Available online at www.sciencedirect.com
Geomorphology 97 (2008) 237 – 248
www.elsevier.com/locate/geomorph
Ice-walled-lake plains: Implications for the origin of hummocky
glacial topography in middle North America
Lee Clayton a,⁎, John W. Attig a , Nelson R. Ham b , Mark D. Johnson c ,
Carrie E. Jennings d , Kent M. Syverson e
a
Wisconsin Geological and Natural History Survey, 3817 Mineral Point Road, Madison, Wisconsin 53705, USA
b
Geology Department, St. Norbert College, 100 Grant Street, DePere, Wisconsin 54115, USA
c
Department of Geology, Earth Science Centre, Box 460, SE-405 30 Göteborg, Sweden
d
Minnesota Geological Survey, 2642 University Avenue, St. Paul, Minnesota 55114, USA
e
Geology Department, University of Wisconsin, Eau Claire, Wisconsin, 54702, USA
Received 12 September 2006; received in revised form 4 December 2006; accepted 27 February 2007
Available online 23 June 2007
Abstract
Ice-walled-lake plains are prominent in many areas of hummocky-till topography left behind as the Laurentide Ice Sheet melted
from middle North America. The formation of the hummocky-till topography has been explained by: (1) erosion by subglacial
floods; (2) squeezing of subglacial till up into holes in stagnant glacial ice; or (3) slumping of supraglacial till. The geomorphology
and stratigraphy of ice-walled-lake plains provide evidence that neither the lake plains nor the adjacent hummocks are of subglacial
origin. These flat lake plains, up to a few kilometers in diameter, are perched as much as a few tens of meters above surrounding
depressions. They typically are underlain by laminated, fine-grained suspended-load lake sediment. Many ice-walled-lake plains
are surrounded by a low rim ridge of coarser-grained shore sediment or by a steeper rim ridge of debris that slumped off the
surrounding ice slopes. The ice-walled lakes persisted for hundreds to thousands of years following glacial stagnation. Shells of
aquatic molluscs from several deposits of ice-walled-lake sediment in south-central North Dakota have been dated from about
13 500 to 10 500 B.P. (calibrated radiocarbon ages), indicating a climate only slightly cooler than present. This is confirmed by
recent palaeoecological studies in nearby non-glacial sites. To survive so long, the stagnant glacial ice had to be well-insulated by a
thick cover of supraglacial sediment, and the associated till hummocks must be composed primarily of collapsed supraglacial till.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Ice-walled-lake plain; Glacial hummocks; Supraglacial debris; Pleistocene–Holocene transition
1. Introduction
⁎ Corresponding author. Tel.: +1 608 263 6839; fax: +1 608 262
8086.
E-mail addresses: [email protected] (L. Clayton),
[email protected] (J.W. Attig), [email protected] (N.R. Ham),
[email protected] (M.D. Johnson), [email protected] (C.E. Jennings),
[email protected] (K.M. Syverson).
0169-555X/$ - see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.geomorph.2007.02.045
In many places in the mid-continent area of North
America, ice-walled-lake plains are a conspicuous
component of tracts of hummocky-till topography. The
purpose of this paper is to detail evidence that the icewalled-lake plains are of supraglacial origin, including
palaeontological evidence that they could not have
formed subglacially. Because of the close association of
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L. Clayton et al. / Geomorphology 97 (2008) 237–248
ice-walled-lake plains and the hummocky-till topography, we argue that the hummocky till was also primarily
of supraglacial origin.
Till hummocks have been interpreted to have formed
in several different ways, including by erosion in
subglacial floods (Munro and Shaw, 1997), by squeezing
of subglacial material up into holes through stagnant
glacial ice (Stalker, 1960; Eyles et al., 1999; Boone and
Eyles, 2001), and by slumping of supraglacial material
down into holes through stagnant glacial ice (Gravenor
and Kupsch, 1959; Johnson et al., 1995; Johnson and
Clayton, 2003).
The ice-walled-lake plains in these tracts of hummocky topography have typically been overlooked or
unrecognized, and they are rarely mentioned in regional
reports or textbooks on glacial geology. Yet they are
widespread in areas of thick glacial sediment along the
southern and southwestern marginal zone of the
Laurentide Ice Sheet (Fig. 1). Here we review the morphology and sedimentology of ice-walled-lake plains in
mid-continental North America and evaluate what they
tell us about the origin of hummocky till.
Many ice-walled-lake plains can be identified with
confidence because they have distinctive characteristics.
They typically consist of offshore sediment surrounded by
a ring of shore-zone sediment (Fig. 2a,b). Commonly, the
outward-facing ice-contact face at the edge of the plain
slumped when the support was removed as the surrounding ice melted (Fig. 2b). In many places, the lake sediment
is tens of meters thick. In contrast to typical lake sediment,
the ice-walled-lake sediment today is not in a basin, but is
elevated above the surrounding topography.
A variety of names has been applied to these features,
typically using various combinations of words such as
dead-ice or stagnant-ice, ice-walled or perched, lake
or lacustrine, and plains or plateaus. Parizek (1969)
called them moraine lake plateaus.
2. Sources of information
Much of the information about ice-walled-lake plains
is scattered in local reports on the glacial geology of
specific areas, many not widely available. Our intent
here is to summarize observations from several decades
past and bring them up to date with more recent
findings. Our emphasis is on North Dakota, Minnesota,
and Wisconsin, because this area has abundant icewalled-lake plains, most of which have been inventoried
in reports of the three state geological surveys (Fig. 1),
although we know of numerous examples in Iowa,
South Dakota, Illinois, Manitoba, Saskatchewan, and
Alberta as well as in Denmark, Sweden, Norway,
Fig. 1. Locations of the known ice-walled-lake plains in North Dakota, Minnesota, and Wisconsin, compiled largely from references cited in text.
Each is represented by a spot that has been exaggerated by adding 0.5 km to its periphery. Major scarps are indicated on the inset map by a hachured
line, and the solid line on the inset map is the approximate farthest extent of the Laurentide Ice Sheet during the last part of the Wisconsin Glaciation.
L. Clayton et al. / Geomorphology 97 (2008) 237–248
239
Fig. 2. Schematic cross sections of ice-walled-lake plain while still confined by ice (a) and after ice melted. (b). Vertical ruling in sloughs represents faintly
bedded Holocene sediment; horizontal ruling in non-glacial lake represents latest Pleistocene and earliest laminated fossiliferous Holocene sediment.
Poland, Lithuania, Germany, and Russia (Johnson and
Clayton, 2003; Johnson et al., 2004).
The purpose of those reports was to map and describe
the geology of an area, generally a county or regional
study. Some of these contain detailed discussions of icewalled-lake plains and were based on interpretations of
aerial photographs and large numbers of observations of
shallow exposures, generally roadcuts, with some drill
holes and excavations.
3. Description
3.1. Abundance
In North Dakota (Fig. 1), 170 ice-walled-lake plains
larger than about 1 km have been shown on a 1:500 000
state-wide compilation (Clayton, 1980; many smaller
ones exist) derived from 1:125 000 maps in bulletins
published by the North Dakota Geological Survey in the
1960s and 1970s. All occur on either the Turtle
Mountains, a 60-km-wide upland in northern North
Dakota and southern Manitoba, or on the Coteau du
Missouri (Missouri Coteau), an upland 20 to 50 km wide,
extending 1000 km southeast across Saskatchewan, North
Dakota, and South Dakota (Fig. 1). Numerous ice-walledlake plains are shown on 1:125 000 maps in bulletins of
the South Dakota Geological Survey (Koch, 1975;
Christensen, 1977; Leap, 1988; Hammond 1991). All
are on either the Coteau du Missouri or the Coteau des
Prairies (Prairie Coteau), a triangular upland that extends
into Minnesota (Fig. 1).
In Minnesota, hundreds of ice-walled-lake plains
formed in broad zones along the west flank of the Des
Moines Lobe on the Coteau des Prairies in southwestern
Minnesota (Patterson, 1992, 1995, 1997; Patterson et al.,
1999) and along its east flank in the southeast and southcentral part of the state (Patterson and Hobbs, 1995;
Lusardi, 1997; Lusardi et al., 2002). They also occur, but
less abundantly, in other parts of the state, where they tend
to be more subtle and of lower relief than those formed on
the flanks of the Des Moines Lobe (Fig. 1; Meyer, 1985,
1993a, 1993b; Hobbs et al., 1990; Meyer et al., 1990,
2001; Patterson, 1992; Meyer and Hobbs, 1993; Harris
and Knaeble, 1999; Meyer and Lusardi, 2000; Patterson
and Knaeble, 2001; Lusardi et al., 2002; Syverson et al.,
2005). About 330 ice-walled-lake plains have been
identified and shown on 1:100 000 maps of the Wisconsin
Geological and Natural History Survey (Clayton, 1986,
1987, 2001; Johnson, 1986, 2000; Mickelson, 1986; Attig
and Muldoon, 1989; Attig, 1993; Ham and Attig, 1997;
Mickelson and Syverson, 1997).
3.2. Morphology
Like present-day lakes, ice-walled lakes tend to have
rounded outlines, especially the smaller ones. Few icewalled-lake plains are more than 5 km in diameter. Many
less than 0.5 km have been recognized but have generally
not been mapped. Many ice-walled-lake plains are as
high as, or even twice as high as, the surrounding till
hummocks (in the concluding remarks, we argue that the
correlation between the height of hummocks and the
height of ice-walled-lake plains is evidence that the
hummocks are of supraglacial origin).
Ice-walled-lake plains form a morphologic continuum with low to high relief (Clayton and Cherry, 1967;
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L. Clayton et al. / Geomorphology 97 (2008) 237–248
Ham and Attig, 1996, 1997; Syverson et al., 2005). The
lower ones, less than about 20 m high, tend to be
slightly dished, gently sloping downward from an outer
rim into a central level area. Many low ones have a
distinct ridge around the edge of the plain, some of
which are several meters above the middle of the plain
(Figs. 3 and 4). The higher ice-walled-lake plains (20 to
50 m high) tend to be flat to slightly convex, gently
sloping from a central flat area down past a rim that
lacks a ridge (Fig. 5a,b). Rim ridges might have originally existed but slumped away from the higher icecontact faces.
Some ice-walled-lake plains appear to have had more
than one phase of development (Johnson, 1986; Attig and
Muldoon, 1989; Attig, 1993). For example, in Fig. 4a, ice
walls apparently at first surrounded the lake plain above
Fig. 3. Two low-level ice-walled-lake plains with rim ridge beaches, surrounded by hummocky collapsed till. Dotted lines mark the crest of icecontact faces; 8 km north of Medford, Taylor County, Wisconsin; sec. 36, T. 32 N., R. 1 E.; from Attig, 1993, Figs. 24 and 25. (a) View to west.
(b) Part of the U.S. Geological Survey Medford 7.5-minute topographic quadrangle; contour interval 3 m (10 ft). The large dashed V symbol marks
the view of the photograph. Star symbols show location drill holes mentioned in text. North is to right in the photograph. Photograph by John Attig.
L. Clayton et al. / Geomorphology 97 (2008) 237–248
241
Fig. 4. (a) Two-phase terraced ice-walled-lake plain. Part of the U.S. Geological Survey Rosholt NW 7.5-minute quadrangle. Contour interval is 10 ft
(∼ 3 m). Located in sec. 34, T. 26 N., R. 9 E. in Marathon County, Wisconsin (Attig and Muldoon, 1989). (b) Ice-walled-lake plain, cultivated,
surrounded by hummocky till. Dotted line marks the crest of the ice-contact face around the lake plain. Located 19 km southwest of Ross, North
Dakota, in sec. 6 and 7, T. 154 N., R. 94 W. (Clayton, 1972, Plates 1 and 2). U.S. Department of Agriculture aerial photograph BAL-4 V45.
(c) Oblique aerial photograph of ice-walled-lake plain surrounded by till hummocks with central depressions, located 11 km south-southwest of
Stanley, North Dakota, in sec. 23 and 26, T. 155 N., R. 92 W. The plain is 0.5 km wide. Dotted line marks the crest of the ice-contact face. Photograph
by Lee Clayton.
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L. Clayton et al. / Geomorphology 97 (2008) 237–248
Fig. 5. Diagram (a) and photograph (b) of a section through a small delta in the rim of an ice-walled-lake plain. The topset and foreset beds are slightly
gravelly sand, and the bottomset beds are silty. This section was exposed in a sand and gravel pit 2 km south of the town of Turtle Lake, Barron
County, Wisconsin, in the SE1/4 NW1/4 SE1/4 sec. 6, T. 33 N., R. 14 W. From Johnson (1986, Fig. 31). (c) Small ice-walled-lake plain, 18 m high,
11 km northeast of Napoleon, Logan County, North Dakota, in sec. 24, T. 136 N., R. 72 W. (Clayton, 1961, Plate 1). Photograph by Lee Clayton.
the 1280 ft (390 m) contour. The ice walls receded and a
lower lake plain formed at the 1230 ft contour.
3.3. Sedimentology
The central part of most low-lying lake plains is
underlain by offshore clay, silt, and fine sand, grading to
coarser near-shore sand, with beach sand and gravel at
the rim (Figs. 5a,b and 6a,b; Clayton and Cherry, 1967;
Johnson, 1986; Attig, 1993; Ham and Attig, 1997;
Johnson, 2000; Syverson et al., 2005). The offshore
sediment is horizontally laminated, and near the margin
the bedding in the coarser sediment dips at a few degrees
toward the centre. Faults are commonly observed near
the ice-contact faces of the lake plains.
Some of the lower-level ice-walled-lake plains have
rim ridges with steep outer slopes (ice-contact faces) but
with gentle inner ones. These rim ridges are typically
composed of beach sediment, but some have poorly
sorted debris flows interbedded with the shore or near-
shore sediment (Fig. 6a,b; Attig, 1993; Attig et al.,
1998). The gently sloping rims of some ice-walled-lake
plains have bodies of sand and gravel interpreted to be
deltas with high-angle foreset beds dipping toward the
centre of the lake plain (Fig. 5a,b). A delta extending
into a lake plain is visible in the southwest part of the
lake plain shown in Fig. 4a.
Other low-lying ice-walled-lake plains have rim
ridges that are steep on both sides. They are composed
of material that is similar to the till surrounding the icewalled-lake plain and is thought to be material slumped
off the surrounding ice.
3.4. Thickness of lake sediment
Few ice-walled-lake plains have been systematically
investigated, but the thickness of lake sediment in
outcrops or drill holes has been noted in several regional
studies. In North Dakota and Wisconsin, the lake
sediment is often reported to be as thick as or thicker
L. Clayton et al. / Geomorphology 97 (2008) 237–248
243
Fig. 6. (a) Cross section of the rim ridge of an ice-walled-lake plain. Slumped ice-contact face is on the right (southeast) side of ridge. Shore and near-shore
gravelly sand, till, and offshore silt and fine sand interfinger on the left (northwest) side of ridge. (b) Close-up view of till on top of offshore sediment, located
at the rectangle in the middle of the outcrop. (c) Part of U.S. Geological Survey Perkinstown 7.5-minute topographic quadrangle. This exposure is in a small
gravel pit indicated by a star. The crest of the ice-contact face around the lake plain is marked with a dotted line. Contour interval is 3 m (10 ft). Located
1.5 km south of Perkinstown, Taylor County, Wisconsin. From Attig (1993, Fig. 19) and Attig et al. (1998, Fig. 1.10). Photographs by Nelson Ham.
than the height of the plain above adjacent inter-hummock
depressions. The following paragraphs detail the observations of thickness known to us in these two states. Lake
sediment is 15 to 30 m thick in drill holes in ice-walledlake plains of North Dakota where typical till hummocks
are 5 to 15 m high, with some 30 m high (Clayton, 1962;
Clayton and Cherry, 1967; Clayton and Freers, 1967).
Clayton (1962) mapped an ice-walled-lake plain in the
same area with more than 23 m of horizontally laminated
clay and silt exposed in a roadcut on one side and a similar
thickness of stratified sand, gravel, and lacustrine clay and
silt with numerous gravity faults in gravel pits on its other
side. Royse and Callender (1967) found about 5 m of lake
sediment in the concave middle of three large but lowrelief ice-walled-lake plains in northwest North Dakota.
They found more than 17 m in the middle of a large highlevel ice-walled-lake plain in the same area, where the
surrounding till hummocks are up to 20 m high.
In northwest Wisconsin, Johnson (1986) reported
drill holes with 7 to 10 m of lacustrine silt with interbedded debris-flow sediment in low-level ice-walledlake plains, where the surrounding till surface has about
10 to 15 m of local relief. Cahow (1976) reported 23 m
of lake silt in a drill hole in an ice-walled-lake plain in
northwestern Wisconsin, where the local till relief is
‘tens of feet’. Attig (1993; unpublished data) drilled
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through 3 to 4 m of laminated clayey silt and fine sand
overlying gravelly sand and till at three sites in the
middle part of the lake plain shown in Fig. 3, and 4 m of
gravelly sand in the rim ridge. Twelve additional sites
with 3 to more than 20 m of laminated lake sediment
observed in drill holes in ice-walled-lake plains are
known to us (Ham, 1994; Attig et al., 1998).
The presence of thick offshore sediment in highrelief ice-walled-lake plains indicates an abundant
source of silt and clay up the slope above lake level.
These observations of lake-sediment thickness confirm
that lake processes shaped these features, which are not
flat-topped till hills with only a thin layer of postglacial
sediment on top.
4. Identification
Ice-walled-lake plains generally have a distinctive
morphology (Fig. 2). They are commonly first recognized because they are much flatter than surrounding
areas of hummocky till (Figs. 2, 3, 4, 5c and 6c), and
because they are perched well above the present-day
wetlands in the surrounding depressions, and because
farm fields are free of pebbles, cobbles, and boulders
(Figs. 3a,b, 4a–c, 5c and 6c). Many are conspicuous
because they are cultivated land surrounded by grazing
land (Fig. 4b) or by woodland (Fig. 3a).
Hummocky collapsed supraglacial outwash or supraglacial lake deposits are common in some hummocky-till
regions (Attig, 1993). Their morphologies are similar
and may be hard to distinguish from each other and from
hummocky till without subsurface information. In one
roadcut through till hummocks in north-central Wisconsin, Attig (1993) noted that crude bedding plains could
be seen to dip downward toward the high points in the
landscape. He interpreted this to indicate that at least the
upper parts of the hummocks were of supraglacial origin
and that all the hummocks are gradational with icewalled-lake plains.
5. Fossils, climate, and chronology
The period of treeless tundra vegetation and permafrost in the proglacial environment during active-ice
deglaciation at the end of the Wisconsin Glaciation is
poorly known in the mid-continent area, because of the
scarcity of wood for radiocarbon dating (Clayton et al.,
2001). However, this generally frigid scene ended during
the better-dated Pleistocene–Holocene transition, perhaps several hundred years after active-ice deglaciation,
with a marked warming of the climate (Fig. 7; Yansa,
2006). This is significant in understanding ice-walled-
Fig. 7. Chronology of the Pleistocene transition in south-central North
Dakota based on calibrated radiocarbon dates (refer to text).
lake plains because many persisted through the transition
and into earliest Holocene time.
In hummocky-till areas of the northeast Great Plains
this striking change has been documented in two sets of
palaeoecologic studies. One is based on mollusc shells
preserved in the youngest ice-walled-lake deposits in the
hummocky-till area of south-central North Dakota. The
other is based on organic remains in non-glacial lake
deposits of the same age and in the same region as the
ice-walled-lake plains.
Together these two suites of fossils show that some of
the ice-walled lakes persisted for hundreds or thousands
of years in a temperate climate. This indicates an
insulating blanket of thick debris on top of the
surrounding stagnant glacial ice.
5.1. Ice-walled lake suite
At the time of the Pleistocene–Holocene transition, the
climate warmed, and the ice-walled lakes that survived
were colonized by a variety of organisms. The shells of 28
species of mollusc were recognized in ice-walled-lake
sediment, as well as in hummocky collapsed supraglacial
lake sediment and in hummocky collapsed supraglacial
outwash at 40 sites in south-central North Dakota
(Clayton, 1961, 1962; Tuthill, 1967). Most of these
fossils are unleached and retain well-preserved morphologic details, permitting identification to species level.
Also present are the shells of at least ten species of
ostracods and the oogonia (reproductive bodies) of
charophytes (a bushy green alga). Generally, only
calcareous fossils have been preserved; lime has been
leached from only the upper several centimeters of soil in
this area. Most non-calcareous fossils have been
destroyed because all the fossil sites are above the water
table and are well oxidized, but the presence of fish is
indicated by the presence of the glochidia (larvae) of
mussels, which are obligate parasites of fish. Recently,
Brandon Curry (Illinois State Geological Survey) has
L. Clayton et al. / Geomorphology 97 (2008) 237–248
found shells of ostracods that lived in ice-walled lakes in
Illinois.
Most of these fossil sites were roadcuts, and all were
well above any nearby bodies of water. All the fossils
were found in sediment that could only have been deposited in depressions that were walled or floored by
stagnant glacial ice. Few of the shells show any evidence
of transport; many of the mussel shells were still articulated, indicating they remain where they lived, died, and
were subsequently buried by lake or stream sediment.
According to Tuthill et al. (1964) and Tuthill (1967),
the climate at the time molluscs lived in the ice-walled
and supraglacial environments in south-central North
Dakota was moist enough to support a mixed conifer
and deciduous woodland and was only slightly cooler
than at present. It may have been like present-day northcentral Minnesota, 400 km to the northeast, where freshwater lakes have outlets and stable water levels. In
contrast, the environment of the Coteau du Missouri
today and through most of Holocene time had dry hot
summers with prairie vegetation. Modern sloughs and
most lakes are ephemeral, lack outlets, and are alkaline,
and harbour a different association of molluscs than
those that lived in the ice-walled lakes.
Mussel shells in seven of the ice-walled deposits have
been radiocarbon dated: 10 123 ± 399; 10 966 ± 365;
11 298 ± 195; 11 531 ± 494; 11 711 ± 505; 12 944 ± 279;
13 497 ± 302 B.P. (Moran et al., 1973). In addition,
radiocarbon dates of 12 088 ± 482 and 12 119 ± 477 B.P.
from white-spruce wood in hummocky till on the Coteau
du Missouri in northwestern North Dakota probably
mark the time of tree burial during the collapse of
supraglacial till (Pettyjohn, 1967). All dates presented
here have been calibrated using the method described by
Fairbanks et al. (2005).
5.2. Non-glacial-lake suite
The fossils just described from the ice-walled and
supraglacial stream and lake deposits on the Coteau du
Missouri are now in high positions in the landscape,
well above, and unrelated to, present-day sloughs, lakes,
and streams. However, complementary climatic evidence comes from studies of pollen and macrofossils
collected from non-glacial lake sediment underlying the
modern slough deposits, in the same general area as the
ice-walled-lake plains.
Most sloughs in south-central North Dakota contain
several meters of Holocene sediment, which consists of
faintly bedded or non-bedded silt and clay with few noncalcareous fossils as a result of bioturbation and
oxidation during widely fluctuating Holocene water
245
levels. Below the faintly bedded sediment in many
sloughs is finely laminated organic lake clay, containing
abundant well-preserved plants and animals that lived at
the same time as the molluscs in the ice-walled lakes.
This lake sediment occurs under the slough sediment
in less-hummocky places where the stagnant glacial ice
melted earlier than the ice around nearby betterinsulated ice-walled lakes. This lake sequence was
deposited in deeper water with more-stable water levels
and less-oxidizing bottom conditions, resulting in better
fossil preservation than in the later Holocene sloughs. At
one site in south-central North Dakota (Cvancara et al.,
1971), more than 160 species have been identified,
including beaver, muskrats, frogs, molluscs, numerous
yellow perch and other fish, 81 species of beetles, aspen
leaves, and white-spruce cones and needles, with beavergnawed wood radiocarbon dated at 11155 ± 182B.P.
(calibrated date). Yansa (2002, 2006) studied several
sites like this and concluded that south-central North
Dakota may have had tundra vegetation for a short time
following deglaciation, which was replaced by a whitespruce parkland (savanna) from about 14 500 to about
13 500 B.P. (calibrated radiocarbon dates), followed by
a deciduous parkland until about 11 500 B.P., which in
turn was replaced by grassland through the Holocene. The
parkland vegetation resembled the aspen parkland existing today 400 km to the north and northeast in northwest
Minnesota and southern Manitoba.
This corroborates Tuthill's (1967) interpretation that
the climate that exists today just northeast of the prairie–
woodland boundary is similar to the climate that existed in
south-central North Dakota during at least the final stages
of many of the ice-walled lakes. This is also the conclusion
reached by Florin and Wright (1969) and Wright (1980) in
northern Minnesota, where forest-floor litter occurs at the
bottom of Holocene pond and lake sequences, indicating
that buried ice lasted into a time of moderate climate and
that trees growing on the debris-covered ice were let down
into the final collapse depressions as the ice melted.
Although the fossil preservation in these non-glacial
lakes was very different from the preservation in the icewalled lakes, the two suites of organisms were contemporaneous and lived in the same general area, and
for that reason their ecological conditions were very
similar. Together they indicate that the ice-walled lakes
persisted into the early Holocene warm period and were
not subglacial.
6. Discussion
Reasoning by analogy with permafrost thaw lakes
gives some insight into the formation of supraglacial
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L. Clayton et al. / Geomorphology 97 (2008) 237–248
and ice-walled lakes (Attig, 1993; Ham, 1994; Ham and
Attig, 1998). Both ice-walled lakes and permafrost thaw
lakes (thermokarst lakes) occur where ice is present
below the surrounding land surface. The basin of a thaw
lake deepens because the ground ice continually thaws
beneath the lake if it is too deep to freeze solid during
the winter. Where unfrozen, the average annual temperature at the bottom must stay above the freezing point.
The flow of heat downward from the lake water and
upward from within the earth will cause the permafrost
or stagnant glacial ice to gradually melt away beneath
the lake. However, if everything is frozen solid up to the
land surface or lake surface during the winter, conduction of heat to the atmosphere will prevent the permafrost or glacier ice from melting. Therefore, once a
supraglacial lake is deep enough to remain unfrozen
during the winter, the lake will eventually melt its way
through underlying stagnant glacial ice, initially resting
on a small area of solid ground, eventually becoming an
ice-walled lake, and then perhaps merging with adjacent
ice-walled lakes before final drainage.
An explanation is needed for the way the lake is
initiated and becomes deep enough for the bottom water
to remain unfrozen in the winter. A starter lake might
appear where the thickness of supraglacial debris is
irregular, causing differential insulation and melting,
which could result in depressions with lakes too deep to
freeze solid. However, the distribution of individual icewalled-lake plains within a group of ice-walled-lake
plains generally has no obvious relationship to any
known pattern of debris irregularity. Some ice-walledlake plains are arranged in rows parallel to rows of lakes
in ice-block depressions. However, their distribution
generally seems to be haphazard, depending on the
irregular arrangement of glacial debris on stagnant
glacial ice. If the initial surface of the supraglacial debris
is too flat for supraglacial starter lakes, such as on a
supraglacial outwash plain, no ice-walled lakes can
form. The presence of collapse hummocks composed of
supraglacial lake sediment between many ice-walledlake plains indicates that these supraglacial lakes were
too shallow or short lived to melt their way to the bottom
of the ice (Attig, 1993).
Permafrost may have played an important role in the
development of ice-walled-lake plains by inhibiting the
melting of the debris-covered ice (Ham and Attig,
1996). For example, Attig (1993), Clayton et al. (2001)
and Attig et al. (2003) argued that some of the highest
ice-walled-lake plains, 4 km from the outermost limit of
the late Wisconsin glacial advance, at least 60 m above
the limit, could not have existed unless the drainageways through the glacier and through the coarse-grained
till under the ice were frozen shut. Ham et al. (1993) and
Ham and Attig (1996) also proposed a landscape model
for the deglaciation of hummocky areas in northern
Wisconsin, suggesting that permafrost delayed the
melting of stagnant buried glacial ice for a few thousand
years after active glaciation and stabilized the ice-cored
landscape, allowing the formation of large, welldeveloped ice-walled lakes.
7. Conclusions
In the previous sections, we have described the
morphology, sedimentology, palaeontology, and chronology of these flat-topped hills in enough detail to
demonstrate that they are ice-walled-lake plains. Their
elevated position above the surrounding till hummocks
and the distribution of lake-sediment facies within them
are possible only if the sediment was deposited within
ice walls that have subsequently melted. The presence of
molluscs in the lakes and the radiocarbon dates indicate
that the glacial ice was well-insulated from the
surrounding ice. The insulating material could only
have been the same supraglacial till that makes up the
hummocks around the lake plains.
Any explanation of hummocky-till topography must
accommodate associated ice-walled-lake plains. Molluscs could not live under a glacier, and ice walls could
not have survived during hundreds or thousands of years
of temperate climate unless they were well-insulated.
Although it is certainly possible that there is some
subglacial component of hummocky-till topography in
the mid-continent area, the geomorphology and stratigraphy of ice-walled-lake plains argue for a primarily
supraglacial origin for the lake plains and much of the
surrounding hummocky topography.
Acknowledgments
We thank Armand LaRoque and John Hiemstra for
their many suggestions for improving the manuscript.
Lee Clayton thanks Jon Rau for introducing him to icewalled-lake plains.
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