Journal of the Iowa Academy of Science: JIAS

Volume 95 Number Article 3

1988

Reevaluating Evidence for the Pre-lllinoian Entrenchment of Northeastern Iowa River Valleys

Curtis M. Hudak University of Iowa

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Recommended Citation Hudak, Curtis M. (1988) "Reevaluating Evidence for the Pre-lllinoian Entrenchment of Northeastern Iowa River Valleys," Journal of the Iowa Academy of Science: JIAS, 95(4), 109-113. Available at: https://scholarworks.uni.edu/jias/vol95/iss4/3

This Research is brought to you for free and open access by the Iowa Academy of Science at UNI ScholarWorks. It has been accepted for inclusion in Journal of the Iowa Academy of Science: JIAS by an authorized editor of UNI ScholarWorks. For more information, please contact [email protected]. ]our. Iowa Acad. Sci. 95(4): 109-113, 1988

Reevaluating Evidence for the Pre-lllinoian Entrenchment of Northeastern Iowa River Valleys 1

CURTIS M. HUDAK2

Department of Geology, University of Iowa, Iowa City, Iowa 52242

Previous workers have proposed a pre-Illinoian age for the deepest bedrock entrenchment of northeastern Iowa valleys. Lines of evidence used ro support a pre-Illinoian age of entrenchment are: ( 1) deep bedrock-entrenchment interpreted ro take "long" periods of time, (2) "valleys" filled with "red-weathered drift", which was interpreted as "old", (3) differences in cross-valley profiles supposedly related to glacial advances, (4) thick beds of gravel could only be deposited by outwash streams, and (5) glacially buried (drift-filled) bedrock valleys exist outside of the Paleozoic Plateau ro the west and south. These inferences are questionable because: ( 1) erosion (entrenchment) rates are variable, (2a) "valleys" were interpreted as anything below the controversial peneplains or highest uplands, (2b) "red" oxidation colors can occur quickly in coarse-grained sediments, (2c) "drift" is an ambiguous term meaning till and/or outwash (stratified sediments), (3) cross­ valley profiles are related to the erosional resistance of the bedrock, (4) gravels can be deposited by non-glacially derived waters, and (5) rivers of the Paleozic Plateau cut across (and are therefore younger than) the glacially buried valleys to the west. More recent evidence indicate that the river valleys of Iowa's Paleozoic Plateau may have been at least partially entrenched after the last local pre-Illinoian glaciers receded. INDEX DESCRIPTORS: alluvium, "Driftless Area", fluvial geomorphology, incised river valleys, Paleozoic Plateau, peneplains, Quaternary landscapes

Both the uplands and bedrock entrenched river valleys of northeast­ used during the late 1800's and early 1900's. Rates of erosion or ern Iowa and surrounding regions have been interpreted as pre­ deposition are now recognized as too variable for reliable estimates of Illinoian in age since the late 1800's. This paper summarizes and time (Datt and Batten, 1988: 10-11). evaluates the evidence and concepts used to support the pre-Illinoian Unfortunately, early geologists also lacked a standardized particle­ interpretation and gives reasons why the Quaternary history of major size scale. The silt-day distinction varied between different authors valleys in northeastern Iowa needs revision. The purpose of this paper and papers of the same author, making one suspicious of their usage is not to discredit the pioneer geologists/naturalists, for they did an (e.g., "loess clays" in Calvin, 1896:63). Even after Wentworth ( 1922) excellent job given the older concepts and technologies of their era. established a standardized scale, field identifications have remained Instead, I hope to encourage modern studies regarding the Quaternary difficult for the finer sizes. This difficulty led to unstandardized and landscape development of these major valleys by offering alternate variable descriptions of both upland and valley sediments. hypotheses, given the same data that the pioneering scientists used. Documenting the upland Quaternary geology is an important srep towards determining the valley's relative age through correlation of QUATERNARY GEOLOGY AND LANDSCAPES strata, soils, and geomorphic surfaces. Orr (1907), Williams (1923), Historical Setting Trowbridge (1966), and Hallberg (1980: 101-102) documented pre­ The Paleozoic Plateau (Prior, 1976; Hallberg et al., 1984) and Illinoian till and erratics on the previously interpreted "Driftless Area" surrounding regions have received much attention since the first uplands across all of northeastern Iowa. Where pre-Illinoian till is geologic expedition in 1839, when D.D. Owen commented on the absent it must have either been eroded after the last local pre-Illinoian possibility of a driftless district in the Midwest (Fig. 1). glacial advance, or was never deposited in localized areas. Chamberlin and Salisbury ( 1886) published the first derails of the Prior ro rhe mid-1960's two to three glacial advances during the Driftless Area's geology while McGee was writing a comprehensive "Iowan", "Kansan", and "Nebraskan" stages were thought to have report ( 1891) on the Quaternary geology of northeastern Iowa. traversed northeastern Iowa, but this stage terminology is now Chamberlin and Salisbury ( 1886) mapped all but the northeastern tip defunct (Ruhe et al., 1968; Hallberg, 1980, 1986). The concepts and of Allamakee County as drift-covered, while McGee ( 1891), Calvin technologies available to pre-1950's workers utilized isolated outcrops (1895), Calvin and Bain (1900), Leonard (1906), and many workers and failed to demonstrate both the third dimension of each stratum, to follow were more influenced by the rugged bedrock valleys, and and the preserved Quaternary sequence from surface to bedrock. These they mapped larger "Driftless Areas." conceptual differences resulted in the recognition and counting of only These early studies were developed prior to and during the infancy those tills exposed (two, when fortunate). Assuming two tills com­ of the age of radiogenic dating when scientists used geologic evidence prised the entire local glacial stratigraphy, the early workers naturally to determine ages of both rocks and the earth, among which include correlated to other two-till exposures, which may not have been the the rates of erosion and sedimentation (see Holmes, 1913 ). Although same two tills. early geologic estimates for the earth's age were not old enough, it The third dimension of Quaternary deposits in northeastern Iowa is would be just as easy for these pioneering scientists to overestimate the now better understood. Ruhe et al. ( 1968) demonstrated by drilling age of geologic events (especially the younger events) because of rhe and tracing geomorphic surfaces from upland to lowland, aided by relatively slow erosional and depositional rates observed by scientists radiocarbon dating the overlying basal loess materials, that the "Iowan over small increments of time. Although never explicitly stated in the drift" was an erosion surface cut into the "Kansan" till plain. Hallberg pertinent literature of eastern Iowa, these concepts were apparently and Boellstorff (1978), and Hallberg (1980) recognized more than three glacial tills at classic "Nebraskan" and "Kansan" type localities in western Iowa, and also at the classic "Kansan" ("Iowan") localities 1lowa Quaternary Studies Group Contribution no. 21. studied by Ruhe er al. ( 1968) in northeastern Iowa (see Hallberg, 2Present address: BRW, Inc., 700 Third Sr. S., Minneapolis, Minnesota 55415 1986, for a review). 110 JOUR. IOWA ACAD. SCI. 95(1988)

Traditional Evidence for the Age of Valleys occur, especially when related to glaciation (e.g., Baker, 1981; Evidence for pre-Illinoian incision(s) of northeastern Iowa valleys Wright, 1972:536-539). The ages of northeastern Iowa valleys includes: (1) deep bedrock entrenchment, (2) valleys filled with red­ should be reevaluated using radiogenic dates and other current weathered drift (usually "Kansan" and/or "Iowan"), while the uplands technologies. are either driftless or covered by patches of the "Nebraskan (oldest)" 2) The statement "valleys filled with red-weathered drift" contains till, (3) thick beds of gravel could only be deposited by outwash three questionable items: a) "drift" is an ambiguous term meaning till streams, (4) valley cross-profile changes supposedly related to glacial and/or outwash, b) iron oxidation, which causes "red" coloration, can advances, and (5) existence of many glacially buried (drift-filled) occur quickly in porous and permeable sediments, and c) "valleys" bedrock valleys to the south and west of the Paleozoic Plateau. Each were interpreted to be anything below the peneplains or highest line of evidence is discussed below. uplands. 1) Deep bedrock entrenchment suggesting an "old" landscape age a) "Drift" was generally described as "stratified sands and gravels." is probably related to the pre-radiogenic dating concepts of geologic Deposits described as "tills" were mostly from uplands while "drift" time mentioned above. Several authors hint at the time it must have deposits were both deep in the valleys and on the uplands. Valley taken to erode through bedrock (e.g., Norton, 1901:291; Calvin, "drift" deposits within the Paleozoic Plateau region were never 1907:214; Alden and Leighton, 1917:71). Savage (1905:445) on the positively identified as till or sediments in contact with till (McGee, landscape of Fayette County suggested 1891; Calvin, 1895, 1906, 1907; Calvin and Bain, 1900; Leonard, {t)he topography of this region has been developed through the 1906; Savage, 1905; Trowbridge, 1915, 1921, 1966; Williams, erosion of the streams which, during long geological ages, have 1923; Kay and Apfel, 1929; Kay and Graham, 1943). cut through the floor of the Niagara {Silurian) limestone and The literature reveals that no tills were ever found in the valleys of have carved their channels deeply into the underlying beds. the Paleozoic Plateau as suggested by the following: This broad plateau has been profoundly trenched by the waters ... the {upland) hybrid {till) deposit alone suggests that the of the Turkey river and of its chief affluents ... marginal ice crept into the valleys and therefore moved so Contrary to the interpreted slow rates of erosion are many lines of sluggishly as to slightly grind the cliffs and crags without evidence that indicate that relatively quick bedrock entrenchment can carrying into them the far-traveled erratic debris ... {McGee,

TURKEY RIVER PROJECT AREA

I Des Moines Lobe ·--r---,I I I I I I I I

I Southern Iowa Drift Plain ~ I ' -- ...,_J.- --- -,- i_ -- :~ -- -I I I I I I I I I I I

Fig. 1. Physiographic provinces and rivers of northeastern Iowa. The so-called "Driftless Area" is the ruled part of the Paleozoic P\ateau \as modified by Hudak, 1987c after Prior, 1976). NORTHEASTERN IOWA RIVER VALLEYS 111

1891:514) Although the "Lancaster (lower) peneplain" near the Turkey River Furthermore, if a till had been located in a valley of Iowa's so-called Valley is continuing to grow in upstream directions it is neither "flat" Driftless Area, it likely would have become as famous as the Bridge­ nor "close to [the present) base level". Therefore, contrary co what port Till of southwestern Wisconsin (see below). Calvin (1895, 1907), Trowbridge (1921, 1966), Kay and Apfel Tills were described from northeastern Iowa valleys near their (1929), Knox (1982), and Hedges and Alexander (1985) allude to, headwaters in Howard and Bremer counties outside of the Paleozoic "peneplains" should not be regarded as a single surface across the Plateau (Calvin, 1903; Norton, 1906). Recent work shows the Upper Mississippi drainage basin; nor are these disjunct, accordant headward erosion of the current valleys is exhuming the rill (Ruhe et uplands (which are interpreted as a "single peneplain") necessarily al, 1968; Hallberg, 1980). Bedrock elevation decreases co the west remnants of the pre-Pleistocene landscape. Such an interpretation and south of the Paleozoic Plateau region while the Quaternary should have stratigraphic evidence. Determining where the valley sediments thicken in a wedge-like manner (cf. U.S.G.S. topographic leaves off and the upland begins becomes more difficult without the maps with Hansen 1972, 1975, 1977). A post-glacial valley would peneplain concept because the pre-Illinoian landscape probably had an exhume progressively older Quaternary units in the headwater regions irregular topography. just as the downstream portions exhumed the progressively older 3) Calvin ( 1905) thought stratified gravels must have been deposit­ Paleozoic bedrock. ed as glacial outwash. However, incerfingering of upland tills and b) Weathering played an important role in age-determination of valley "oucwash" gravels and sands has never been demonstrated in sediments. Calvin ( 1898: 207, 241-244; Calvin and Bain, 1900: eastern Iowa. We of course now recognize chat gravels can be 463-470; etc.) thought the red ferruginous zones found on tills and transported by non-glacially derived waters. Carmen ( 1931) and Kay gravels (Buchanan gravels) were caused from intense weathering over and Miller (1941) suggested that gravels may be erosionally derived long periods. He used the ferruginous zones as a "marker bed" and from the tills (which is currently the favored theory for eastern Iowa; correlated from upland gravel units associated with an upland "Kan­ Hallberg, 1980), however oucwash was usually assumed responsible san" till to other gravel units in uplands and valleys alike. for the thick gravel accumulations. Alden and Leighton (1917) reinterpreted many of the "Kansan" 4) Cross-profile changes within northeastern Iowa valleys, from gravelly terraces along the northeastern Iowa rivers as "Iowan ouc­ gentle side-slopes in upstream areas (interpreted as more recently wash" because of their relatively fresh appearances. However, the glaciated pare of valley) to steep side-slopes farther downstream "Iowan" gravels "were considerably more weathered and eroded than (interpreted as not so recently glaciated), were used as evidence for Wisconsinan gravels." They did state chat more weathered "Kansan" glacial advances (Alden and Leighton, 1917:72). However, B. Witzke gravels may exist below the "Iowan" gravels. Kay and Miller ( 1941) and G. Ludvigson (1987, pers. comm., Iowa Geological Survey lacer discussed the danger of correlating on the basis of red oxidation Bureau), Hallberg et al. ( 1984), and ongoing work by the author colors, especially with sand and gravel deposits not in contact with one indicate cross-profile differences are caused by changes in bedrock or more till units (although they continued to use the peneplains, lichologies (erosional resistance). Although Alden and Leighton also depth of leaching, and geographic location as correlation tools). recognized chis face, they apparently felt chat because these landscapes Iron precipitation/oxidation can occur quickly in coarse-textured are rugged they are old. Already biased, Alden and Leighton's sediments, and is usually concentrated at or near the main and perched argument becomes circular and confusing. water tables and seeps associated with sedimentary textural changes 5) Many authors apparently equated the gorge-like valleys of the (see Hallberg et al., 1978 for general review and discussion on iron Paleozoic Plateau with the buried "preglacial" (and interglacial) behavior in sediments). gorge-like valleys found from deep wells south and west of the c) In 1889 Davis defined the peneplain as a nearly flat land surface Paleozoic Plateau (e.g., Norton, 1895; Alden and Leighton, of considerable area, produced by long-continued subaerial erosion 1917:75; Kay and Apfel, 1929:29-32). There is no evidence to nearly to base level, in a humid stream environment. In northeastern suggest that these valleys are related; in fact, most modern valleys of Iowa there were traditionally two major peneplains recognized. The Iowa, including the headwaters of the northeastern Iowa rivers, cut highest and oldest peneplain was called the "Dodgeville plain", while across the glacially buried (drift-filled) valleys, although on occasion the lowest and youngest major peneplain was called the "Lancaster they are coincident with the old valleys (see Hansen, 1972, 1975, plain" (Trowbridge, 1921). "The location of the {'Nebraskan') drift on 1977; and Figure 4 in Bettis and Littke, 1987). the flattish uplands and not in the valleys ... " and the so-called "Kansan oucwash" deep within the valleys of the Paleozoic Plateau, Early Counter Evidence to the Pre-Illinoian led Williams (1923: 103) to believe "the dissection of the lower or Age of the Valleys Lancaster {pene)plain" succeeded the "Nebraskan" ice and "preceded Alden and Leighton ( 1917: 13 3) were surprised "not to see {gravels) the coming of the Kansas ice." Trowbridge (1954) later concluded more markedly developed ... where there were ... direct lines of chat the "Dodgeville plain" was not a peneplain. drainage from the {so-called} Iowan ice front." Their surprise indicates The peneplain concept is highly questionable because coo many that cill-oucwash interfingering between upland and valley deposits factors can reset the erosion cycle and cause landscape rejuvenation was not observed. Also, they thought (pp. 132-134) sediments from' (e.g., changes in climate, base-level, etc.). Also, no "peneplain" of the the Turkey River were not very weathered compared to the "Buchanan Upper Mississippi Valley cuts across major lichologic boundaries (lace Kansan? age oucwash) gravels". Therefore, Alden and Leighton within one of the disjunct accordant uplands (G. Ludvigson, 1987, assigned the Turkey River sediments to the post-Illinoian and pre­ pers. comm.), which might be expected of a stream environment Wisconsinan "Iowan" glacial epoch, even though a Wisconsinan eroding to base level. These so-called "peneplains" are the result of proboscidean was found in the deposits (Hay, 1914: 433; Hudak, differential resistance to erosion of the bedrock strata. Ongoing work 1987c). Although Alden and Leighton were meticulous in their by the author near the Turkey River Valley and ocher eastern Iowa reporting of facts, they apparently favored the more popular evidence valleys indicates the "Lancaster (lower) peneplain" is continuing to discussed above. grow in the headward directions of smaller screams by carving out lesser resistant strata, largely at the expense of the higher geomorphic Bridgeport Till surface (Dodgeville? plain). At the same time the lower reaches of The Bridgeport Till makes up pare of the Bridgeport Terrace (or these streams dissect the more resistant bedrock strata chat make up scrach) deposits found relatively high in the bedrock valley at the the "Lancaster peneplain" (see e.g. Place 1 in Hudak, 1987c). confluence of the Mississippi and Wisconsin Rivers (southwestern 112 ]OUR. IOWA ACAD. SCI. 95( 1988)

Wisconsin border with northeastern Iowa; Knox et al., 1982). The till (Hallberg, 1980, 1986). Detailed systematic field-studies are needed is the only documented pre-Illinoian ice-contact deposit along the to better determine the history of landscape development in northeast­ Upper Mississippi drainage basin which demonstrably sits within a ern Iowa valleys and the Upper Mississippi River Valley. modern bedrock river valley (see also Hudak, 1987c, and Baker, 1986 for discussion on valley deposits of west-central Wisconsin). Eastward ACKNOWLEDGEMENTS fining sand units within the Wisconsin River Valley (present up­ stream direction) have been interpreted as "outwash related to the Drafts of this report were critically reviewed and improved by Bridgeport Till" (Knox et al., 1982). These sand units of the disjunct Richard G. Baker, E. Arthur Bettis III, Lon D. Drake, George R. Bridgeport Terrace have been interpreted as correlative and traced Hallberg, Holmes A. Semken, Jr., Graham Tobin, and two anony­ deep within the Wisconsin Valley (Knox et al., 1982), and are used as mous reviewers. E. Arthur Bettis and George R. Hallberg of the Iowa the main evidence for a pre-Illinoian maximum incision along the Geological Survey Bureau kindly provided me with discussion con­ Upper Mississippi Valley (Knox, 1982, 1986). If the alternate cerning interpretations and theories. Modern fieldwork and radiocar­ hypothesis regarding Illinoian and younger ages of at least partial bon dating were partially funded by the University of Iowa Depart­ bedrock entrenchment in the Upper Mississippi drainage basin is to ment of Geology (Littlefield Fund), Iowa Geological Survey Bureau, be eliminated, then datable evidence (indisputably pre-Illinoian) is Iowa Science Foundation (Grant #ISF-86-6), Geological Society of needed from deep within the valleys (see a review of the Bridgeport America (Research Grant #3449-85), and a University of Iowa Terrace/strath and other possible evidence regarding the valley ages in Collegiate Action Council Grant. Hudak, 1987 c; Hudak et al., ms.). Modern Counter Evidence to the Pre-Illinoian REFERENCES Age of the Valleys ALDEN, W.C., and LEIGHTON, M.M. 1917. Thelowandrift, a review of The first reported systematic, geomorphic study on a major valley the evidences of the Iowan stage of glaciation. Iowa Geological Survey (Turkey River) in the Paleozoic Plateau of Iowa revealed pre-lllinoian Annual Report 26:49-212. tills bearing "Sangamon" soils on the present-day uplands (Hudak, BAKER, R. W. 1986. Evidence for an early Pleistocene minimum age for the 1987c; Hudak et al, ms.; Figure 1). Within Hudak's (1987c) project Upper Mississippi Valley. Geological Society of America Abstracts with area, tills were not found in the Turkey Valley, while the "Sangamon" Programs 18(6):532. soil was traced to moderate valley depths. Because till is apparently BAKER, V. R. (editor) 1981. Catastrophic flooding: the origin of the truncated by the Turkey Valley at upland or valley-crest positions and Channeled Scabland. New York: Dowden, Hutchinson, and Ross. "Sangamon" soils are traced from upland positions to moderate depths BETTIS, E.A., Ill, and HALLBERG, G.R. 1986. Evolution of valleys and within the valley, early major entrenchment(s) is (are) interpreted to valley fills in eastern Iowa: Insights into Mississippi Valley history. Geological Society of America Abstracts with Programs 18(6):540. post-date the last local pre-lllinoian glaciation and pre-date, or is (are) BETTIS, E.A., Ill, and LITTKE, J.P. 1987. Holocene alluvial stratigraphy equal to, "Sangamon" soil formation (Hudak, 1987c; Hudak et al., and landscape development in Soap Creek watershed, Appanoose, Davis, ms.) Inset below the "Sangamon" soils are several terrace alluvial fills. Monroe, and Wapello Counties, Iowa. Iowa Department of Natural These alluvial fills, previously reported as pre-Illinoian ("Kansan" and Resources, Geological Survey Bureau Open File Report 87-2. "Iowan") outwash, contained wood with finite radiocarbon dates and CALVIN, S. 1895. Geology of Allamakee County. Iowa Geological Survey are therefore reinterpreted as being mid-Wisconsinan in age or Annual Report 4:35-120. younger (Hudak, 1987c; Hallberg et al., 1984; Hudak et al., ms.). CALVIN, S. 1896. Geology of] ones County. Iowa Geological Survey Annual Report 5:33-112. SUMMARY CALVIN, S. 1898. Geology of Buchanan County. Iowa Geological Survey Annual Report 8:201-254. Published stratigraphic descriptions from the Paleozoic Plateau CALVIN, S. 1903. Geology of Howard County. Iowa Geological Survey indicate that valley deposits are mostly stratified alluvium, with Annual Report 13: 21-80. minor amounts of unsorted and sorted colluvium, and massive loess. CALVIN, S. 1905. The Aftonian gravels and their relatons to the drift sheets in the region about Afton Junction and Thayer. Davenport Academy of Most of the Paleozoic Plateau valley deposits were referred to as Science 10: 18-31. "drifi:", "glacial materials", "outwash", "glacio-fluvial debris", etc., CALVIN, S. 1906. Geology of Winneshiek County. Iowa Geological Survey while the upland deposits were referred to as both "drift" or "till." Annual Report 16: 3 7 -146. "Valley till" was described or located from only the headwater regions CALVIN, S. 1907. Some features of the channel of the Mississippi River or areas south of the Paleozoic Plateau. Great care was taken to between Lansing and Dubuque, and their probable history. Proceedings of describe and locate upland tills from northeastern Iowa (Orr, 1907; the Iowa Academy of Science 14:213-220. Williams, 1923; Trowbridge, 1966), and yet no valley till was CALVIN, S., and BAIN, H.F. 1900. Geology of Dubuque County. Iowa documented in the same manner from the Paleozoic Plateau (with the Geological Survey Annual Report 10:381-652. lone exception of the anomalous Bridgeport Till). CARMEN, J.E. 1931. Further studies of the Pleistocene geology of north­ western Iowa. Iowa Geological Survey Annual Report 35:15-194. Most of the evidence stated above for a pre-lllinoian age of deep CHAMBERLIN, T.C., and SALISBURY, R.D. 1886. Preliminary paper on bedrock incision is interpretive, and leaves many questions and the Driftless Area of the Upper Mississippi Valley. United States Geolog­ hypotheses open regarding the evolution of the Upper Mississippi ical Survey 6th Annual Report: 199-322. River and its tributaries. Prior to Hudak (1987c), no detailed DOTI, R.H., JR., and BATTEN, R.L. 1988. Evolution of the earth. systematic field-study of Quaternary strata within a major northeast­ Fourth edition. McGraw-Hill. ern Iowa valley was reported. Most of the pre-Illinoian evidence came HALLBERG, G.R. 1980. Pleistocene stratigraphy in east-central Iowa. Iowa from isolated outcrops and was not placed in its proper geomorphic Geological Survey Technical Information Series 10. context. HALLBERG, G. R. 1986. Pre-Wisconsin glacial stratigraphy of the central Alternate interpretations for northeastern Iowa valleys (e.g., Hall­ plains region in Iowa, Nebraska, Kansas, and Missouri. (In) Quaternary Glaciations in the Northern Hemisphere, edited by V. Sibrava, D.Q. berg et al., 1984; Bettis and Hallberg, 1986; Hudak, 1987a, b, c; Bowen, and G.M. Richmond. Quaternary Science Reviews 5: 11-15. Hudak et al., ms.) are constructed using: ( 1) more recent technologies HALLBERG, G.R., BETTIS, E.A., Ill, and PRIOR, J.C. 1984. Geologic (e.g., cosr-effecrive drilling, isotope dating), (2) more recent concepts overview of the Paleozoic Plateau region of northeastern Iowa. Proceed­ in landscape development (e.g., Ruhe et al., 1968; Ruhe, 1969), and ings of the Iowa Academy of Science 91(1):5-11. (3) a revised lithostratigraphic and chronostratigraphic framework HALLBERG, G.R., andBOELLSTORFF,J.D. 1978. Stratigraphic "confu- NORTHEASTERN IOWA RIVER VALLEYS 113

sion" in rhe region of rhe rype areas of Kansan and Nebraskan deposirs. Programs 18(6):660. Geological Sociery of America Absrracrs wirh Programs 10(6):255-256. KNOX, J.C., ATTIG, J.W., JOHNSON, M.D. 1982. Pre-Wisconsinan HALLBERG, G.R., FENTON, TE., and MILLER, G.A. 1978. Srandard deposirs in rhe Bridgeporr Terrace of rhe lower Wisconsin River Valley. wearhering zone rerminology for rhe descriprions ofQuarernary sedimenrs (In) Quarernary hisrory of rhe Drifrless Area. Wisconsin Geological and in Iowa. (In) Srandard procedures for evaluarion ofQuarernary marerials in Narural Hisrory Survey Field Trip Guidebook 5: 103-118. Iowa, edired by G.R. Hallberg. Iowa Geological Survey Technical LEONARD, A.G. 1906. Geology of Clayron Counry. Iowa Geological Informarion Series 8:75-109. Survey Annual Reporr 16:213-317. HANSEN, R.E. 1972. Bedrock ropography of easr-cenrral Iowa. Unired McGEE, W.J. 1891. The Pleisrocene hisrory of norrheasrern Iowa. Uni red Srares Geological Survey Miscellaneous Geologic Invesrigarions Series Srares Geological Survey l lrh Annual Reporr: 189-577. Map 1-717. NORTON, W.H. 1895. Geology of Linn Counry. Iowa Geological Survey HANSEN, R. E. 1975. Bedrock ropography of norrheasr Iowa. U nired Srares Annual Reporr 4:121-196. Geological Survey Miscellaneous lnvesrigarions Series Map 1-933. NORTON, W.H. 1901. Geology of Cedar Counry. Iowa Geological Survey HANSEN, R. E. 1977. Bedrock ropography of norrh-cenrral Iowa. Uni red Annual Reporr 11:279-396. Srares Geological Survey Miscellaneous lnvesrigarions Series Map 1-1080. NORTON, W.H. 1906. Geology of Bremer Counry. Iowa Geological HAY, 0. P. 1914. The Pleisrocene mammals oflowa. Iowa Geological Survey Survey Annual Reporr 16:319-406. Annual Reporr 23: 1-662. ORR, E. 1907. Exposures of Iowan and Kansan(?) drifr, easr of rhe usually HEDGES, J., and ALEXANDER, E.C., Jr. 1985. Karsr-relared fearures of accepred wesr boundary line of rhe Drifrless Area. 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