Sedimentary Geology, 62 (1989) 407-428
Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
407
Bedforms of the Keewatin Ice Sheet, Canada
J.M. AYLSWORTH and W.W. SHILTS
Terrain Sciences Division, Geological Survey of Canada, 601 Booth Street, Ottawa, K1A OE8 (Canada)
Received June 1988; revised version received January 1989
Abstract
Aylsworth, J.M. and Shilts, W.W., 1989. Bedforms of the Keewatin Ice Sheet, Canada. In: J. Menzies and J. Rose
(Editors), Subglacial Bedforms--Drumlins, Rogen Moraine and Associated Subglacial Bedforms. Sediment. Geol.,
62: 407-428.
By compiling glacial bedforms on a map that covers most of one sector of the Laurentide Ice Sheet, it is possible to
make some suggestions about their genesis based largely on spatial relationships.
It can be concluded that drumlins and ribbed moraine form at the base of actively flowing ice under similar
dynamic conditions. For either landform to exist, however, there must have been enough sediment available in the base
of the glacier to leave or form a feature large enough to be recognizable. The presence or absence of sufficient load is
related to the geology of the glacier bed and has little to do with regionally changing dynamics of the ice-water system.
Likewise, given sufficient load, it is evident that whether drumlins formed or whether ribbed moraine formed in a
certain area is a function of the physical nature of the load which is, again, related to geology of the source outcrops.
Whether the physical characteristics come into play after the sediment has been released from the ice and is being
reshaped by basal drag, streamlining, etc., or whether the nature of the load while entrained changes the behaviour of
the basal part of the ice is unclear.
Physical characteristics of the basal sediment load have apparently promoted internal thrusting of coherent slabs of
entrained debris and ice to form ribbed moraine on melting, whereas drumlins may reflect moulding of plastic
subglacial debris or erosional streamlining of both the unconsolidated glacial substrate and bedrock. The observation
that many eskers cross drumlin fields at nearly right angles to their orientation suggests that conditions producing
streamlining and those pertaining to subglacial drainage are separated in time and circumstance.
The general occurrence of drumlins and eskers throughout the sediment-rich portions of the Keewatin Ice Sheet,
from Zone 1 to its edge, is difficult to reconcile with the restriction and intimate association of these forms with ribbed
moraine almost exclusively in Zone 2. Because such a zonal relationship exists to some extent around other ice divides,
at least in Labrador/Nouveau Quebec and Newfoundland, it seems that some condition changed or existed in this zone
throughout or at some specific time during the existence of the Keewatin Ice Sheet. Possibly around the last ice divides,
reactivation of the ice sheet by alimentation associated with climatic deterioration may have promoted flow of thin,
brittle ice. This may have "shattered" the glacier, forming stacked thrust plates of ice and debris. With subsequent
stagnation, the thrust plates may have melted out without substantial deformation.
Introduction
outflow for the Keewatin sector of the Laurentide
Ice Sheet, have been systematically mapped
For
the past
15 y e a r s ,
glacial deposits
and
b e d f o r m s a r o u n d t h e K e e w a t i n I c e D i v i d e ( L e e et
1). T h e o r i g i n a l i m p e t u s f o r t h e c o m p i l a t i o n
al., 1957), w h i c h
to provide background
was the late glacial centre of
Quaternary
formation for environmental
pipeline route and
Geological Survey of Canada Contribution No. 30388.
0037-0738/89/$03.50
© 1989 Elsevier Science Publishers B.V.
and
compiled for an area of about 1,300,000 km 2 (Fig.
exploration
assessment of a gas
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Fig. 1. Location map.
greenstone belts of eastern District of Keewatin
and the uranium exploration areas around Baker
Lake, in central Keewatin. In the course of carrying out these projects, observations made at over
8000 sediment sampling sites, covering about
15500 km 2, were integrated with air photo interpretation of associated landforms and periglacial
features to develop an interpretive map legend
that was used to draw reconnaissance maps of
landform/sediment associations. These maps,
while supported by a dense network of ground
observations in eastern Keewatin, are almost totally compiled from data interpreted from air photos for much of the area west of the Keewatin Ice
Divide.
The principal glacial bedforms depicted on these
maps include drumlins * and other streamlined
features, unorganized, low-relief h u m m o c k y
moraine, ribbed (rogen) moraine * *, and eskers.
When these features are abstracted from the larger
scale maps, together or individually onto smallscale maps, it can be seen that they form distinc-
tive areal patterns and occur in configurations
that are more or less clearly related to the ice
divide (Figs. 2, 3, 4). It was on the basis of these
patterns that Shilts and Aylsworth (in Shilts et al.,
* In this paper the term drumlin will be used to describe a
range of streamlined features from those with classical
"inverted spoon" shape to elongated flutes on till plains,
crag and tail features, etc. In fact, although it is our
intention to treat drumlins as constructional features, at the
scale of mapping it is not possible to tell how much
bedrock, streamlined by erosional processes, has been included in our interpretation. To the extent that drumlins
may originate by erosional streamlining of preexisting features, bedrock or drift, perhaps the distinction is not so
important.
** The term " r i b b e d moraine", as used in this paper, includes
landforms with the classical rogen moraine morphologies of
the Lake Rogen area in Sweden (Lundqvist, 1969), as well
as a variety of forms that are commonly associated with
classical rogen forms on the Canadian Shield. In most cases
the two terms can be used interchangeably because areas
mapped as ribbed moraine are dominated by the classical
rogen bedform.
409
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drumlin fiel
ribbed mora
Fig. 2. Laterally alternating and mutually exclusive trains of ribbed moraine and drumlins radiate outwards from the area of the
Keewatin Ice Divide. (Modified from Shilts et al., 1987.)
1987) suggested a series of landform/sediment
zones around the Keewatin Ice Divide. In that
paper we suggested that the zonation resulted
from the complex interplay between dynamic conditions at the glacier's base and geology of the
glacier bed. The principal thesis of this paper is
that the large scale patterns of landforms and
sediments of the Keewatin ice sheet place important constraints on possible inferences about
their origin. Such inferences are more commonly
drawn from studies of individual features or small
groups of features outside the context of regional
glacial history (Lundqvist, 1969; Aario, 1977;
Shaw, 1979, 1985; Bouchard, 1980; etc.).
Landform/sediment zones
Four zones that are roughly concentric about
the Keewatin Ice Divide are defined by landform
assemblages (Fig. 4; Shilts et al., 1987). Zone 1,
including the Divide, is characterized by a lack of
eskers or oriented landforms and includes significant areas of low relief hummocky moraine. Sedim e n t / l a n d f o r m assemblages in this zone reflect
stagnation of a very thin ice sheet during deglaciation and low glacier flow velocities just prior to
deglaciation.
Zone 2 is delimited by the distribution of fibbed
(rogen) moraine, but also includes extensive
drumlin fields and well-developed esker systems.
There is a close lateral relationship between ribbed
moraine and drumlins, and their distribution is
commonly related to specific source areas. Although both may be formed during active glacier
flow under similar dynamic conditions, their distribution within this zone is probably more a
function of how basal ice dynamics is affected by
the amount and type of entrained debris, rather
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Fig. 4. Landform/sediment zones around the Keewatin Ice Divide. Gray areas represent hummocky moraine, the characteristic
landform of the Ice Divide. (Modified from Shilts et al., 1987; Aylsworth and Shilts, 1989.)
411
than how basal dynamic conditions changed along
flow lines from the centre of ice dispersal.
Zone 3 is characterized by fairly continuous,
commonly drumlinized, drift cover. This cover
thins outward, probably reflecting dispersal of
debris from easily eroded outcrops lying near the
ice dispersal centre. Many eskers cross Zone 3, but
they too decrease in size and frequency outward.
Zone 4 comprises large tracts of bedrock outcrop with minimal drift cover. The sediment-poor
ice that traversed this region was apparently unable to erode local bedrock effectively and carried
only small amounts of debris from more easily
eroded outcrops which lie closer to the central
part of the ice sheet. Thus, the paucity of constructional landforms in this zone reflects a dearth
of the glacial sediment necessary to build them.
Sediment supply
Although there is no concensus about the genesis of drumlins and ribbed moraine, it is certain
that where they are found, a sufficient volume of
sediment must have been entrained in the glacier
to build them and any other landforms composed
of unconsolidated debris. For instance, where the
glacier was carrying only enough sediment to leave
behind a metre-thick layer of till over bedrock, it
is not likely that it could have been effective in
forming 10-m-high drumlins or ribbed moraine
ridges. Thus, the lack of subglacial bedforms in
some areas is not necessarily attributable to the
lack of favourable dynamic conditions for their
formation, but may be due simply to a lack of
sufficient sediment to build or to be eroded or
shaped into a landform.
This concept is probably best illustrated in
Zone 4, between Great Bear and Great Slave
Lakes, a region of extensive bedrock outcrop with
negligible drift cover along the outer (westernmost) edge of the Canadian Shield. Within this
zone, eskers, which are profusely developed to the
east in the more heavily drift-covered terrain of
Zones 2 and 3, are virtually non-existent (Fig. 3).
However, at the outer (with respect to ice flow)
edge of Zone 4, on unmetamorphosed Paleozoic
sedimentary bedrock down ice from the Paleozoic/Precambrian contact, eskers and other gla-
cial bedforms appear abruptly and are well developed in the direction of glacial flow away from the
Shield. This suggests that while the subglacial
drainage conditions required to form eskers probably existed over Zone 4, as well as up-ice and
down ice from it, the basal sediment load was so
low that no recognizable depositional forms were
produced. Where the basal sediment load production was higher from more easily eroded lithologies on either side of Zone 4, eskers are well
represented. Naturally, other bedforms formed directly by ice, such as drumlins, whether erosional
or depositional in genesis, would not have been
developed in Zone 4. In other words, while dynamic conditions at the base of the ice may have
been such that bedforms could have been constructed, for them to have expression, sufficient
unconsolidated sediment must have been present
in or under the glacier for their formation, whether
by accretion, moulding, or erosion.
A second geological factor influencing the nature of the landform/sediment association beneath the Keewatin Ice Sheet is the physical nature of the load itself. If, for instance, drumlin
formation required an easily deformable sediment
in the basal zone, sandy sediment reworked from
proglacial fluvial deposits or a bouldery sediment
derived from outcrops particularly disrupted into
felsenmeer might not form drumlins, even if the
dynamic conditions at the base of the ice were
appropriate. The texture and physical properties
of sediments that form drumlins and ribbed
moraine are quite variable, and it is our impression that the only difference between the two
types of features is the higher concentration of
coarse boulders a n d / o r reworked sorted gravelly
sediments in ribbed moraine areas. Bouchard
(1989) discusses textural properties of sediment
forming rogen (ribbed moraine) ridges more thoroughly.
In summary, the presence of fields of landforms in one area and lack of them in another
does not necessarily reflect differing glacier dynamics. Their presence or absence just as likely
reflects low concentrations of basal glacial sediment down ice from bedrock particularly resistant
to glacial erosion or debris loads with physical
characteristics that did not permit deformation or
41
erosional shaping into one discrete landform as
opposed to another. Their presence or absence
may reflect the amount of basal debris which in
turn is related to erodability of source outcrops;
or, their distribution may be controlled by physical characteristics of the basal load, whether it or
the ice in which it was suspended was deformable
or not.
Drumlins
The origin of drumlins and related streamlined
features has been debated for over a century, and
it is not the authors' intention to summarize the
postulated origins of drumlins. Comprehensive
mapping of patterns of drumlin occurrence from
the centre nearly to the edge of a single, continental-scale ice mass, however, allows us to place
certain constraints on these discussions. We believe that the spatial relationships of drumlins to
the geology and topography of the glacier's bed
and to changing flow conditions of the glacier
itself, deduced from other bedforms such as eskers,
rule out some current hypotheses and support
others. We further believe that local studies of
drumlin characteristics should not be extrapolated
to the general unless they are integrated with the
history and configuration of the whole sector of
the ice sheet under which they were formed.
('omposition
Holes 0.5-1.0 m deep were dug in many drumlins in eastern Keewatin to provide samples for
the drift geochemistry project mentioned above.
These drumlins were covered by mudboils, round
to oval periglacial features, ca. 2 m in diameter,
which characteristically develop on silty, clayey
diamictons, such as till (Shilts, 1978). In the holes
that were dug in drumlins (ca. 50) and in the only
section that was found through a drumlin, we
encountered only till or a periglacial diamicton
derived from till. The almost universal presence of
mudboils on drumlin surfaces in the perenially
frozen terrain north of the tree line and south of
about 66 ° latitude (much of the area formerly
covered by the Keewatin lce Sheet) attests to their
characteristic composition of till or derived diamicton (Fig. 5a, b). For comparison,surfaces of
eskers ridges in the same area, composed of sorted
sand and gravel, do not resemble nearby drumlins
in any way, having much more sparse vegetation
and being disrupted by orthogonal frost cracks
with no or very rare mudboils (Fig. 6). Because of
the preferential development of different patterned ground forms on different sediment types,
there is very little difficulty in guessing the composition of at least the upper 2 m of sediment
composing drumlins or other bedforms in the
southern and central parts of the Canadian tundra.
a
Fig. 5. Drumlins covered by mudboils, south (a) and southwest (b) of Kaminak Lake. In contrast, low wet areas between drumlins
may accumulate a heavy organic cover upon which frost cracks develop. (a: GSC 203011-E; b: GSC 204658)
413
Fig. 7. Light snow cover enhances the pattern of frost cracks
on the surface of drumlins in the Thelon sedimentary basin,
northwest of Aberdeen Lake. (Portion of NAPL airphoto
A15050-66)
Fig. 6. In contrast to mudboil covered till surface, esker ridge
(shown here) or other features composed of sand and gravel
characteristically develop orthogonal frost cracks. (Portion of
NAPL airphoto A19625-143)
Notwithstanding the above generalization, some
drumlins and associated ribbed moraine in the
region within or just down ice from the large basin
underlain by poorly consolidated, kaolin-cemented, late Precambrian orthoquartzites of the
Thelon sedimentary basin are disrupted by orthogonal frost cracks and are nearly free of
mudboils and vegetation (Fig. 7). McDonald (personal communication, 1968) reported these drumlins to be covered by a dense layer of sandstone
slabs. Examination of ribbed moraine ridges with
these same characteristics showed them to be composed almost wholly of orthoquartzite so thoroughly disaggregated as to give the appearance of
an unconsolidated, well-sorted pebbly sand (Fig.
8). Very little detritus from outside the Thelon
basin was mixed with the sediment. The lack of a
fine-grained component in this till accounts for
the absence of mudboils, the sparse vegetation,
and the frost cracks on bedforrns composed of
detritus from the Thelon sedimentary basin.
Just south of the treeline, in north-central
Manitoba, the authors have recently observed
well-developed, large-scale polygonal patterns on
most drumlins (Fig. 9). Mudboils are not found in
the treed areas, regardless of sediment type, and
the significance of these frost polygons with respect to sediment composition is not known although the till is known to be sandy (Dredge et
al., 1985).
Fig. 8. Train of fibbed moraine composed almost wholly of
disaggregated orthoquartzite, north of Aberdeen Lake. Surface
is unvegetated. (Portion of NAPL airphoto A15390-23)
414
Distribution patterns
Fig. 9. Frost cracks on surface of drumlin, northern Manitoba
Note depressions thought to be plunge pools on drumlin side.
(GSC 204658-C)
In summary, drumlins over most of this terrain
are composed, at least in their upper 2 m, of till.
In and adjacent to the poorly consolidated sandstones of the Thelon Basin, drumlins may be
composed of sediment that appears to be sorted
but, in actual fact, the sediment owes its apparent
sorting to the glacial disaggregation, with little
grain crushing, of a well sorted parent material.
The parent material is not mixed with the poorly
sorted debris that is characteristically produced by
crushing and abrasion of clasts from the more
crystalline parts of the Shield.
Patterns of distribution and morphological
characteristics of drumlins may help us understand their genesis. These characteristics can provide clues that, although presently undecipherable,
may, in conjunction with detailed studies, provide
support for or constraints on future interpretations.
Apart from the important and widespread spatial relationships of drumlin fields (1) to other
landforms, (2) to areas of low or high sediment
productivity, and (3) to a lesser degree, to the
configuration of flow of the Keewatin Ice Sheet,
individual drumlins or groups of drumlins may
deviate from the norms of shape or composition in
such a way as to provide insight into their genetic
relationship to other bedforms. Perhaps the most
important of these deviations are the rare cases
where drumlins show signs of transition to ribbed
moraine forms (Fig. 10) or where ribbed moraine
ridges are drumlinized (Fig. 11). These cases, already discussed by Shilts and Aylsworth (in Shilts
et al., 1987) clearly indicate that the two forms are
spatially related, and may form under similar basal
glaciodynamic conditions.
The locations of drumlin fields are neither random nor are they always clearly related to specific
phases of flow or other regionally variable characteristics of the Keewatin Ice Sheet. They are
Fig. 10. Drumlins or flutes, partially broken up into ribbed moraine north of Dubawnt Lake. (Portion of NAPL airphoto A15066-19)
415
Fig. 11. Fluted or drumlinized ribbed moraine deposited by ice
flowing southwestwards, southwest District of Keewatin. (Portion of airplioto NAPL A17181-76)
rare in Zone 1, where sediment of which they
normally would be formed appears to be deposited instead in irregular areas of hummocky
moraine (Fig. 12). Zone 1 is thought to mark the
area to which a long-lived Wisconsinan ice divide
migrated from the north or northwest (Cunningham and Shilts, 1977). When the area now
characterized as Zone I was east of the original ice
divide, basal ice flow velocities across it were
sufficient to erode, entrain, and transport debris, a
deduction supported by the lack of distortion of
southeastward trending dispersal trains across the
divide. The divide must have migrated through the
ice sheet, reshaping subglacial or already entrained debris according to ice flow patterns that
migrated with it. Thus, the present drumlin pattern and orientations represent flow configuration
just before the ice sheet became effectively dead.
Because of low flow velocities, debris near its
centre was never shaped into linear features.
Drumlins occur in intimate association with
ribbed moraine in Zone 2. Although the transition
from one form to the other may procede rarely in
the direction of glacial flow, the normal transition
is an abrupt lateral one (Fig. 13). In some cases
ribbed moraine ridges are drumlinized or, less
commonly, drumlins seem to be broken up into
incipient ribs (Figs. 10, 11). In rare instances
drumlins exist as individual ridges within a train
of ribbed moraine. Because of the close lateral
relationship between these two landforms, it is
thought that similar glaciodynamic conditions existed during their formation. However, drumlins
occur throughout the area formerly covered by the
Keewatin Ice Sheet, whereas ribbed moraine occurs only sporadically outward from Zone 2. Thus,
although similar glaciodynamic conditions were
probably responsible for both forms, the universal
presence of drumlins contrasts markedly with the
Fig. 12. Low relief, rounded, hummocky moraine--a typical landform in area of Keewatin Ice Divide. (a) Portion of NAPL airphoto
A17779-147; (b) oblique view near Yathkyed Lake (GSC 203540-R).
Fig. 13. Sharp lateral contact between ribbed moraine and
drumlins north of Dubawnt Lake. (Portion of NAP[_ airphoto
A 15066-1 l 0)
restricted occurrence of ribbed moraine to an area
near the ice divide. Similar relationships of ribbed
moraine, drumlins, and ice divides have been observed in Labrador (Klassen, in Shilts et al., 1987).
Elsewhere in the area formerly covered by the
Keewatin Ice Sheet, drumlins can be found at any
distance from the ice divide, except in Zone 4
where low englacial and basal debris concentrations generally precluded their formation.
Drumlin fields tend to occur in ribbon-shaped
landform trains of scale and shape similar to
glacial dispersal trains (Shitts, 1976, 1977). The
trains in many cases trail down ice from specific
rock types or from lake basins, particularly those
areas where outcrops, because of their "soft" lithology, structure, or susceptibility to frost
shattering, have caused the base of the glacier to
become heavily laden with debris. Probably the
best examples of this phenomenon are the spectacular fields of drumlins trailing away from the
poorly consolidated outcrops of the Thelon sedimentary basin. These fields, often figured in texts
and articles on landforms, are clearly related to a
heavy glacier load with enough debris to permit
the shaping of a thick sheet of basal till. As
mentioned above, the composition of these drumlins, where observed, comprises almost entirely
erratics of Thelon Sandstone and fine d•asts and
sand produced by its disaggregation.
From such observations, we conclude that
drumlin patterns were determined in large part by
the geology of the glacier bed. Why certain geological features have triggered drumlin formation
is not, however, altogether clear. Presently, we
think that geological features on the glacial bed
initiated drumlin formation in one or more of the
following ways: (1) subjected to glacial erosion,
certain types of outcrops may have provided
abundant sediment with physical characteristics
suitable for moulding or eroding into streamlined
forms; (2) sudden influxes of sediment into the
base of the glacier over certain outcrops may have
created basal dynamic conditions suitable for
drumlin formation; and (3) topographic irregularities such as basins or ridges may have altered
basal dynamics in such a way as to have created
drumlin-forming conditions.
Whatever the reason for drumlin formation, the
authors believe that beneath the Keewatin Ice
Sheet as well as elsewhere, there is often an easily
recognizable geological or topographic anomaly at
the head of a train (or field) of drumlins and that
the anomaly can often be seen to be related to or
to be the cause of particularly intense glacial
erosion and sediment production. Perhaps drumlins trailing away from lakes can be regarded as
disaggregated and shaped analogues of the " p o p
out" hills of bedrock that rest on the down ice
sides of lake basins from which they were glacially
thrust in Alberta (Fenton, in Osterkamp et al,,
1987).
For example, individual drumlins within a train
of drumlins trailing southeastward from Kaminak
Lake are composed of sandy till with a significant
component of clasts derived from the granitic
pluton that underlies the basin forming the western part of the lake. Till on the up-ice (northwest)
side of the basin, which was sampled in detail, is
not shaped into drumlins and is dominated by far
travelled, red, fine-gained debris derived from the
Thelon basin. Thus, it might be concluded (1) that
the shallow ( < 30 m) depression now occupied by
417
the lake somehow initiated basal dynamic conditions favourable for forming drumlins down ice,
or (2) that excavation of the basin provided a
large quantity of debris with physical characteristics appropriate to form drumlins. The latter,
largely geological explanation is favoured by the
authors for this drumlin train.
West of Baker Lake, clusters of drumlins with
northerly oriented axes are isolated within a field
of drumlins oriented at fight angles to them. They
are in low-relief terrain with no obvious topographic feature to protect them from the later
westward flow. In this region, northward and
westward oriented drumlins are intermingled in a
complex way, always without obvious topographic
protection of the older (northward) drumlins from
later flow. In northernmost Keewatin and Somerset Island there is preserved a thick regolith, and
glacial deposits are rare, presumably because that
part of the Keewatin (or M'clintock; Dyke, 1984)
Ice Sheet was frozen to its base and unable to
erode. It may be that this general freezing is
attenuated to freezing of increasingly small patches
of the base progressively southward. Perhaps some
fields of preserved older drumlins represent such
frozen patches which came into existence before
local flow shifted radically. Not much can be said
about the temporal relationships among the hypothesized basal freezing, dual flow directions,
and migration of the ice divide, but a study aimed
at answering these questions may provide significant insights into mechanisms of drumlin formation.
In summary, we believe that the primary control on drumlin distribution is geology of the
glacier bed, since dynamic conditions for drumlin
formation seem to have existed throughout the
Keewatin Ice Sheet. The specific locations and
shapes of drumlin fields suggest that they represent trains of glacial debris with properties appropriate for being shaped into drumlins or for
influencing the flow regime of the base of the
glacier in such a way that drumlins were formed.
The lack of drumlins in the vicinity of the ice
divide suggests that although sufficient basal glacial sediment was available for their formation,
evidenced by extensive fields of thick till in the
form of hummocky moraine, flow velocities were
not great enough to shape it into streamlined
forms. In other areas low ice flow velocities typical of an ice divide (Boulton et al., 1985) would
mitigate against erosion and entrainment of debris
from the substrate. Since Zone 1 was at one time
some distance east or south of the ancestral
Keewatin Ice Divide, however, it was loaded, because of the formerly higher basal flow velocities
on the limb of a divide, with debris that eventually
ceased to move, due to steadily diminishing velocities as the divide migrated through the ice to its
final location in the central part of Zone 1.
lce streams
Ice streams, high velocity "rivers" of ice flowing within a more sluggishly moving ice sheet,
with or without control by bed topography, have
long been proposed for the Laurentide Ice Sheet
(Hughes, 1987), based on analogies with ice
streams in the Antarctic and Greenland ice sheets.
Geological evidence of ice streaming across
Boothia Peninsula, described by Dyke (1984), first
drew attention to the potential for mapping ice
streams on the Canadian Shield. More recent work
by Dyke and Morris (1988), Hicock (1988), Kristjansson and Thorleifson (1987), and Kaszycki (in
Shilts et al., 1987) has documented geological evidence of ice streaming in the high Arctic and away
from the Hudson Bay Lowlands southwest of
Hudson Bay.
Geological evidence of ice streaming consists of
trains or belts of thick drift, commonly heavily
fluted or drumlinized. The drift is generally compositionally distinct from till in adjacent terrain in
that it contains high concentrations of far travelled
(up to 100's of km) debris with little local component. Where ice streams have been hypothesized,
the drift they deposited is generally derived from
bedrock or drift that was highly susceptible to
glacial erosion. The boundaries of the belts of
exotic debris are very sharp, the concentrations of
far travelled components dropping from several
tens of percent to zero within a very short distance, usually less than a few kilometers. The
fluting, so characteristic of thick drift in the ice
streams, has similarly abrupt boundaries and in
Ontario terminates down ice against well devel-
Keewatin Ice Divide was northwest of Baker Lake
(Fig. 14). G o o d examples of these trains of fluted,
drumlinized terrain are depicted in Figs. 7. 10, 13.
These trains trend through and away from the
easily eroded, poorly consolidated sandstones of
the Thelon Basin, just as the Ontario trains trail
down ice from and are composed of easily eroded
drift and Paleozoic bedrock of the Hudson Bay
Basin.
Drumlin fields and major dispersal trains east
of the Keewatin Ice Divide traditionally have been
considered to be the products of long term, normal ice sheet erosion and deposition (Shilts et al.,
1979; Shilts, 1980), but perhaps assumptions about
them should be reexamined. Whatever their origin,
oped, arcuate end moraines. The fluted terrain is
commonly crossed by a quasi-parallel system of
eskers which are also confined to the area of thick
drift and terminate in Ontario against end
moraines. The esker-meltwater systems are clearly
superimposed on the fluted surfaces, suggesting
that where the ice stream ceased to flow, the mass
of ice of which it was composed stagnated.
The authors do not know to what extent ice
streams are responsible for the trains of drumlins
that occur throughout the area formerly covered
by the Keewatin Ice Sheet. Many of the trains
originating in the Thelon Basin appear to be ice
streams, clearly bypassing areas of ice that protected drumlinized terrain formed when the
65°N
I
\
"-\
\
\
\ \'~,\ ~ \\\\
\"
,
.'X
Fig. 14. Trains of streamlined landforms in the Thelon basin seem to bypass an area in which older southward trends are preserved
west of Dubawnt Lake. Circle pattern represents area of low relief hummocky moraine. Dashed line represents a prominent ice
marginal position marked by numerous parallel short esker segments and coalescing ice contact fans or deltas. (From Aylsworth and
Shilts, 1989.)
419
drumlin fields east and west of the Keewatin Ice
Divide seem to trail away from specific lithologies
or topographic irregularities and are closely assod a t e d with ribbed moraine only in Zone 2.
At this writing it is difficult to relate drumlin
formation to patterns caused by ice streams, partially because the concept has been so recently
accepted that possible areas of drift deposited by
ice streams have not been investigated. The
authors, while accepting the important role of ice
streaming in creating trains of streamlined landforms, would caution that not all large scale dispersal trains or trains of landforms are related to
rapid flow of ice streams during deglaciation.
M a n y are surely the product of sustained, normal
flow over broad fronts from discrete ice divides.
Ribbed moraine
Ribbed moraine typically consists of short ( < 1
km long) sinuous ridges less than 10 m high
(Table 1). Like drumlins or flutes, the ridges occur
in fields which form ribbon-shaped patterns
elongated in the direction of ice movement (Fig.
15). The ridges are often asymmetric with a proximal (up-ice) side noticeably less steep than the
distal (down-ice) side. The authors have previously
noted (Shilts, 1977; Shilts et al., 1987) that in
places this asymmetry is so marked that the terrain has a "fish-scale" texture on high altitude
photographs (Fig. 15). In these terrains, the ridges
look like plates of sediment thrust one on the back
of the other (see also, Bouchard, 1989).
Fig. 15. Long narrow train of ribbed moraine west of Rankin Inlet. Asymmetrical profile of ridges gives "fish-scale" appearance on
photo. Ice movement was to southeast. Several scales of ribbed moraine are visible on photo. (NAPL airphoto A14703-55)
42~
I ABI.E 1
Varieties of ribbed moraine
'" Classic" Rogen moraine
Blocky ribbed moraine
Fish-scale ribbed moraine
Dune-shape ribbed moraine
C o m m o n in Zone 2; forms
landform trains in association with rogen moraine.
Commonly found on higher
or steeper areas of rogen
train or flanking rogen train
although not restricted to
these locales.
Locally important in Zone
2; forms belts or parts of
belts in association with
other varieties of ribbed
moraine. C o m m o n l y occupies lowest parts of topography.
Locally important in Zone
2; occurs in trains, generally not in association with
other types of ribbed
moraine. Developed above
and below marine limit.
Till, with heavy boulder
cover on surface.
Till, with boulders on surface.
Till or gravel without
boulder cover
Irregular, more-or-less equidimensional angular or
"'blocky" h u m m o c k s that
seem to line up in trends
transverse to ice flow creating appearance of broken
up rogen ridges. Similar
scale as associated rogen
ridges.
Short, strongly asymmetric
curvilinear ridges with platey or "fish-scale" appearance on airphotos. Long,
gentle stoss side is bounded
by a curved or scalloped
sharp lee side. Plates suggest thrusting of till beds,
one on the back of the
other.
Short, curvilinear, asymmetric ridges with transverse dune or barchan form.
Similar in size and form to
fish-scale moraine, but individual ridges are isolated
from each other on otherwise featureless till plain.
Same as for rogen moraine.
Same as for rogen moraine.
May be associated with outwash or other gravelly deposits that have been overridden and reworked.
Occurrence
Very common in Zone 2,
both above and below
marine or lake limit. Rare
in Zone 3, not found in
Zones 1 &4. Forms trains of
landforms in narrow belts
oriented parallel to regional
ice flow.
Presumed composition
Till, commonly with large
boulders on surface. Gravelly in places, without
boulder cover
a
Form and size
Sinuous ridges oriented
transverse to regional ice
flow and to adjacent drumlins; up to 3 k m but generally 1 km long, and generally less than 10 m high;
sometimes with prominent
knobs projecting above
crestline; knobs m a y be
fluted. Ridges are asymmetric with gentle stoss and
steep lee sides. These features are very similar to
rogen moraine ridges from
Lake Rogen area of Sweden
(Lundqvist, 1969). Forms of
2 or more scales may occur
together one set into the
other.
Associations
Passes laterally abruptly
into drumlin fields that
form belts with similar
orientations. May be associated with parallel belts of
flat, boulder-covered terrain. Commonly intermingled with Blocky moraine;
less commonly with Fishscale moraine.
421
Composition of ribbed moraine has been little
studied in the area of the Keewatin Ice Sheet.
With little or no population in regions where
ribbed moraine occurs, there is little chance of
observing sections through the ridges, a problem
even in the populated "rogen" areas of Sweden
and Finland. Bouchard (1989) has observed and
described sediment associations in rogen moraine
ridges excavated during construction east of James
Bay. We assume that his descriptions of internal
structural and sediment facies are typical of ribbed
moraine in similar geological terranes of the
Canadian Shield, including the ribbed moraine
west of Hudson Bay.
Numerous shallow excavations and qualitative
observations have been made of ribbed moraine in
the region between Kaminuriak Lake and Rankin
Inlet (Fig. 16). At least the upper metre of these
Fig. 17. Ribbed moraine with extensive cover of granite
boulders derived from outcrops west of Rankin Inlet (see Fig.
16) Note helicopter at extreme right for scale. (GSC 203315-F)
ridges is composed of till with a mix of far-travelled
(Dubawnt Group) and local clasts not unlike that
of till in adjacent till plains or drumlinized till
911 o
9'3 o
9'5 0
U
Inlet
i
\
LEGEND
Inlet
Rogen moraine
~
Frost-shattered
granite
~
Drumlins or flutings
Hudson Bay
50km.
62o
9,1 °
Fig. 16. Trains of ribbed moraine extend from outcrops of granite particularly susceptible to frost-shattering. Surface of ribbed
moraine is covered with granite boulders. Intervening boulder-free areas have drumlin fields. Note drumlin train extending from
Kaminak Lake. (After Shilts et al., 1987.)
422
plains. Unlike adjacent drumlinized or featureless
till plains, however, the ribs are mantled with a
cover of coarse ( > 1 m diameter) rounded boulders
which are in some places so numerous that they
touch each other, in many cases inhibiting access
to the underlying finer material of which the ridges
are built (Fig. 17). Throughout the area of ribbed
moraine west of Rankin Inlet, the coarse boulder
cover is dominated by granites that appear to have
originated in outcrops of fluorite-bearing granite
in the region southeast of Baker Lake, around
Kaminuriak Lake (Shilts et a1.,1987).
Flying over the Rankin Inlet trains at altitudes
over 1000 m it is possible to see that fields of
fibbed moraine form within trains of boulders
which head at specific outcrops in the Kaminuriak
Lake area. While these outcrops have not been
checked on the ground by the authors, they are
assumed to be highly susceptible to frost shattering
based on descriptions of geologists who mapped
the bedrock of that area. If so, they would have
provided a source of bouldery sediment (felsenmeer) to be incorporated by a glacier advancing
from the ancestral Keewatin Ice Divide at the
onset of the last glaciation.
Not all of the boulder trains in the Kaminuriak
region are formed into ribbed moraine, however.
Along the southwestern edge of the region of
alternating fields of boulder-free drumlinized till
plain and bouldery ribbed moraine, there are wide
strips of terrain where dense concentrations of
boulders lie on thin till or bedrock with virtually
no constructional irregularities (Fig. 18). It is
thought that the glacial sediment associated with
the boulders is too thin to be shaped into ridges.
This assumption is probably also applicable to
other boulder streams in the Rankin Inlet field
which likewise have no associated ribbed forms.
Ribbed moraine investigated north of Aberdeen
Lake, just north (down-ice) of the Thelon Basin, is
composed almost wholly of disaggregated Thelon
sandstone. Because of the lack of fines in the
sandy debris, these ridges are cut by frost polygons and lack a cover of vegetation (Fig. 8).
Farther north (lat. 65 °15', long. 98030 ') ribs that
formed down ice from an Aphebian orthoquartzite
are covered by a dense layer of orthoquartzitic
boulders. The ridges themselves are composed of
l-ig. 18. Boulder field forming tilt plain with no ~ssociated
ribbed forms, northeast of Kaminak Lake. Figure For scale
~(}S( 204422-H)
till with a significant amount of Thelon Basin
debris, similar to till in adjacent till plains.
Some areas of ribbed moraine in southern
Keewatin, particularly northwest of Thaolintoa
Lake, appear to be composed of gravelly sediments with few surface boulders (Fig. 19). Where
so constituted, the ridges have forms that depart
considerably from the classic rogen shape. In some
cases they are similar to aeolian forms, such as
barchan dunes, but are clearly the product of ice
deposition.
Fields of ribbed moraine often seem to bear
evidence of two phases of formation. In some
cases a second phase is suggested by development
of drumlin-shaped nodes on the high points of a
ridge (Fig. 11). More common are areas where two
scales of ribbed moraine coexist. In such areas, the
Fig. 19. Crescentic, barchan-like ribbed moraine formed on
gravel and sand northwest of Thaolintoa Lake; ice flowed from
right to left. (GSC 203540-0)
423
i)
Fig. 20. North of Aberdeen Lake, smaller scale ribbed moraine
appears to be superimposed on larger ribbed moraine of the
classical rogen form, note esker. (NAPL airphoto A15390-31)
larger set generally has classical rogen form while
the smaller set may have a broken up, "blocky"
appearance. In some places north of Aberdeen
Lake the later, smaller ridges have a "fish-scale"
appearance and appear to be oriented more or less
parallel to a nearby esker (Fig. 20). Perhaps late
glacial movement toward the esker conduit (Shilts,
1984), induced by thermal erosion of the conduit's
ice walls, reoriented ice flow toward the esker,
forming the heavy load of englacial debris into
rogen ridges roughly parallel to the esker. In this
area, however, the general orientation of all sizes
of individual ridges and of trains of ridges is
clearly related to northwestward regional ice flow
and not to local eskers. The eskers trend as much
as 20 o to the regional ice flow direction and cut
through the ribs, indicating deposition after their
formation (Fig. 20).
Perhaps the most important and constraining
aspect of ribbed moraine development in the area
formerly covered by the Keewatin Ice Sheet is the
distribution of ribbed terrain largely within a
horseshoe-shaped zone around the south end of
the Keewatin Ice Divide (Shilts and Aylsworth, in
Shilts et al., 1987). A similar zonation is described
around the Labrador-Nouveau Quebec Divide
(Klassen, in Shilts et al., 1987; Bouchard, 1989),
and the Glacial Map of Canada (Prest et al., 1968)
shows what appears to be a zone of ribbed moraine
around the former centre of the Newfoundland ice
cap. Minor ribbed moraine is present in New
Brunswick, south of the Miramichi ice divide, and
extensive belts of rogen moraine are well known
around the Fennoscandian ice divide. In Europe,
however, zonation such as that reported by the
authors has not been described.
It is clear that there is an intimate relationship
of ribbed moraine not only to specific lithologic or
topographic characteristics of the glacier bed, but
also to other bedforms and to the centre of the ice
sheet. Whereas fields of drumlins may occur anywhere that substrate conditions are favourable,
from the centre to the edge of the Keewatin Ice
Sheet, fibbed moraine terrain is concentrated in a
zone, 200-250 km wide, around the central ice
divide zone. It is rare elsewhere in the ice sheet.
Any explanation of the genesis of ribbed moraine
must accord with these geographical and geological constraints.
In Sweden and in Quebec (Lundqvist, 1969;
Bouchard, 1980) it has been suggested that rogen
moraine forms only above marine limit and in
depressions in the landscape. In the region covered
by the Keewatin Ice Sheet, ribbed moraine is
equally well-developed above and below marine
limit (Fig. 21). In fact, its occurrence seems to be
largely independent of any water body, fresh or
marine. Likewise, fibbed moraine is developed
across the landscape with little topographic con-
i
Fig. 21. Ribbed moraine is well developed both above (south)
and below (north) the marine limit beach south of Schultz
Lake. (Portion of NAPL airphoto A15404-102)
424
trol, albeit this part of the Canadian Shield is
rather flat. A good illustration of the lack of
topographic control is in the Deep Rose Lake area
(NTS 66G) north of Aberdeen Lake, where northwesterly trending fields of ribbed moraine cross
drainage basins with no deflection and are well
developed on the low interfluences between
streams and lakes. In some parts of Zone 2 and
beyond the area of heavy rib development, in
Zone 3, ribbed moraine can be found preferentially in depressions that trend parallel to ice
flow.
In Sweden, Lundqvist (1969) has developed a
widely reproduced model showing the relationship
of drumlins and rogen moraine to be one in which
one form passes through a transition zone into the
other down ice. In this model the drumlins occupy
the higher parts of the landscape (zones of extensive glacial flow) and rogen moraine occupies the
depressions (zones of compressive flow). Such
down-ice transitions are rare in Keewatin Ice
Sheet. Generally the rogen/drumlin transition is
an abrupt lateral change from one type of terrain
to the other.
Why there should be such apparent discrepancies in the relationship of drumlinized and ribbed
moraine terrain between the Fennoscandian and
Keewatin ice sheets is not presently known, but
the answer probably lies in a difference in geology
and topography between the areas.
To summarize, the morphology and physical
characteristics of rogen moraine seem to be similar
in areas where they have been studied in Canada
and Europe. The composition of sediment that
forms ribbed moraine in the area covered by the
Keewatin Ice Sheet seems to be related to specific
geological features on the glacier bed, including
topographic expression, lithology and structure of
source outcrops and nature of the unconsolidated
debris that was concentrated at or near the glacier's
sole. Such relationships b e t w e e n ice dynamics and
the geology of the glacial bed are not known to
have been studied regionally for other ice sheets.
The distribution of ribbed moraine in a clearly
defined zone around the centre of ice flow is
c o m m o n to sectors of the North American ice
sheets and may be similarly developed around the
Fennoscandian ice sheet.
In contrast to these similarities, there are scveral
striking differences among the features described
around the various European and Canadian ice
sheets. The down-ice transition from drumlins to
rogen moraine and back to drumlins, etc., is rarely
noted in the area of the Keewatin Ice Sheet, where
sharp, lateral transitions are the norm. Ribbed
moraine in Fennoscandia and Q u e b e c / L a b r a d o r
(Bouchard, 1989) is generally found in depressions
while ribs in Keewatin are often strewn across the
countryside in a pattern largely independent of
topography. It should be remembered, though,
that the relief under the Keewatin Ice Sheet is
slight, being less than 10 m locally and less than
50 m, on the average, regionally. The relief of the
beds of the Fennoscandian and L a b r a d o r / Q u e b e c
ice sheets is considerably sharper, possibly contributing to localization of rogen moraine in valleys. Finally, the rogen areas of Sweden and of
Quebec have been developed exclusively above the
limits of late and postglacial marine submergence,
whereas ribbed moraine in Keewatin is equally
well developed above and below marine limit (Fig.
21).
Any hypothesis that purports to explain the
genesis and distribution of ribbed or rogen moraine
must provide a satisfactory explanation of not
only the discrepancies in their setting in various
ice sheets, but the similarities in their spatial distribution, as well.
Eskers
Eskers are not glacier bedforms in the sense
that they were formed by running water and not
by moving ice. Nevertheless, because of their intimate association with some fields of bedforms,
their mode of formation can both constrain and
enhance interpretations of bedform genesis.
Eskers of the Keewatin Ice Sheet have been
shown to form an integrated drainage system with
tributaries as high as 4th order (Shilts et al., 1987).
The drainage radiates, as do the trains of landforms, from the outer boundary of Zone 1 which
surrounds the Keewatin Ice Divide (Fig. 3). The
authors (Shilts et al., 1987) have suggested that
this pattern represents the trace of late glacial
surface drainage of the Keewatin Ice Sheet, eskers
425
themselves representing the depositional reaches
of basal ice tunnels, formed where surface drainage
plunged through crevasses or moulins some dis-
tance from the backwasting glacier edge, tapping
the basal, subglacial and englacial sediment load.
To have formed such massive ridges, exceeding 30
m height in some places, esker streams must have
incorporated basal sediment from a broad zone
adjacent to the esker tunnel, either by ice flow
toward thermally eroding tunnel sides or through
subglacial tributaries. Although esker tunnels were
probably short at any given time, they must have
migrated by thermal erosion headward, following
the trace of the drainage net on the surface of the
ice. Conditions within the ice sheet must have
been fairly inactive in the vicinity of the short,
headward migrating tunnels because the esker
ridges are undeformed and cross or are set into
streamlined features that reflect late glacial flow
patterns (Fig. 22a, b). From this model for esker
deposition, discussed more thoroughly by Shilts
(1984), we conclude that the Keewatin Ice Sheet
retreated principally by wasting from the surface
downward, notwithstanding the fact that sedimentological evidence for such wasting is expressed by esker deposits clearly formed at the
glacier's base. The widespread integrated surface
drainage system which the esker patterns reflect
suggests that while it was still quite large, much of
the Keewatin Ice Sheet was below the equilibrium
line during deglaciation. This further suggests that
during its retreat phase, the ice was quite inactive,
which not only accounts for preservation of the
esker ridges, but partially m a y account for the
preservation of easily destroyed internal sediment
masses such as those preserved along thrust planes,
i.e. sediment that forms ribbed moraine ridges.
Conclusion
Fig. 22. (a) Esker draped over drumlin at fight angles to
drumlin trend just south of Keewatin/Manitoba border. Note
frost polygons suggestiveof sandy, gravellytexture of sediment
of which drumlin is composed. (b) esker superimposed on
drumlin field suggesting sluggish or no ice flow at time of
deposition, west of Rankin Inlet. (a: GSC 204658-B; b: portion
of NAPL airphoto A14302-17)
From the discussion above, it is obvious that
we do not have an easily testable model for formation of glacial bedforms beneath the Keewatin Ice
Sheet. Nevertheless, by compiling these forms on
a map that covers most of one sector of the
Laurentide Ice Sheet, we have been able to place
certain constraints on others' hypotheses and to
propose some suggestions about genesis based
largely on spatial relationships of the bedforms.
We conclude that drumlins and ribbed moraine
form at the base of actively flowing ice under
42~
similar dynamic conditions. For either landform
to exist, there must have been enough sediment
available in the base of the glacier to leave or form
a feature large enough to be recognizable, a common-sense observation, but one that is often overlooked when discussing the dynamics of the continental ice sheet (i.e. White, 1972; Sugden, 1977;
Hughes et al., 1985). The presence or absence of
sufficient load can be related confidently to the
geology of the glacier bed and has little to do with
regionally changing dynamics of the ice-water
system. Likewise, given sufficient load, it is evident that whether drumlins form or whether ribbed
moraine forms in a certain area is somehow a
function of the physical nature of the load which
is, again, related to geology of the source outcrops.
Whether the physical characteristics come into
play after the sediment has been released from the
ice by lodgment or meltout processes and is being
reshaped by basal drag, streamlining, etc. or
whether the nature of the load while entrained
changes the behaviour of the basal part of the ice
is unclear.
From our observations, we feel that for ribbed
moraine, the load has somehow promoted internal
thrusting of coherent blocks of entrained debris
bands and ice, whereas drumlins may reflect
moulding of plastic subglacial debris or erosional
streamlining of both the unconsolidated glacial
substrate and bedrock. Probably the streamlined
features are a combination of both moulding and
of erosion, both hypotheses being equally entertained in the literature. Here as elsewhere where
we have observed streamlined features, there is no
evidence that drumlins owe their existence to subglacial fluvial processes or have any relationship
to glaciofluvial systems, other than a quasi-parallel orientation with them. In fact, the observation
that many eskers cross drumlin fields at nearly
fight angles to their orientation suggests that conditions producing streamlining and those pertaining to subglacial fluvial drainage are separated in
time and circumstance. This is not to say that
pre-existing glaciofluvial deposits cannot be
sculpted into streamlined forms by overriding ice,
as apparently happened in the Thaolintoa ribbed
moraine field, described previously.
Finally, the general occurrence of drumlins and
eskers throughout the sediment-rich portton.~ of
the Keewatin Ice Sheet, from Zone 1 to its edge, is
difficult to reconcile with the restriction and intimate association of ribbed moraine with these
forms almost exclusively in Zone 2. The further
observation that such a zonal relationship exists
around other ice divides, at least in L a b r a d o r /
Nouveau Quebec and Newfoundland, leads us to
think that some condition changed or existed in
this zone throughout or at some specific time
during the existence of the Keewatin Ice Sheet.
Although glaciologists may be able to propose
scenarios that would generate such conditions,
based on physics of the ice-water system in a fully
developed, vigorously flowing continental glacier,
we think it more logical and simple to assume that
a change of conditions within the ice sheet occurred when it had melted back approximately to
the outer edge of Zone 2. A possible explanation
may be that the Keewatin Ice Sheet was reactivated abruptly by a general deterioration of
climate with associated alimentation of the glacier
at the time when it had shrunk to the approximate
dimensions of the outer edge of Zone 2. Such a
climatic event would probably be hemispheric in
scale, if not world wide. The effects of such reactivation would be similar around other ice divides still in existence at this late(?) date, and
would be rare around older centres of ice dispersal
that did not experience reactivation, such as those
in the mountainous and maritime regions around
the outer margins of the Laurentide ice sheet.
Although reactivation of sectors of the North
American ice sheets, resulting in readvances and
surges, occurred during various climatic changes
or because of marine or lacustrine drawdown
throughout the period of deglaciation, only around
the last ice divides, where glacier profiles were low
and the ice thin, would such reactivation have
"shattered" the glacier into stacked thrust plates
of ice, englacial, and subglacial sediment. If such
shattering occurred in parts of the glacier where
concentrations and physical properties of sediment and ice conspired to cause internal thrusting,
melting under subsequent stagnant conditions
would have exposed sediment beds draped one
over another just as they were stacked in the ice.
Certainly the "fish-scale" moraine and general
427
a s y m m e t r y o f r i b b e d m o r a i n e ridges suggest that
their form derives f r o m m e l t i n g out, with little
d e f o r m a t i o n , of s o m e such i n t e r n a l structure. T h e
r e a s o n for s h a t t e r i n g o n l y late in the K e e w a t i n Ice
Sheet's h i s t o r y m a y b e r e l a t e d to the low g r a d i e n t
a n d thin ice t h a t existed as a result of d o w n w a s t ing of a largely s t a g n a n t ice mass. Steeper gradients a n d thicker ice d u r i n g r e a d v a n c e s that t o o k
p l a c e when the ice sheets were " h e a l t h i e r " , earlier
in the d e g l a c i a t i o n cycle m a y n o t have p r o m o t e d
s h a t t e r i n g a n d stacking of b a s a l thrust plates of
ice a n d sediment. Even if thrust stacking d i d t a k e
p l a c e in these earlier events, c o n t i n u e d internal
flow m a y have o b l i t e r a t e d it.
T h e a b r u p t i n n e r t e r m i n a t i o n of Z o n e 2 m a y
m a r k the i n n e r limit of critical ice thickness or
flow velocity necessary to p r o d u c e brittle shear at
the base of the ice. T h e b o u l d e r s a n d gravelly
s e d i m e n t s that are associated with till in r i b b e d
m o r a i n e trains b o t h m a y have p r o m o t e d shattering a n d thrust stacking b y their n o n - p l a s t i c b e h a v i o u r b e n e a t h or within the b a s a l ice. W h e r e
these c o m p o n e n t s were absent, the reactivated ice
m a y have flowed m o r e n o r m a l l y , f o r m i n g d r u m lins o r i e n t e d in directions c o n t r o l l e d b y ice sheet
c o n f i g u r a t i o n (shapes of its edges a n d l o c a t i o n s of
centres of outflow) at the onset of reactivation.
A f t e r the flow that was g e n e r a t e d b y the reactivation of thin ice ceased, the internal structures
m e l t e d out o r were u n c o v e r e d with little dist u r b a n c e , preserving the r i b b e d form. T h e d u r a tion o f flow need n o t have b e e n long, p o s s i b l y
b e i n g m e a s u r e d in weeks, m o n t h s or years. Also, it
m a y have b e e n cyclical, a c c o u n t i n g for the m u l t i p l e scales a n d forms of r i b b e d moraine, s u p e r i m p o s e d one on the other. Cyclical reactivation p o s sibly c o u l d a c c o u n t for the d r u m l i n i z a t i o n of
r i b b e d m o r a i n e ridges, a l t h o u g h this m a y equally
well r e p r e s e n t s o m e t r a n s i t i o n a l flow regime or
s e d i m e n t c o m p o s i t i o n with p h y s i c a l characteristics
i n t e r m e d i a t e b e t w e e n those t h a t p r o d u c e d one
b e d f o r m o r the other, d e p e n d i n g on m i n o r changes
in flow conditions.
I n conclusion, we feel that the m o d e l s p r o p o s e d
here, which are largely b a s e d o n the r e g i o n a l dist r i b u t i o n o f b e d f o r m s of the K e e w a t i n Ice Sheet,
explain, o r at the very least, shed light on c o n d i tions a n d m e c h a n i s m s for b e d f o r m d e v e l o p m e n t
b y c o n t i n e n t a l ice sheets. T h e r e r e m a i n m a n y details to c o n f i r m or explain, b u t with the f r a m e w o r k p r o v i d e d b y recent m a p p i n g these m a y b e
c o n f i r m e d at a rate o n l y c o n s t r a i n e d b y the availab i l i t y o f funds a n d i n t e r e s t e d researchers.
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