (beach plum); Lathyrus mnritimus (beach pea); Euphorbia ■pohgonifoLta (seaside spurge). This list of plants is kindly furnished by Dr. H. C. Cowles of the University of Chicago. It should be stated, however, that Dr. Cowles gives little credence to them asevidence of marine conditions here, rather considering it as begging the question to regard plants with such a wide range along the interior lake shores as strictly sea-shore plants. *The. Flora of Cook County, Illinois, and a part of Lake County, Indiana. Wm. K. Hig- ley and Charles S. Raddin, Bull. Chicago Acad. Sci., Vol. II, No. i, p. 15. iSgi. 46 THE GEOCRArHY OF The presence of the fossils mentioned above might be accounted for by artificial introduction. They might have been thrown there by white men, or introduced in a fertilizer used on the soil. The well-known trading of the Indians of the northern interior with the south and east coast might account for their hav- ing been left here, before the coming of white men. They might have been left on the beach of Lake Chicago by the Indians of that time, and have been water-worn and buried by the waves of its shore. It should also be noted that the physical relations indicate that the Calumet beach marks the border of a lake which seems to have stood sufficiently above sea level to maintain a strong current through its outlet, which seems incompatible with the occurrence of marine life in its waters. On the other hand, the water-worn and fragmental condition of a large part of the marine shells found on the Calumet beach, the thorough perforation of many specimens by sea-borers, the occurrence of very delicate, tiny shells in the sand filling the coils of the larger gasteropod shells, together with the statements of Mr. Welch, that he himself cleared the ridge of its native trees and underbrush, broke the sod, and has lived there for nearly thirty years, that he never used any fertilizer containing shells, that the only evidence of Indian residence he has ever found was a single arrowhead, that he has plowed up and gathered the shells ever since the ground was broken — all these facts are against the idea of an artificial introduction of.the shells, and favor the idea of deposition in situ by marine waters. The southern range of all the species found would also seem to preclude the idea of their intro- duction from the north or northeast, for the shells found by Drs. Bell and Ells are all of Arctic species. If the shells be evidence of an incursion of the sea, their occurrence, so far as known on this second, or Calumet beach only, would indicate this stage (or a part of it) as the time of the incursion, and the southern range of all these species at the present day would indicate that the incur- sion was not from the northeast through the St. Lawrence embay- ment, but from the south, through a Mississippi embayment. In view of these apparently conflicting considerations, final judgment concerning the interpretation of the shells must be sus- pended until further evidence is forthcoming. 7'hc Third or Tollcston beach. — Following the Calumet stage of CHICAGO AND ITS ENVIRONS. 47 the lake was a stage during which the waters stood at a level but 20 feet above the present lake. The lowering of the lake from the Fig. 18. Shows the relations of land and water about Chicago at the Tolleston stage of Lake Chicajfo. Shaded areas, land. 48 THE GEOGRAPHY OF preceding stage may have been due to a re-opening of the outlet to the north, or to a rapid cutting down of the outlet. The change of level may have been less sudden than the position of the two shore-lines might lead us to infer. At this stage of the lake a third beach was developed, called the Tolleston beach, from the village of Tolleston in northwestern Indiana. The relations of land and water at this stage, so far as the vicinity of Chicago is concerned, are shown in Fig. i8. Remnants of a terrace at a level corresponding to this shore- line have been seen at various points in Wisconsin. Such a ter- race has been seen north of Milwaukee, but between Milwaukee and Kenosha, it has been destroyed by the encroaching of the lake on the laud. From Kenosha to Waukegan it is well developed, and is fol- lowed closely by the line of the Chicago and Northwestern railway. Thence southward to Evanston, the advancing shore of the lake has removed all trace of this beach, as of those of the earlier stages. At Evanston, on the grounds of the Northwestern Uni- versity, this beach appears at the present shore-line, and runs southward along the eastern border of the Rose Hill bar formed at the preceding stage of the lake (PI. II, p. 8). The low area to the west of Rose Hill was probabl}^ flooded for a time at this stage, but the shore-line from Rose Hill to near Hawthorne is poorly marked. Traces of it are seen near Milwaukee avenue and North avenue, at the rock elevation near the intersection of Chicago and Western avenues, and south- westward from the corner of Douglas and Central Park boule- vards. From the rock elevation at Hawthorne, however, which marks the north side of the head of the outlet at this stage, to the Des Plaines river one mile north of Summit, the shore of the Tolleston stage of the lake is well defined by a sandy beach. From Summit toward Willow Springs, the shore of the outlet at this stage is marked by the fifteen to twenty-foot drift bluff now followed by Archer road. From Summit eastward, the south shore of the outlet, is marked by a low bluff. From one- half mile west of the corner of Western avenue and Garfield boule- vard, the shore-line is marked by a strong ridge of sand and gravel which swings in a broad curve southeastward through Auburn Park to South Englewood. In the earlier j)art of the Tolleston stage the shore-line seems CHICAGO AND ITS ENVIRONS. 49 to have swung off to the south and southeastward from near the intersection of South Halsted and Eighty-seventh streets, and passing through Fernwood at Stewart avenue and One Hundred and Third street, turned southward along the drift cliff, passing through Kensington to the Calumet river at Riverdale. The Sag outlet, probably not a very active line of discharge at this stage, seems to have occupied the present line of the Calumet river (reversed) between Riverdale and Blue Island. The head of the outlet was narrow, and seems soon to have been blocked by a bar formed from material borne by the shore currents southward along the Kensington cliff. West of Blue Island the channel was divided as before by Lane's Island, now considerably enlarged by the lowering of the surrounding waters. From Dolton, southeastward into Indiana, the shore-line is marked by extensive deposits of beach sand and gravel, now much covered by accumulations of dune sand. From the position of Rose Hill cemetery, where the Rose Hill bar was deflected to the southwest, the shore currents con- tinued southward, depositing their material in a great reef o-wer most of that part of the North Side of the City of Chicago which lies between the North Branch of the Chicago river, and the pres- ent shore of Lake Michigan (Fig. 6, p. 15). The slight elevation which this deposit made is traversed by North Clark street, and about midway of its length is Graceland cemetery. As this broad reef-like deposit extended southward and increased in height, it finally became the shore-line, cutting off whatever bay lay to the west. This reef is readily traceable to a point about a mile south of Lincoln Park. It is said to have been nearly continu- ous through the City of Chicago, but it is now scarcely recognizable for a distance of nearly four miles to the southward. In this stretch, grading has destroyed it ; but from Groveland Park, at Cottage Grove avenue and Thirty-fourth street, it extends southwestward a distance of seven miles through the northwestern part of Wash- ington Park, Englewood and Auburn Park, to South Englewood, where it unites with the shore-line just described. This broad reef appears to have been built southwestward as a series of overlapping hooks which were turned into the bay at the west in the manner already described (p. 36). 50 THE GEOGRAPHY OF The advance of the reef constricted the channel of free flow toward the outlet, and the drainage in that direction was shifted more and more to the south. At the same time the lowering of the lake level seems to have diminished the outflow in that direc- tion, so that the current was feeble, and finally destroyed, and the bar was completed across the bay to the farther shore at South Englewood (Fig. i8). This is the most notable instance within the area studied of the process of cutting off embayments, and the con- sequent simplification of the shore-line. The southern part of the area behind (west of) this bar eventually drained out to the east- ward through the depression now occupied by the Auburn Park lagoon, and the establishment of the Chicago river probably drained the remainder. It was probably while this reef was being built and the over- flow to the west diminished, that the present outlet of the lake to the north was being established. As the outflow to the north increased, that via Summit diminished. At the Tolleston stage of the lake. Stony Island had begun to emerge as an island or a reef (Fig. i8), and its position gave it a controlling influence on the currents. Under its protection, the currents shifted southward by the extension of the reef already referred to began to work upon the gentle drift slope of the land along the west shore of the lake, and a low terrace, surmounted by a sandy beach, was developed from South Englewood, through Burnside to the lee of Stony Island. These southeasterly currents were here met by the westward currents about Stony Island, and turned abruptly southwestward toward the present site of Pullman. As a result of the reefs made by these currents, the shore-line was shifted eastward, and the original line through Fernwood was abandoned. Stony Island, — Stony Island is an elevation of rock. Its strata have quaquaversal or periclinal dips, /. e., the strata, on all sides of the ridge, dip outward (Fig. 7). The angle of dip ranges from 30° to 42 '\ At first thought, the "island" appears not to be an erosion remnant, but due to a local elevation of the rock strata. Gentle undulations of the rock-beds are seen at other exposures, but none so abrupt as this. No very satisfactory evidence is at hand by which the date of the uplift which deformed the beds can be, fixed. If it preceded the later part of the erosion which CHICAGO AND ITS ENVIRONS. 51 affected the limestone before the glacial period, the erosion rem- nant (that is, the island) happened to correspond in position with the center of the uplift. There is some evidence in the rock itself that its deformation took place while the layers which are now exposed were under great weight. If this be so, the great weight was probably the weight of other beds since eroded away. The large island to the west of Stony Island. — The large island of the Calumet stage, made by the union of the Blue Island and Mount Forest islands of the Glenwood stage, was still larger dur- ing the Tolleston stage (Fig. 18. Compare Figs. 14 and 17). This was the necessary result of the lowering of the waters of the lake. Evidences of life at the Tolleston stage. — In striking contrast with the Glenwood and Calumet beaches, the Tolleston beach con- tains abundant traces of life closely related to the life of Lake Michigan, if not identical with it. Changes in topography effected by Lake Chicago. — Aside from the phenomena of the shore-lines set forth in the preceding pages, certain changes in the topography of the Chicago plain were effected by the waters of Lake Chicago. First and last, the level of Lake Chicago fluctuated from its maximum 640 A. T. to the present level of Lake Michigan, 581 feet A. T. The shore was at some time or other at all levels between these extremes, and the horizontal cutting of its waves therefore affected all parts of the Chicago plain. By this cutting, the inequalities of the surface of the drift of the plain, as left by the ice, were almost entirely obliterated, changing a plain that was at least slightly undulating, to one which is exceptionally flat. The Blue Island ridge was doubtless the highest drift swell, and was not entirely removed, though probably much narrowed by the waves. During the Glenwood stage of the lake, the Val- paraiso moraine was cut back to its present position, leaving the lake plain with a border which is, in many places, abrupt. Most of the debris resulting from the erosion of the shores of the lake was carried out through the outlet, though some of it is seen in the beaches, and some of it was spread as stratified drift over certain parts of the plain. 52 THE GEOGRAPHY OF RECENT CHANGES. Lake Michigan beach. — With the diversion of the waters of the lake from the outlet to the north, the history of Lake Chicago may be considered as passing into the history of Lake Michigan, so that the series of beaches and bars lying between the Tolleston shore-line of Lake Chicago and the present shore of Lake Michi- gan, mark the closing stages of the history of Lake Chicago, and the earliest stages of Lake Michigan. During this stage, sO much of the Chicago plain as was still submerged was being built up by deposits of sand and gravel brought to the head of the lake by the southward drift of the littoral currents. In the northern part of the city, as far south as Lincoln Park, there is a close-set series of sand and gravel ridges lo to 15 feet high, between the Tolleston beach and the present shore of the lake. These ridges are often capped with a little dune sand. Southward from Thirty-fifth street (PI. II) the deposits of this stage cover a considerable area. Northeast and east of Washington Park, there is a series of 10 to 12 low ridges. These were built as subaqueous ridges by drift from the north. They have a generally parallel direction, some- times branch, and vary in length from one to six miles, running out into the sandy plain. Their southern ends are usually turned slightly to the west, as in hook formations. The longest and most prominent of these ridges is that passing through the campus of the University of Chicago, where its structure was well seen before destroyed by grading. It continues southward through the western part of Oakwood cemetery, terminating one mile north of Burnside. The existence of the basin of Lake Calumet is probably due, in part, to the influence of Stony Island which deflected the currents about its eastern end, whence they continued southward, depositing sand and gravel along their course, and leaving the area of the shallow lake unfilled. Like ridges enclosed Hyde Lake, Wolf lake and Lake George (PI. II), as well as the adjacent marshy areas. Between these lakes and the Tolleston beach to the south is a remarkable series of parallel ridges, so closely set that they can- not all be separately represented on the map. Including those indicated on the map as belonging to the Tolleston stage, there are, from Hammond north to the south end of Lake George, 90 of these ridges, ranging from three to ten feet in height. They are CHICAGO AND ITS ENVIRONS. 53 separated, in many cases, by narrow marshy belts. The ridges running southward between these lakes break up into several nar- row ridges, and curve to the eastward to form a part of the whole series. These ridges are composed of sand with little gravel, and taken together have the form of a great depositional terrace. This extensive filling, together with a slight lowering of the water level, brought the lake shore to its present position. The drift of the sand at the head of the lake and its accumulation there is still in progress. Shore erosion. — The opposite phase of lake shore work, namely, wave erosion, is also well shown near the city. From Evanston northward, the waves of the lake are cutting into the bluffs, and driving the shore-line farther and farther west (Fig. i6). Locally and very recently this advance of the water on the land Fig. 19. Ulitt on the shore near ijlencoe. There is a narrow beach at the base of the clitt. (Harms.) has been stayed by various human devices, but the process by which cliffs and wave-cut terraces are developed is still clearly shown. Fig. 16 illustrates the changes which have taken place where the shore of the lake is a bluff. The surface, as left by the ice, occupied some such position as A D. The waves have cut back, reaching the bluff at D'. This is bordered by a submerged wave-cut terrace to the right of D'. THE GEOGRAPHY OF CHICAGO AAD ITS ENVIRONS. 55 Fig. 21. Figure illustrating changes in the position of the debouchure of the Chicago river. The position of the outlets in 1S30, and at the present time, are shown, and also the posi- tion of the sand-bar which caused the deflection.-(Adapted from map of Col. T.J. Crane, U. S. Corps of Engineers.) 56 THE GEOGR.IP//V OF CHICAGO AND ITS ENVIRONS. 57 58 THE GEOGRAPHY OF The material eroded by the waves from the bluffs has been shifted southward. The fact of this southerl}' transportation may be seen on the north side of every pier extending into the lake, and in the spits wrapping around the ends of these piers from north to south below the water surface. As at earlier stages in the history of the lake, so now, bars are constantly forming across the river mouths, and must be repeatedly removed by dredging to keep the harbors open. Before the improvement of the present harbor at Chicago there was a bar across the outlet of the Chicago river, which shifted the debouch- ure southward nearly one-half mile from its present position (see Fig. 2i), or opposite the foot of Madison street. The Calumet river has undergone similar changes (see Figs. 22 and 23). That part of the stream east of Hegewisch (Fig. 23) has been reversed by the dredging of a channel from Hegewisch to the outlet of Lake Calumet. •J4. Dune near Ouni' Faik. (Cowles.) CHICAGO AND ITS ENVIRONS. 59 Some years ago Dr. Edmund Andrews' discussed the present beach of Lake Michigan, and compared its strength with that of the beaches of Lake Chicago. His paper is now out of print, but the computations have been reproduced and supplemented by Mr. Leverett in his paper already referred to. The computations were made for the purpose of obtaining a measure of post-glacial time, from the rate of shore erosion to the north, the rate of littoral *^k-. " — ^. ^a /W l»^ ■^™^ T' _^^^^-r*«-=«^^^''^^^^' ,..- .. , Fig. 25. A dune in process of destruction. Sand has been Ijlown away, except where lield by the roots of the trees. (Cowles.) See also Fig. 28. transportation to the south, and the amount of filling already ac- complished since the withdrawal of the ice. While there are many unknown and undeterminable factors in such a problem, the results were, as Dr. Andrews remarked, useful in showing that it is impossible to allow, even on the most liberal estimates, any such duration of post-glacial time as 100,000 years, which, at that time, had often been claimed. iThe North American Lalces Considered as Chronometers of Post-Glacial Time. Edmund Andrews, M.D., Trans. Chi. Acad. Sci., Vol. II, 1870, pp. 1-23. 6o THE (JEOGK.U'HY OF The dunes. — The formation of sand dunes by the blowing up of the fine sand from the beach into ridges and hills has been going on, perhaps since the birth of Lake Chicago; but the most striking results have been accomplished since the shore of the lake reached approximately its present position. Small dunes, but dunes which illustrate all essential principles of dune forma- tion, may be seen at Windsor Park, near the foot of Seventy-ninth street. At Dune Park and Millers, dunes are to be seen in all stages of development, from the little drifts of sand in the lee of stumps or shrubs, to great shifting hills of sand loo to 200 feet in height. The position of the more important dune€ about Chicago is indicated on Plate II (p. 8). The formation of the dunes is easily understood. As the brisk wind which is carrying sand passes an obstructing object, such as a tree, a shrub, or a tuft of grass, its current is inter- rupted, and in tlie quieter area in the lee of the obstruction some Fig. 26. Dune covered wiili vegctatiun in the backjrround. Fresh dune in tiie foreground to the loft. This dune is migrating toward the forest, and will bury it, if the advance continues. (Cowles.) CHICAGO AND ITS ENVIRONS. 6l of the sand is dropped. A little pile or drift of sand accumulat- ing in such a position is the beginning of a dune. Hundreds of them may be seen along the shore at the present time. Where a drift of sand becomes appreciable, it itself becomes an obstruction, against and beyond which more sand lodges. Thus the dune grows, and under favorable conditions may attain great dimensions (Fig 24). Fig. 27. Trees discovered after burial. > In the background- to the right are hving trees which have grown since the present surface was established. (Cowies.) But destruction goes hand in hand with construction (Fig. 25). The wind takes up sand not only from the beach, but from the surface of the dunes. It is gathered up from the windward side, and carried up over the crest only to be dropped on the leeward slope. So the dune may be shifted, inch by inch, from windward to lee- ward. This movement, which in the course of time may be great, is known as the tnigration of dunes (Fig. 26). In places dunes have moved inland great distances, burying vegetation and devastating ^For an excellent discussion of the vegetation of the dunes, see articles by Dr. Henry C. Cowies in the Botanical Gazette, Vol. XXVII, 1898. 62 THE GEOGRAPHY OF fields. Some of the dunes about the head of the lake are now far from shore, but it is not always possible to say how far their posi- tion is due to their migration inland, and how far to the recession of the shore from them, as the result of shore-filling. Dunes are likely to be migratory until vegetation gets a foot-hold on them. When this is done the sand ceases to be blown, and the dune ceases its travels. Thus the dunes along the west side of Blue Island ridge, and between Hammond and Thornton, are fixed, being covered and held by vegetation, while the dunes about Millers are still shifting. Fig. 2S. Sand which once buried vegetation has been blown on, exposing the dead wood. After a dune has become clothed with vegetation, sand may accumulate upon it, being lodged by the shrubs and trees. If the sand accumulates more rapidly than the trees grow upward, they will be buried. This has been done at various points about the head of the lake. In its migration the moving dune may bury trees (Fig. 26). If the sand which buries the forests be blown on again before vegetation gets a foot-hold on it, the dead forest may be again discovered, — resurrected, but not to life (Fig. 27). This CHICAGO AND ITS ENVIRONS. 63 also is to be seen about the head of the lake at various points near Millers. Stream erosion. — Stream erosion has not accomplished much on the Chicago plain since the withdrawal of the lake waters from it; but young valleys are being developed in the highland along the lake bluff to the north of Evanston, in the slopes of the Blue Island ridge, in Mount Forest Island, in the sides of the outlet, and in general wherever the slopes are considerable. Excellent examples of post-glacial erosion (young valleys) are to be seen in the vicinity of Winnetka and Glencoe, about one mile west of Palos Springs, and at various points along the outlet. Fig. 29. Post-glacial valleys, shown by contours. The trifling amount of erosion which the plain itself has suf- fered is to be seen along the Chicago river (Fig. 30), where dredging has not been carried on, along the Des Plaines river, along Salt creek near Riverside, and along Thorn creek. The small amount of Fig. 30. Cross-section of the Chicago river valley near Jefferson Park. Stream erosion, the poorly developed drainage, the lakes and extensive marshy areas, are evidence of the extreme youth of the topography of the area under discussion; yet it is to be remem- bered that much of the plain is too low for stream erosion tobe effective. 64 THE GEOGKA/'I/y OF CHICAGO AND ITS ENVIRONS. Weathering. — Since the glacial drift was deposited, perhaps 6,000 to 10,000 years ago, it has been changed to some slight extent by weathering. The change is most obvious in the altera- tion of color which its surface has undergone. While the body of the unstratified drift is gray, the upper part just beneath the soil is huffish or brownish. This change in color is primarily the result of oxidation of the iron compounds originally in the drift. The formation of the soil. — So soon as the ice melted from the region, weathering began to prepare the surface for the support of plant life. When vegetation began to grow and die and decay, organic matter was contributed to the mineral matter which sup- ported the first vegetation, and the carbonaceous matter made the soil black. The same changes affected the area temporarily occu- pied by the lake, so soon as the water was drawn off. The growth of the vegetation has, in turn, furthered the surface changes in the drift. UNIVERSfTY OF ILLINOIS-URBANA 3 0112 079553720