^kd^^ STATE OP ILLINOIS WILLIAM G. STRATTON, Governor DEPARTMENT OF REGISTRATION AND EDUCATION VERA M. BINKS, Director DIVISION OP THE STATE GEOLOGICAL SURVEY JOHN C. FRYE, Chief URBANA REPORT OF INVESTIGATIONS 185 STRATIGRAPHIC AND SEDIMENTOLOGIC ASPECTS OF THE LEMONT DRIFT OF NORTHEASTERN ILLINOIS BY LELAND HORBERG and PAUL EDWIN POTTER » in^» **•»* / NOV 1 4 1996 !L btUL ©urtVtY PRINTED BY AUTHORITY OF THE STATE OF ILLINOIS URBANA, ILLINOIS 1955 , L UNOIS STATE GEOLOGICAU SURVEY 3 3051 00007 1070 STATE OF ILLINOIS WILLIAM G. STRATTON, Governor DEPARTMENT OF REGISTRATION AND EDUCATION VERA M. BINKS, Director DIVISION OF THE STATE GEOLOGICAL SURVEY JOHN C. FRYE, Chief URBANA REPORT OF INVESTIGATIONS 185 STRATIGRAPHIC AND SEDIMENTOLOGIC ASPECTS OF THE LEMONT DRIFT OF NORTHEASTERN ILLINOIS LELAND HORBERG and PAUL EDWIN POTTER i »«*Np* S mv 1 4 m PRINTED BY AUTHORITY OF THE STATE OF ILLINOIS URBANA, ILLINOIS 195 5 ORGANIZATION STATE OF ILLINOIS HON. WILLIAM G. STRATTON, Governor DEPARTMENT OF REGISTRATION AND EDUCATION HON. VERA M. BINKS, Director BOARD OF NATURAL RESOURCES AND CONSERVATION HON. VERA M. BINKS, Chairman W. H. NEWHOUSE, Ph.D., Geology ROGER ADAMS, Ph.D., D.Sc, Chemistry ROBERT H. ANDERSON, B.S., Engineering A. E. EMERSON, Ph.D., Biology LEWIS II. TIFFANY, Ph.D., Pd.D., Forestry W. L. EVERITT, E.E., Ph.D. Representing the President of the University of Illinois DELYTE W. MORRIS, Ph.D. President of Southern Illinois University GEOLOGICAL SURVEY DIVISION JOHN C. FRYE, Ph.D., D.Sc, Chief (16026— 2M— 5-55) STATE GEOLOGICAL SURVEY DIVISION Natural Resources Building, Urbana JOHN C. FRYE, Ph.D., D.Sc, Chief M. M. LEIGHTON, Ph.D., D.Sc, Chief, Emeritus Enid Townley, M.S., Geologist and Assistant to the Chief Velda A. Millard, Junior Assistant to the Chief Helen E. McMorris, Secretary to the Chief RESEARCH (not including part GEOLOGICAL RESOURCES Arthur Bevan, Ph.D., D.Sc, Principal Geologist Frances H. Alsterlund, A.B., Research Assistant Coal Jack A. Simon, M.S., Geologist and Head G. H. Cady, Ph.D., Senior Geologist and Head, Emeritus Charles E. Marshall, Ph.D., Visiting Research Scientist Robert M. Kosanke, Ph.D., Geologist Raymond Sieyer, Ph.D., Geologist John A. Harrison, M.S., Associate Geologist Paul Edwin Potter, Ph.D., Associate Geologist Harold B. Stonehouse, Ph.D., Associate Geologist Margaret A. Parker, M.S., Assistant Geologist (on leave) M. E. Hopkins, M.S., Assistant Geologist Kenneth E. Clegg, M.S., Assistant Geologist Oil and Gas A. H. Bell, Ph.D., Geologist and Head Lester L. Whiting, B.A., Associate Geologist Virginia Kline, Ph.D., Associate Geologist Wayne F. Meents, Assistant Geologist Margaret O. Oros, B.A., Assistant Geologist Kenneth R. Larson, A.B., Research Assistant Jacob Van Den Berg, B.S., Research Assistant Petroleum Engineering Paul A. Witherspoon, M.S., Petroleum Engineer and Head Frederick Squires, A.B., B.S., D.Sc, Petroleum Engi- neer, Emeritus Industrial Minerals J. E. Lamar, B.S., Geologist and Head Donald L. Graf, Ph.D., Geologist James C. Bradbury, A.M., Assistant Geologist Meredith E. Ostrom, M.S., Assistant Geologist Donald L. Biggs, M.A., Assistant Geologist Clay Resources and Clay Mineral Technology Ralph E. Grim, Ph.D., Consulting Clay Mineralogist W. Arthur White, M.S., Associate Geologist Herbert D. Glass, Ph.D., Associate Geologist Charles W. Spencer, M.S., Research Assistant Groundwater Geology and Geophysical Exploration Arthur Bevan, Ph.D., D.Sc, Acting Head Merlyn B. Buhle, M.S., Associate Geologist Robert E. Bergstrom, Ph.D., Assistant Geologist John W. Foster, M.S., Assistant Geologist James E. Hackett, M.S., Assistant Geologist Margaret J. Castle, Assistant Geologist Draftsman (on leave) Wayne A. Pryor, M.S., Assistant Geologist Lidia Selkregg, D.N.S., Assistant Geologist Robert C. Parks, Technical Assistant Engineering Geology and Topographic Mapping George E. Ekblaw, Ph.D., Geologist and Head William C. Smith, M.A., Assistant Geologist Stratigraphy and Areal Geology H. B. Willman, Ph.D., Geologist and Head David H. Swann, Ph.D., Geologist Elwood Atherton, Ph.D., Geologist Charles W. Collinson, Ph.D., Associate Geologist Donald B. Saxby, M.S., Assistant Geologist T. C. Buschbach, M.S., Assistant Geologist Howard R. Schwalb, B.S., Research Assistant Frank B. Titus, Jr., B.S., Research Assistant Charles C. Engel, Technical Assistant Joseph F. Howard, Assistant Physics R. J. Piersol, Ph.D., Physicist, Emeritus Topographic Mapping in Cooperation with the United States Geological Survey. time personnel) GEOCHEMISTRY Frank H. Reed, Ph.D., Chief Chemist Grace C. Johnson, B.S., Research Assistant Coal Chemistry G. R. Yohe, Ph.D., Chemist and Head Earle C. Smith, B.S., Research Assistant Guey H. Lee, M.S., Research Assistant Physical Chemistry J. S. Machin, Ph.D., Chemist and Head Juanita Witters, M.S., Assistant Physicist Tin Boo Yee, Ph.D., Assistant Chemist Daniel L. Deadmore, B.S., Research Assistant Fluorine Chemistry G. C. Finger, Ph.D., Chemist and Head Robert E. Oesterling, B.A., Assistant Chemist Carl W. Kruse, M.S., Special Research Assistant Raymond H. White, B.S., Special Research Assistant Richard H. Shiley, B.S., Special Research Assistant Chemical Engineering H. W. Jackman, M.S.E., Chemical Engineer and Head R. J. Helfinstine, M.S., Mechanical Engineer and Supervisor of Physical Plant B. J. Greenwood, B.S., Mechanical Engineer James C. McCullough, Research Associate (on leave) Robert L. Eissler, B.S., Assistant Chemical Engineer Walter E. Cooper, Technical Assistant Edward A. Schaede, Technical Assistant Cornel Marta, Technical Assistant X-Ray W. F. Bradley, Ph.D., Chemist and Head Analytical Chemistry O. W. Rees, Ph.D., Chemist and Head L. D. McVicker, B.S., Chemist Emile D. Pierron, M.S., Associate Chemist Donald R. Dickerson, B.S., Assistant Chemist Francis A. Coolican, B.S., Assistant Chemist Charles T. Allbright, B.S., Research Assistant William J. Armon, B.S., Research Assistant Joseph M. Harris, B.A., Research Assistant JoAnne E. Kunde, B.A., Research Assistant Joan M. Cederstrand, Research Assistant George R. James, Technical Assistant Frances L. Scheidt, Technical Assistant MINERAL ECONOMICS W. H. Voskuil, Ph.D., Mineral Economist W. L. Busch, A.B., Assistant Mineral Economist Ethel M. King, Research Assistant EDUCATIONAL EXTENSION George M. Wilson, M.S., Geologist and Head Dorothy E. Rose, B.S., Assistant Geologist RESEARCH AFFILIATES IN GEOLOGY J Harlen Bretz, Ph.D., University of Chicago John A. Brophy, M.S., Research Assistant, State Geological Survey Stanley E. Harris, Jr., Ph.D., Southern Illinois Uni- versity C. Leland Horberg, Ph.D., University of Chicago M. M. Leighton, Ph.D., D.Sc, Research Professional Scientist, State Geological Survey Heinz A. Lowenstam, Ph.D., California Institute of Technology William E. Powers, Ph.D., Northwestern University Paul R. Shaffer, Ph.D., University of Illinois Harold R. Wanless, Ph.D., University of Illinois J. Marvin Weller, Ph.D., University of Chicago CONSULTANTS Geology, George W. White, Ph.D., University of Illinois Ralph E. Grim, Ph.D., University of Illinois L. E. Workman, M.S., Former Head, Subsurface Division Ceramics, Ralph K. Hursh, B.S., University of Illinois Mechanical Engineering, Seichi Konzo, M.S., University of Illinois April 15, 1955 GENERAL ADMINISTRATION (not including part-time personnel) LIBRARY Anne E. Kovanda, B.S., B.L.S., Librarian Ruby D. Frison, Technical Assistant MINERAL RESOURCE RECORDS Vivian Gordon, Head Margaret B. Brophy, B.A., Research Assistant Sue J. Cunningham, Technical Assistant Betty Clark, B.S., Technical Assistant Jeanine Climer, Technical Assistant Kathryn Brown, Technical Assistant Marilyn W. Thies, B.S., Technical Assistant Hannah Fisher, Technical Assistant LaRoy Peterson, Technical Assistant Patricia L. Luedtke, B.S., Technical Assistant Genevieve Van Heyningen, Technical Assistant PUBLICATIONS Barbara Zeiders, B.S., Assistant Technical Editor Meredith M. Calkins, Geologic Draftsman Marlene Ponshock, Assistant Geologic Draftsman TECHNICAL RECORDS Berenice Reed, Supervisory Technical Assistant Marilyn DeLand, B.S., Technical Assistant Mary Louise Locklin, B.A., Technical Assistant GENERAL SCIENTIFIC INFORMATION Ann P. Ostrom, B.A., Technical Assistant Jill B. Cahill, Technical Assistant April IS, 1955 OTHER TECHNICAL SERVICES Wm. Dale Farris, Research Associate Beulah M. Unfer, Technical Assistant A. W. Gotstein, Research Associate Glenn G. Poor, Research Associate* Gilbert L. Tinberg, Technical Assistant Wayne W. Nofftz, Supervisory Technical Assistant Donovan M. Watkins, Technical Assistant FINANCIAL RECORDS Velda A. Millard, In Charge Leona K. Erickson, Clerk-Typist III Virginia C. Sanderson, B.S., Clerk-Typist II Irma E. Samson, Clerk-Typist I CLERICAL SERVICES Mary Cecil, Clerk-Stenographer III Mary M. Sullivan, Clerk-Stenographer III Lyla Nofftz, Clerk-Stenographer II Lillian Weakley, Clerk-Stenographer II Sharon Ellis, Clerk-Stenographer I Barbara Barham, Clerk-Stenographer I Mary Alice Jacobs, Clerk-Stenographer I Lorraine Cunningham, Clerk-Stenographer I Irene Benson, Clerk-Typist 1 Mary J. de Haan, Messenger-Clerk I AUTOMOTIVE SERVICE Glenn G. Poor, In Charge* Robert O. Ellis, Automotive Mechanic Everette Edwards, Automotive Mechanic David B. Cooley, Automotive Mechanic's Helper *Divided time CONTENTS Page Introduction 7 Acknowledgments 7 Glacial geology 9 General 9 Lemont drift 9 Buried soil on Lemont drift 10 Wisconsin (Cary) deposits 11 Sedimentology 12 Fraction analyses 12 Texture 15 Crystalline and shale pebbles 16 Age of the Lemont drift 18 Origin of the Lemont drift 18 References 20 Appendix 1 — A test of all contrasts in analysis of variance 21 Appendix 2 — Location of sample localities 23 ILLUSTRATIONS Plate Page 1. Buried soil and Lemont drift 11 Figure 1. Distribution of Lemont drift and sample locations 8 2. Facies relations of Lemont drift 9 3. Fraction analyses of Lemont till 13 4. Fraction analyses of Lemont till 14 5. Textural contrast between Lemont till and Valparaiso till 15 TABLES Table Page 1. Comparative dolomite roundness 15 2. Tests of two contrasts 16 3. Homogeneity of Valparaiso and Lemont crystalline pebbles 17 Digitized by the Internet Archive in 2012 with funding from University of Illinois Urbana-Champaign http://archive.org/details/stratigraphicsed185horb STRATIGRAPHIC AND SEDIMENTOLOGIC ASPECTS OF THE LEMONT DRIFT OF NORTHEASTERN ILLINOIS BY LELAND HORBERG* and PAUL EDWIN POTTER ABSTRACT The Lemont drift occurs southwest of Chicago. Field and laboratory studies have provided additional information as to its age and source and contributed to knowledge of its environment of deposition. Two fraction analyses of the Lemont till show that dolomite is dominant in size ranges from 0.5 mm. to at least 256 mm., chert has a restricted size range, crystallines are in negligible abundance, and the clay fraction is composed of illite and quartz. This evidence supports a hypothesis of local derivation for most of the Lemont drift larger than 0.5 mm. The Lemont till is clearly differentiated from the Valparaiso till by its low clay content. As shown by Scheffe's form of the analysis of variance, roundness of 16 to 32 mm. dolomite is similar, however. Dolomite roundness in both tills is significantly less than that of associated outwash and esker gravel. Crystalline pebble counts indicate that the Lemont and Valparaiso tills had similar patterns of drift dispersion from a common source region in the Canadian Shield. Differ- ences in texture and shale-pebble content, however, most probably resulted from differ- ences in depositional environment and changes in the local source area caused by cover- ing of pre-Cary drift. Truncated remnants of a buried soil show a maximum depth of leaching of 78 inches which indicates a probable Sangamon age. Although morphologic soil zonation is pro- nounced, clay-mineral contrasts between parent material and B horizon are minor. Favored by retreat down the backslope of the Niagara cuesta and possibly by an ameliorating late Ulinoian climate, conditions for ponding and slackwater deposition were provided, and the Lemont complex of water-laid drift with minor amounts of till was deposited. INTRODUCTION The Lemont drift of the Chicago region has long been of interest because of uncer- tainties as to its age and origin. It is one of the oldest known drifts in the region, under- lying the surficial drift of Cary age, and is the only drift that is thoroughly oxidized and has remnants of a buried soil profile at its top. Discussion of age has centered on whether the drift is Tazewell or Uli- noian. Problems of origin arise from dif- ferences in sedimentary properties of the Lemont and Cary drifts: the Lemont in- cludes a complex of silty till and water- laid silt, gravel, and sand, whereas the Cary drifts are composed dominantly of clayey till ; the Lemont contains very few Devonian shale pebbles from the basin of Lake Michigan, whereas they are abun- dant in the Cary tills. The Lemont was named by Bretz (1939, 'University of Chicago. p. 53) from exposures in abandoned quar- ries in Niagaran dolomite west of Lemont (locality 13, fig. 1). Possible equivalent deposits, which extend southward along the DesPlaines Valley for about 12 miles into the Joliet region, have been described by Goldthwait (1909, p. 42), Fisher (1925, p. 79-87), and Horberg and Emery (1943). Acknowledgments The technical assistance and advice of many members of the Illinois Geological Survey staff is gratefully acknowledged. We are particularly indebted to Arthur Bevan, M. M. Leighton, and H. B. Will- man for their careful editing and helpful suggestions. Colin R. Blyth, Department of Mathe- matics, University of Illinois, called our attention to and provided essential help in application of Scheffe's analysis of variance method. [7] ILLINOIS STATE GEOLOGICAL SURVEY LEMONT DRIFT GLACIAL GEOLOGY General The area of study is situated in south- western Cook County and includes ad- joining parts of DuPage and Will coun- ties. The area may be divided into ( 1 ) the DesPlaines and Sag valleys, which were outlet channels of glacial Lake Chicago; (2) the north-south trending belt of Cary moraines with the Valparaiso moraine sys- tem on the west and the Tinley end mo- raine and ground moraine on the east; and (3) the lake plain of Lake Chicago (fig. 1). Most of the area is covered with deposits of Cary age, but about a dozen exposures of Lemont drift and some outcrops of the underlying Silurian Niagaran dolomite oc- cur along the DesPlaines and Sag valleys. Glacial striae on the Niagaran dolomite below Valparaiso and Tinley drift indicate that the Cary ice sheet moved in a south- westerly direction across the area (fig. 1). Striae below Lemont drift 2 miles north- east of Lemont (fig. 1) trend S 55° W and record a similar direction of movement for the Lemont ice (Bretz, 1955, p. 68). Lemont Drift The Lemont drift is differentiated from overlying Cary drifts on the basis of its yellowish-brown color (caused by oxida- tion), the silty character of the till and as- sociated water-laid deposits, and its low content of Devonian shale pebbles. It in- cludes both massive and rudely bedded ("water-laid") types of stony tills, slack- water and lacustrine silts, outwash sand and gravel with openwork gravel layers, deltaic gravel with foreset beds and cobbles and boulders that indicate deposition close to the ice front, and at one locality, lam- inated lacustrine clay. The complex facies relations are best shown in the gravel pit west of Worth (locality 8, fig. 1). As shown in figure 2, the sediments here include, from oldest to youngest : a lower till, a relatively thick sequence of bedded silt with sandy layers that in places show current ripples and cut-and-fill structures, a discontinuous wedge of lacustrine clay, and an upper unit of till with associated deltaic gravel. At the type locality near Lemont (locality 13, fig. 1 ) the deposit contains more till, as indicated in the section below. Thick- ness {feet) Wisconsin (Cary) — Valparaiso Till, clayey, gray, calcareous ex- cept for upper 2 feet .... 4 Illinoian — Lemont drift Silt, sandy, bedded, yellow brown, calcareous, irregular contact with overlying till 4 Gravel, sandy, silty, yellow brown, calcareous, in places in- durated with calcareous cement 2 Silt, sandy, bedded, yellow brown, calcareous 2 Till, silty, yellow brown and gray, rudely bedded, calcareous . . 2 Till, silty, massive, yellow brown with irregular masses of unoxi- dized light gray till, calcareous, more stony than the Valparaiso 18 Silt, as above, base not exposed . 4 Depth ifeet) 10 12 14 32 36 In contrast to both of the above expo- sures, the Lemont at the Dolese-Shepard W Fig. 2. — Diagram showing facies relations of Lemont drift in gravel pit at Worth, 111. (locality 8). Wt, Tinley till; Wv, Valparaiso till; S, Sangamon soil on Lemont drift; Lt, Lemont till; Lg, Lemont gravel; Lc, Lemont clay; Ls, Lemont silt with some sand. Vertical interval about 30 feet; horizontal distance about 800 feet. 10 ILLINOIS STATE GEOLOGICAL SURVEY quarry near McCook (locality 4, fig. 1 ) is composed mainly of gravel, as shown by the section below. Thick- ness (feel) Wisconsin (Cary) — Tinley till Till, very clayey, yellow gray to gray, calcareous 15 Illinoian — Lemont gravel with weathered zone Gravel, coarse, average about 2 inches, yellow brown, oxidized, noncalcareous, many dolomite pebbles soft and rotten, limonite stains, discontinuous; horizon 3 of weathering profile . . . 4.5 Gravel, as above, coarser in places with cobbles up to 8 inches, large- ly dolomite, rounded till pebbles with granule armor common, brown, oxidized, calcareous; hori- zon 4 of weathering profile . . 8 Gravel, sandy, average about 0.5 inches, poorly sorted, cut-and-fill structures, openwork gravel with manganese-coated pebbles pres- ent in places, prominent foresets with average dip direction about S 5° W, oxidized, yellow brown, calcareous; horizon 4 . . . .4 Gravel, clayey, composed of over 50 percent armored till pebbles, unoxidized, gray, sharp contact with oxidized gravel above and below, calcareous 2.5 Depth {feet) 15 19.5 27.5 31.5 34 Gravel, composed largely of dolo- mite slab-stones 0.5 to 1 foot, poorly sorted, imbrication shows current moving S 40° W, yellow brown, calcareous 2 Gravel, sandy, average about 0.5 inches, poorly sorted, foreset and horizontal beds, yellow brown, calcareous 5 36 41 Silurian Niagaran dolomite The Lemont is not well exposed at the remaining localities shown in figure 1. It is composed in most places of bedded silt with thin gravel seams and associated gravel layers that locally are indurated; in a few places the silt is massive and loess- like. Buried Soil on Lemont Drift Remnants of a truncated soil profile on Lemont drift are exposed at localities 4, 8, and 9 (fig. 1). The most nearly complete section was observed in the gravel pit near Worth (locality 8) where remnants were noted at five places in the pit (fig. 2). A section exposed in 1954 in the northeastern corner of the pit is given below. Horizon A B, B, 3 a 3 b Description Recent soil on Tinley till: Loam, silty, noncalcareous, gray humus Loam, clayey, noncalcareous, dense. Till, noncalcareous, joints with colloid fill- ings Till, clayey, calcareous, brownish gray . Buried soil on Lemont sand and gravel: Gumbo gravel, dense, gravelly, rare weathered dolomites in lower part, largely fine gravel particles in a clayey matrix, noncalcareous, dark reddish brown to dark yellowish brown Gravel, clayey, silty, dolomite residuals, noncalcareous, more pebbly than above, Mn stain in lower 10 inches, dark yellow- ish brown to yellowish brown . . . . Gravel, coarse, silty clay matrix, rotten dolomites, matrix noncalcareous but some pebbles react with acid, yellowish red ("ferretto zone") Silt, clayey, bedded, calcareous with poorly developed veinlets and lime disseminations in upper 3 inches, gray with brown mot- tlings Color (Munsel) 5 YR 3/4 to 10 YR 4/4 10 YR 4/4 to 10 YR 5/4 5 YR 5/8 P H 7.0-7.8 6.5-7 7.5-7. Thickness (inches) 30 25 23 Depth (inches) 6 15 24 38 30 55 78 86 WISCONSIN (CARY) DEPOSITS 11 At other places in the gravel pit, remnants of the soil are marked by shallower leached horizons and horizons of reddish-brown iron-oxide concentrations. At locality 9, in a borrow pit near the junction of State highways 7 and 83, the - i '^fc • f ret •. ■fHr Plate 1. — Buried soil and Lemont drift. A. Buried soil on Lemont gravel below Tinley till, gravel pit, Worth, 111. (locality 8, fig. 1). Base of Tinley and top of horizon 2 is at hammer; base of horizon 2 is at rule; upper part of "ferretto zone," horizon 3b is at shovel. B. Deltaic gravels and overlying silts. Soil profile (A) is approximately 75 feet beyond right edge of picture. buried soil occurs on sandy stratified Le- mont silt below thin Cary gravel. It con- sists of a reddish-brown leached zone two feet thick (horizon 3) underlain by four feet of yellowish-brown calcareous silt (horizon 4). The paleosol at locality 4, the Dolese- Shepard quarry near McCook, is described above in the geologic section for that local- ity. It is distinguished by a limonite- stained leached horizon 54-inches thick on Lemont gravel (horizon 3) and is over- lain by 15 feet of calcareous unoxidized Tinley (Cary) till. Three samples of the 2-micron fraction from horizons 2, 3, and 4 of the buried soil at locality 8 were analyzed by H. D. Glass of the Illinois State Geological Sur- vey. All consist of an illite of muscovite crystallization with fairly good crystallin- ity and a considerable amount of quartz. The amount of quartz increases downward in the profile. At Lemont (locality 13) another sample of the 2-micron fraction from oxidized but otherwise unweathered till also contained an illite-quartz associa- tion. Thus except for the decrease in quartz, little or no mineralogic change ac- companied profile development. According to Jackson and Sherman (1953, p. 262), the decrease in 2-micron quartz is the first ex- pression of the intermediate stages of soil weathering. The uniformity of illite in horizons 2, 3, and 4 confirms the observa- tions of Grim (1942, p. 259) and Jack- son and Sherman (1953, p. 263) that the illite present in many soils is usually in- herited from parent material. Wisconsin (Cary) Deposits Deposits of Cary age in the area include the Valparaiso and Tin'ey drifts and de- posits related to glacial Lake Chicago. The Valparaiso morainic system in the area is composed of at least three north- south trending end moraines, crowded to- gether without intervening ground mo- raines and, in places, superposed. Most of the drift consists of light-gray to yellow- gray clayey till with pebbles and boulders dominantly of dolomite. The average depth of leaching of the till is about 3 feet. 12 ILLINOIS STATE GEOLOGICAL SURVEY The Tinley drift is differentiated from the Valparaiso primarily on the basis of morphology and areal relations (Bretz, 1939, p. 50). In a few places, such as in the Worth pit, the two tills occur together stratigraphically ; here they are differenti- ated by an unconformity with basal gravel and the greater abundance of Devonian shale pebbles in the Tinley. Deposits related to glacial Lake Chicago include discontinuous lacustrine clays and silts in beach gravels on the lake plain and glacial-river gravels along the DesPlaines and Sag outlet valleys. SEDIMENTOLOGY Compared with other sediments, few data are available for glacial deposits, es- pecially for tills. Contributions to the sed- imentology of glacial deposits have been made by Udden (1914), Krumbein (1933), Lundqvist (1935), Wentworth (1936), Davis (1951), Swineford and Frye (1951), Holmes (1941 and 1952), Shepps (1953), Dreimanis and Reavely (1953), Murray (1953), and others. In this study, objectives of sedimentologi- cal investigation were : 1 ) to contribute to the general sedimentology of glacial de- posits, and 2) to clarify the origin of the Lemont drift through integration of sedi- mentologic and field evidence. We be- lieved that the best way to achieve the sec- ond objective was by comparing the Le- mont drift with the overlying clayey Val- paraiso till. Fraction Analyses Preliminary to such comparison, how- ever, two fraction analyses (size versus min- eralogy) were made of the Lemont till. As early demonstrated by Trowbridge and Shepard (1932), mineralogical composition is size dependent. Subsequent examples of this dependence in the clay, silt, sand, and gravel fractions have been provided by Correns (1938), Cogen (1935, p. 3-5), Grim (1950, p. 13-21), Van Andel (1950, p. 18-21), Potter (1955, p. 7-8), White (1955, personal communication), and oth- A fraction analysis for glacial till has been made by Davis (1951). The dependence of mineralogic composition on size for the Lemont till is clearly shown in figures 3 and 4. Figure 3 was obtained by sizing two sam- ples of till (localities 8 and 13) and count- ing 100 to 200 particles per size grade. In figure 3 mineralogic abundance is ex- pressed in number percent ; composition of each size grade totals 100 percent. Figure 4 shows the same data expressed as weight percent of the entire sample. The following relationships are indi- cated for figures 3 and 4: 1. dominance of dolomite especially in the range 0.5 mm. to more than 256 mm. 2. rapid decrease in dolomite in the smaller sand grades (<0.5 mm.). 3. chert largely restricted to the range 0.5 mm. to 64 mm. 4. crystallines in negligible abundance. 5. a clay fraction composed of illite and finely ground quartz. A hypothesis of local derivation for much of the Lemont till best explains the above relationships. The scarcity of dura- ble crystalline rocks of Canadian Shield derivation shows that most of the Lemont drift larger than 0.5 mm. was derived from Paleozoic sediments. That this Paleo- zoic source was essentially local (perhaps 80 percent derived from distances less than 20 miles) is indicated by the abundance and lithologic similarity of the dolomite peb- bles and cobbles in the till to the immedi- ately underlying Niagaran bedrock. The size range of chert pebbles further sub- stantiates this conclusion. The bedrock of the Chicago region contains chert nodules and layers rarely more than 50 to 75 mm. thick. The observed upper size limit of chert pebbles in the Lemont till, 64 mm., corresponds well to the thickness of the cherts in the bedrock. For the sand fraction less than 0.5 mm., the progressive and rapid decrease in car- bonates is best explained by a combination of local derivation and decreased resistance to abrasion. Glacial erosion of a carbonate bedrock incorporates boulders, cobbles, and FRACTION ANALYSES 13 roo 90H 80- 70- v- z lu 60 o ttc UJ °- 50H (K UJ § 40- O 2 30-1 20- 10- 0- tr ■'. <' .3 . O • O :*z- ■: ^ c >^ C -a .« T3 *-> T3 n c 3 rt a 3 cd C 3 o o o O o O J P4 hJ Pd hJ tf 3 0.37 13 0.34 7 0.45 1 0.26 17 0.38 8 0.53 5 0.34 15 0.35 14 0.51 2 0.34 16 0.39 12 0.52 8 0.36 8 0.40 9 0.40 11 0.30 6 0.29 8 0.53 10 0.30 Average 0.33 0.36 0.49 in an environment that prevented deposi- tion of most of the clay fraction. In Le- mont till deposition, there is implied a greater role for meltwater. What effect did more abundant meltwater have on abrasion of the larger clastic components? Comparison of the roundness of 16-to-32 mm. dolomite pebbles collected from Le- mont till, Valparaiso till, and outwash and esker gravels provides one index. Sam- ples of 40 pebbles were collected at 19 lo- calities and roundness estimated with the Krumbein chart (1941, p. 68-70). Table 1 shows the results of this sampling. The significance of the mean values 0.32, 0.36, and 0.49 was tested with a form of the analysis of variance (Scheffe, 1953). A brief explanation and examples of the calculations are given in Appendix 1. Geologically, two hypotheses are of in- terest : 1. Are the means of dolomite roundness in the Valparaiso and Lemont tills equal (Hi : (*i = fj. 2 ) ? 2. If so, is their average value signifi- cantly different from that of outwash and *)? esker gravel Ml + M2 (H 2 : ~ The hypotheses were tested at the five percent level. The difference between dolo- mite roundness in the two tills (H^) is 16 ILLINOIS STATE GEOLOGICAL SURVEY not significant (table 2). The thirteen samples from Valparaiso and Lemont tills can be considered to have been obtained from the same population. Hence they were combined, tested against dolomite roundness in outwash and esker gravel (H 2 ), and found to be significantly dif- ferent. What geologic hypothesis best ac- cords with this evidence? Plumley (1949, p. 558-559) found that, depending on stream gradient, 16-to-32 mm. limestone gravel reaches a roundness of 0.49 in as little as 3 to 6 miles. This contrasts sharply with glacial transport which only produces angular to subangular (0.34) dolomite pebbles. Whereas the presence of blunted and faceted pebbles in till indicates abrasional impact, this impact is not only less frequent in tills, but involves particles capable of little rotation. Hence progressive abrasion could not lead to well-rounded fragments. Lack of rotation is also the prob- able reason for the greater role of crushing in glacial transport. Thus, because both impact and rotation in response to impact are less than in stream-bed deposits, gla- cial transport produces blunted, faceted, angular to subangular fragments. The similarity of dolomite roundness in Lemont and Valparaiso tills indicates, in spite of pronounced differences in clay matrix, that this process was essentially identical for both tills. It suggests that the greater abundance of silt in Lemont till is more probably the result of incorporation of pro- Lemont silts than removal of clay-size par- ticles during deposition. Crystalline and Shale Pebbles The areal relations of the Valparaiso morainic system (Bretz, 1939, p. 46) clearly indicate a regional pattern of drift dispersion and a restricted source region in the Canadian Shield. Because the Lemont is known at only a few exposures over a limited area, a comparable morphologic ap- proach to its pattern of drift dispersion and source region in the Canadian Shield is not possible. We have an indirect method for estimating this pattern, however, by com- paring the pre-Cambrian rock types (com- monly referred to as crystallines) in the Le- mont with those in the Valparaiso till, with its known pattern of drift dispersion. Sim- ilar pebble suites for the Lemont and Val- paraiso would indicate a common crystal- line source region in the shield and would lend support to a hypothesis of a common pattern of drift dispersion. Crystalline pebbles were collected from three exposures of Valparaiso till (locali- ties 1, 10, and 18) and from four expo- sures of Lemont till and gravel (localities 8, 13, 15, and 16). Because crystallines are much less abundant in the Lemont than in the Valparaiso, it was necessary to select a rather wide size range (8 to 32 mm.) and to sample Lemont outwash as well as till. A total of 1295 pebbles was collected and classified into five groups : phaneritic acid igneous, phaneritic basic igneous, dia- base, metamorphic, and volcanic. As shown in table 3, the homogeneity of the Valpa- raiso and Lemont crystallines was tested by forming a contingency table (Mood, 1950, p. 273-281). The lack of signifi- cance at the five percent level indicates that there are less than five chances out of 100 that the two samples were not derived from the same population. This indicates that, with respect to these variables of classification, the Lemont and Table 2. — Tests of Two Contrasts Hypotheses A e *, 2 r) degrees of free- dom, r being the number of degrees of freedom on which s 2 is based. For a given problem, S is constant no matter how many contrasts are tested. Selecting a level of significance a, the probability is 1 — a that all the contrasts satisfy e s^ e 0. Using this rule, our probability of de- ciding that even one of the many possible zero contrasts is not zero, is at most a. *For the data of table 1, s 2 is computed as: 2 _ 2 x H 2 — mxi + 2 x 2i 2 — nixt + 2 x 3i 2 — n 3 x 5 0.0340 16 n - 3 = 0.00213 22 ILLINOIS STATE GEOLOGICAL SURVEY Application of the Method Using (6) to compute S, and selecting a = .05, we have s 2 = 2 (3.63) = 7.26 S = 2.69. To test hypothesis H x : /xi the contrast = CifJLi + C2M2 M2 form where 2 Q = d + c 2 = 1 + (-1) = 0. i The estimate of this contrast is 9 = cxi + c 2 x 2 = 0.234 - 0.358 = - 0.034 and the variance of this estimate is ., k = s ^ 1 n * (-i)v crs- c 2 -s- ni n 2 7 + = 0.0006592. Forming the confidence region (7) and substituting we have 0.103