J ^- ^angenhefm, Jr. Dept. Geo/. Univ. III. ^^^na, III. STATE OF ILLINOIS WILLIAM G. STRATTON, Governor DEPARTMENT OF REGISTRATION AND EDUCATION VERA M, BINKS, Director DIVISION OF THE STATE GEOLOGICAL SURVEY M. M. LEIGHTON, Chief URBANA REPORT OF INVESTIGATIONS— NO. 174 EXTENSION OF TAZEWELL GLACIAL SUBSTAGE OF WESTERN ILLINOIS AND EASTERN IOWA BY PAUL R. SHAFFER A Cooperative Study with the Iowa Geological Survey Reprinted from the Geological Society of America Bulletin Vol. 65, pp. 443-456, 1954 PRINTED BY AUTHORITY OF THE STATE OF ILLINOIS URBANA, ILLINOIS 1954 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 ADAIVIS, Ph.D., D Sc, Chemistry R. H. ANDERSON. B.S,, Engineering A. E. EMERSON, Ph.D., Biology LEWIS H. TIFFANY, Ph.D., Forestry W. L. EVERITT, E.E.. Ph.D., Representing the President of the Uni- versity of Illinois GEOLOGICAL SURVEY DIVISION M. M. LEIGHTON, Ph.D., Chief BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA VOL. 66. PP. 443-456 MAY 1954 EXTENSION OF TAZEWELL GLACIAL SUBSTAGE OF WESTERN ILLINOIS INTO EASTERN IOWA By Paul R. Shaffer Abstract The Green River lobe of Tazewell (Shelbyville) ice extended west of the Mississippi River to the eastern border of the Goose Lake Channel in Clinton County, Iowa. It extended northwestward to within 13^ miles of the village of Goose Lake, across an area previously mapped as lowan and Kansan drift. A small area of lUinoian drift is present between the Shelbyville drift and the Goose Lake Channel. The lowan drift is separated from the western border of the Goose Lake Channel and the Shelbyville drift by a small strip of Kansan drift on the north and a small area of Illinoian drift on the south. When the Shelbyville ice reached the "big bend" of the Illinois River in Bureau County, IlHnois, it blocked the eastward-flowing drainage of the old Mississippi River and formed the first stage of glacial Lake Milan which extended from the "big bend" to the rock-defended outlet near Andalusia, Illinois. Later when the Shelby- ville ice had advanced westward to the west side of the present Rock River valley, glacial Lake Milan was restricted to the lower Rock River valley, and glacial Lake Cordova formed north and northwest of the ice dam. The outlet for glacial Lake Cordova was over a divide composed of Illinoian drift between Cordova, Illinois, and Le Claire, Iowa. Later when the Shelbyville ice crossed the present Mississippi River valley, glacial Lake Savanna was formed to the north. Its waters rose until they found an outlet through the Goose Lake Channel to the Wapsipinicon River, down the Wapsipinicon and through the new valley past Cordova and Le Claire. When the Shelbyville ice withdrew, the present course of the Mississippi River between Fulton, Illinois, and Keokuk, Iowa, had been estabHshed. Two sets of loess-covered terrace remnants, deposited under different controlling con- ditions, are present between Geneseo and Lomax, IlHnois. One set between Geneseo and Andalusia, IlHnois, rises to a nearly uniform height and resulted from quiet-water depo- sition in glacial Lake Milan. Below Muscatine, Iowa, He remnants of deposits made up largely of fine sands and silts, whose deposition was controlled by a constriction in the valley of the Mississippi River near Keokuk. When the great volume of water from the upper Mississippi drainage was added to that already flowing past Keokuk, constriction caused higher water levels upstream with areas of relatively quiet water in marginal por- tions of the valley and in the ponded tributaries. The possible contemporaneity of the ShelbyviUe terrace on the lower Iowa with the high terrace of Lake Calvin upstream caHs for a re-examination of the terraces along the Iowa River and raises the question of the possibility that the Shelbyville and the lowan were contemporaneous. CONTENTS TEXT Page Page Wisconsin drift 448 Introduction 444 lowan 448 Acknowledgments 444 TazeweU (ShelbyviUe) in Iowa 448 Evidence for extension of Shelbyville ice into Tazewell (Shelbyville) in IlHnois 448 eastern Iowa 444 Drainage history 449 Green River lowland 444 General features 449 Garden Plain esker 447 Glacial Lake Milan 449 Older drifts 447 Glacial Lake Cordova 449 Kansan drift 447 Glacial Lake Savanna 450 IlHnoian drift in Iowa 447 Tazewell terraces 452 Illinoian drift in IHinois 448 References cited 455 443 444 P. R. SHAFFER— TAZEWELL SUBSTAGE IN EASTERN IOWA ILLUSTRATIONS Figure Page 1. Index map 444 2. Bedrock topography of part of northwestern Illinois 445 3. Glacial Lake Calvin 445 4. Glacial geology of area studied in north- western Illinois and east-central Iowa 446 Figure Page 5. Drainage pattern during Sangamon and lowan 450 6. Glacial Lake Milan (restricted) and low- water stage of glacial Lake Cordova 451 7. Glacial Lake Savanna 452 8. Tazewell terrace remnants 453 Introduction For more than 60 years the invasion of ice from Iowa into northwestern Illinois during the lowan substage of the Wisconsin stage has been IOWA Scale of Miles 9 59 I QQ 2Q INDIANA Figure L— Index Map Area studied shaded a moot question. Leverett (1899, p. 151) felt that evidence favored the invasion but left the question open for subsequent study. Leighton (1923, p. 267) concluded that "A post-Illinoian ice sheet invaded the Green River basin. . . . The evidence is insufl5cient to determine whether this ice lobe, now called the Green River lobe, is lowan in age or a correlative of the Shelbyville drift of Early Wisconsin age." Subsequent work by Templeton (1939, unpub. thesis) and Ekblaw (1942, p. 154) led them to believe that the Green River lobe was related to the Shelbyville moraine of the Tazewell sub- stage of the Wisconsin stage. In 1948 Leighton examined some new open- ings in the Garden Plain esker (Leverett, 1899, p. 79) near the bluff of the Mississippi valley. The fresh character of the materials of the esker and the conformable relations of the overlying loess to the gravels, plus evidence of the dam- ming of the Mississippi River found by him at Savanna, Illinois, led to further search in Illi- nois and Iowa for other evidence of a westward extension of the Green River lobe beyond the tentative boundary proposed by him in 1923. The first report on these studies (Leighton and Shaffer, 1949) was made before the maximum westward and northwestward extent of the Green River lobe was known, and before the Tazewell drainage history of the Mississippi Valley between Clinton and Keokuk, Iowa, had been completely determined. The area covered by the writer is shown in Figure 1. Acknowledgments These studies were made under the auspices of the Illinois State Geological Survey and the Iowa Geological Survey. Dr. M. M. Leighton, Chief, Illinois State Geological Survey, sug- gested the possibility of a western extension of the Green River lobe and aided greatly in the field work and with the preparation of this re- port. Dr. H. G. Hershey, Director, Iowa Geo- logical Survey, and Professor A. C. Trowbridge, State University of Iowa, served as consultants in the studies. Their aid, advice, and counsel have been extremely valuable. Dr. H. B. Will- man and Dr. George E. Ekblaw of the Illinois State Geological Survey contributed valuable suggestions and data. Professor George W. White, University of Illinois, made helpful sug- gestions during the preparation of the report. Mr. Herbert G. Stewart, Jr., and Mr. Vincent C. Shepps served as research assistants for one summer. Their aid is gratefully acknowledged. Evidence for Extension of Shelbyville Ice into Eastern Iowa Green River Lowland A lowland which had persisted from pregla- cial time was present between Fulton, Illinois, EVIDENCE FOR EXTENSION OF SHELBYVILLE ICE INTO EASTERN IOWA 445 Figure 2. — Bedrock Topography of Part of Northwestern Illinois Showing preglacial bedrock valley between Fulton and "big bend" of Illinois River at Bureau, Bureau County. Reproduced from Horberg's "Bedrock Topography of Illinois": 111. State Geol. Survey Bull. 73. and the Illinois River during the advance of the Tazewell lobe. The lowland was mentioned by Udden (1899, PI. VII, p. 322) as a possible course for the preglacial Mississippi River. It extended southeastward from Clinton through the Meredosia Channel and Green River Basin to the "big bend" of the Illinois River near Bureau (Fig. 2). Horberg's (1946, p. 191) extended work on the bedrock surface and the drainage systems of Illinois has shown that in Sangamon time the Mississippi had a course southeastward from Fulton to what is now the Illinois River in Bu- reau County, Illinois. Later Horberg (1950, PI. I, sheet 1) presented a contour map of the bed rock surface of Illinois showing this major bed- rock valley (Princeton Valley) from Fulton to the "big bend" of the Illinois River (Fig. 2). Trowbridge et al. (1941, p. 296) and Leverett (1941, p. 1920) had previously recognized that the lake which formed in front of the western margin of the Illinoian ice in Iowa, called Lake Calvin (Fig. 3), was of short duration, and that, Figure 3. — Glacial Lake Calvin After Schoewe (1924, pi. VI, p. 56) following the retreat of the Illinoian ice, the upper Mississippi River drainage returned to its southeastward course to the Illinois River and continued there during the Sangamon inter- glacial stage. The presence of this broad valley in post- Sangamon time facilitated the western exten- 446 P. R. SHAFFER— TAZEWELL SUBSTAGE IN EASTERN IOWA EVIDENCE FOR EXTENSION OF SHELBYVILLE ICE INTO EASTERN IOWA 447 sion of the Shelby ville lobe, also called the Green River lobe. Garden Plain Esker Leverett (1899, p. 79) described a gravel-pit exposure in an esker in the western part of sec. 22, Garden Plain Township, Whiteside County, Illinois, about 2}^ miles east of its western terminus, and traced the esker westward to the bluff of the Mississippi River (Fig. 4). Leigh ton observed additional and more recent openings in the esker farther west in sec. 20. Other open- ings in the esker system are known as far east as Cattail Creek in sec. 19 of Union Grove Town- ship, Whiteside County. The cross-bedding dips westward, indicating deposition by a west- ward-flowing, subglacial stream. The gravels are extremely fresh where overlain by calcare- ous and fossiliferous loess. Near the western end, where the gravels are overlain with only a few feet of loess, there is practically no staining, and the limestone pebbles and cobbles are fresh and firm. Near the eastern end of the eskerine tract the gravel, which underlies about 6 feet of sand, is rusty brown, the coloring, however, being the result of ground-water oxidation. Some of the limestone pebbles are partially dis- solved, while adjacent ones are fresh. Leverett (1899, p. 79) felt "that the esker must be re- ferred either to the lUinoian or to an extension of the lowan ice westward from the Rock River Basin." To refer the esker to the lUinoian re- quires truncation of the Sangamon profile of weathering just prior to loess deposition. The Garden Plain esker, however, lies within an eskerine tract. The esker is regarded as Shelby- ville in age, formed by a stream flowing beneath the ice of the Green River lobe, because (1) there is no interval of weathering between the fresh gravels of the esker and the overlying cal- careous and sometimes fossiliferous loess; (2) the esker occurs in a region of constructional topography; and (3) till younger than Illinoian occurs in the surrounding area and is similar to Shelbyville till of the Green River lobe farther east. Older Drifts Kansan Drift North, west, and northwest of the Shelby- ville drift in Iowa, the Kansan drift plain ex- tends across northern Clinton County. Present mapping (Fig. 4) west of the Goose Lake Chan- nel is similar to that of Udden (1905, p. 417) and summarized by Kay and Apfel (1928, opp. p. 15). The most significant revision is the recognition of a narrow strip of Kansan drift between the eastern limit of the lowan drift and the Goose Lake Channel. The present study shows that in an area of approximately 25 square miles east of the Goose Lake Channel the surface material is Shelby- ville. In sec. 11, T. 82 N., R. 5 E., 2^ to 3 feet of Shelbyville drift overlies Kansan gumbotil. The Kansan drift plain is thoroughly dis- sected, has a relief of more than 100 feet, and is covered with loess that averages 10-15 feet thick on the divides. The Kansan drift plain is more dissected, and the drift is more deeply leached than the Illinoian drift, and is much more dissected than the lowan or Shelbyville drift plains. The latter, except near the border, are covered with thin loess or the loess is ab- sent. The oxidized and leached portions of the Kansan drift are chocolate brown to yellow; the unoxidized and unleached portions are blue gray, jointed, and compact. An outstanding but not unusual feature of the Kansan drift in this area is the presence of a distinctive reddish-brown ferretto zone that is developed on the drift beneath loess, where gumbotil did not develop or was removed by erosion. A noteworthy section of the Kansan drift which shows the development of silttil as de- fined by Leighton and MacClintock (1930, p. 41) is present in a road cut in SE3'^ of SEJ^^ of sec. 12, T. 82 N., R. 6 E. The sUttil lies above a 2-3 foot ferretto zone and is overlain by Farm- dale loess which in turn is overlain by Peorian loess. In other places the gumbotil has been changed to second-cycle silttil. Illinoian Drift in Iowa Norton (1899, p. 492) indicated Illinoian drift overlain by loess over about three-fourths of Scott County. Exposures and auger borings indicate Illinoian till overlain by Farmdale loess, which is overlain by Peorian loess, on the south bluffs of the Wapsipinicon River in sec. 34, T. 80 N., R. 5 E., northwest of Princeton. 448 P. R. SHAFFER— TAZEWELL SUBSTAGE IN EASTERN IOWA Udden (1905, p. 418) suggested that the II- linoian might exist in Clinton County, but found no proof of its presence. Carman (1909, p. 38) noted that a small area in southeastern Clinton County was probably covered by II- linoian ice. Auger borings indicate two areas of Illinoian drift in Clinton County. One area Hes between the southeastern margin of lowan drift and the Goose Lake Channel (Fig. 4). Another area hes between the southwestern border of the Shelby- ville drift and the Goose Lake Channel. The Illinoian drift in Clinton County is more deeply weathered than the lowan or the drift of the Green River Lobe. Depth of leaching ranges from 9 to more than 17 feet. The till has none of the ferret to zones so common in the Kansan. Illinoian Drift in Illinois Illinoian drift is present north of the Shelby- ville from Fulton to Sterling (Fig. 4). Occasion- ally gumbotil has developed, and at many places the drift is overlain by Farmdale loess. The lihnoian occupies a higher, more dissected, and more heavily loess-covered area than the area of Shelbyville drift to the south. lUinoian drift occurs at Cleveland, East Moline, Moline, Hampton, Rapid City, and as far north as sec. 17, T. 19 N., R. 2 E. Between Hampton and Rapid City gumbotil, overlain by Farmdale loess, with Peorian loess above, is present in the valley walls of the Mississippi River. Southwest of Hillsdale in sec. 36, T. 19 N., R. 2 E., compact tiU containing many frag- ments of coal is overlain by terrace deposits of sands, silts, and clays which have an altitude of 640 feet A.T. Wisconsin Drift lowan The lowan drift of Clinton County, Iowa, was deposited by a long, narrow lobe of ice which extended across the central part of the county from the main body of ice to the west. The ice lobe, which extended across portions of southern Jones and northern Cedar counties before reaching Clinton County, was previously mapped eastward to the alluvial flat along the Mississippi River. Present mapping (Fig. 4) based on road-cut exposures and numerous auger borings indicates that the narrow lobe of lowan ice did not extend east of the Goose Lake Channel. A narrow strip of Kansan drift lies between the lowan drift and the west side of the Goose Lake Channel to the north while a small area of Illinoian drift Hes between the lowan drift and the western side of the Goose Lake Channel to the south. Along its borders the lowan drift lies beneath a thick cover of loess. Leaching of carbonates from the loess has generally proceeded to a depth of 7 or 8 feet, and calcareous loess over- lies calcareous lowan till. Away from the bor- ders the loess is thin or absent and leaching ranges from 18 inches to 5 feet. A pebble band at the top of the lowan is common throughout the area, and granite boulders are numerous and in some cases large. lowan drift is present over bedrock, adjacent to Kansan, and adja- cent to Illinoian, but at no place was the Shelby- ville found above the lowan. Tazewell (Shelbyville) in Iowa The Green River lobe of Shelbyville ice ex- tended west of the Mississippi River to the eastern border of the Goose Lake Channel in Clinton County and northwestward to within about IH miles of the village of Goose Lake (Fig. 4). The ice crossed the Mississippi River between North Clinton (Lyons) and Princeton, Iowa, into an area previously mapped as lowan and Kansan drift. The southwestern portion of the extension is separated from the Goose Lake Channel by a small area of Illinoian drift. A critical exposure in the above interpreta- tion occurs in sec. 11, T. 82 N., R. 5 E., where calcareous Shelbyville till overlies Kansan gum- botil (Fig. 4). Tazewell (Shelbyville) in Illinois North of Geneseo as far as Fulton, the sur- face material is Shelbyville drift, except where it is covered by later alluvium in the wide low- lands of the Rock River valley, the Meredosia Channel, and the Cattail Channel (Fig. 4). Shelbyville drift is present north of Geneseo at numerous places in the Cordova quadrangle — namely. Pink Prairie, Sharon, Spring Hill, WISCONSIN DRIFT 449 Hillsdale, south of Albany, east of Fenton, on the east side of Cattail Creek, and at Union Grove. The northern boundary of the Shelby- ville extends from Fulton across the southern portion of the Morrison and Sterling quadran- gles where the eastern limit of this study joins the boundary determined by Temple ton (1939, unpub. thesis, Univ. 111., pi. II). The lowan and Shelbyville drifts are similar in several respects. Where the loess is very thin or absent, the depth of leaching of the Shelby- ville till ranges from 2>^ to 6 feet as it does in the lowan till under similar conditions. Where the Shelbyville drift is covered with 10 or more feet of loess, leaching of carbonates from the loess has proceeded for 8 or 9 feet, and calcare- ous loess overHes calcareous till. Where lowan drift is under similar loess cover, the depth of leaching in the overlying loess is similar. The lowan and Shelbyville drifts are dissimi- lar in several respects. No pebble band was found on top of the Shelbyville, and the nu- merous, large, granitic boulders characteristic of the lowan were not found in the areas of Shelbyville drift. Drainage History General Features In the area between Fulton and Keokuk, two drainage systems operated during both the Sangamon interglacial stage and the lowan substage (Fig. 5). The northern drainage flowed through the Meredosia Channel southeastward to the Illi- nois River in Bureau County. A tributary which headed in a rock divide just west of Andalusia flowed eastward and northward along the course of the present Rock River valley to join the old Mississippi (Fig. 5). A divide also ex- isted near Cordova from which a small stream flowed north to the old Mississippi and a stream flowed southwestward past Rock Island, Illi- nois, to join the stream in the lower Rock River valley. The southern drainage system included the Iowa and Skunk rivers which flowed southward through the Keokuk rock valley. The old Iowa included a southward -flowing tributary which headed in the rock divide just west of Andalusia and received the waters of Copperas Creek, Eliza Creek, and Edwards River from Illinois and numerous tributaries from the west in Iowa. Glacial Lake Milan With the onset of Tazewell (Shelbyville) gla- ciation, the first blocking of the northern drain- age system occurred at the "big bend" of the present Illinois River (Fig. 5). Here the ice ponded the old Mississippi to form the first stage of a glacial lake, herein called Lake Milan (Fig. 6). The lake, which in the initial stage ex- tended from the Illinois River to the outlet over the rock divide west of Andalusia, was later re- stricted to the lower Rock River valley when the ice reached the western walls of the present Rock River valley. The outlines of the first stage of Lake Milan are not shown because the ad- vancing ice progressively covered the area be- tween the "big bend" and Geneseo. The bedrock topography map (Fig. 2) gives the general out- hnes of the area inundated. Glacial Lake Cordova Wlien the Tazewell (Shelbyville) ice reached the west wall of the Rock River valley south- east of Cordova, glacial Lake Cordova was formed (Fig. 6). Glacial Lake Milan was then restricted to the lower Rock River valley and received meltwater from the ice, which may have already reached Geneseo, Illinois, or was advancing toward Geneseo from the east. The diagram of Lake Cordova, based on present topography, represents the earliest minimum extent and lowest level (641 ft. A.T.) which occurred promptly after the ice blocked the old upper Mississippi flow into glacial Lake Milan. The level of glacial Lake Cordova subsequently rose about 15 feet, the waters spilled over the divide near Cordova and cut the Cordova gorge. The relationship between this cutting and the Green River lobe was pointed out by Leighton (1923, p. 281). The initial cutting was undoubt- edly accomplished quickly because the material removed was lUinoian drift which is at least 50 feet thick in this area. Exposures in the valley wall near Hampton and Rapid City, IlHnois, show that gumbotil developed on the upper surface of the Illinoian till at an elevation of about 655 feet above sea level. The bedrock 450 P. R. SHAFFER— TAZEWELL SUBSTAGE IN EASTERN IOWA I— I ) \J0 DAVIESS j j ... Goose Loke Chonnel CARROLL ._.X Coftoil ^honnel / Rock Creek ; t ' )» Fulfon I — \ ^yt^^^y^-^ vdT^ " ivTfE s I. 1^ ^L t >— Channel Corddvo J|^ V^ "^^>' j S C T T^-§-^ ) — 1 , Divide JT^ ^ MUSCATI NE loovenpoM^^^ .^ """'v. i ,-^^^_ " Muscotine }^ ^ M^^juv^ ^ — Hslond LEE -I, Princetoo Volley j AnddlJsio' , ^ R 0_C_^ , B U RAE A U L U I 1 MERCER I- LEE 2 O Or O H DES MOINES Burlington ^' J HANCOCK ^'Keokuk HENRY J L_IJ f-Big Bend of Illinois I River .j-r •i r Scale of Miles O 9 10 2p Figure 5. — Drainage Pattern During Sangamon and Iowan surface is at 600 feet or a few feet less in the valley. The highest terrace deposits of Lake Milan are found at 641 feet, thus the maximum height of the outlet of the lake was near 641 feet in elevation. About 15 feet of cutting through the lllinoian till therefore established southward flow through the Cordova gorge. Glacial Lake Savanna When the advancing Shelbyville ice crossed the Mississippi valley into eastern Iowa, glacial Lake Savanna (Leighton and Shaffer, 1949, p. 1904) formed north of Fulton. The outflow of this lake was by way of the Maquoketa River bay, through the Goose Lake Channel to the Wapsipinicon, and through the Cordova valley. The diagram for Lake Savanna (Fig. 7) is based on present topography and on the highest ter- race deposits found (679 feet A.T.). The records of this lake are found in sedi- ments that rise to the approximate level of the low col in the Goose Lake diversion channel 10 miles west of Clinton, Iowa. The following sec- tion, discovered and described by Leighton, is exposed in a side hill pit in the city of Savanna, DRAINAGE HISTORY ssysL 451 Scale of Miles S_l t-3 1-J) 1 Figure 6. — Glacial Lake Milan (Restricted) and Low-Water Stage of Glacial Lake Cordova Generalized from present topography- northeast of the intersection of Gilbert and Pierce Streets. Thickness Top of Section 699 feet A.T. {Feet) Loess, calcareous 20 Bedded deposits of variegated silts, sands, and clays. The sand is rusty streaked, the re- mainder blue, gray, and maroon. All cal- careous 8 Green clay shale (Maquoketa) 2 (exposed) Another side hill excavation occurs at the same elevation about 300 feet west and north- west of this section. In the southwest corner of this excavation there is more than 4 feet of bedded clay containing lime concretions, over- lain by 1^ feet of fine sand and silt with clay partings. All the material is calcareous and is overlain by calcareous loess. In the NEM of sec. 8, T. 23 N., R. 4 E., along the east-west road through the section, there is an excellent exposure of water-laid sediments. The exposure includes more than 20 feet of horizontally bedded sands, rusty near the top, with layers of silt 1-2 inches thick in the upper 2 feet. Limy streaks and lime concretions are prominent in the top 4 feet. At the base is a layer of rubble composed of fragments of the bedrock below. The top of the bedded deposits is between 675 and 680 feet above sea level. Northeast of Fulton, on a small tributary to Otter Creek, in the NWM of sec. 17, T. 22 N., R. 4 E., the following materials are exposed: Top of Section 650 feet AAr\ r t A rr Thickness 660 feet A.T. (peet) Loess, sandy, fossiliferous in lower portion, leached to a depth of 2-3 feet 15 Sand, with fine sand and a few silt beds. Ar- ranged like foreset delta beds 40 The water-laid sediments in the sections are too high for the "Mankato" terrace deposits in the Mississippi valley and its tributaries and, unlike the "Mankato" terrace deposits, are cov- ered with rather thick loess. The waters of Glacial Lake Savanna joined those of glacial Lake Milan (restricted), flowed through the rock valley west of Andalusia, and lowered the outlet fairly rapidly. With the 452 P. R. SHAFFER— TAZEWELL SUBSTAGE IN EASTERN IOWA 90**30' — 1 9(f20 90' 10* 90* 00* Otter Creek Scole of Miles 2 4 6 8 Figure 7. — Glacial Lake Savanna At first, highest-level stage. Generalized from present topography lowering of this outlet, the intrenchment of the Tazewell Terraces Cordova gorge and the withdrawal of the Green jwo sets of loess-covered Tazewell terrace River lobe to the east, the two drainage systems remnants, deposited under different controlling were integrated to form the present Mississippi conditions, are present between Geneseo, Illi- River valley between Fulton and Keokuk. is^nois, and Keokuk (Fig. 8). TAZEWELL TERRACES 453 — ^1 GOOSE LAKE CHANNEL '\\ I ^ C E D A I >C L I N T N .^f' R -^ Clintoi 1^ »Fulton <^ I WHITESIDE CORDOVA GORGE '/^^^^'^ ?/V«Z. SCOTT V,') O \ "1 I MUS^CATINE z^^*^^"^ ^/~^ I y^ AN DALUSIA GORGE _ *^^)^\30-§2^^.,y^^^^^ ■i I I LOUISA ANDALUSIA( Muscatineti^^&^^^^^^ . 63i' ^'■^'tT^^^'^^^^^^^ Andalusia M E RCX-5--^ MTSOrr HENRY LEE k<* ^ ,5Keokuki.(>, HANCOCK ] ^-7^eO