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

FRANK G. THOMPSON, Director

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

REPORT OF INVESTIGATIONS—NO. 118

PREGLACIAL EROSION SURFACES IN ILLINOIS

LELAND HORBERG

Reprinted from The Journal of Geology, Vol. LIV, No. 3, 1946

|tUNO' Q 3G1CAL SUR v ARY

PRINTED BY AUTHORITY OF THE STATE OF ILLINOIS

URBANA, ILLINOIS

1946 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 NORMAN L. BOWEN, Ph.D., D.Sc, LL.D., Geology ROGER ADAMS, Ph.D., D.Sc, Chemistry LOUIS R. HOWSON, C.E., Engineering CARL G. HARTMAN, Ph.D., Biology EZRA JACOB KRAUS, Ph.D., D.Sc, Forestry ARTHUR CUTTS WILLARD, D.Engr., LL.D.

President of the University of Illinois

GEOLOGICAL SURVEY DIVISION

M. M. LEIGHTON, Chief

[500-7-46

ILLINOIS STATE GEOLOGICAL SURVEY

3 3051 00005 7335 PREGLACIAL EROSION SURFACES IN ILLINOIS 1

LELAND HORBERG

Illinois State Geological Survey, Urbana ABSTRACT

A generalized map of buried erosion surfaces in Illinois, compiled from detailed maps of the bedrock topography, provides regional perspective for recognition and correlation of erosion surfaces within the state. A correlation of the Lancaster peneplain of northwestern Illinois, the Calhoun peneplain of western Illinois, and the Ozark peneplain of northern Illinois is suggested; an independent lower surface which developed on the weak rocks of the Illinois basin is described and named the "Central Illinois peneplain"; and possible straths along major preglacial valleys are recognized and named the "Havana strath."

INTRODUCTION tures of the bedrock surface. The original Because of the cover of glacial drift, maps for about half the state were com- studies of erosion surfaces in the Missis- piled on topographic sheets and con- sippi Valley region have been restricted toured with intervals of 10, 20, and 25 largely to the Driftless Area and to the feet. For the remainder of the state, uplands lying outside the limits of where less control was available," 50-foot glaciation. Studies in Illinois, which con- contours were drawn on a base map hav- cern (i) the Driftless Area of north- ing a scale of 4 miles to the inch. The western Illinois, (2) Calhoun County in final map of the entire state was drafted western Illinois, and (3) the southern un- on the latter scale. glaciated section, are summarized briefly The generalized map of the erosion in the following pages. A few attempts surfaces was drawn from the detailed have been made to correlate buried bed- map by connecting points of equal eleva- rock uplands in local areas within the tion on interstream areas. It thus shows glaciated region, but there has been no the general form of the uplands before regional treatment of the problem. In the dissection and could be considered an at- present study a groundwork for regional tempted reconstruction of a late Tertiary interpretation was provided by a de- land surface somewhat modified by later tailed bedrock-surface map of Illinois, fluvial and glacial erosion. The element which was compiled in connection with of personal judgment enters into the se- ground-water studies. A generalized con- lection of broad valleys that are to be tour map of the erosion surfaces (Fig. 1) shown or eliminated, otherwise the con- summarizes the essential features re- tours are closely controlled by numerous vealed by the detailed map. points on the divides. This method ap- The detailed bedrock-surface map was plied to the upland terraces of southern compiled from about 60,000 records of New England was found to show the borings within the state and from all available data on the location of bedrock same major features as those revealed by exposures. In areas where the drift is thin, multiple projected profiles of the same 2 the present topography, shown by topo- area. graphic maps, revealed the major fea- 2 George F. Adams, "Upland Terraces in South-

1 Published with the permission of the chief of ern New England," Jour. Geo!., Vol. LIII (1945), the Illinois Geological Survey, Urbana, Illinois. p. 309.

179 i8o LELAND HORBERG

PREVIOUS STUDIES OF EROSION In the Illinois section of the Driftless SURFACES IN ADJOINING Area two prominent upland surfaces are DRIFTLESS AREAS present. The upper surface, termed the 6 "Dodgeville peneplain," is represented NORTHWESTERN ILLINOIS by the tops of narrow ridges and isolated The upland surfaces in the Driftless mounds capped by Silurian dolomite Area of the upper Mississippi Valley which are 1,000-1,150 feet above sea- have been studied intensively for the last level and 350-450 feet above drainage. fifty years and are better known than The lower surface, called the "Lancaster other erosion surfaces in the interior of peneplain," 7 lies about 200 feet below the the continent. 3 Yet there is no general Dodgeville plain and is of greater extent agreement as to the number of cyclical than the older surface. It coincides close- surfaces present or their age. Interpreta- ly with the top of the Ordovician Galena tions have ranged from noncyclical de- dolomite and slopes southwestward from velopment during a single period of about 950-800 feet above sea-level 4 erosion to recognition of five distinct 8 (Figs. 3 and4). cyclical surfaces. 5 Most writers have Gravels of the Windrow formation are identified remnants of either one or two found on remnants of the Dodgeville peneplains. peneplain in adjoining states, 9 but only

3 O. H. Hershey, "Preglacial Erosion Cycles in scattered pebbles, occurring at various Illinois," Amer. Geol., Vol. XVIII 10 Northwestern elevations, have been found in Illinois. (1896), pp. 72-100, and "The Physiographic develop- Nebraskan drift is reported to be present ment of the Upper Mississippi Valley," ibid., Vol. XX (1897), pp. 246-68; U. S. Grant and E. E. on both upland surfaces in Iowa but has Burchard, "Lancaster-Mineral Point Folio, Wis- not been discovered in valleys eroded consin-Iowa-Illinois," U.S. Geol. Surv. Geol. Atlas, below the Lancaster plain. This relation Folio 145 (1907), p. 2; A. C. Trowbridge, "Some 11 Partly Dissected Plains in Jo Daviess County, led Trowbridge to conclude that the Illinois," Jour. Geol., Vol. XXI (1913), pp. 731-42; lower surface was undissected at the E. W. Shaw and A. C. Trowbridge, "Galena- Elizabeth Folio, Illinois-Iowa," U.S. Geol. Surv. Geol. 6 Named from the upland near Dodgeville, Wis- Atlas, Folio 200 (1916), pp. 9-10; A. C. Trowbridge consin, by Trowbridge (p. 64 of ftn. 3 [1921]). Trow- and E. W. Shaw, "Geology and Geography of the bridge now regards the Dodgeville plain as part of Galena and Elizabeth Quadrangles," 77/. Geol. Surv. the lower Lancaster peneplain rather than as an in- Bull. 26 (1916), pp. 136-46; U. B. Hughes, "A Cor- dependent erosion surface (personal communication relation of tjie Peneplains of the Driftless Area," [1946]). Proc. Iowa Acad. Sci., Vol. XXIII (1916), pp. 125- 32; A. C. Trowbridge, "The Erosional History of the 7 Named from Lancaster, Wisconsin, by Grant Driftless Area," Iowa Univ. Studies, 1st ser., No. 40 and Burchard, p. 2 of ftn. 3. ("Studies in Nat. Hist.," Vol. IX, No. 3 [1921]), 8 Both surfaces are well shown on the Galena and of the problem; PP- 55—127 (the most detailed study Elizabeth quadrangle maps. contains a complete bibliography up to 192 1); 9 "Preglacial on the Lawrence Martin, "Physical Geography of Wiscon- R. D. Salisbury, Gravels Quartzite Range near Baraboo, Wisconsin," Jour. sin," Wis. Geol. Surv. Bidl. 36 (2d ed., 1932), pp.69- Geol, Vol. Ill pp. 665-67; Trowbridge, 77; A. C. Trowbridge, in Kansas Geological Society (1895), ftn. F. T. Thwaites and Guide Book, Ninth Annual Field Conference, Upper pp. 111-13 of 3 (192 1); W. H. Twenhofel, "Windrow Formation: An Upland Mississippi Valley (Wichita : Kansas Geological Soc, Gravel Formation of the Driftless and Adjacent 1935), pp. 62, 75; F.T. Thwaites, in Kansas Geological Areas of the Upper Mississippi Valley," Bull. Geol. . . . . Bates, Society Guide Book , pp. 105-26; R. E. "Geomorphic History of the Kickapoo Region, Soc. Amer., Vol. XXXII (1920), pp. 293-314.

Wisconsin," Bull. Geol. Soc. Amer., Vol. L 10 (1939), Trowbridge, p. 112 of ftn. 3 (1921); H. B. 820-41. pp. Willman, personal communication (1946).

4 Martin, ftn. 3. "Pp. 123-25 of ftn. 3 (1921) and pp. 62, 75 of 5 Hershey, ftn. 3. ftn. 3 (1935). PREGLACIAL EROSION SURFACES IN ILLINOIS 181 time of Nebraskan glaciation. However, lower upland about 600 feet above sea- the possibility of subsequent removal of level south of the flexure in the southern 14 the drift from the valleys cannot be tip of the county (Fig. 1). This relation eliminated altogether, and the surface raises the question of whether there was may have been dissected at an earlier important movement along the Cap date. This alternative is indicated by the au Gres fault after deposition of the occurrence of Kansan and possibly gravels, as concluded by Rubey, 15 or Nebraskan drift within deep bedrock whether gravels, possibly of different valleys of the ancient Mississippi and its age, were deposited on two distinct sur- tributaries (Fig. 5) in the central part of faces. the state. 12 A compromise view that the SOUTHERN ILLINOIS deep valley of the ancient Mississippi in The driftless section of southern the Driftless Area is younger than the Illinois roughly includes the area lying deep valley to the south has not been south of the Pennsylvanian (Caseyville) supported preliminary subsurface by escarpment, which extends east-west studies in the intervening area. completely across the southern tip of the

state (Fig. 1). Within this region there CALHOUN COUNTY, WESTERN ILLINOIS are two important uplands which are The narrow upland peninsula between structurally and topographically dis- the Mississippi and the Illinois rivers tinct: (1) the Shawnee section, which above their junction in western Illinois is includes the Pennsylvanian cuesta and driftless except for loess deposits, and adjoining lower surfaces on the south, two erosion surfaces in the area have and (2) an isolated segment of the Salem been described by Rubey. 13 An W. W. plateau of the Ozark region margining upper surface, forming the crest of the the Mississippi trench along the west upland at an elevation of 700-750 feet, 16 side of the area. The upland surfaces in was named the "Calhoun peneplain," the Shawnee section occur at several and an intermediate level, 125-250 feet levels, on various bedrock formations lower, was described as a postmature which have been extensively faulted and surface (Fig. 1). The Lincoln Hills, in the probably include undifferentiated cyclical adjoining area in Missouri, rise 100-200 and structural surfaces. The uplands of feet above the higher surface. The higher the Salem plateau, in contrast, rise to ac- surface is largely on Osage (Mississip- cordant levels about 700 feet above sea- pian) limestones, and to the north it level and are underlain almost exclusive- bevels structure. Gravels of "Lafayette ly by deeply weathered Devonian cherts. type," composed of chert, quartz, and No detailed studies have been made of quartzite in a ferruginous matrix, are the entire district, and attempted cor- present on the upper surface north of the x 4 Rubey, ftn. R. D. Salisbury, "On the Cap au Gres faulted flexure and on a 13; Northward and Eastward Extension of the Pre- Pleistocene Gravels of the Mississippi Basin," Bull. I2 Leland Horberg, "A Major Buried Valley in Geol. Soc. Amer., Vol. Ill (1892), pp. 183-86; Stuart East-central Illinois and Its Regional Relation- Weller, "Notes on the Geology of Northern Calhoun ships," Jour. Geol., Vol. LIII (1945), PP- 353-55; County," ///. Geol. Surv. Bull. (1907), p. 231. unpublished sample study records in the files of the 4 Illinois Geological Survey. J s Ftn. 13.

J 3 "Geology and Mineral Resource of the Hardin- 16 N. M. Fenneman, Physiography of Eastern Brussels Quadrangles, Illinois," unpublished manu- United States (New York: McGraw-Hill Book Co., script (1931). Inc.. 1938), pp. 438-39, 651-52, PI. VI. .

182 LELAND HORBERG

relation must be based on studies of two pieted sometime during the Tertiary. It local areas and on broad regional rela- is significant that the Devonian forma- tions. As the result of studies by R. D. tions of the Salem Plateau have been Salisbury 17 in Hardin County at the weathered and leached to depths of eastern edge of the Shawnee Hills, and about 400 feet and that this unusual 18 by J. E. Lamar in the Carbondale depth of alteration has been ascribed by 22 quadrangle in the western part, four J. M. Weller to a possible prolonged erosion surfaces ranging in elevation alteration under peneplain conditions. from 500 to 900 feet have been de- The generalized contour map of the scribed, as shown by Table 1. The sur- upland surfaces (Fig. 1) suggests (1) the face 700-760 feet above sea-level, de- presence of a widespread surface about scribed by Lamar, appears to be most ex- 700 feet above sea-level that is correlative tensive, and it is probably this surface with the Ozark peneplain, and (2) re-

TABLE 1 Erosion Surfaces in Southern Illinois

Hardin County Carbondale Quadrangle (Salisbury) (Lamar)

Name Elev. Elev. Name

Present flood plains . 320-340 320-350 Present flood plains Elizabethtown plain 400-420 McFarlan plain 500-540 500-560 McFarlan plain(?) Karbers Ridge plain 600-640 000-650 Karbers Ridge plain(?)

] 700-760 Unnamed Buzzards Point plain 860-900 800-860 Buzzards Point plain(?)

which R. F. Flint19 recognized as part of stricted higher-summit areas which may the domed Ozark peneplain and correlat- represent remnants of the Buzzards ed with the Highland Rim surface in Point plain or monadnocks on the lower Kentucky and Tennessee, and which surface. Lower surfaces, 500-550 feet and N. M. Feneman, 20 in addition, correlated 600-650 feet in elevation, in the central with the Lexington peneplain of Ken- part of the area may represent the Mc- tucky and the Lancaster peneplain in the Farlan and Karbers Ridge plains, re- Driftless Area. The surface is believed spectively, or they may be primarily by Flint21 to transect Wilcox (Eocene) structural plains. strata and therefore to have been com- Lafayette-type gravel occurs on the 23 J Plateau in Union County, on 7 Stuart Weller, Charles Butts, L. W. Currier, Salem and R. D. Salisbury, "Geology of Hardin County," the Pennsylvanian escarpment in Galla- 77/. Geol. Surv. Bull. 41 (1920), pp. 47-52. tin County, 24 and on the crests of hills 18 "Geology and Mineral Resources of the Car- 22 "Devonian System in Southern Illinois," ///. bondale Quadrangle," 77/. Geol. Surv. Bull. 48 Geol. Surv. Bull 68A: Devonian Symposium (1944), (1925), pp. 152-54. pp. 101-2. 19 of "Ozark Segment Mississippi River," Jour. 2 ^ J. M. Weller, personal communication (1945). Geol., Vol. XLIX (1941), pp. 634-36, 640. 2 4 Charles Butts, "Geology and Mineral Re- 20 Pp. 441, 504, and 660 of ftn. 16. sources of the Equality-Shawneetown Area," 77/. 21 Pp. 639-40 of ftn. 19. Geol. Surv. Bull. 47 (1925), p. 52. .

PREGLACIAL EROSION SURFACES IN ILLINOIS 183

and ridges at lower elevations south to dolomite, although in only a few places the Ohio valley (Fig. 1). Because of the does it coincide closely with the upper presence of a weathered zone up to 6 feet part of the formation. Comparison of the thick on the Lafayette and below the generalized bedrock-surface contours 25 loess, J. M. Weller has concluded that with structure contours of similar inter- deposition of Lafayette gravels was fol- val on the top of the Galena (Figs, i and lowed by a long period of stable condi- 2) shows that in the western half of the tions, under which weathering pro- upland the rock surface is generally 150 gressed, and that the major period of feet or more below the top of the dolo- bedrock erosion giving rise to the present mite, and that to the east the surface topography occurred during the final crosses onto the overlying Maquoketa part of the post-Lafayette preglacial shale and lies ioo feet or more above the interval. top of the Galena dolomite. In various local areas the surface is eroded on beds BURIED EROSION SURFACES ranging in age from Silurian (Niagaran GALENA UPLAND SURFACE dolomite) to Cambrian (Trempealeau Description. The summit areas of the — dolomite) preglacial Galena upland in north- When the structural and topographic central and northwestern Illinois form a trends are compared, a further lack of remarkably uniform plain, which slopes coincidence is revealed. The regional southeastward from 950 feet above sea- slope of the upland is roughly S. 30 E., level at the edge of the Driftless Area to whereas the three major structural about 800 feet at the south edge of the trends are north-south along the Wis- upland (Lancaster peneplain, Fig. 1). consin arch, east-west along the Sa- The gradient is about 3 feet per mile, and vanna-Sabula anticline, and N. 6o° W. local relief on the restored surface in along the positive element between the most places is less than 50 feet. A broad, Sandwich fault zone and the LaSalle upland crest, which extends southeast- anticline (Fig. 2). Locally there are ward across the southwestern part of the several places where beveling of struc- upland, coincides closely with the pre- ture is indicated: (1) across the Savanna- glacial divide between the ancient Missis- Sab ula anticline and the syncline to the sippi and Rock River drainage systems north, (2) across the northwest flank of (Fig. 5). Along the south and east mar- the LaSalle anticline near its juncture gin of the upland there is a break in up- with the Savanna-Sabula anticline in land profiles between elevations of 800 southwestern Ogle County (3) across the feet on the north and 650 feet on the northern end of the Sandwich fault zone south. The zone along which the change and related structures in southeastern occurs is about 20 miles wide and is Ogle and northeastern Lee counties, and independent of structure. Along it is (4) across numerous minor folds which drawn the boundary between the Lan- plunge down regional dip on the east caster and Central Illinois peneplains flank of the Wisconsin arch. (Fig. 1). It is concluded that there is only a Relation to structure.—The upland gross relation between structure and the plain is developed largely on the Galena upland surface and that, in detail, the 25 "Geology and Oil Possibilities of Extreme transgressions of structure are so numer- Southern Illinois," 77/. Geol. Surv. Rept. Investiga- tions 71 (1940), p. 45. ous and important that the surface can- -

LEGEND

HAVANA STRATH

CENTRAL ILLINOIS PENEPLAIN (MCFARLANE PLAIN ? v\w\^ LANCASTER -CALHOUN ^^ OZARK PENEPLAIN r DODGEVILLE (BUZZARD'S POINT ? PENEPLAIN

A REPORTED OCCURRENCES OF PREGLACIAL LAFAYETTE - TYPE GRAVELS

CONTOUR , INTERVAL 50 FEFT

10 20 30 40 50 60 MILES

Fig. i. —Generalized contour map of the erosion surfaces in Illinois PREGLACIAL EROSION SURFACES IN ILLINOIS 185 not be considered a simple structure of projected profiles in the Driftless plain. Area that the Lancaster peneplain of Correlation.—The Galena upland plain Trowbridge is a stripped structural plain within the drift-covered area was recog- rather than a cyclical surface. This con- nized previously 26 and interpreted as clusion appears to be justified by rela- modified remnants of what is now gen- tions in the Illinois section of the Drift- erally regarded as the Lancaster or late less' Area, where the Lancaster peneplain

Fig. 2.—Structure contours on top of the Galena dolomite in northern Illinois. W, Wisconsin arch; S-S, Savanna-Sabula anticline; S, Sandwich fault zone; L, LaSalle anticline.

Tertiary peneplain. This conclusion is coincides closely with the top of the substantiated by the present study. Galena dolomite. However, if it is the The alternative—that the surface Lancaster surface that extends south- represents the higher Dodgeville pene- eastward below the drift and bevels plain—is possible if downwarping is structure on the Galena upland, the postulated; and it is supported indirectly cyclical origin of the surface finds new by Bates, 27 who concluded from a study support. In fact, one of the strongest

26 evidences of beveling is found just south Hershey ftn. 3 (1896) (1897); J H. Bretz, "Geology and Mineral Resources of the Kings of the drift margin, where the Maquo- Quadrangle," III. Geol. Surv. Bull. 43 (1923), pp. 273- keta shale and Silurian dolomite along R. S. Knappen, "Geology and Mineral Re- 77; the syncline north of the Savanna- sources of the Dixon Quadrangle," III. Geol. Surv. Bull. 4Q (1926), pp. 90-93. Sabula anticline are truncated by the

2 ? Pp. 833-37 of ftn. 3. bedrock surface (Fig. 3). i86 LELAND HORBERG

In an attempt to determine whether UPLAND SURFACES IN WESTERN ILLINOIS this bedrock surface represents the Lan- Description.—The bedrock uplands in caster or the Dodgeville surface, a western Illinois are less continuous and generalized contour map covering most more variable in elevation than those of of the Driftless Area in Illinois was con- the Galena upland to the north and the structed from topographic sheets Pennsylvanian lowland to the east,

(Fig. 4). The reconstruction indicates They appear to represent a broad, ma- (1) that the Dodgeville upland in its ture surface, on which there are possible northern part is delimited from Lan- remnants of a summit peneplain 700- caster surfaces to the east and west by 800 feet above sea-level and possibly a clearly defined escarpments but that to lower surface 600-650 feet above sea- the south there is no sharp break to a level (Fig. 1). The upper surface is lower surface; (2) that possible isolated interrupted by two broad valleys, so that remnants of the Dodgeville peneplain in it is not continuous from north to south. the south half of the Elizabeth quad- Possible remnants of the lower surface

Fig. 3.— Cross-section showing Dodgeville {upper) and Lancaster {lower) surfaces in northwestern Illi- nois along A-A' of Fig. 1. Qgl, glacial drift; Su, Silurian dolomite; Om, Maquoketa shale; Ogp, Galena- Platteville dolomite; Osp, St. Peter sandstone. rangle are distinct from the Lancaster are present only along the northern and surface on the north but descend more eastern margin of the upland. gradually to a lower surface on the In the western part of the area the south; (3) that the Lancaster surface in upper surface is continuous from weak the northern half of the Elizabeth quad- Pennsylvanian beds on the east to resist- rangle continues eastward below the ant Mississippian (Meramec and Osage) drift and possibly southward onto the dolomites to the west. Actually, the uplands of the Savanna quadrangle. highest elevations are on Pennsylvanian

Relations 1 and 2 suggest the possibility strata near the northwest corner of the that the Dodgeville peneplain is down- upland. The high elevation of the upland warped to the south and that the Lan- as a whole compared with the lowland to caster could be a local southwest-sloping the east cannot be explained by differ- structural plain. However, it appears to ential erosion, as there are no important be the Lancaster surface which continues differences in the compositon of the eastward into the area where the buried Pennsylvanian rocks in the two areas. Galena upland surface is best developed Nor is there any marked difference in

(Fig. 1). Because of this, the Galena up- drainage position, since the higher up- land surface is correlated with the Lan- land is essentially surrounded by major caster peneplain. preglacial valleys. One possible explana- Fig. 4.— Generalized contour map of upland surfaces in the Driftless Area, northwestern Illinois LELAND HORBERG tion may be found in the higher struc- Green River bedrock lowland, which tural position and diminished thickness separates the uplands of western Illinois of the Pennsylvanian strata to the west, from the Galena upland (Fig. 1), a corre- so that resistant Mississippian lime- lation with the Lancaster peneplain is stones floor the lower portions of many possible. Lower surfaces along the eastern large bedrock valleys. The local base- and northern margins of the upland are levels provided by these resistant beds probably extensions of the Central Illinois may have been effective in retarding peneplain from the preglacial Pennsyl- erosion of the uplands. This explanation, vanian lowland to the east. however, would not apply to the north- UPLAND SURFACE IN SOUTHWESTERN ILLINOIS ern third of the upland, where Mississip- pian beds are absent, nor would it explain Description.—The bedrock upland why erosion of the lower surface to the northwest of the southern Illinois drift- east was not also retarded by similar less area in Monroe, Randolph, and local baselevels along its western margin. Jackson counties is covered by relatively For these reasons it is believed that thin drift and forms a prominent feature structural control was not a major factor of the present topography (Fig. 1). A and that the escarpment between the central ridge which rises to a maximum two areas represents the junction of a elevation of over 700 feet extends along higher and a lower peneplain. the length of the upland and is bordered Preglacial gravels of Lafayette type by extensive flattish summit areas. The have been reported from Pike, Adams, summit areas have a general elevation of Hancock, Fulton, Peoria, Tazewell, War- 600-700 feet north of Kaskaskia bedrock ren, Henderson, and possibly Rock Is- valley and 600-650 feet south of the val- 28 is Missis- land counties (Fig. 1). ley. The upland underlain by Correlation.—Summit areas in the sippian (Osage, Meramec and Chester) southern part of the upland are continu- strata and represents a segment of the ous with the Calhoun peneplain of the Salem plateau of the Ozarks isolated by driftless area of western Illinois described the Mississippi River. by Rubey 29 and those in the northern Correlation.—The upland . was con- part are close to the required level. If the sidered a part of the Ozark topographic 30 the basis of surface is projected northward across the dome by R. F. Flint, and on projected profiles was interpreted as an 28 A. H. Worthen, "Geology of Hancock Coun- It thus forms a ty," Geol. Surv. of III, Vol. I (1866), p. 330; "Ge- upwarped peneplain. ology of Pike County," ibid., Vol. IV (1870), p. 37; northward continuation of the Ozark "Geology of Fulton County," ibid., p. 91; H. M. peneplain from the driftless area to the Bannister, "Geology of Tazewell, McLean, Logan, surface and Menard Counties," Geol. Surv. of III., Vol. IV south. Northwestward, the ap- (1870), p. 179. Occurrences in Adams, Henderson, pears to continue across the Mississippi and possibly Rock Island county are reported by 31 River into the St. Louis area and to R. D. Salisbury in "A Further Note on the Ages of of the Orange Sands," Amer. Jour. Sci., Vol. XLII merge with the Calhoun peneplain (3d ser., 1891), pp. 252-53, and in ftn. 14. The oc- western Illinois. currence in Peoria County was noted by J. A. Udden in "The Geology and Mineral Resources of the s° Pp. 634-36 of ftn. 19. Peoria Quadrangle," U.S. Geol. Surv. Bull. 506 3 X N. M. Fenneman, "Physiography of the St. (191 p. 50, and possible occurrences in southern 2), Louis Area," 77/. Geol. Surv. Bull 12 (1909), pp. 52, Pike and eastern Warren counties were reported by 57; "Geology and Mineral Resources of the St. (personal communication J. E. Lamar [1945]). Louis Quadrangle, Missouri-Illinois," U.S. Geol.

2 ' Ftn. 13. Surv. Bull. 438 (191 1), pp. 43-44- PREGLACIAL EROSION SURFACES IN ILLINOIS 189

CENTRAL ILLINOIS PENEPLAIN of the drift in a diamond-drill boring; 32

Description.—This modified surface is (2) near Sullivan, about 30 miles north- believed to extend over most of the east in central Moultrie County, where Pennsylvanian lowland and to be widely about 15 feet of ferruginous conglomer- developed on the Niagara cuesta of ate with quartz and chert pebbles, mixed northeastern Illinois. It is thus con- with weathered Pennsylvanian siltstone sidered the most extensive erosion sur- at the base of the drift, were identified by face within the state. The surface is un- the writer from well sample cuttings; and usually uniform in elevation; and, (3) near Wedron, in LaSalle County, in throughout the northern three-quarters the northern part of the area, where 2-4 of the area, upland elevations in most feet of limonitic conglomerate and sand- places lie between the 600- and the stone with polished chert pebbles direct- 650-foot contours; to the south they ly overlies the bedrock. 33 The two latter descend, to about 500 feet (Fig. 1). The deposits occur below the highest part of highest sections of the upland rise about the upland at elevations of about 485 and 100 feet above the general level in the 550 feet, respectively, and may be re- east-central part of the state and form worked from a former extensive upland broad crests which conform closely to gravel. preglacial drainage divides (Figs. 1 and Correlation.—The surface is believed

5). Small remnants of the surface may be to represent a single peneplain developed present in the Pennsylvanian upland to largely on the weak rocks of the Illinois the west, as previously noted, and also basin below the level, of older erosion southwest of the Galena upland in surfaces to the north, west, and south. northwestern Illinois. The surface was recognized previously in

There is a general coincidence of the various parts of southern Illinois 34 and in surface with the Illinois structural basin, most cases was ascribed to a "third but a general lack of adjustment to local cycle" of erosion, completed in the structures. Transection of structure is Tertiary, following the two cycles repre- especially apparent along the LaSalle up- sented by upland plains in the driftless lift, which extends northwest-southeast areas of southern and northwestern across the northern half of the area Illinois. 35 In the northern part of the area

(Fig. 2). In the LaSalle region, beds 32 Frank Leverett, "The Illinois Glacial Lobe," ranging in age from Pennsylvanian to U.S. Geol. Surv. Mono. j8 (1899), p. 107.

Ordovician (Shakopee dolomite) are 33 H. B. Willman and J. N. Payne, "Geology and beveled by the surface. Further lack of Mineral Resources of the Marseilles, Ottawa, and Streator Quadrangles," III. Geol. Surv. Bull. 66 adjustment to structure is indicated by (1942), pp. 204-5. the possible extension of the surface onto 34 E. W. Shaw and T. E. Savage, "Murphysboro- the hard rocks of the Niagara cuesta of Herrin Folio, Illinois," U.S. Geol. Surv. Atlas, Folio northeastern Illinois. 185 (191 2), p. 12, and "Tallula-Springfield Folio, Three possible occurrences of pre- Illinois," U.S. Geol. Surv. Atlas, Folio 188 (1913), p. 10; E. W. Shaw, "New Athens-Okawville Folio, gravel glacial Lafayette-type are known Illinois," U.S. Geol. Surv. Atlas, Folio 213 (1921), Illinois," within the area (Fig. 1) : (1) on the bed- p. 8, and "Carlyle-Centralia Folio, U.S. Atlas, Folio 216 Wallace rock upland at Pana, in Christian Coun- Geol. Surv. (1923), p. 7; Lee, "Gillespie-Mt. Olive Folio, Illinois," U.S. ty, a thin bed of ferruginous con- where Geol. Surv. Atlas, Folio 220 (1926), p. 1. glomerate with angular and rounded 3s This is the conclusion of E. W. Shaw and T. E. chert pebbles was penetrated at the base Savage in ftn. 34 (191 2), (1913); and Shaw (1921), 190 LELAND HORBERG the surface has been clearly identified in bedrock lowland near the junction of the LaSalle County and correlated with the Mahomet (Teays) and ancient Missis- Galena upland surface and the Dodge- sippi bedrock valleys. 36 ville peneplain. This correlation is not supported by the present study because ENTRENCHED PREGLACIAL VALLEYS of the physiographic discontinuity of the The deep-stage valleys (Fig. 5) are LaSalle County surface with the Galena entrenched ioo feet or more below the upland plain and the possibility that the Havana straths and probably represent latter is continuous with the Lancaster the final episode in preglacial erosional peneplain rather than with the Dodge- history. Although the preglacial age of ville. the upper Mississippi bedrock valley STRATHS ALONG MAJOR PREGLACIAL VALLEYS may be open to question, there is evi- dence that Kansan and, possibly, Ne- A broad-valley stage preceding stream braskan drift is present in bedrock val- entrenchment is evidenced by possible leys of the ancient Mississippi, Rock, and straths along the preglacial Mahomet Mahomet (Teays) systems, indicating (Teays), ancient Mississippi, Kaskaskia, that these valleys, and probably most of and Wabash drainage systems (Figs. 1 the deep bedrock valleys, were eroded to and 5) . The existence of these surfaces is their present depths before the glacial interpreted entirely from subsurface period. data, and they are related only indirectly to the alluvial plains which floor some of SUMMARY OF EROSIONAL HISTORY these valleys at present. The cyclical The oldest erosion surface in the state origin of some of the surfaces is uncertain may be represented by remnants of the because local baselevels and glacial Dodgeville peneplain in northwestern changes may have been important fac- Illinois and the Buzzards Point plain in tors in their development. Also, in some southern Illinois. Below these levels the places the bedrock surface could not be Lancaster-Calhoun-Ozark peneplain ap- delineated accurately because of in- pears to have developed as an extensive sufficient data. It is significant, however, regional surface in late Tertiary time. that the cumulative data indicate rock Following completion of the peneplain, benches, which seem to descend regu- and probably prior to the making of the larly from 550 feet on the north to 450 central Illinois peneplain, Lafayette-type feet and less toward the south, and that gravels were spread over its surface, 37 the valleys are much wider than those and it appears likely that the positions of resulting from late preglacial entrench- ment. Because of these features, an 37 These gravels present numerous unsolved erosion level intermediate between the problems. Their age is indefinite ''Tertiary." and it is probable that similar deposits of various ages are to Central Illinois peneplain and the "deep- be found in the upper Mississippi Valley as well as in stage" valleys is postulated. It is named the Gulf states. Similarities in composition could be the "Havana strath" from the extensive explained by the re-working of an originally wide- spread deposit or by the repeated access to similar source materials. It is uncertain whether the gravels (1923). Wallace Lee 1, 11 of ftn. correlated (pp. 34) on the Lancaster-Calhoun-Ozark surface are older the surface with the lower (Lancaster) surface in the than the deposits at lower elevations in central and Driftless Area and suggested its possible completion southern Illinois or whether they are essentially con- in the Mesozoic and uplift in the Tertiary. temporaneous and therefore, record important de- 3 6 Willman and Payne, pp. 204-5 of ftn. 33. formation of the peneplain. PREGLACIAL DRAINAGE

PRESENT ORAINAGE

Fig. 5.—Major preglacial drainage systems in Illinois 192 LELAND HORBERG major preglacial drainage lines were de- which followed, a local peneplain was termined. With uplift of the peneplain, eroded on the weak beds of the Illinois the larger streams in many places were basin. The extent of the surface to the established transverse to structure in east and northeast is not known, but to superimposed valleys or in antecedent the north and west it is believed to valleys on the upwarped peneplain sur- terminate against remnant uplands of face. Outstanding transverse valleys in the older land surface. The ancestral the state include the courses of (1) the Mississippi and Mahomet (Teays) were ancient Mississippi across the west flank probably the major streams, although of the Wisconsin arch and Savanna- their courses may not have been estab- Sabula anticline in northwestern Illinois, lished until the close of the cycle.

(2) the ancient Rock across the positive During the Havana cycle the main element between the Sandwich fault zone preglacial drainage lines were present and LaSalle anticline in north-central and broad valleys were eroded along the

Illinois (Figs. 2 and 5), (3) the ancient ancient Mississippi, Mahomet, Kaskas- Iowa and ancient Mississippi across the kia, and Wabash systems. A rejuvena- Cap au Gres structure in the Calhoun tion of drainage initiated the deep- County area of western Illinois, and valley stage, which continued until

(4) the ancient Mississippi across the interruption of the cycle by glaciation. east flank of the Ozark dome in south- This last cycle may have opened with the western Illinois. 38 earth movements at the close of the During the third cycle of erosion Tertiary, in which event the main bed- rock valleys could have been entrenched 3 8 Considered antecedent to the doming of the Ozark peneplain by R. F. Flint (ftn. 19). during the preglacial Pleistocene.