Surface Form of the Southern Laurentide Ice Sheet and Its Implications to Ice-Sheet Dynamics

Surface form of the southern Laurentide Ice Sheet and its implications to ice-sheet dynamics

PETER U. CLARK Department of Geosciences, Oregon State University, Corvallis, Oregon 97331-5506

ABSTRACT ka).1 An accurate reconstruction of the Lauren- surface morphology from the elevation of lateral tide Ice Sheet, therefore, has significant implica- and terminal moraines. Because these moraines Reconstructions of the ice-surface mor- tions for interpreting global sea-level history, the were deposited only along the ice margin, they phology of several lobes of the southern amount of ice volume recorded in the deep-sea do not provide direct information on the form of Laurentide Ice Sheet reinforce previous ar- oxygen-isotope record, the effect of the ice sheet the interior of the ice sheet. Nevertheless, the guments that this sector of the ice sheet was on atmospheric circulation, and mechanisms of reconstructed ice-surface morphology within thin and low sloping. Driving stresses, esti- ice-sheet growth and collapse. The form, extent, 500 km of its margin should distinguish between mated from the geometry of the recon- and dynamics of the ice sheet, however, remain several distinct and contrasting models of the structed ice surfaces, are 0.7-4.3 kPa for the topics of considerable debate (Hughes and oth- Laurentide Ice Sheet (compare Boulton and 14 ka Des Moines Lobe, 0.9-1.2 kPa for the ers, 1977; Shilts, 1980; Denton and Hughes, others, 1985; Fisher and others, 1985; Hughes, 14 ka James Lobe, 0.9-1.7 kPa for the 18-20 1981; Andrews, 1982, 1987; Dyke and others, 1987). Furthermore, such reconstructed mor- ka Lake Michigan Lobe, 1.8-2.9 kPa for the 1982; Fisher and others, 1985; Boulton and oth- phologies offer important glaciological data re- 15-18 ka Chippewa Sublobe, and 17-22 kPa ers, 1985; Hughes, 1987; Dyke and Prest, flecting the behavior and dynamics of the ice for the 15-18 ka Green Bay Lobe. Previous 1987a, 1987b). sheet that provide critical boundary conditions estimates of rates of ice-margin advance One means of constraining the form of the for ice-sheet modeling. (450-2,000 m/yr) indicate moderate-to-fast Laurentide Ice Sheet is by reconstructing ice- Several attempts have been made at recon- ice velocities for the ice lobes. Reconstructed structing the surface morphology of the southern driving stresses and velocity estimates of the 1 AH age estimates are based on the radiocarbon Laurentide Ice Sheet, either from slopes of mo- Des Moines, James, and Lake Michigan time scale. raines (Wright, 1972; Mathews, 1974) or by Lobes are analogous to the distal ends ("ice plains") of low-sloping (0.4 x 10~3) but fast moving (500 m/yr) West Antarctic ice streams, whose dynamics have been attrib- uted to sliding and/or subglacial sediment deformation by pervasive shear. These reconstructions support recent models of the Lau- rentide Ice Sheet which include movement by sliding or by subglacial sediment deformation along its southern, western, and northwestern sectors; evidence for either mechanism should be represented in the sedimentologic and geomorphic records. Thin ice in these regions indicates that the Laurentide Ice Sheet contained less ice volume and represented less of an orographic obstacle to atmospheric circulation than has been considered in models of the ice sheet on a rigid bed with steep profiles.

INTRODUCTION

The Laurentide Ice Sheet was the largest of the Northern Hemisphere ice sheets that devel- Figure 1. Major lobes along the southern margin of the Laurentide Ice Sheet between Illinois oped during the last glacial maximum (ca. 18 and Montana.

Geological Society of America Bulletin, v. 104, p. 595-605, 10 figs., May 1992.

595

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specifying the driving stress of the ice (Fisher ern (1-14 kPa; Beget, 1987) sectors of the Lau- then contoured by projecting elevations from and others, 1985; Hooke and Mooers, 1986; rentide Ice Sheet. Reconstructions from parts of moraines parallel to the form lines (Fig. 2D). Mooers, 1989). These studies have given rise to the northeastern sector of the ice sheet (Buckley, At its maximum extent, the lobate southern two distinct views of this sector of the ice sheet: 1969; Clark, 1988; Klassen and Fisher, 1988) margin of the Laurentide Ice Sheet deposited (1) that it was thin and low-sloping, and thus indicate higher driving stresses (30-125 kPa). moraines that can be traced nearly continuously characterized by low values of driving stress, or Several previous workers have estimated as one system from Illinois to Montana. Esti- (2) that it was characterized by high values of former ice thicknesses and ice-surface slopes of mates of the ages of these moraines suggest that driving stress, and the ice was correspondingly various lobes of the southern Laurentide Ice they range from 14 to 21 ka (see reviews by thicker with steeper surface slopes. Sheet. Using moraine elevations along the mar- Clayton and Moran, 1982; Mickelson and oth- The purpose of this paper is to reconstruct the gin of the James Lobe and the land elevation in ers, 1983). Because ice-surface reconstructions ice-surface morphology of the southern Lauren- the central axis of the lobe, Flint (1955) esti- are made for individual lobes, where moraine tide Ice Sheet from Illinois to Montana (Fig. 1) mated a minimum ice thickness of 500 m. Sim- ages are more nearly the same, I assume that the and to discuss implications to dynamics of the ilarly, Flint (1971) used the elevation of moraines from each lobe were deposited former ice sheet. This reconstruction, based on marginal deposits of the Green Bay Lobe and contemporaneously. moraine elevations and flowlines, supports pre- the depth of Lake Michigan to estimate a min- Because significant differences in depositional vious estimates of low longitudinal ice-surface imum thickness of 580 m for the Lake Michigan environments existed along the southern margin profiles and driving stresses for parts of this sec- Lobe 400 km from its terminus. of the ice sheet, moraines marking the limit of tor of the ice sheet (Wright, 1972; Wright and Wright (1972) estimated the ice thickness of that margin have complex and varied morphol- others, 1973; Mathews, 1974; Beget, 1986). the Superior Lobe during the late-glacial Autom- ogies and sediments (Mickelson and others, ba phase as ranging up to 900 m over the Lake 1983, 1986; Attig and others, 1989). Moraines REGIONAL SETTING Superior basin, 270 km from the terminus. Sur- deposited by each lobe, however, generally have face slopes were -1.2 x 10"3. a similar origin. Moraines from the Lake Michi- As the Laurentide Ice Sheet advanced south- Mathews (1974) estimated low driving gan Lobe have "low local relief.. . composed ward, ice flow was increasingly influenced by stresses (0.9-2.3 kPa)2 for ice-surface slopes de- mainly of subglacial till"; moraines of the Green pre-existing topographic lowlands, leading to the rived from the elevation of ice-marginal deposits Bay Lobe are comprised of "thick, sandy, and development of major ice lobes such as those of the James and Des Moines Lobes. loamy supraglacial till forming high-relief hum- examined here (Lake Michigan, Green Bay, Des Hooke and Mooers (1986) reconstructed the mocky topography"; and moraines of the James Moines, Chippewa, and James Lobes) (Fig. 1). Des Moines Lobe based on a specified driving and Des Moines Lobes have low- to high-relief Association of the major lobes with pre-existing stress of 60 kPa, arguing that it "seems to have hummocky topography underlain by clayey su- lowlands has long been known (Chamberlin, been distinctly higher than suggested by Ma- praglacial till (Mickelson and others, 1983, 1883; Leverett and Taylor, 1915; Horberg and thews (1974)" (p. 34). p. 13). Therefore, because ice surfaces are Anderson, 1956). The Lake Michigan Lobe reconstructed for individual lobes, the origin of flowed south out of the Lake Michigan basin in METHODS, ASSUMPTIONS, AND the moraine(s) associated with each lobe is not Illinois, the Green Bay Lobe advanced out of the POTENTIAL ERROR SOURCES considered to be a significant variable in the Green Bay lowland in Wisconsin, and the Chip- reconstruction. pewa Sublobe advanced down the Chippewa Ice-Surface Reconstructions Moraine elevations are commonly used to de- River valley. The Des Moines Lobe moved termine the surface elevation of former glaciers down the Minnesota River valley and then Ice-surface reconstructions presented here are (Mathews, 1967, 1974; Pierce, 1979; Beget, crossed a low divide and continued down the based on moraine elevations and flowlines re- 1987; Klassen and Fisher, 1988; Clark, 1988; shallow Des Moines River valley into Iowa. The constructed from ice-flow indicators, based on Denton and others, 1989; Bockheim and others, James Lobe advanced down the broad James the following assumptions: 1989). In order to measure elevations of mo- River valley in South Dakota. (1) the highest moraine elevation in any one raines of the southern margin of the Laurentide area corresponds to the ice-surface elevation at Ice Sheet that have varied relief and morphol- PREVIOUS WORK that point, ogy, I assumed that, for any given area, the (2) ice-flow indicators used to constrain highest elevation of a moraine within 2 km (ar- Low driving stresses have been inferred from flowlines for any one lobe formed contempo- bitrarily assigned) of the distal edge of the mo- reconstructed ice surfaces of the southwestern raneously, and raine corresponded to the ice-surface elevation (0.4-4.5 kPa; Mathews, 1974)2 and northwest- (3) formlines on the ice surface are perpen- in that area. Elevations obtained in this way may dicular to flowlines. differ from the absolute elevation of the former I used four basic steps in reconstructing ice- ice surface, depending on how the moraine 2 Mathews (1974) reported higher values of driving surface morphology (Fig. 2). First, I compiled formed, whether ice-collapse lowered the mo- stresses (7-22 kPa), but these were incorrectly calcu- the basic geologic data available from geologic raine surface following deposition, etc., but they lated from a value A derived from moraine slopes, maps, principally ice-flow indicators (for exam- are likely to be within 10-30 m of the actual where A = h/x'A, where x is horizontal distance and h ple, drumlins, striae) and ice extent (as recorded ice-surface elevation. More importantly, this is the difference in elevation over x. Driving stress (rd) 2 simplifying approach provides a relatively con- is determined from A by: rd = A pg/2, where p is the by moraines) (Fig. 2A). Elevations of moraines density of glacial ice and g is the acceleration of gravity were obtained from topographic maps (see sistent means of measuring and comparing abso- (Hollin, 1962). The values of driving stress referred to below). Flowlines were reconstructed from lute differences between ice-surface elevations here are calculated from Mathews' values of A. Clay- available ice-flow indicators (Fig. 2B). Form- along a moraine. The low variability between ton and others (1985, p. 239) also reported these cor- lines of the ice surface were drawn at right an- adjacent measured elevations (<10 m; Fig. 3) rected values of driving stress as determined from supports this assertion. This method thus results Mathews' values of A. gles to flowlines (Fig. 2C). The ice surface was

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A B which case ice-surface slopes reconstructed here and driving stresses calculated from them are maximum values.

Calculation of Driving Stress

In this paper, driving stresses are calculated from the reconstructed ice surfaces of the various ice lobes (Mathews, 1967, 1974; Beget, 1987; Klassen and Fisher, 1988; Clark, 1988), as opposed to reconstructions where ice-surface geometry was based on specified values of driving stress (Fisher and others, 1985; Hooke and Mooers, 1986; Mooers, 1989; Ackerly, 1989; Ridky and Bindschadler, 1990).

Driving stress (rd) of a glacier is balanced by the basal resistive drag (rb), with corrections made for longitudinal-stress gradients (G) and stress from side drag (S) (Paterson, 1981; Alley and others, 1987a; Whillans, 1987; van der Veen and Whillans, 1989): rd = rb + 2G + S (equation 1), where t¿ = p ghsina (equation 2), where p is density of ice, g is acceleration of gravity, h is thickness of the ice, and a is ice- surface slope. In this paper, calculated driving stresses are Figure 2. Schematic illustrations of methods used to reconstruct ice surfaces. A. Moraine based on ice-surface reconstructions over hori- marking extent of lobe, moraine elevations recorded every 2-6 km (small arrows), and ice-flow zontal distances > 10-20 times the ice thickness. indicators (short lines). B. Flowlines reconstructed from ice-flow indicators. C. Form lines Where driving stresses are >10 kPa, longitudi- drawn at right angles to flowlines. D. Ice surface contoured by extrapolating moraine eleva- nal stress gradients (G) are considered negligible tions parallel to form lines. over these distances (Robin, 1967; Paterson, 1981; Cooper and others, 1982). Where driving stresses are < 10 kPa, however, ignoring G can in a reasonably accurate representation of the face slopes of glaciers (Pateison, 1981). Given cause errors of tens of percent or larger for the ice-surface slope along the margin. the same elevations along a moraine crest, signif- very lowest driving stresses. Error analyses indi- Moraine elevations were measured from U.S. icantly different ice-surface gradients can be ob- cate that driving stresses as calculated here with- Geological Survey topographic maps (primarily tained depending on the configuration of flow- out considering longitudinal stress gradients may 1:24,000 quadrangles). One or two elevations lines (Fig. 4). Therefore, ice-surface gradients can be minimum values for a given area with a well- were obtained from each topographic map be accurately reconstructed only from moraine lubricated bed and maximum values where the along the moraine system from Illinois into elevations where flowlinesar e available. bed is frozen (R. B. Alley, February 1991, per- Montana. In total, one elevation was measured Previous estimates of surface slopes of several sonal commun.). Driving stresses calculated for every 2-6 km for most of the >3,500 km of ice lobes (James, Des Moines, Superior) along all but the Green Bay Lobe are <5 kPa, how- moraine system investigated (>1,000 measure- the southern Laurentide Ice Sheet (Wright, ever, indicating that driving stresses are unlikely ments) (Fig. 3). 1972; Mathews, 1974) were based on moraine to increase significantly (>10 kPa) with incor- Flowlines were reconstructed for each ice elevations only and did not attempt to account poration of G. Driving stresses of the Green Bay lobe from the orientation of available ice-flow for the orientation of flowlines. Lobe are >15 kPa, indicating that G can be indicators, assuming that the ice-flow indicators Because of the importance of flowlines in de- considered negligible. formed contemporaneously. termining ice-surface formlines, ice-surface re- Side drag is relatively insignificant except for Based on ice-sheet reconstructions where constructions in this paper are better constrained valley glaciers (Nye, 1965; Raymond, 1980) or basal shear stress was specified at values of for the Green Bay, Des Moines, and Chippewa for ice streams flowing through more slowly 30-60 kPa, Mooers (1986, 1989) argued that Lobes, where ice-flow indicators are wide- moving ice sheets (Whillans, 1987; Alley and drumlins associated with the Superior and spread, than for the Lake Michigan and James others, 1987a).

Green Bay Lobes could not have formed at the Lobes, where these indicators are poorly From these considerations, driving stress (rd) same time and thus could not be used to con- represented. was calculated from the reconstructed ice sur- strain regional ice-flow direction. As discussed I assumed that formlines on the ice-lobe surface of each lobe by using equation 2. further below, however, the results of this study faces were perpendicular to flowlines (Paterson, The driving stress was calculated iteratively indicate that contemporaneous drumlin forma- 1981). For gentle ice-surface slopes and low up flowlines to account for variable bed topog- tion is possible if driving stresses are significantly driving stresses (<10 kPa), however, flowlines raphy (Schilling and Hollin, 1981). No correc- lower than specified by Mooers. may be oblique to formlines (compare Bind- tion was made for isostatic depression; this Flowlines constrain formlines for most sur- schadler and others, 1987, Figs. 2 and 8), in would reduce reconstructed surface slopes and

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Figure 3. Locations (small dots) of moraine elevations measured from topographic maps. Representative profiles illustrate change in elevation along moraine crests associated with the Des Moines Lobe (F-F'), the Green Bay Lobe (G-G'), and the Lake Michigan Lobe (H-H').

calculated driving stresses by 5% to 30% (Pierce, Iowa, and Matsch (1972) described "extra- bel and others, 1983), although the Pine City 1979; Schilling and Hollin, 1981), depending on morainic" till in southwestern Minnesota as advance may be younger (Clayton and Moran, whether isostatic equilibrium was achieved be- marking an ice margin also dating ca. 20 ka 1982). neath the ice lobes. (compare Clayton and Moran, 1982). Ice-flow indicators associated with the 14 ka The reconstructed ice surface of the Des advance are relatively common in the area cov- ICE-SURFACE MORPHOLOGIES AND Moines Lobe (Fig. 5B) is based on elevations ered by the Des Moines Lobe. Surface till of the DRIVING STRESSES measured from the Bemis Moraine. Because ice- southern half of the former ice lobe is character- flow indicators (described below) are available ized by small, parallel ridges aligned transverse Des Moines Lobe only for deposits associated with the younger to ice movement (Gwynne, 1942; Ruhe, 1969; Bemis Moraine, the ice surface associated with Palmquist and Connor, 1978; Kemmis and oth- The Des Moines Lobe was the most conspic- deposition of "Tazewell" and "extra-morainic" ers, 1981) that formed subglacially (Foster and uous of the lobes projecting southward from the tills cannot be reconstructed. Elevations on mo- Palmquist, 1969; Kemmis and others, 1981; Laurentide Ice Sheet (Fig. 1). The lobe was raines marking the outer "Tazewell" and "extra- Stewart and others, 1988). >500 km long and up to 250 km wide. morainic" ice margin are similar to those on the Isopleths of shale content in till in the Minne- The best-developed moraine deposited by the adjacent Bemis Moraine, however, indicating sota River valley, southern Minnesota, indicate Des Moines Lobe is the Bemis Moraine (Fig. that the 20 ka ice surface may have been similar that ice flow was nearly transverse to the central 5A), which dates from an advance ca. 14 ka to the 14 ka ice surface reconstructed here. axis of the valley and to the axis of the Des (Ruhe, 1969; Kemmis and others, 1981). De- For the reconstruction of the 14 ka Des Moines Lobe that flowed in it (Matsch, 1972). posits dating ca. 20 ka are preserved only on the Moines Lobe, I assumed continuity of the north- Formlines in this region should thus parallel western side of the lobe. Ruhe (1969) mapped eastern Bemis margin with the Pine City these isopleths. moraines marking the outer distribution of Moraine of the Grantsburg Sublobe (Wright As noted by Wright and others (1973) and "Tazewell" till on the margin of the lobe in and Ruhe, 1965; Wright and others, 1973; Goe- Mathews (1974), the western segment of the

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flow indicators of the 14 ka Des Moines Lobe area (Flint, 1955; Flint and others, 1959). Flint 1000 were contoured at 50-m intervals from moraine (1955) described striae orientations from the elevations. The ice-surface reconstruction identi- central region of the south-central part of the ice fies a low-sloping, thin ice lobe, with an ice lobe. J. Gilbertson (unpub. data) measured thickness of only 300 m 275 km up-ice from the striae orientations on clast pavements at the base terminus (Fig. 5C). Cross sections (Fig. 6) indi- of the regional till sheet from the northeastern cate that the lateral extent of the lobe was in part of the lobe. places constrained by topography. Additional control on flowlines and formlines Driving stresses calculated 275 km up the is indicated by the nearly horizontal elevation of central flowline of the ice-surface reconstruction moraines near the terminus, indicating flowlines (Fig. 5C) are 0.7-1.7 kPa. Higher driving that intersected these moraine segments at right stresses were calculated along two flowlines of angles. Finally, assuming a constant rate of thin- the reconstruction, one on the western flank ning around the margin of the lobe, the margin (H-H'; 1.4-2.6 kPa) and one on the eastern should retreat parallel to formlines, and thus re- flank (I-I'; 2.6-4.3 kPa) of the lobe (Fig. 5). cessional moraines should also closely parallel Given the flow law of glacial ice, where strain formlines. Therefore, I used the orientation of n rate (e) = f(r) where n = 1 for rd < 10 kPa prominent recessional moraine ridges (Flint, (equation 3) (Weertman, 1983), the differences 1955; Flint and others, 1959) as a guide to in strain rates and associated velocities (compare drawing formlines where other ice-flow indica-

Paterson, 1981, p. 87) at the low values of rd tors were not available. calculated in this paper will be negligible. Clayton and others (1980) described ice-flow indicators in North Dakota associated with the James Lobe 14 ka margin used here. Surface formlines were drawn perpendicular The James Lobe, the next large ice lobe west to flowlines and contoured at 50-m intervals of the Des Moines Lobe (Fig. 1), advanced from moraine elevations (Fig. 7B). Driving down the James River valley to the confluence stresses calculated from the two ice-surface re- of the James and Missouri Rivers in South constructions are 0.9-1.2 kPa (see Fig. 7C), Dakota. similar to those for the Des Moines Lobe. The margin for the James Lobe at the last glacial maximum is not well known (Clayton Chippewa Sublobe and Moran, 1982). As drawn here (Fig. 7A), it corresponds primarily to the extent of "Cary" The Chippewa Sublobe was a small lobe of drift mapped by Flint (1955) and the "advance ice in western Wisconsin that advanced down 2" position described by Lemke and others the Chippewa River valley to its maximum ex- (1965) (compare Mickelson and others, 1983). tent at the Chippewa moraine between 16 and The northwestern margin joins the margin 18 ka (Fig. 8A) (Clayton, 1984; Attig and oth- Figure 4. Schematic illustrations demon- marked by the "Zeeland" and "Long Lake" ers, 1985). Numerous ice-flow indicators, pri- strating effect of changing flowlines on re- drifts in North Dakota (Clayton, 1966; Clayton marily drumlins (Goebel and others, 1983), constructing ice-surface topography, where and others, 1980). suggest that flowlines were nearly parallel to moraine elevations remain the same. Strongly An additional reconstruction was done for the each other and that ice advanced from the north- divergent flowlines (upper) versus nearly readvance of the James Lobe 14 ka, based on east (Fig. 8A). Attig and Clayton (1990) parallel flowlines (middle) result in signifi- the margin suggested by Clayton and Moran described southeasterly directed ice-flow indica- cantly different ice-surface longitudinal pro- (1982). The northeastern margin in South Da- tors from the southeastern portion of the lobe files (X-X' versus Y-Y') (lower). kota joins with the Bemis Moraine of the Des (where none is indicated in Fig. 8A). They sug- Moines Lobe, and the northwestern margin in gested that these indicators formed in association South Dakota is correlated with the 14 ka posi- with older, debris-covered ice, however, and tion drawn by Clayton and Moran (1982) in thus would not affect the ice-surface reconstruc- Bemis Moraine decreases in elevation fairly con- North Dakota. Elevations along this margin tion presented here. sistently from <640 m in northeastern South were measured into Montana to the Canadian The western continuation of the reconstructed Dakota to 290 m near Des Moines, Iowa, >500 border (Fig. 3). ice surface corresponds to the St. Croix moraine km to the south (compare Fig. 3, profile F-F'). Ice-flow indicators are poorly represented in (Attig and others, 1985). A similar trend exists for the moraine marking the area covered by the James Lobe (Flint and Driving stresses calculated from the recon- the outer "Tazewell" till border. In contrast, the others, 1959). Gwynne (1951) described small structed ice surface along a 50-km flowline (Fig. elevation of the eastern segment of the moraine ridges similar in morphology and composition 8C) are 1.8-2.9 kPa. rises from 290 m at its southern end to only 400 to those described from the Des Moines Lobe m 50 km north of the Iowa-Minnesota border, (Gwynne, 1942), although a subglacial origin Green Bay Lobe and it then decreases to 336 m where it inter- has not been determined. As noted by Gwynne sects the St. Croix Moraine (Fig. 5A). (1951), however, the trend of the ridges does not The maximum extent of the Green Bay Lobe Surface formlines drawn perpendicular to ice- parallel moraine ridge crests mapped in the same in Wisconsin is recorded by the Johnstown and

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Figure 5. A. Major moraines of the Des Moines Lobe (after Flint and others, 1959). B. Reconstructed ice-surface morphology of the 14 ka Des Moines Lobe. See text for discussion of profiles H-H' and I-I'. Contour interval on ice surface is 50 m. C. Longitudinal ice-surface profile for 14 ka Des Moines Lobe.

Outer or Hancock moraines (Fig. 9A) (Flint and Ice-flow indicators, primarily drumlins, are Lobe was at nearly right angles to its terminus others, 1959; Attig and others, 1985). These abundant in the area covered by the Green Bay along much of its margin. may date from 15 to 18 ka (Clayton and Moran, Lobe (Flint and others, 1959; Lineback and oth- Formlines drawn at right angles to flowlines 1982; Mickelson and others, 1983; Attig and ers, 1983; Farrand and others, 1984) (Fig. 9A). indicate that ice-surface contours are nearly par- others, 1985). The elevation of this moraine sys- Flowlines are drawn from these indicators as- allel to the ice margin. Contouring of the ice tem increases from -300 m at its southeastern suming they were formed contemporaneously surface is therefore limited to a maximum dis- end to —500 m at its northern end, nearly 350 (see, however, Mooers, 1986). Reconstructed tance of <20 km up-ice from the margin (Fig. km away (Fig. 3, profile G-G'). flowlines indicate that ice flow of the Green Bay 9B). Within these constraints, calculated driving

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120 160 320 (km)

Figure 6. Cross sections across the upper (G-G') and lower (F-F') segments of the 14 ka Des Moines 400 Lobe (right), illustrating thin ice '300 that is in part controlled by topography.

stresses are 15-17 kPa along the southern mar- right angles. The elevation of the terminal mo- 10A). The moraine reaches an elevation of 343 gin and 22 kPa along the northwestern margin raine increases north of 41 °30', where the orien- m at the Illinois-Wisconsin border, of the lobe, or the highest values of rd of any of tation of the moraine crest changes abruptly These factors indicate that flowlines of the the lobes evaluated in this paper. I assumed that from north-south to northeast-southwest (Fig. Lake Michigan Lobe diverged out of the Lake these values could be extrapolated 50-70 km up the respective flowlines in order to contour further the ice surface (Fig. 9C). B 100° 99° 98° 97°

Lake Michigan Lobe

The maximum extent of the Lake Michigan Lobe in Illinois during the last glaciation is recorded by the Shelbyville, Bloomington, and Marengo moraines (Fig. 10A) (Willman and Frye, 1970; Lineback, 1979; Johnson and others, 1986). In reconstructing the ice surface, I assumed that these moraines record one lobe event. Because they were deposited by separate sublobes of the Lake Michigan Lobe, however, they may differ in age by as much as 2,000 yr (18-20 ka) (Willman and Frye, 1970; Johnson, 1976; Mickelson and others, 1983). With the exception of a few till fabrics, ice- flow indicators are absent in the area covered by the Lake Michigan Lobe (Flint and others, 1959; Lineback, 1979; Lineback and others, 1983). Nevertheless, several indirect lines of evidence constrain ice-flow directions near the margin. Elevations along most of the moraine system from the Illinois/Indiana border to ~41°30'N range only between 205 m to 270 m, with the higher elevations associated with re-entrants along the moraine (equivalent to the location of junctures of sublobes; Willman and Frye, 1970). 200 (km) The outermost sections of the moraine system range in elevation from 205 m to 217 m (Fig. 3, Figure 7. A. Major moraines of the James Lobe (after Flint and others, 1959). B. Recon- profile H-H'), indicating that they paralleled a structed ice-surface morphology of the James Lobe. Contour interval on ice surface is 50 m. contour and that flowlines intersected them at C. Longitudinal ice-surface profile for 14 ka James Lobe.

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Michigan basin and approached the terminal moraines at nearly right angles along most of its margin (Lineback and others, 1983). The surface of the Lake Michigan Lobe was reconstructed by contouring surface formlines, drawn perpendicular to inferred flowlines, from moraine elevations (Fig. 10B). This was best ac- complished from the re-entrants associated with moraines of the sublobes (Bloomington, Shelby- ville) (Fig. 10A). Surface formlines contoured in this way extended only up to 25 km up-flowline; I extrapolated these values another 40 km up inferred flowlines in order to further contour the ice surface (Fig. 10B). Driving stresses calculated from ice-surface profiles over this distance are 0.9-1.7 kPa (Fig. 10C).

Figure 8. A. Major moraines and drumlins (long axes shown by short lines) of the Chip- DISCUSSION pewa Sublobe (after Farrand and others, 1984). B. Reconstructed ice-surface morphology of the 15-18 ka Chippewa Sublobe. Contour interval on ice surface is 50 m. C. Longitudinal Comparison to Previous Work ice-surface profile for 15-18 ka Chippewa Sublobe. Ice-surface reconstructions presented in this paper support and reinforce previous arguments for low-sloping, thin ice along the southern margin of the Laurentide Ice Sheet (Wright, 1972; Mathews, 1974; Boulton and Jones, 1979; Clark, 1980; Boulton and others, 1985; Fisher and others, 1985; Clark and Bruxvoort, 1989). Although Mathews (1974) used a more approx- imate means of estimating driving stresses (that is, he did not constrain ice-surface slopes from flowlines), his values for the Des Moines and James Lobes (0.9-2.3 kPa)3 are virtually identi- cal to those calculated here (0.7-4.3 kPa). Furthermore, these low values are now calculated for the Chippewa and Lake Michigan Lobes. Estimated values of driving stress of the Des Moines Lobe (0.7-4.3 kPa) are significantly lower than the value (60 kPa) specified for the lobe by Hooke and Mooers (1986). Their ice- surface reconstruction, based on a constant, specified driving stress of 60 kPa, results in the lobe having an elongate dome. This reconstruction accounts in part for the distribution of shale in till of the Minnesota River valley (Matsch, 1972), but it results in flowlines that are highly inconsistent with ice-flow indicators at the southern end of the lobe. I also reconstructed the Des Moines Lobe by specifying higher (10-100 kPa) driving stresses, but only along the central flowline of the lobe. Figure 9. A. Major moraines and drumlins (long axes shown by short lines) of the Green The ice surface was contoured only by con- Bay Lobe (after Flint and others, 1959; and Lineback and others, 1983). B. Reconstructed straining the orientation of contours from mo- ice-surface morphology of the 15-18 ka Green Bay Lobe. Contours are dashed where eleva- raine elevations and those elevations calculated tions are inferred from extrapolated longitudinal ice-surface profile. Contour interval on ice surface is 50 m. C. Longitudinal ice-surface profile for 15-18 ka Green Bay Lobe, dashed where extrapolated beyond control of moraine elevations. 3See footnote 2.

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the relation between longitudinal velocity by internal deformation and driving stress (Paterson, 1981, p. 87), the low driving stresses associated with the ice lobes of the southern Laurentide Ice Sheet indicate that mean longitudinal velocities arising from internal deformation must have been negligible. Several studies, however, have estimated that rates of ice-margin movement along the southern Laurentide Ice Sheet ranged from 450 m/yr to 2,000 m/yr (Willman and Frye, 1970; Mickelson and others, 1981; Clay- ton and others, 1985; Johnson and Hansel, 1987). These rapid rates, in association with the low driving stresses, indicate that sliding, subglacial sediment deformation, or both were in- volved in dynamics of the ice lobes. The reconstructed physical characteristics (surface slopes, driving stresses) and rapid rates of movement of the Des Moines, James, and Lake Michigan Lobes are analogous to the distal ends of ice streams ("ice plains") presently draining the West Antarctic ice sheet. These "ice plains," described as "hybrid[s] between an ice stream and an ice shelf' (Bindschadler and others, 1987, p. 8894), have ice-surface slopes and (m) 1 c driving stresses (2 to 7 kPa) similar to those of 300 an ice shelf, but the ice is grounded just above hydrostatic equilibrium, and velocities are >500 250- Lake Michigan Lobe A-A' m/yr, similar to an ice stream (Shabtaie and 200- Bentley, 1987; Bindschadler and others, 1987; —I— —i— Shabtaie and others, 1987). o 10 20 30 40 50 60 (km) If rates of movement of the Green Bay Lobe were similar to those of the adjacent Lake Mich- Figure 10. A. Major moraines deposited by the Lake Michigan Lobe in Illinois. Lake igan Lobe, the reconstructed characteristics of Michigan shown by dashed-Iine pattern (after Willman and Frye, 1970; and Johnson and this lobe indicate that it was similar to the upper others, 1986). B. Reconstructed ice-surface morphology of the 18-20 ka Lake Michigan Lobe. regions of some ice streams draining the West Contour interval on ice surface is 50 m. C. Longitudinal ice-surface profile for 18-20 ka Lake Antarctic ice sheet, where driving stresses are Michigan Lobe. 20-25 kPa (Alley and others, 1987a). The dynamics of rapidly flowing but low- sloping ice streams in the West Antarctic ice from the specified driving stresses along the trast, the lower driving stresses calculated (as sheet have been related to the presence of a sub- flowline. In all cases, ice-surface contours differ opposed to specified) in this paper for the Green glacial deforming layer (Blankenship and others, significantly from those based on ice-flow indi- Bay Lobe by assuming contemporaneous drum- 1986; Alley and others, 1986, 1987a; Mac- cators. Furthermore, unreasonably large spatial lin formation suggest reasonable spatial var- Ayeal, 1989). Engelhardt and others (1990) variations in driving stress and associated strain iations in driving stress (15-22 kPa). Because argued that, in addition to subglacial sediment rates (see equation 3, where n = 3 for rj > 10 contemporaneous drumlin formation can be deformation, basal sliding due to high basal kPa; Weertman, 1983) occur along the flanks of accommodated at driving stresses lower than water pressure beneath ice stream B may be a the lobe. those specified by Mooers (1986, 1989), strati- factor in ice-stream mechanics. By specifying relatively high driving stresses graphic evidence must be presented in order to The dynamics of the low-sloping but rapidly (30-60 kPa), Mooers (1986, 1989) recon- support his model of time-transgressive drumlin moving lobate southern margin of the Lauren- structed the ice surfaces of the Superior and formation. tide Ice Sheet have also been ascribed to subgla- Rainy Lobes, which lie just east of the Des cial deforming sediment (Boulton and Jones, Moines Lobe, and the Green Bay Lobe. In as- Dynamics of Lobes 1979; Boulton and others, 1985; Fisher and oth- suming these higher driving stresses, Mooers ers, 1985). Recent work in this region has identi- argued that unreasonable spatial variations in Glacier movement may arise as a result of fied sedimentological evidence that may identify driving stress occurred if drumlins associated internal deformation of ice, sliding of the glacier diagnostic criteria of subglacial sediment defor- with the lobes formed contemporaneously dur- over its base, deformation of underlying sedi- mation by pervasive shearing (Johnson and ing the maximum extent of the lobes. In con- ment, or a combination of these mechanisms. In Hansel, 1990; Clark, 1991). In addition, Alley

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(1991) and Johnson and others (1991) calcu- sal centers, of which the most recent interpreta- Richard Alley's interest and suggestions have lated high subglacial sediment fluxes for the tion is that of Dyke and Prest (1987a, 1987b). clarified the paper. Hilt Johnson and Tore southern margin that are best explained by sed- Models of the Laurentide Ice Sheet that eval- Vorren reviewed an earlier version of this paper, iment transport in a deforming bed. uated the significance of subglacial deforming and their comments and those by Bulletin re- In contrast, Clayton and others (1985, 1989) sediment to the dynamics and form of the ice viewer Lee Clayton and three anonymous Bul- have invoked surging behavior of the Des sheet (Fisher and others, 1985; Boulton and oth- letin reviewers significantly improved and fo- Moines and James Lobes due to high subglacial ers, 1985) identified several dispersal centers. cused material in this paper. pore-water pressure that supported the overlying Furthermore, a relatively thin carapace of ice, ice. Although abundant evidence exists for local whose surface profile was adjusted to the as- REFERENCES CITED deformation of glacial sediments in North Da- sumed low strength of underlying deforming Ackerly.S. C.,1989, Reconstructions of mountain glacier profiles, northeastern kota (Clayton and others, 1980; Bluemle and sediments, fringed a high region centered over United States: Geological Society of America Bulletin, v. 101, Clayton, 1984), Moran and others (1980) and rigid bedrock of the Precambrian Shield. p. 5« 1-572. Alley, R. B., 1991, Deforming-bed origin for southern Laurentide till sheets?: Clayton and others (1985, p. 239) argued that Reconstructions of surface morphology from Journal of Glaciology, v. 37, p. 67-76. Alley, R. B., Blankensbip, D. D., Bentley, C. R„ and Rooney, S. T., 1986, subglacial sediments deposited in this region moraine elevations, such as done here for the Deformation of till beneath ice stream B, West Antarctica: Nature, "can normally withstand a shear stress greater ». 322, p. 57-59. southern margin, provide a critical test of the Alley, R. B, Blankenship, D. D., Bentley, C. R., and Rooney, S. T., 1987a, Till than 100 kPa," and the sediments were thus not distinct models of the flow and form of the ice beneath ice stream B: 3. Till deformation: Evidence and implications: Journal of Geophysical Research, ». 92, p. 8921-8929. subject to pervasive shear deformation (Clayton sheet. Surface reconstructions from southern Alley, R. B., Blankenship, D. D„ Rooney, S. T, and Bentley, C. R, 1987b, and others, 1989). Clayton and others (1985, Continuous till deformation beneath ice sheets: International Associa- (this study), southwestern (Mathews, 1974), tion of Hydrological Sciences Publication no. 170, p. 81-91. 1989) further suggested that the geologic evi- northwestern (Beget, 1987), and northeastern Andrews, J. T., 1982, On the reconstruction of Pleistocene ice sheets: A review: Quaternary Science Reviews, v. 1, p. 1-30. dence from the southern Laurentide Ice Sheet (Clark, 1988; Klassen and Fisher, 1988) sectors 1987, The Late Wisconsin glaciation and déglaciation of the Laurentide (intact till stratigraphy, sharp lower contacts, Ice Sheet, in Ruddiman, W. F„ and Wright, H. E„ Jr., eds„ North of the former ice sheet support an asymmetric, America and adjacent oceans during the last déglaciation: Geological lenses of sorted sediment in till) argues against a relatively thin, multi-domed ice-sheet model Society of America, The Geology of North America, Volume K-3, p. 13-37. deforming bed. (Andrews, 1987; Beget, 1987), although An- Attig, J. W., and Clayton, L., 1990, Strike-slip features and possible Late Wisconsin surging of the Chippewa Lobe, north-central Wisconsin: Despite these arguments, little geomorphic or drews (1987) suggested that a single-domed ice American Quaternary Association, Program and Abstracts, v. 11, p. 13. sedimentologic evidence has been presented in sheet may have existed briefly during the last Attig, J. W„ Clayton, L„ and Mickelson, D. M„ 1985, Correlation of late Wisconsin glacial phases in the western Great Lakes area: Geological support of arguments that the dynamics of the glacial maximum. Surface reconstructions pre- Society of America Bulletin, v. 96, p. 1585-1593. Attig, J. W, Mickelson, D. M„ and Clayton, L„ 1989, Late Wisconsin land- southern margin of the Laurentide Ice Sheet sented here demonstrate not only thin ice along form distribution and glacier-bed conditions in Wisconsin: Sedimentary were associated with widespread subglacial the southern margin during the last glaciation, Geology, ». 62, p. 399-405. Beget, J., 1986, Influence of till rheology on Pleistocene glacier flow in the water between a rigid sediment bed and the base but also the physical implausibility of thick ice southern Great Lakes area, U S A,: Journal of Glaciology, v. 32, having existed in this area, at least at the mar- p. 235-241. of the ice. Furthermore, Alley (1991) recently 1987, Low profile of the northwest Laurentide Ice Sheet: Arctic and argued that many of these well-known strati- ginal positions represented by moraines. Alpine Research, v. 19, p. 81-88. Bindschadler, R. A., Stephenson, S. N., MacAyeal, D. R., and Shabtaie, S., graphic characteristics of till sheets from this The presence of a thin carapace of ice along 1987, Ice dynamics at the mouth of ice stream B, Antarctica: Journal of Geophysical Research, ». 92, p. 8885-8894. region are better explained by a deforming bed the northwestern, southwestern, and southern Blankenship, D. D., Bentley, C. R-, Rooney, S. T., and Alley, R. B., 1986, mechanism. Seismic measurements reveal a saturated porous layer beneath an active margins of the Laurentide Ice Sheet indicates Antarctic ice stream: Nature, v. 322, p. 54-57. The ice-surface reconstructions presented here that the global component of ice volume repre- Bluemle, J. P., and Clayton, L., 1984, Large-scale glacial thrusting and related processes in North Dakota: Boreas, v. 13, p. 279-299. do not distinguish between sliding and subgla- sented by the ice sheet is significantly less than Bockheim, J. G-, Wilson, S. C., Denton, G. H., Andersen, B. G., and Stuiver, M., 1989, Late Quaternary ice-surface fluctuations of Hatherton Gla- cial sediment deformation. Identifying which for an ice sheet resting on a rigid bed with steep cier, Transantarctic Mountains: Quaternary Research, v. 31, mechanism determined the dynamics of the profiles (Fisher and others, 1985; Boulton and p. 229-254. Boulton, G. S., and Jones, A. S., 1979, Stability of temperate ice caps and ice southern margin of the Laurentide Ice Sheet will others, 1985). In addition, a thinner ice sheet sheets resting on beds of deformable sediment: Journal of Glaciology, v. 24, p. 29-43. require geophysical modeling using reconstruct- may have represented less of an orographic ob- Boulton, G. S., Smith, G. D., Jones, A. S„ and Newsome, J., 1985, Glacial ed physical parameters (driving stress, rates of stacle to atmospheric circulation than has been geology and glaciology of the last mid-latitude ice sheets: Geological Society of London Journal, v. 142, p. 447-474. movement, duration of events) as boundary considered in recent atmospheric general-circu- Broccoli, A. J., and Manabe, S., 1987, The contributions of continental ice, atmospheric C02, and land albedo to the climate of the last glacial conditions. In addition, further detailed investi- lation models (Manabe and Broccoli, 1985; maximum: Climate Dynamics, v. 1, p. 87-99. gation of the sedimentologic and geomorphic Kutzbach and Guetter, 1986; Broccoli and Buckley, J. T., 1969, Gradients of past and present outlet glaciers: Geological Survey of Canada Paper 69-29, 13 p. record is necessary to identify unequivocal evi- Manabe, 1987; Rind, 1987; Lautenschlager and Chamberlin, T. C., 1883, Preliminary paper on the terminal moraine of the second glacial epoch: U.S. Geological Survey, 3rd Annual Report, dence of sliding, sediment deformation by per- Herterich, 1990). Finally, the sensitivity and re- p. 291-402. vasive shear, or both. sponse of a thin ice sheet to climatic change may Clark, J. A., 1980, The reconstruction of the Laurentide Ice Sheet of North America from sea level data: Method and preliminary results: Journal of be significantly different than for a thick ice Geophysical Research, ». 85, p. 4307-4323. Clark, J. A., and Bruxvoort, J. B., 1989, Reconstruction of the Laurentide Ice Implications for the Laurentide Ice Sheet sheet, particularly in investigating the coupling Sheet from tilted glacial lake shorelines and sea-level data: An inverse of sediment deformation to deterministic proc- calculation: Geological Society of America Abstracts with Programs, v. 21, p. 7. esses that may be unrelated or only indirectly Clark, P. U., 1988, Glacial geology of the Tomgat Mountains, Labrador Cana- Debate on the form, extent, and dynamics of dian Journal of Earth Sciences, v.25,p.U84-1198. the Laurentide Ice Sheet during the last glacial related to climate (see Alley and others, 1987b). 1991, Striated dast pavements: Products of deforming subglacial sediment?: Geology, ». 19, p. 530-533. maximum has focused on two contrasting mod- Clayton, L., 1966, Notes on Pleistocene stratigraphy of North Dakota: North Dakota Geological Survey Report of Investigation 44, 25 p. els: (1) a single-domed ice sheet centered over ACKNOWLEDGMENTS 1984, Pleistocene geology of the Superior region, Wisconsin: Wisconsin Hudson Bay, with internal ice-sheet components Geological and Natural History Survey Information Circular 46,40 p. Clayton, L., and Moran, S. R., 1982, Chronology of late Wisconsinan glacia- similar to those described from the present Ant- Marcus Greek and Mark Schneider assisted in tion in middle North America: Quaternary Science Reviews, v. 1, p. 55-82. arctic Ice Sheet (Hughes and others, 1977; data collection. Jay Gilbertson and Tim Kem- Clayton, L., Moran, S. R„ Bluemle, J. P., and Carlson, C. G„ 1980, Geologic Denton and Hughes, 1981; Hughes, 1987), and mis provided important ice-flow data from the map of North Dakota: U.S. Geological Survey, scale 1:500,000. Clayton, L., Teller, J. T., and Attig, J. W., 1985, Surging of the southwestern (2) a multi-domed ice sheet with several disper- James and Des Moines Lobes, respectively. part of the Laurentide Ice Sheet: Boreas, v. 14, p. 235-242.

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Clayton, L., Mickelson, D. M., and Attig, J. W., 1989, Evidence gainst perva- Johnson, W. H., and Hansel, A. K., 1987, Fluctuations of the late Wisconsin Lobe, Wisconsin: Geological Society of America Abstracts with Pro- sively deformed bed material beneath rapidly moving lobes of the (Woodfordian) Lake Michigan Lobe in Illinois, U.S.A.: International grams, v. 18, p. 699. southern Laurentide Ice Sheet: Sedimentary Geology, v. 62, Union for Quaternary Research, International Congress, 12th, Program 1989, Drumlin formation: A time transgressive model: Boreas, v. 18, p. 203-208. with Abstracts, p. 195. p. 99-108. Cooper, A.P.R., Mclntyre, N. F., and Robin, G. de Q., 1982, Driving stresses in 1990, Multiple Wisconsinan glaagenic sequences at Wedron, Illinois: Moran, S. R., Clayton, L., Hooke, R. LeB., Fenton, M. M., and Andriashek, the Antarctic ice sheet: Annals of Glaciology, v. 3, p. 59-64. Journal of Sedimentary Petrology, v. 60, p. 26-41. L. D., 1980, Glacier-bed landforms of the Prairie region of North Denton, G. H., and Hughes, T. J., 1981, eds.. The last great ice sheets: New Johnson, W. H , Moore, D. W., and McKay, E. D , III, 1986, Provenance of America: Journal of Glaciology, v. 25, p. 457-476. York, John Wiley, 484 p. late Wisconsinan (Woodfordian) till and origin of the Decatur sublobe, Nye, J. F., 1965, The flow of a glacier in a channel of rectangular, elliptic or Denton, G. H., Bockheim, J. G., Wilson, S. C., Leide, J. E., and Andersen, east-central Illinois: Geological Society of America Bulletin, v. 97, parabolic cross section: Journal of Glaciology, v. 5, p. 661-690. B. J., 1989, Late Quaternary ice-surface fluctuations of Beardmore p. 1098-1105. Palmquist, R., and Connor, K., 1978, Glacial landforms: Des Moines drift Glacier, Transantarctic Mountains: Quaternary Research, v. 31, Johnson, W. H., Hansel, A. K., and Stiff, B. J., 1991, Glacial transport rates, sheet, Iowa: Association of American Geographers Annals, v. 68, p. 183-209. late Wisconsinan Lake Michigan Lobe in central Illinois: Implications p. 166-179. Dyke, A. S., and Prest, V. K., 1987a, Late Wisconsinan and Holocene history for transport mechanisms and ice dynamics: Geological Society of Pateison, W.S.B., 1981, The physics of glaciers: New York, Pergamon Press, of the Laurentide Ice Sheet: Geographic physique et Quatemaire, v. 41, America Abstracts with Programs, v. 23, p. A61. 380 p. p. 237-264. Kemmis, T. J., Hallberg, G. R., and Lutenegger, A. J., 1981, Depositional Pierce, K. L., 1979, History and dynamics of glaciation in the northern Yellow- 1987b, Paleogeography of northern North America, 18,000-5,000 environments of glacial sediments and landforms on the Des Moines stone National Park area: U.S. Geological Survey Professional Paper years ago: Geological Survey of Canada, Map 1703A. Lobe, Iowa: Iowa Geological Survey Guidebook Series 6,138 p. 729-F, 90 p. Dyke, A. S., Dredge, L. A., and Vincent, J.-S., 1982, Configuration and dynam- Kiassen, R. A., and Fisher, D. A., 1988, Basal-flow conditions at the northeast- Raymond, C. F., 1980, Temperate valley glaciers, in Colbeck, S. C„ ed.. Dy- ics of the Laurentide Ice Sheet during the late Wisconsin maximum: em margin of the Laurentide Ice Sheet, Lancaster Sound: Canadian namics of snow and ice masses: New York, Academic Press, p. 79-139. Geographie physique et Quatemaire, v. 36, p. 5-14. Journal of Earth Sciences, v. 25, p. 1740-1750. Ridky, R. W., and Bindschadler, R. A., 1990, Reconstruction and dynamics of Engelhardt, H., Humphrey, N., Kamb, B., and Fahnestock, M., 1990, Physical Kutzbach, J. E., and Guetter, P. J., 1986, The influence of changing orbital the Late Wisconsin "Ontario" ice dome in the Finger Lakes region, conditions at the base of a fast moving Antarctic ice stream: Science, parameters and surface boundary conditions on climate simulations for New York: Geological Society of America Bulletin, v. 102, v. 248, p. 57-59. the past 18,000 years: Journal of the Atmospheric Sciences, v. 43, p. 1055-1064. Farrand, W. R., Mickelson, D. M„ Cowan, W. R , and GoebeL J. E., 1984, p. 1726-1759. Rind, D., 1987, Components of the ice age circulation: Journal of Geophysical Quaternary geologic map of the Lake Superior 4C*6° quadrangle. Lautenschlager, M., and Herterich, K., 1990, Atmospheric response to ice age Research, v. 92, p. 4241-4281. United States and Canada: U.S. Geological Survey Miscellaneous conditions: Climatology near the earth's surface: Journal of Geophysical Robin, G. de Q., 1967, Surface topography of ice sheets: Nature, v. 215, Investigations Series Map 1-1420 (NL-16). Research, v. 95, p. 22547-22557. p. 1029-1032. Fisher, D. A., Reeh, N., and Langley, K., 1985, Objective reconstructions of the Lemke, R. W., Laird, W. M., Tipton, M. J., and Lindvall, R. M., 1965, Ruhe, R. V., 1969, Quaternary landscapes in Iowa: Ames, Iowa, Iowa State late Wisconsinan Laurentide Ice Sheet and the significance of deforma- Quaternary geology of northern Great Plains, in Wright, H. E., Jr., and University Press, 255 p. bte beds: Geographie physique et Quatemaire, v. 39, p. 229-238. Frey, D. G., eds., The Quaternary of the United States: Princeton, New Schilling, D. H., and Hollin, J. T., 1981, Numerical reconstructions of valley Flint, R. F., 1955, Pleistocene geology of eastern South Dakota: U.S. Geologi- Jersey, Princeton University Press, p. 15-28. glaciers and small ice caps, in Denton, G. H., and Hughes, T. J., eds.. cal Survey Professional Paper 262,173 p. Leverett, F., and Taylor, F. B., 1915, The Pleistocene of Indiana and Michigan The last great ice sheets: New York, John Wiley and Sons, p. 207-221. 1971, Glacial and Quaternary geology: New York, John Wiley and and the history of the Great Lakes: U.S. Geological Survey Monograph Shabtaie, S., and Bendey, C. R., 1987, West Antarctic ice streams draining into Sons, 892 p. 53, 529 p. the Ross Ice Shelf: Configuration and mass balance: Journal of Geo- Flint, R. F., Colton, R. B., Goldthwait, R. P., and Witlman, H. B„ 1959, Glacial Lineback, J. A., 1979, Quaternary deposits of Illinois: Illinois State Geological physical Research, v. 92, p. 1311-1336. map of the United States: New York, Geological Society of America. Survey Map, scale 1:500,000. Shabtaie, S., Whiilans,!. M., and Bentley, C. R., 1987, The morphology of ice Foster, J. D., and Palmquist, R. C., 1969, Possible subglacial origin for "minor Lineback, J. A., Bleur, N. K., Mickelson, D. M.. Farrand, W. R., and Gold- streams A, B, and C, West Antarctica, and their environs: Journal of moraine" topography: Iowa Academy of Science Proceedings, v. 76, thwait, R. P., 1983, Quaternary geologic map of the Chicago 4°*6° Geophysical Research, v. 92, p. 8865-8883. p. 296-310. quadrangle, United States: U.S. Geological Survey Miscellaneous Inves- Shilts, W. W., 1980, Flow patterns in the central North American ice sheet: Goebel, J. E„ Mickelson, D. M., Farrand, W. R., Clayton, L., Knox, J. C., tigations Series Map 1-1420 (NK-16). Nature, v. 286, p. 213-218. Cahow, A., Hobbs, H. C., and Walton, M. S., Jr., 1983, Quaternary MacAyeal, D. R., 1989, Large-scale ice flow over a viscous basal sediment: Stewart, R. A., Bryant, D., and Sweat, M. J., 1988, Nature and origin of geologic map of the Minneapolis 4°x6° quadrangle, United States: U.S. Theory and application to ice stream B, Antarctica: Journal of Geophys- corrugated ground moraine of the Des Moines Lobe, Story County, Geological Survey Miscellaneous Investigations Series Map 1-1420 ical Research, v. 94, p. 4071-4087. Iowa: Geomorphology, v. 1, p. 111-130. (NL-15). Manabe, S., and Broccoli, A. J., 1985, The influence of continental ice sheets on van der Veen, C. J., and Whiilans, I. M„ 1989, Force budget: I. Theory and Gwynne, C. S., 1942, Swell and swale pattern of the Mankato lobe of the the climate of an ice age: Journal of Geophysical Research, v. 90, numerical methods: Journal of Glaciology, v. 35, p. 53-60. Wisconsin drift plain in Iowa: Journal of Geology, v. 50, p. 200-208. p. 2167-2190. Weertman, J., 1983, Creep deformation of ice: Annual Reviews of Earth and 1951, Minor moraines in South Dakota and Minnesota: Geological Mathews, W. H., 1967, Profiles of late Pleistocene glaciers in New Zealand: Planetary Sciences, v. 11, p. 215-240. Society of America Bulletin, v. 62, p. 233-250. New Zealand Journal of Geology and Geophysics, v. 10, p. 146-163. Whiilans, I. M., 1987, Force budget of ice sheets, in van der Veen, C. J., and Hollin, J. T., 1962, On the glacial history of Antarctica: Journal of Glaciology, 1974, Surface profiles of the Laurentide Ice Sheet in its marginal areas: Oerlemans, J., eds., Dynamics of the West Antarctic Ice Sheet: Amster- v. 4, p. 173-195. Journal of Glaciology, v. 13, p. 37-43. dam, The Netherlands, D. Riedel, p. 17-36. Hooke, R. LeB., and Mooers, H. D., 1986, Glaciology of some mid-continent Matsch, C. L., 1972, Quaternary geology of southwestern Minnesota, in Sims, Willman, H. B., and Frye, J. C., 1970, Pleistocene stratigraphy of Illinois: ice lobes: Problems and possible avenues for research: American Qua- P. K., and Morey, G. B., eds., Geology of Minnesota: A centennial Illinois State Geological Survey Bulletin 94,204 p. ternary Association, Program and Abstracts, v. 9, p. 34-36. volume: Minneapolis, Minnesota, Minnesota Geological Survey, Wright, H. E., 1972, Quaternary history of Minnesota, in Sims, P. K., and Horberg, C. L., and Anderson, R. C., 1956, Bedrock topography and Pleisto- p. 548-560. Morey, G. B., eds., Geology of Minnesota: A centennial volume: Min- cene glacial lobes in central United States: Journal of Geology, v. 64, Mickelson, D. M., Acomb, L. J., and Bentley, C. R., 1981, Possible mechanism neapolis, Minnesota, Minnesota Geological Survey, p. 515-547. p. 101-116. for the rapid advance and retreat of the Lake Michigan Lobe between Wright, H. E., Jr., and Ruhe, R. V., 1965, Glaciation of Minnesota and Iowa, in Hughes, T. J., 1987, Ice dynamics and deglaciation models when ice sheets 13,000 and 11,000 years BP: Annals of Glaciology, v. 2, p. 185-186. Wright, H. E., Jr., and Frey, D. G., eds., The Quaternary of the United collapsed, in Ruddiman, W. F., and Wright, H. E„ Jr., eds., North Mickelson, D. M., Clayton, L, Fullerton, D. S., and Boms, H. W., Jr., 1983, States: Princeton, New Jersey, Princeton University Press, p. 29-42. America and adjacent oceans during the last deglaciation: Geological The late Wisconsin glacial record of the Laurentide Ice Sheet in the Wright, H. E„ Jr., Matsch, C. L., and Cushing, E. J., 1973, Superior and Des Society of America, The Geology of North America, Volume K-3, United States, in Porter, S. C., ed., Late-Quaternary Environments of Moines lobes, in Black, R. F , Goldthwait, R. P., and Willman, H. B„ p. 183-220. the United States: Minneapolis, Minnesota, University of Minnesota eds., The Wisconsinan Stage: Geological Society of America Memoir Hughes, T. J., Denton, G. H . and Grosswald, M. G., 1977, Was there a Press, p. 3-37. 136, p. 153-185. late-Wurm ice sheet?: Nature, v. 266, p. 596-602. Mickelson, D. M„ Clayton, L., and Muller, E. H., 1986, Contrasts in glacial Johnson, W. H„ 1976, Quaternary stratigraphy in Illinois: Status and current landforms, deposits, glacier-bed conditions, and glacier dynamics along problems, in Mahaney, W. C., ed., Quaternary stratigraphy of North the southern edge of the Laurentide Ice Sheet: American Quaternary MANUSCRIPT RECEIVED BY THE SOCIETY APRIL 1,1991 America: Stroudsburg, Pennsylvania, Dowden, Hutchinson & Ross, Association Abstracts with Program, v. 9, p. 28-30. REVISED MANUSCRIPT RECEIVED OCTOBER 28,1991 Inc., p. 161-196. Mooers, H. D., 1986, Computer reconstruction of the Pleistocene Green Bay MANUSCRIPT ACCEPTED NOVEMBER 1,1991

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