STATE OF DWIGHT H. GREEN, Governor DEPARTMENT OF REGISTRATION AND EDUCATION FRANK G. THOMPSON, Director

DIVISION OF THE STATE GEOLOGICAL SURVEY M. M. LEIGHTON, Chief URBANA

RKPORT OF INVESTIGATIONS- NO. 129

PHYSIOGRAPHIC DIVISIONS OF ILLINOIS

M. M. LEIGHTON, GEORGE E. EKBLAW, and LELAND HORBERG

Reprinted from The Journal of Geology

Vol. 56, No. 1, January, 1948

PRINTED BY AUTHORITY OF THE STATE OF ILLINOIS

URBANA, ILLINOIS

1948 ORGANIZATION

STATE OF ILLINOIS HON. DWIGHT H. GREEN, Governor DEPARTMENT OF REGISTRATION AND EDUCATION

HON. FRANK G. THOMPSON, Director

BOARD OF NATURAL RESOURCES AND CONSERVATION

HON. FRANK G. THOMPSON, Chairman W. H. NEWHOUSE, Ph.D., Geology ROGER ADAMS, Ph.D., D.Sc, Chemistry LOUIS R. HOVVSON, C.E., Engineering

A. E. EMERSON, Ph.D., Biology LEWIS H. TIFFANY, Ph.D., Forestry GEORGE D. STODDARD, Ph.D., Litt.D, LL.D., L.II.D. President of the University of Illinois

GEOLOGICAL SURVEY DIVISION

M. M. LEIGHTON, Chief SCIENTIFIC AND TECHNICAL STAFF OF THE STATE GEOLOGICAL SURVEY DIVISION 100 Natural Resources Building, Urbana

M. M. LEIGHTON, Ph.D., Chief Enid TOWNLET, M.S., Assistant to the Chief Helen E. McMorris, Secretary to the Chief Velda A. Millard, Junior Asst. to the Chief Elizabeth Stephens, B.S., Geological Assistant

GEOLOGICAL RESOURCES Mineral Resource Records Arthur Bevan, Ph.D., D.Sc, Principal Geologist in Vivian Gordon, Head Charge Ruth R. Warden, B.S., Research Assistant Dorothy F. Spencer, B.S., Technical Assistant Mary Burnett, Technical Assistant Coal Harriet C. Daniels, B.A., Technical Assistant G. H. Cady, Ph.D., Senior Geologist and Head GEOCHEMISTRY R. J. Helfinstine, M.S., Mech. Engineer Robert M. Kosanke, M.A., Assoc. Geologist Frank H. Reed, Ph.D., Chief Chemist John A. Harrison, B.S., Asst. Geologist Grace C. Johnson, B.S., Research Assistant Jack A. Simon, M.S., Asst. Geologist (on leave) Raymond Siever, M.S., Asst. Geologist Coal Mary E. Barnes, B.S., Asst. Geologist Margaret Parker, B.S., Asst. Geologist G. R. Yohe, Ph.D., Chemist and Head D. Robert Scherer, B.F.A., Technical Assistant Ruth C. Wildman, M.S., Research Asnstanl Clare O. Gordy, B.A., Technical Assistant Industrial Minerals

Oil and Gas J. S. Machin, Ph.D., Chemist and Head Tin Boo Yee, M.S., Assistant Chemist Ph.D., Geologist and A. H. Bell, Head Paulene Ekman, B.A., Research Assistant Frederick Squires, B.S., Engineer David H. Swann, Ph.D., Assoc. Geologist Virginia Kline, Ph.D., Assoc. Geologist Fluorspar Paul G. Luckhardt, M.S., Asst. Geologist G. C. Finger, Ph.D., Chemist and Head Wayne F. Meents, Asst. Geologist Horst G. Schneider, B.S., Special Research Assistant Richard J. Cassin, B.S., Research Assistant Robert E. Oesterling, B.A., Special Research Asst. Nancy McDurmitt, B.S., Research Assistant Richard Blough, B.A., Research Assistant

Industrial Minerals Chemical Engineering H. W. Jackman, M.S.F.., Chemical Engineer and Head E. Lamar, B.S., Geologist and Head J. P. W. Henline, M.S., Assoc. Chemical Engineer Robert M. Grogan, Ph.D., Assoc. Geologist B. J. Greenwood, B.S., Mechanical Engineer Raymond S. Shrode, B.S., Research Assistant James C. McCullough, Research Associate

Clay Resources and Clay Mineral Technology X-ray and Spectrography Ralph E. Grim, Ph.D., Petrographer and Head W. F. Bradley, Ph.D., Chemist and Head William A. White, B.S., Asst. Geologist Analytical Chemistry Groundwater Geology and Geophysical Exploration O. W. Rees, Ph.D., Chemist and Head L. D. McVicker, B.S., Chemist Carl A. Bays, Ph.D., Geologist and Engineer, and Head Howard S. Clark, A.B., Assoc. Chemist Robert R. Storm, A.B., Assoc. Geologist Emile D. Pierron, B.S., Research Assistant Merlyn B. Buhle, M.S., Assoc. Geologist Elizabeth Bartz, A.B., Research Assistant M. W. Pullen, Jr., M.S., Asst. Geologist Gloria J. Gilkey, B.S., Research Assistant Gordon W. Prescott, B.S., Asst. Geologist Wm. F. Loranger, B.A., Research Assistant Robert N. M. Crash, B.S., Asst. Geologist Ruth E. Koski, B.S., Research Assistant Margaret J. Castle, Asst. Geologic Draftsman Annabelle G. Elliott, B.S., Technical Assistant

Engineering Geology and Topographic Mapping MINERAL ECONOMICS W. H. Voskuil, Ph.D., Mineral Economist George E. Ekblaw, Ph.D., Geologist and Head W. L. Busch, Research Associate Richard F. Fisher, M.S., Asst. Geologist Nina Hamrick, A.M., Research Assistant Ethel M. King, Research Assistant Areal Geology and Paleontology EDUCATIONAL EXTENSION H. B. Willman, Ph.D., Geologist and Head Heinz A. Lowenstam, Ph.D., Assoc. Geologist Gilbert O. Raasch, Ph.D., Assoc. Geologist Dorothy Ranney, 6.S., Technical J. S. Templeton, Ph.D., Assoc. Geologist Assistant LIBRARY Subsurface Geology Anne E. Kovanda, B.S., B.L.S., Librarian L. E. Workman, M.S., Geologist and Head Ruby D. Frison, Technical Assistant Elwood Atherton, Ph.D., Assoc. Geologist Paul Herbert, Jr., B.S., Asst. Geologist PUBLICATIONS Marvin P. Meyer, M.S., Asst. Geologist Dorothy E. Rose, B.S., Technical Editor Donald Saxby, M.S., Asst. Geologist M. Elizabeth Staaks, B.S., Robert C. McDonald, B.S., Research Assistant Assistant Editor Meredith M. Calkins, Geologic Draftsman Ardis D. Pye, Assistant Geologic Draftsman Physics Wayne W. Nofftz, Technical Assistant Leslie D. Vaughan, Assoc. Photographer R.J. Piersol, Ph.D., Physicist Emeritus Beulah M. Unfer, Technical Assistant

Consultants: Ceramics, Ralph K. Hursh, B.S., University of Illinois; Mechanical Engineering, Seichi Konzo, M.S., University of Illinois Topographic Mapping in Cooperation with the Geological Survey. January 15, 1948 PHYSIOGRAPHIC DIVISIONS OF ILLINOIS

M. M. LEIGHTON, GEORGE E. EKBLAW, AND LELAND HORBERG ABSTRACT

The classification proposes minor modifications in Fenneman's divisions and recognizes subdivisions of the and sections, based largely on glacial features. The boundaries and characteris- tic features of the subdivisions are described, and their origin and relations to glacial features and topography are discussed.

INTRODUCTION The report is the outgrowth of geo-

During the last sixty years increasing morphic and glacial investigations car- attention has been given to physio- ried on by Leighton since 1919, by Ekblaw since and of recent studies graphic classification ; and the broad out- 1923, lines first conceived by W. M. Davis, of the bedrock topography and subsur- face deposits by Horberg. J. W. Powell, and others have crystal- lized into a widely accepted classification Copies of a preliminary draft of the map for the United States. The establishment of physiographic divisions of Illinois of this classification was due largely to were prepared in 1944 and furnished to comprehensive studies by the late N. M. the Committee on Drainage Basins and Fenneman (1914, pp. 84-134; 1928, pp. of the Illinois Post-war Planning Commission and to the Illinois 261-353; 1 931; 1938). In these studies it was recognized that, with the progress of Legislative Flood Investigating Commis- topographic and geologic mapping and sion. the advances in geomorphic knowledge, GENERAL DESCRIPTION AND REGIONAL numerous refinements and revisions RELATIONS would be made. It is the purpose of the

present report to make such adjustments Illinois is essentially a , of Fenneman's regional boundaries in and, compared with many other states, Illinois as are warranted by present in- it presents few striking physiographic formation and to delineate smaller sub- contrasts. The relief over most of the

divisions which distinguish physiograph- state is moderate to slight and is not suf-

ic differences that can be shown on large- ficient to exert a marked effect on cli- scale maps and which can be used in mate. Situated in the south-central part

regional studies within the state (table of the great Central Lowland (fig. 2) and

1 and fig. 1). These subdivisions, in near the confluence of major lines of turn, may be broken down later into still drainage, it is the lowest of the north- smaller units which may be used as a central states. The mean elevation is basis for description in quadrangle re- about 600 feet above sea-level, compared ports and other local studies. Local phys- with 700 feet for Indiana, 1,050 feet for iographic areas of this type have been , 1,100 feet for , and 800 described by F. M. Fryxell (1927, pp. feet for (Gannett, 1892, p. 1-53) and by H. B. Willman (1942, pp. 289). The total relief of the state is 973 31-37). In the present paper only the feet, the highest point, 1,241 feet above distinctive characteristics of the larger sea-level, being in the subdivisions are summarized. northwest corner of the state, and the

16 PHYSIOGRAPHIC DIVISIONS OF ILLINOIS 17 lowest point, 268 feet above sea-level, the , and Coastal Plain—lie almost junction of the and Mississippi entirely outside the glacial boundary in rivers. The greatest local relief is near the southern and southwestern Illinois. major valleys, especially within the FACTORS DETERMINING PHYSIOGRAPHIC driftless uplands of northwestern and CONTRASTS , and reaches a maxi- mum of 775 feet between Williams , The physiographic contrasts between 1,065 feet above sea-level, and the Ohio various parts of Illinois are due to the River , 290 feet above sea-level, in following factors and conditions: topog-

TABLE 1

Physiographic Classifications of Illinois

Classification by Leighton, Ekblavv Classification by Fenneman and Horberg

Central Lowland province Central Lowland province

f Great Lake section

Great Lake section < Chicago Lake Plain

[ Wheaton Morainal Country

Till Plains section Kankakee Plain Bloomington Ridged Plain Till Plains section. Rock River Hill Country Green River Lowland Galesburg Plain Springfield Plain Mount Vernon Hill Country section Wisconsin Driftless section. Wisconsin Driftless section

Ozark Plateaus province /Ozark Plateaus province \ Lincoln section

Salem section Salem Plateau section

Interior Low Plateaus province Interior Low Plateaus province Shawnee section section

Coastal Plain province Coastal Plain province

Pope County of southern Illinois. The raphy of the bedrock surface; extent of local relief in most counties in the state, the several glaciations; differences in however, is less than 200 feet. glacial morphology; differences in age of Although large-scale relief features are the uppermost drift; height of the glacial absent, the physiographic divisions of plain above main lines of drainage; the state are readily apparent and as- glaciofluvial aggradation of basin areas; sume great local significance. More than and glaciolacustrine action. nine-tenths of the state lies within the TOPOGRAPHY OF THE BEDROCK SURFACE Central Lowland, all of which is glaci- ated except the Wisconsin Driftless sec- Recently acquired knowledge of the tion in . The other topography of the bedrock surface of provinces—Ozark Plateaus, Interior Low Illinois, so widely obscured by the glacial TILL PLAINS SECTION, GREAT LAKE SECTION

CENTRAL LOWLAND PROVINCE !\ °, I

CO, \Plain

CENTRAL LOWLAND PROVINCE

INTERIOR LOW PLATEAUS PROVINCE COASTAL PLAIN PROVINCE ILLINOIS STATE GEOLOGICAL SURVEY

Horberg Fig. i. —Physiographic divisions of Illinois, by Leighton, Ekblaw, and 18 19 20 M. M. LEIGHTON, G. E. EKBLAW, AND L. HORBERG deposits, emphasizes the great impor- present topography (fig. 3). The bed- tance of this factor in the shaping of the rock control that characterizes the broad physiographic features of the state Mount Vernon Hill Country and most of (Horberg, 1946, pp. 179-192). Prior to the Rock River Hill Country is due tc glaciation an extensive lowland, eroded the fact that these have but one on the weak rocks of the glacial mantle (the ), whereas , covered most of central the remainder of the glaciated area of the Illinois. Bordering it on the north, west, state was buried beneath either two or and south were uplands that had been three drift-sheets. Nebraskan, Kansan, developed, for the most part, on the and Illinoian drifts are present in western more resistant older Illinois, the first two from the Keewatin and dolomites. This ancient pattern is field; Kansan, Illinoian, Wisconsin, and reflected to an important degree in the possibly Nebraskan drifts from the present land surface. The extensive low- Labradorean field are present in north- land of provided condi- eastern Illinois; and Kansan and Illinoi- tions for the thickest accumulation of an drifts (Labradorean) are present in glacial deposits and the development of west-central and south-central Illinois. the prairie plains of this portion of the state. The higher uplands of north- DIFFERENCES IN GLACIAL MORPHOLOGY western Illinois and of southern Illinois Two contrasting types of topography (the Shawnee Hills) prevented the fur- the Wheaton Morainal Country and ther movement of the potent Illinoian — the Bloomington Ridged Plain impres- glacial lobe and caused striking physio- — sively illustrate differences in glacial graphic juxtapositions. morphology. When the Wheaton Mo- EXTENT OF THE SEVERAL GLACIATIONS rainal Country was formed, the ice lobe was more confined to the deep Lake The effect of the extent of several gla- Michigan basin, and the are ciations is most apparent in the physio- closely huddled together. In molding the graphic contrasts between the glaciated Bloomington Ridged Plain, the and nonglaciated areas of the state—the was more extensive and more widely Wisconsin Driftless section and the bor- radiating, and during its receding and re- dering Rock River Hill Country in the advancing substages it formed moraines northwest and the Shawnee Hills section widely spaced, alternating with nearly and the Mount Vernon Hill Country in featureless ground- plains. The the south. This emphasis upon glacia- moraines are also generally smoother tion, however, should be modified by than the bold moraines of the Wheaton that measure of difference which already Morainal Country. existed in the preglacial landscape. Even so, there is no denying the fact that a DIFFERENCES IN AGE OF THE UPPERMOST topographic revolution resulted from DRIFT glaciation. The successive superposition of Physiographic contrasts have also younger drift-sheets upon older in some been produced by differences in age of parts of Illinois and the deposition of the uppermost drift, as in the case of the only one drift-sheet in other parts also Bloomington Ridged Plain compared produced physiographic contrasts in the with the Springfield Plain or the Gales- 21

an- tral line ard, }hic mo- ong line and re- iine COASTAL KANKAKEE WHEATON CHICAGO In- PLAIN P L Al N MORAINAL CNTRY. LAKE PL. 1 ) lary l L_ ,n's. UNGL DRIFT ast- lan- Chicago EL E. VAT ION sec- 1 ^ FEET 800 ited har- 600 ains 400 daw lilar 200 ling

- SEA- LEVEL the :lfne —Sandwich Fault Chi- eing )od) Fig. 3. —Diagrarresent and bedrock surfaces, and (4) succession and structure of bedrock formations. :ago dess i part Om —Maquoketa formation 1. It j part Og — and Platteville formations 4 Os —St. Peter der- ntly each nds, vays Sag The spits re or utlet iver.

c is- utlet bove COASTAL SHAWNEE MOUNT VERNON HILL COUNTRY SPRINGFIELD PLAIN BLOOMINGTON RIDGED PLAIN KANKAKEE WHEATON CHICAGO PLAIN HILLS PLAIN MORAINAL C'NTRY. LAKE PL.

ELEVATION Chicago elevation IN FEET m feet 800

Sandwich Fault

APPROXIMATE HORIZONTAL SCALE

In.. Diagrammatic cross section 3. from Cairo north to Bloomington and thence northeast to Chicago, showing (i) physiographic divisions, (2) glacial deposits, (3) profiles of present and bedrock surfaces, and (4) succession anil JttUI hire oi bedrock formations. Note that the ratio of the vertical scale to the horizontal scale, 462: r, exaggerates the dip of the bedrock formations.

KEY TO BEDROCK FORMATIONS

T —Tertiary system Pmi— McLeansboro group. Pv —CaseyviUe group Mm —Meramec group (hit — Maquokcta formation A' — system Pc —Carbondale group Mc2—Chester group, upper .!/<' - -1 i^ase group Og —Galena and I'htltvilli (umh.iii..h Pm3—McLeansboro group, 1 Pi —Tradewater group Met —Chester group, lower Mk — Kinderbook group Os —St. Peter sandstone PHYSIOGRAPHIC DIVISIONS OF ILLINOIS 21

burg Plain. The more youthful age of the Hampshire ridge south from the Wiscon- Bloomington Ridged Plain is responsible sin-Illinois state line to a point in central for the preservation of its glacial land- Kane County where the Elburn moraine other two is turns westward and southwestward, scape ; the greater age of the responsible for their erosional features. easterly along an arbitrary topographic boundary to the Cary Valparaiso mo- HEIGHT ABOVE MAIN LINES OF DRAINAGE raine in western Du Page County, along The effect of height above main lines the outer edge of the Valparaiso moraine of drainage on physiographic contrasts is southward to the Will County line, and the basic factor in the differentiation of along the outer edge of the closely re- the Galesburg Plain from the Springfield lated Rockdale-Manhattan moraine Plain. The Galesburg Plain is sufficiently southward and southeastward to the In- higher above the Illinois River so that its diana-Illinois state line. This boundary valleys are more sharply incised than coincides, in general, with Fenneman's. were those of the Springfield Plain, and The only significant difference is in east- they have different stream regimens. ern Kankakee County, where the "Kan- kakee swamp" is excluded from the sec- GLACIOFLUVIAL AGGRADATION OF tion, because the area is an alluviated A BASIN AREA glacial drainageway and lacks the char- The Green River Lowland is an ex- acteristics of the true lacustrine plains ample of the glaciofluvial aggradation of found elsewhere in the section (Ekblaw basin area. a and Athy, 1925, pp. 417-428). A similar revision of the boundary in the adjoining GLACIOLACUSTRINE ACTION part of Indiana is implied.

The Chicago Lake Plain is an example The section is subdivided into the of glaciolacustrine action. Wheaton Morainal Country and the Chi- cago Lake Plain, the boundary being PHYSIOGRAPHIC DIVISIONS marked by the highest (Glenwood) shoreline of glacial . GREAT LAKE SECTION Chicago Lake Plain.—The Chicago The Great Lake section of the Central Lake Plain is the well-known featureless Lowland province is separated from the " prairie" of early writers of the area. It

Till Plains section to the south because is characterized by a flat surface, under-

of the bold encircling moraines of Lake lain largely by till, which slopes gently Michigan basin, the greater prominence lakeward and is interrupted by low beach of lakes, and the extent of lacustrine ridges, morainic headlands and islands, plains in this area. In Illinois a purely and by two large glacial drain ageways topographic boundary between the two along the Des Plaines River and Sag sections was drawn by Fenneman along Channel (Bretz, 1939, pp. 43-59). The "the outer edge of certain late Wisconsin beach ridges include well- developed spits moraines," which, south of the Kankakee and bars, which parallel the lake shore or River, form "the western rim of the funnel southwestward toward the outlet 'Kankakee swamp ....'" (1928, p. 315). channel along the Des Plaines River. In this report the boundary is drawn as Blue Island, a prominent morainic is-

follows: Along the west border of the land, is situated just east of the outlet Tazewell Marengo ridge (fig. 4) and the channel and rises about 50 feet above Fig. 4. —Glacial map of northeastern Illinois PHYSIOGRAPHIC DIVISIONS OF ILLINOIS 23

the plain. Dunes, which are so con- curs below the Wisconsin deposits along spicuous along the lake shore farther the outer margin of the section in Kane east, are scarcely recognizable and are and McHenry counties; but elsewhere found in only a few scattered localities. this older drift appears to have been re- Originally much of the lake plain was moved before the deposition of the Wis-

swampy and poorly drained. Its three consin drift (fig. 3). rivers—the Chicago, the Calumet, and Postglacial has been slight and the Des Plaines—are without true val- is restricted largely to youthful valleys leys and have courses determined largely along the Fox and Des Plaines rivers. by beach ridges. Locally along the Des Plaines Valley Wheaton Morainal Country. —The near Lemont the Valparaiso drift-sheet Wheaton Morainal Country is character- is thin, and its major features are deter- ized by glacial morainic topography mined by a pre-Valparaiso valley system (mostly of the Cary substage), which is eroded into older underlying drift (Bretz,

more complex in detail and has more 1939, P. 5 2 )- lakes and swamps than do the open Geologic history.—The outer moraines stretches of the adjoining Bloomington in the Great Lake section, including the Ridged Plain. It includes a series of Marengo, Hampshire, and Cropsey, were broad parallel morainic ridges, which en- formed during the Tazewell substage of circle . In detail the to- the , whereas the pography is complicated by a variety of Valparaiso, Tinley, and Lake Border mo- elongated hills, mounds, basins, sags, and raines were deposited during the succeed- valleys. The area is dominated by the ing Cary substage. The Fox River Valley Valparaiso moraine, which has the high- and the valleys of the east and west forks est elevation and, except where inter- of Du Page River were trenched, appar- rupted by valleys, is continuous from ently by torrential waters, before the ad- Wisconsin to Indiana. With the excep- vance of the Cary ice. During glacial tion of the Tinley moraine, all other mo- recession, large blocks of stagnant ice raines are discontinuous geographic fea- became buried and, upon melting, left tures—those in front of the Valparaiso the large basins now occupied by Fox moraine are overridden by it and those Lake and associated lakes. behind are either interrupted by the Chi- As the Cary glacier withdrew into the cago Lake Plain or merge with ground basin of Lake Michigan, Lake Chicago moraines. Karnes, terraces, kettles, was impounded behind the Tinley and basins, and , although not abun- Lake Border moraines, and its discharge dant, occur more commonly than else- waters eroded the outlet channel along where in the state. Fox Lake and associ- the Des Plaines Valley. With retreat of ated lakes are conspicuous water bodies. the ice beyond the Straits of Mackinac, Small basins of extinct lakes and ponds this lower eastward outlet into the At- underlain by stratified silts and clays are lantic Ocean drained Lake Chicago, and found throughout the area. the Chicago Lake Plain emerged with

The topography is determined essen- essentially its present aspect. Unlike the tially by thick Wisconsin drift of the beaches north of , the beaches Lake Michigan ice lobe, the deposits of of the Chicago Lake Plain were not sub- which completely buried the underlying sequently tilted by differential uplift of bedrock surface. Older Illinoian drift oc- the . 24 M. M. LEIGHTON, G. E. EKBLAW, AND L. HORBERG

TILL PLAINS SECTION kakee region scarcely conceals the bed- The Till Plains section, which covers rock surface. about four-fifths of Illinois, is by far the Bloomington Ridged Plain.—The largest physiographic division in the Bloomington Ridged Plain includes most state, and in it seven subdivisions are of the Wisconsin moraines of Tazewell recognized: the Kankakee Plain, Bloom- age and is characterized by low, broad ington Ridged Plain, Rock River Hill morainic ridges with intervening wide Country, Green River Lowland, Gales- stretches of relatively flat or gently un- burg Plain, Springfield Plain, and the dulatory ground moraine. In many Mount Vernon Hill Country. The section places the major moraines rise with

is characterized by broad till plains, gentle slopes, and, although they are which are uneroded or in a youthful conspicuous from a distance, they be- stage of erosion in contrast with the come less so near at hand, whereas the maturely eroded Dissected Till Plains on minor moraines are prominent locally. the older drift-sheets to the west. The It was in this district more than in any outer boundary coincides closely with the other that the grass-covered stretches of margin of the Illinoian drift. rolling prairie and extensive swamps, de- Kankakee Plain. —The Kankakee scribed by early settlers, were most typi- Plain is a level to gently undulatory cally and extensively developed (Poggi, plain, with low morainic islands, gla- 1934, pp. 1-124). The outer boundary of cial terraces, torrent bars, and dunes. the district follows the outer border of

It is partially fluviolacustrine in origin, the Shelbyville moraine from the Indiana

but it differs from the lake plains of the line westward and northward to the

Great Lake section in that the lakes Green River Lowland, beyond which it

which covered it were temporary expan- follows the outer border of the Blooming- sions of glacial floods and did not exten- ton moraine to its intersection with sively alter its surface either by deposi- Hampshire Ridge of the Great Lake tion or by erosion, except along the section. courses of strong currents. It could be The moraines form a series of loops considered a modified intermorainic ba- roughly concentric with the outer bound- sin, floored largely with ground moraine ary of the district and from south to and bedrock. north include the following moraines:

The district is enclosed by the Iro- Shelbyville, Cerro Gordo, Champaign, quois, Manhattan, and Minooka mo- Leroy, Bloomington, Normal, Outer raines of Cary age on the east and north- Cropsey, Middle Cropsey, Inner Crop- east and by the Marseilles and Chats- sey, Farm Ridge, Chatsworth, Mar- worth moraines of Tazewell age on the seilles, and Iroquois. The outer moraines west and south. are more widely spaced than the inner

Most of the region is poorly drained moraines, and a series of re-entrants in by shallow low-gradient streams which the south-central part of the district indi- follow constructional depressions. The cate an interlobate relationship of the two major streams—the Kankakee and Lake Michigan and Lake Erie lobes. A the Des Plaines—occupy glacial sluice- temporary lake, Lake Ancona (Willman ways, which, near Kankakee and Joliet, and Payne, 1942, pp. 213-215), is known are entrenched in Silurian dolomites. to have existed behind the Inner Cropsey

The drift is thick to thin and in the Kan- moraine, but it did not modify the PHYSIOGRAPHIC DIVISIONS OF ILLINOIS 25

ground moraine to any important de- it is only in the western part of the area gree. that it merges with the Cary valley-train The glacial deposits are relatively of the Rock River. Some of the thick throughout the district and com- ridges are in part bars on the outwash pletely conceal the bedrock topography, plain, but many are true longitudinal except locally. Illinoian and older drift dunes with a west-northwest orientation are present below the Wisconsin in most or crescentic parabola dunes. North of places, so that the level aspect of present Geneseo, remnants of the Shelbyville

drift-plains is due largely to the presence can be recognized. At of the older drift-sheets, which rilled in the close of the Cary substage the low- and covered the irregularities of the bed- land was a great swamp in which the two

rock surface (fig. 3). principal rivers, Rock River and Green Drainage development is generally in River, flowed sluggishly along poorly de- the initial stage, and most streams follow fined valleys choked with outwash. and are eroding in constructional depres- The present lowland coincides in large sions, many of which cross morainic part with a broad bedrock lowland which ridges. Undrained basins are much less was occupied by the up

numerous than in the Wheaton Morainal to the time of Wisconsin glaciation ; and a Country and occur mainly along the mo- remnant of the old southern valley-wall rainic ridges. The valleys of principal forms a prominent bluff on the south streams are larger and more numerous side of the present lowland. than in the Great Lake section, owing in Rock River Hill Country.—The Rock part to greater areal extent of this divi- River Hill Country is characterized by sion and to somewhat greater age, and subdued rolling hill-lands in the stage of they have floodplains bordered by valley- late youth to early maturity. It includes train terraces. The Illinois River, the the eroded Illinoian drift-plain north of master-stream of the district, has a broad the Shelbyville moraine and Meredosia flat-bottomed valley with steep walls and Valley and a fringe of early Wisconsin is bordered by numerous narrow steep- drift which lies west of Marengo Ridge.

walled valleys with steep gradients. Be- The Illinoian drift is thin throughout

tween the "Big Bend" and Peoria the most of the district and is not known to valley coincides with the large pre-Wis- be underlain by older till. Thus the major consin valley of the ancient Mississippi uplands and valleys are determined pri- and is wider and has a lower gradient marily by the bedrock surface. The Il-

than the upstream part of the valley, linoian drift is without marked ridging, which is much younger. and constructional forms are very local- Green River Lowland.—The Green ized. In the western part of the district, River Lowland is a low, poorly drained where it borders the Mississippi Valley, plain with prominent sand ridges and thick deposits of and fine sand occur dunes. It is bounded on the north and as broad ridges, paha, and dunes on the south by the Shelbyville moraine of Illinoian . the Green River lobe and on the east by The major streams flow radially from the abrupt front of the Bloomington mo- a central upland into the Mississippi raine (Leighton, 1923, pp. 265-281). Most River on the west and the Rock River on of the district is modified the east and south. Their valleys are rela- related to the Bloomington moraine, and tively broad and steep walled and have 26 M. M. LEIGHTON, G. E. EKBLAW, AND L. HORBERG

terrace remnants of alluvial fill. The and Nebraskan deposits, especially along

Mississippi River and the upper part of buried preglacial valleys. Most of the ir- the Rock River occupy large alluviated regularities of the preglacial surface were valleys. Below the mouth of Kishwaukee filled in with older drift, so that, in con- River, the Rock has cut a post-Illinoian trast with the Rock River Hill Country, rock gorge, which extends south to the only gross features of the bedrock topog- Green River Lowland. Numerous smaller raphy are reflected in the present land- rock gorges are also present along tribu- scape. taries which locally are superimposed on Springfield Plain.—The Springfield spurs of the bedrock upland (Hershey, Plain includes the level portion of the 1893, pp. 314-325). Most of the minor Illinoian drift-sheet in central and south- streams are narrow and V-shaped. central Illinois. It is distinguished mainly Galesburg Plain. —The Galesburg by its flatness and by shallow entrench- Plain in western Illinois includes the ment of drainage as compared with the western segment of the Illinoian drift- more sharply incised valleys of the Gales-

sheet. The till plain is level to undulatory burg Plain. The southern boundary of

with a few morainic ridges and is in a the district, which coincides closely with

late youthful stage of erosion. It is a similar division made by Paul McClin-

bounded by Meredosia Valley and the tock in 1929 (fig. 1, p. 28), is drawn Wisconsin drift border on the northeast, along a line south of which the drift thins by the Illinois Valley on the southeast, and bedrock topography becomes a con-

and by the Illinoian drift boundary on trolling factor ; the western boundary fol- the southwest. On the northwest a con- lows the edge of the Illinoian drift. tinuation of the district across the Mis- Although the greater part of the dis-

sissippi River into Iowa is implied. Four trict is a flat till plain, the morainic fea- morainic ridges can be recognized in the tures in the western part of the region are district—two occur near the drift border, much more conspicuous than elsewhere a third lies near and roughly parallel to on the Illinoian drift-sheet. They include the Illinois Valley, and the fourth—the the Jacksonville and Buffalo Hart mo- Buffalo Hart moraine—extends north- raines and an extensive area of ridged ward through the central part of the re- drift in the Kaskaskia drainage basin. gion. Locally, the Buffalo Hart moraine The moraines are low and broad, but

is a prominent physiographic feature. they are readily recognized because of

The district is drained by streams their continuity and the associated which flow from a central upland west- and kame terraces. The Kaskaskia

ward into the Mississippi River and east- ridges lie just to the east of an interlobate ward and southward into the Illinois area indicated by the moraines and in- River. The larger valleys are steep clude an irregular assemblage of gravelly walled, alluviated, and terraced, except ridges and hills with small intervening for local narrowing along postglacial plains, some of which are old lake basins. gorges. Much of the district is relatively A large proportion of the hills and ridges high above baselevel, so that the minor appear to be kames and crevasse-fillings valleys are numerous, deep, and youth- related to stagnant ice conditions (Ball, ' ful. 1940, pp. 951-970). The Illinoian drift is generally thick Drainage systems are well developed,

and is underlain by extensive Kansan and the district as a whole is in a late PHYSIOGRAPHIC DIVISIONS OF ILLINOIS 27

youthful stage of dissection. The uplands The Illinoian drift in the district is are low with respect to the master- thin, and deposits of underlying older streams, and the valleys are relatively drift are not known to be present except shallow. Most of the principal streams west of Sparta in Randolph County. The have low gradients and occupy broad al- present land surface is primarily a bed-

luviated and terraced valleys; the sec- rock surface of low relief, which is only ondary tributaries have wide V-shaped slightly modified and subdued by a

valleys; and the headwaters, flowing es- mantle of drift (fig. 3). sentially on the till plain, have broad Geologic history of the Till Plains sec- shallow valleys and low gradients. tion. —Prior to glaciation the Till Plains The Illinoian drift is moderately thick section had a long and complex erosional and is underlain by older drift except in history (Horberg, 1946, pp. 179-192). An areas where the bedrock is close to the extensive lowland—the central Illinois surface. Only the larger valleys and up- peneplain—was eroded in the weak lands of the bedrock surface are reflected Pennsylvanian rocks of the Illinois basin

in the present topography (fig. 3). east of the Illinois River; it was bordered Along the southeast side of the Illinois on the west and south by uplands, on

Valley there is a belt of thick loess, with which remnants of an older erosion sur- dune-contours characterizing the bluff- face are extensively preserved. Just margin, but this body of loess thins rap- prior to glaciation a system of deep bed- idly to the southeast. rock valleys, many of which are occupied Mount Vernon Hill Country.—The by present streams, were entrenched be- Mount Vernon Hill Country comprises low the level of the central lowland. The the southern portion of the Illinoian drift- gross features of the section as well as

sheet in Illinois and is characterized by local features in the Rock River Hill mature topography of low relief with re- Country and in the Mount Vernon Hill stricted upland and broad al- Country are determined to a large degree luviated valleys along the larger streams. by this preglacial topography. The Except for a southern extension of the greater relief and higher elevations in the Jacksonville moraine, glacial land forms Rock River Hill Country and in the are essentially absent. The southern and Galesburg Plain are determined by the western boundaries of the district coin- preglacial uplands, whereas the low cide closely with either the outer limits plains in the remaining districts reflect of glaciation or the outer margin of the the central Illinois peneplain. Carbondale group of the Pennsylvanian With the advent of glacial conditions system. and the approach of the Nebraskan A relatively complete drainage system glacier, there was probably a change

is present, and most streams have broad from erosion to aggradation along major terraced valleys and low gradients. Natu- streams as the result of increased load

ral drainage is good throughout the up- and drainage derangements. The oldest land area, but the larger valley bottoms deposits along the ancient Mississippi are poorly drained. Extensive aggraded Valley (middle and lower Illinois) and its lowlands along the Wabash drainage sys- buried eastern fork, Mahomet Valley, tem to the east and the Big Muddy basin appear to represent this stage. This was to the west are outstanding physiograph- followed by the Nebraskan glacial inva- ic features. sion, which is known to have covered at 28 M. M. LEIGHTON, G. E. EKBLAW, AND L. HORBERG least a large part of the upland of western essentially the entire section lies west of Illinois. There is no evidence that the the Mississippi River, and the eastern early fills in the preglacial valleys were boundary of the section was drawn along more than partially removed during the the river by Fenneman. The refinement succeeding Aftonian stage. of the boundary here proposed is of only The Kansan , which followed, local significance. advanced from both the northeast (Lab- The Kansan drift in the area is thin, radorean center) and northwest (Kee- and the topography closely reflects the watin center), probably in that order, ruggedness of the underlying bedrock and together covered most of the district upland. Valley-train terraces along the except for the Rock River and Mount Missisippi Valley and thick loess de- Vernon hill lands. Mahomet Valley and posits on the adjoining bluffs are features its tributaries in the central part of the of secondary importance. The geomor- section were largely buried and, because phic history closely parallels that of the of the diversion of drainage, were not re- adjoining Ozark Plateaus and is dis- excavated during the ensuing Yarmouth cussed later. interglacial stage (fig. 3) (Horberg, WISCONSIN DRIFTLESS SECTION 1945, pp. 349-359)- With the advance of the Illinoian gla- The Wisconsin Driftless section, which cier from the Labradorean center, the ice constitutes the final subdivision of the attained its maximum extent in Illinois, Central Lowland province in Illinois, is a and the entire Till Plains section was ice- submaturely dissected, low plateau bor- covered. Except in the Rock River and dering the outwash-filled valley of the Mount Vernon districts, where older . The eastern Illinoi- drift is largely absent, the ice moved boundary follows the edge of the across a subdued land surface with fills of an drift and is unchanged from Fenne- early drift and during retreat left behind man's classification. a relatively smooth till plain. Following The upland is underlain by the Galena- Sangamon erosion, which was not impor- Platteville and Maquoketa tant except locally, the Wisconsin gla- of age and by outliers ciers of the Tazewell stage covered the of Silurian dolomite. Flat upland areas, northeastern part of the state and formed which coincide closely with the top of the the glacial landscape which is still so Galena-Platteville dolomite, are consid- extensively preserved. ered remnants of the Lancaster pene- plain. 1 A possible higher surface—the DISSECTED TILL PLAINS SECTION Dodgeville peneplain—may be repre- The Dissected Till Plains section in sented by the crests of mounds and nar- western Illinois is represented by a nar- row ridges capped by Silurian dolomite. row isolated segment of the Kansan drift- Benches clearly controlled by structure sheet maturely dissected into an upland occur along the lower reaches of the prin- of high relief. The eastern boundary is cipal valleys, where the Maquoketa shale- determined by the Illinoian drift margin has been stripped from the top of the and the southern boundary by an arbi- Galena-Platteville dolomite. trary line, south of which the drift occurs The plateau is maturely to submature- as patches and is unimportant physio- 1 See Horberg (1946) for bibliography and recent graphically. From a regional standpoint, review of the problem. PHYSIOGRAPHIC DIVISIONS OF ILLINOIS 29 ly dissected by a number of dendritic proposed: (1) the Salem Plateau section drainage systems tributary to the Missis- is expanded northward to include the sippi. The Mississippi Valley has broad partially drift-covered terraced bottom lands and precipitous cuesta in Randolph, Monroe, and St. walls. Most of the minor valleys are Clair counties, which is clearly a part of youthful, with narrow V-shaped valleys the Ozark dome; (2) the and some with incised . There section, first distinguished by E. M. is considerable underground drainage Shepard (1907, pp. 8, 10-n) in Missouri through small and solution chan- and later by W. W. Rubey (1936) in nels, but and other fea- Calhoun County, Illinois, is recognized tures are not conspicuous. The canyon of as a new subdivision. Fenneman included

Apple River is a prominent local feature both these areas in the Till Plains section resulting from glacial diversion of the of the Central Lowland province. former headwaters of Lincoln Hills section. —The Lincoln (Trowbridge and Shaw, 19 16, pp. 95-99). Hills section includes the partially drift- As elsewhere, thick loess deposits mantle covered dissected plateau above the the bluffs of the Mississippi Valley and junction of the Mississippi and Illinois thin eastward. rivers in western Illinois. It is part of a The geomorphic history of the region larger upland which, bisected by the

is largely one of stream erosion and in- Mississippi, lies partly in Missouri. The volves numerous uncertainties. The principal physiographic feature in Illinois broad outlines, however, may be sum- is a maturely dissected central ridge, marized as follows: (1) development of which forms the watershed between the the Dodgeville surface, probably as a Mississippi and the Illinois rivers

peneplain; (2) rejuvenation and erosional throughout the length of the section. As development of the Lancaster surface to previously noted, the northern boundary

a partial peneplain; (3) uplift and en- is arbitrary, and the eastern boundary trenchment of the Mississippi bedrock follows the Illinoian drift border. The valley and some of its larger tributaries; southern boundary with the Salem Pla-

(4) partial filling of the valley by glacial teau is drawn along the Cap au Gres outwash with a maximum thickness of flexure in southern Calhoun County.

more than 300 feet; and (5) postglacial The upland is determined by a sub- erosion. sidiary structure of the Ozark dome, the Lincoln fold, along which the more re- PLATEAUS PROVINCE OZARK sistant pre-Pennsylvanian limestones The Ozark Plateaus province forms a and dolomites crop out. In Illinois the discontinuous upland along the south- plateau is largely underlain by Osage west margin of the state and represents limestones, of which the Burlington lime- the eastern edge of an extensive upland stone is most important physiographi- in southern Missouri and northern Ar- cally; and the boundaries coincide quite

(fig. 2). It includes the driftless closely with the Mississippian-Pennsyl- and thinly drift-covered cuestas on pre- vanian contact. The southern part of the Pennsylvanian rocks which are struc- section is driftless except for loess de- turally and topographically a part of the posits and a single high-channel filling of Ozark dome. Two important modifica- outwash gravel, presumably Kansan. It tions of Fenneman's classification are has long been known as the Calhoun 3° M. M. LEIGHTON, G. E. EKBLAW, AND L. HORBERG

County . Patchy remnants The plateau is submaturely dissected; of Kansan drift are preserved in the and gently rolling summit areas, consid- northern part of the section. ered remnants of the Ozark peneplain, The plateau surface is rugged and occur along the central ridge. Because of broken by closely spaced valleys and the drift mantle, the topography does not ridges. Restricted areas of flattish to appear as rugged as the Lincoln Hills or gently rolling upland representing the the Salem Plateau sections. Karst fea- Calhoun peneplain (Rubey, 1936) are tures, developed primarily on the St. present along the crest of the ridge. The Louis , are present at many valleys of the Mississippi and Illinois places within the area. The central ridge rivers are broad, deeply alluviated, ter- forms the watershed for tributary drain- raced, and have precipitous walls. Most age, but the Mississippi and Kaskaskia of the minor valleys are narrow, V- rivers cross the ridge without regard to shaped, and steeply graded. structure. The valleys of these two major Salem Plateau section. —The Salem streams have broad alluvial flats and Plateau comprises the major part of the steep walls, whereas most of the tribu- Ozark dome in southern Missouri, but taries are youthful. only two small segments, isolated by the The south unglaciated segment of the

Mississippi River, are present in south- Salem Plateau in Illinois is underlain western Illinois. Both segments are ma- largely by a thick succession of deeply turely dissected, partially truncated weathered and cherty cuestas, dominated by a single central limestone formations which on the south ridge. The northern segment is covered are overlapped by Coastal Plain sedi- by thin Illinoian drift, but the southern ments. Structurally, the area is clearly a segment lies south of the glacial bound- part of the Ozark dome, but it is compli- ary. In the northern segment an arbi- cated by a zone of folds and faults trend- trary boundary with the Shawnee Hills ing north-south and northwest-south- is drawn where the and con- east. A clearly defined physiographic glomerates forming the lower Pennsyl- boundary separates the plateau from the vanian escarpment give way to finer sedi- Shawnee Hills to the east and north, the ments and the escarpment dies out, the contrast being marked by more rugged east margin closely follows the overlap- hills, closer drainage texture, absence of ping edge of Pennsylvanian strata, and structural control, and higher elevations the northern boundary coincides with in the plateau section. Most of the pla- the Cap au Gres flexure. The southern teau is maturely dissected by intricate segment is delimited from the Shawnee dendritic drainage, although small rem- Hills to the east along the contact be- nants of a flat upland surface represent- tween Carboniferous and older rocks and ing the Ozark peneplain are preserved follows Fenneman's boundary. throughout the region. The northern

The northern segment is developed on part of the segment is drained by streams Mississippian strata and lies on the back which head in the Shawnee Hills and slope of the Meramec-Osage cuesta, flow westward across the plateau into the which flanks the Ozark uplift on the Mississippi River, whereas in the south- north and east. It is underlain by Mera- ern part a central divide separates the mec limestones on the west and north Mississippi and Cache Valley drainage. and by Chester strata on the southeast. In contrast to other parts of the Ozark PHYSIOGRAPHIC DIVISIONS OF ILLINOIS 31

Plateau in Illinois, most of the larger time and that, with uplift of the pene- tributary valleys, as well as the Missis- plain, streams occupying the Mississippi, sippi Valley, are deeply alluviated, and Illinois, and Kaskaskia valleys became only the secondary tributaries are youth- incised without regard to structure. ful. There is no clear evidence of the succeed- Geomorphic history. —The Ozark Pla- ing central Illinois peneplain and Havana teaus are essentially a preglacial land strath cycles in the region, probably be- surface whose erosional history has con- cause the weaker formations on which tinued during the glacial period. The old- they are elsewhere developed are absent. est landscape features in the province are During the glacial period the preglacial isolated summit areas, over 800 feet topography was modified by alluviation above sea-level, which may be peneplain of the major valleys and by deposition of remnants correlative with the Dodgeville loess on the uplands. peneplain of the Wisconsin Driftless sec- INTERIOR LOW PLATEAUS PROVINCE tion and the Buzzard's Point plain (Salis- bury, in Weller, Butts, Currier, and Shawnee Hills section.—The Interior

Salisbury, 1920, pp. 47~5 2 ) of the Shaw- Plateaus in southern Illinois are repre- nee Hills. An alternative interpretation sented by the western part of the Shaw- 2 is that they are simply monadnocks on nee Hills section and include a complex the lower Ozark peneplain. In either case dissected upland, underlain by Mississip- an extensive surface, called the " Calhoun pian and Pennsylvanian strata of varied peneplain" in the Lincoln Hills section lithology. It is, in the main, the area gen- and the "Ozark peneplain" in the Salem erally referred to popularly as the " Il- Plateau section, was developed below the linois ." The northern margin is level of these isolated remnants and is drawn along a marked topographic responsible for the general accordance of boundary which lies along the inner flank summit levels found throughout the pla- of the lower Pennsylvanian (Caseyville) teau at elevations about 700 feet above cuesta just within the Illinoian glacial sea-level. Near the southern margin of drift boundary, and the southern bound- the plateau the Ozark peneplain is be- ary follows the northern edge of the over- lieved to transect Wilcox (Eocene) strata lapping Coastal Plain sediments. These and therefore to have been completed are essentially Fenneman's boundaries, sometime during the Tertiary (Flint, the only important modification being a 1941, pp. 634-636). The weathering and northwestward extension of the section leaching of the Devonian formations in to include the thinly drift-covered Penn- the southern part of the Salem plateau to sylvanian cuesta in Jackson and Ran- depths of about 400 feet is of unusual in- dolph counties. terest and has been ascribed to pro- The section is situated along the longed alteration under peneplain condi- southern rim of the Illinois basin, so that tions (Weller, 1944, pp. 101-102). Follow- the lower Pennsylvanian cuesta com- ing completion of the peneplain, and 2 The section was originally distinguished by probably prior to erosion of the Central R. F. Flint (1928, pp. 451-457) and named the Illinois peneplain, "Lafayette"-type "Shawnee Hill Section. Fenneman (1938, p. 435) gravels were spread over its surface. It used the name "Shawnee section." The suggested usage of the term "Shawnee Hills section," in the appears likely that the major preglacial present report is proposed purely for descriptive drainage lines were determined at this reasons. 32 M. M. LEIGHTON, G. E. EKBLAW, AND L. HORBERG prises the northern part of the region and the major period of valley-cutting oc- a dissected plateau underlain largely by curred late in the Tertiary (Weller, 1940, Chester (Mississippian) formation com- p. 45). Loess deposition and valley al- prises the southern part. This regional luviation were the principal events structure is complicated by faulting and during the glacial period. folding, which to a varying degree in- COASTAL PLAIN PROVINCE volved a large part of the area. The Pennsylvanian cuesta forms a The Coastal Plain in Illinois includes continuous ridge and watershed, extend- the southern tip of the state and is under- ing completely across the state. In most lain by unconsolidated Cretaceous and places the ridge is maturely dissected by Tertiary sediments, which overlap the youthful valleys, but remnants of flat older Paleozoic rocks to the north. Three upland are locally preserved on narrow physiographic subdivisions are recog- ridge crests throughout the length of the nized: (1) and (2) the coextensive allu- escarpment. vial plain of the Cache and Mississippi

The plateau on Mississippian rocks to valleys and (3) the Cretaceous hills be- the south is maturely dissected, and the tween the Cache Valley and the Ohio larger valleys are alluviated. There are River. The alluvial plains are character- numerous minor escarpments, structural ized by terraces and recent floodplain benches, fault-line scarps, and subse- features. The Cretaceous hills are ma- quent valleys which reflect local struc- turely eroded into a low upland of gently ture and the varied lithology of the bed- sloping knolls and ridges. Outwash and rock. Only small patches of flat upland alluvium extend far up tributary valleys, are present. Karst features in the St. so that the upland is partially buried Louis limestone are present near in and certain segments are essentially Rock, Hardin County, and in south- isolated. central Union County. The earliest events in the geomorphic Geomorphic history. —-The erosional history of the region are indicated by history of the region is similar to that of remnants of "Lafayette "-type gravels the Ozark Plateaus previously outlined. which occur in the Cretaceous hills. The

Remnants of the Ozark peneplain appear erosion surface at their base is evidence to be extensive along the Pennsylvanian of a long period of denudation, during escarpment, and local higher summits, which the Coastal Plain deposits were especially to the east, may represent an lowered and stripped back. This was fol- older (Buzzard's Point plain) cycle (Sal- lowed by deposition of the gravels, their isbury, in Weller, Butts, Currier and weathering under stable conditions, es- Salisbury, 1920, pp. 47-52). Lower sur- tablishment of major drainage lines, and faces on the Mississippian rocks, 500-550 final dissection of the bedrock topogra- feet and 600-650 feet in elevation, are of phy. Prior to glaciation, Cache Valley uncertain origin. Remnants of "La- was occupied by the and the fayette "-type gravels are found both on present Ohio Valley was occupied by the the escarpment and at lower elevations Cumberland and Tennessee rivers. Dur- south to the Ohio River. A deep weath- ing Illinoian or possibly Wisconsin time ered zone on the gravels overlain by loess the valleys were aggraded to the level of indicates that a long period of stable con- the divide between the Ohio and the ditions followed their deposition and that Cumberland rivers at Bay City in south- PHYSIOGRAPHIC DIVISIONS OF ILLINOIS 33

ern Pope County, and the present lower pass through Cache Valley, and it was course of the Ohio was opened. Both only in relatively recent time that the courses were kept open during subse- southern channel became the permanent quent stages, so that flood waters still course of the river.

REFERENCES CITED

Ball, J. R. (1940) Elongate drift hills of southern Illinois: Jour. Geology, vol. 54. Reprinted as Illinois: Geol. Soc. America Bull. 51. Illinois Geol. Survey Rept. Inv. 118. Bretz, J H. (1939) Geology of the Chicago region: Leighton, M. M. (1923) The differentiation of the Illinois Geol. Survey Bull. 65, pt. 1. drift sheets of northwestern Illinois: Jour. Geology, vol. Ekblaw, G. E., and Athy, L. F. (1925) Glacial 31. Kankakee torrent in northeastern Illinois: McClintock, Paul (1929) Physiographic divisions of the Geol. Soc. America Bull. 36. area covered by the Illinoian drift sheet in southern Illinois: Illinois Geol. Survey Rept. Inv. Fenneman, N. M. (1914) Physiographic boundaries pt. 1. within the United States: Annals Assoc. Am. 19. Poggi, E. M. (1934) The Prairie province of Illinois: Gcog., vol. 4. Univ. Illinois Studies in Social Sci., vol. (1928) Physiographic divisions of the United 19. Rubey, W. W. Geology and mineral re- States: ibid., vol. 18. (1936) sources of the Hardin-Brussels quadrangles, (1931) Physiography of western United unpublished manuscript of Illinois Geol. States, New York, McGraw-Hill Book Co. Sur- vey. (1938) Physiography of eastern United Shepard, E. Underground waters States, New York, McGraw-Hill Book Co. M. (1907) of Missouri: U.S. Geol. Survey Water Supply Flint, R. F. (1928) Natural boundaries in the In- Paper 195. terior Low Plateau physiographic province: Trowbridge, A. C, and Shaw, E. W. (1916) Jour. Geology, vol. 36. Geology and geography of the Galena and Eliza- (1941) Ozark segment of Mississippi River: beth quadrangles: Illinois Geol. Survey Bull. 26. Jour. Geology, vol. 49. Weller, J. M. (1940) Geology and oil possibilities Fryxell, F. M. (1927) The physiography of the of extreme southern Illinois: Illinois Geol. region of Chicago, Chicago, University of Survey Rept. Inv. 71. Chicago Press. (1944) Devonian system in southern Illinois: Gannett, Henry (1892) The average elevation of Illinois Geol. Survey Bull. 68A: Devonian the United States: U.S. Geol. Survey 13th Ann. symposium. Rept. Weller, Stuart; Butts, Charles; Currier, Hershey, O. H. (1893) Pleistocene rock gorges of L. W.; and Salisbury, R. D. (1920) Geology of northwestern Illinois: Am. Geologist, vol. 12. Hardin County: Illinois Geol. Survey Bull. 41. Horberg, Leland major buried valley in William, B. Payne, Geology (1945) A H. ; and J. N. (1942) east-central Illinois and its regional relationships. and mineral resources of the Marseilles, Ottawa, Jour. Geology, vol. 53. and Streator quadrangles: Illinois Geol. Survey (1946) Preglacial erosional surfaces in Bull. 66.