JOURNAL OF QUATERNARY SCIENCE (2008) 23(3) 249–272
Copyright ß 2008 John Wiley & Sons, Ltd.
Published online in Wiley InterScience
(www.interscience.wiley.com) DOI: 10.1002/jqs.1141
Landform and sediment imprints of fast glacier
flow in the southwest Laurentide Ice Sheet
DAVID J. A. EVANS,1* CHRIS D. CLARK2 and BRICE R. REA3
1
Department of Geography, University of Durham, Durham, UK
2
Department of Geography, University of Sheffield, Sheffield, UK
3
Department of Geography, University of Aberdeen, Aberdeen, UK
Evans, D. J. A., Clark, C. D. and Rea, B. R. 2008. Landform and sediment imprints of fast glacier flow in the southwest Laurentide Ice Sheet. J. Quaternary Sci., Vol. 23
pp. 249–272. ISSN 0267-8179.
Received 8 January 2007; Revised 19 April 2007; Accepted 9 May 2007
ABSTRACT: Evidence for former fast glacier flow (ice streaming) in the southwest Laurentide Ice
Sheet is identified on the basis of regional glacial geomorphology and sedimentology, highlighting the
depositional processes associated with the margin of a terrestrial terminating ice stream. Preliminary
mapping from a digital elevation model of Alberta identifies corridors of smoothed topography and
corridor-parallel streamlined landforms (megaflutes to mega-lineations) that display high levels of
spatial coherency. Ridges that lie transverse to the dominant streamlining patterns are interpreted as: (a)
series of minor recessional push moraines; (b) thrust block moraines or composite ridges/hill–hole pairs
constructed during readvances/surges; and (c) overridden moraines (cupola hills), apparently of thrust
origin. Together these landforms demarcate the beds and margins of former fast ice flow trunks or ice
streams that terminated as lobate forms. Localised cross-cutting and/or misalignment of flow sets
indicates temporal separation and the overprinting of ice streams/lobes. The fast-flow tracks are
separated by areas of interlobate or inter-stream terrain in which moraines have been constructed at the
margins of neighbouring (competing) ice streams/outlet glaciers; this inter-stream terrain was covered
by more sluggish, non-streaming ice during full glacial conditions. Thin tills at the centres of the
fast-flow corridors, in many places unconformably overlying stratified sediments, suggest that widespread till deformation may have been subordinate to basal sliding in driving fast ice flow but the
general thickening of tills towards the lobate terminal margins of ice streams/outlet glaciers is
consistent with subglacial deformation theory. In this area of relatively low relief we speculate that
fast glacier flow or streaming was highly dynamic and transitory, sometimes with fast-flowing trunks
topographically fixed in their onset zones and with the terminus migrating laterally. The occurrence of
minor push moraines and flutings and associated landforms, because of their similarity to modern
active temperate glacial landsystems, are interpreted as indicative of ice lobe marginal oscillations,
possibly in response to seasonal climatic forcing, in locations where meltwater was more effectively
drained from the glacier bed. Further north, the occurrence of surging glacier landsystems suggests that
persistent fast glacier flow gave way to more transitory surging, possibly in response to the decreasing
size of ice reservoir areas in dispersal centres and also locally facilitated by ice-bed decoupling
and drawdown initiated by the development of ice-dammed lakes. Copyright # 2008 John Wiley &
Sons, Ltd.
KEYWORDS: fast glacier flow; Laurentide Ice Sheet; glacially streamlined landforms; glacial landsystems; palaeo-ice streams.
Introduction
Ice streams and fast-flowing outlet glaciers are the main arteries
by which ice sheets and glaciers transport their mass to the
ocean for calving, or to lower elevations for melting, and their
activity exerts a dominant role on ice sheet mass balance. Their
central role in the dynamics of ice sheets is widely recognised
* Correspondence to: D. J. A. Evans, Department of Geography, University of
Durham, South Road, Durham DH1 3LE, UK.
E-mail: [email protected]
(see Bennett, 2003, for a review) and for palaeo-ice sheets we
have moved from early speculations that they should have
existed (e.g. Denton and Hughes, 1981) to a documentation of
the evidence they leave behind (e.g. Patterson, 1997, 1998;
Ó Cofaigh et al., 2002; Dowdeswell et al., 2006; Jennings,
2006; Wellner et al., 2006) and the reporting of numerous
palaeo-ice stream tracks (see Clark and Stokes, 2003, for a
review). Our understanding of Laurentide Ice Sheet palaeoglaciology has been significantly advanced by the mapping of ice
flow lines, the differentiation of fast-flowing outlets, ice streams
and their lobate outer margins and the deciphering of the
temporal evolution of cross-cutting subglacial landforms
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(e.g. Dyke and Prest, 1987; Boulton and Clark, 1990). Our
knowledge of the subglacial bed conditions conducive to fast
ice flow (e.g. Engelhardt et al., 1990; Bell et al., 1998;
Blankenship et al., 2001) has also improved to the extent that
we can make predictions of where palaeo-ice streams are likely
to have operated.
In an assessment of the potential for fast glacier flow in North
America, Marshall et al. (1996) highlighted the western plains
as a prime location (>90% likelihood) for the former existence
of ice streams or surge lobes within the Laurentide Ice Sheet.
This can be tested by the search for palaeo-ice stream signatures
in the geomorphic and sedimentary record. Most palaeo-ice
streams found thus far have been based on evidence of
subglacial geomorphology (e.g. mega-scale glacial lineations;
Clark, 1993, 1994; Stokes and Clark, 1999, 2001, 2002) or via
associations with large topographic troughs and trough mouth
fans indicative of rapid and voluminous sediment delivery (e.g.
Ó Cofaigh et al., 2003). The signature of fast glacier flow or ice
streaming in terms of terrestrial ice-marginal suites of sediments
and landforms is much less well known and based largely on
interpretations of Quaternary sedimentary sequences. For some
locations along the southern margin of the Laurentide Ice Sheet,
Patterson (1997, 1998) and Jennings (2006) have described the
nature of ice-marginal morainic evidence for a number of
prominent lobes, which are argued to be the product of ice
streaming. Within the British–Irish Ice Sheet, Evans and Ó
Cofaigh (2003) reported the nature of ice-marginal evidence
along the western flank of the former Irish Sea Ice Stream (see
also Scourse, 1991; Hiemstra et al., 2006). In western Canada,
Evans et al. (1999) employed the landsystem approach to draw
comparisons between ancient landform–sediment associations
on the prairies and glacial landsystems at the margins of
modern fast-flowing and surging outlet glaciers of Vatnajokull
in Iceland. In such modern settings, the marginal thickening of
subglacial tills at the snouts of fast-flowing outlet glaciers has
been demonstrated (Evans and Hiemstra, 2005), lending
support to theoretical models of till deposition patterns in
former ice sheets (Boulton, 1996a, 1996b).
In order to assess the potential for palaeo-ice stream or fast
glacier flow/surging activity in the southwest sector of the
Laurentide Ice Sheet, this paper provides a regional assessment
of the glacial geomorphology and glacial geology of the
southern part of the province of Alberta, Canada. This evidence
is significant because it documents the depositional processes
associated with the margins of terrestrially terminating ice
streams. During ice sheet recession from western Canada some
ice streams appear to have terminated in lobes (cf. Jennings,
2006), whose landform and sedimentary characteristics
resemble those of a surging glacier landsystem (Evans et al.,
1999). During the LGM, Laurentide ice streams flowed into the
province of Alberta from the north and west, extending to the
ice sheet limit in northern Montana (Fullerton and Colton,
1986; Fullerton et al., 2004a, 2004b). We here present
evidence for fast glacier flow and possible palaeo-ice streams
in the southeastern and east-central parts of the province of
Alberta and assess the implications of this evidence for ice sheet
palaeodynamics. Like Tulaczyk (2006), we regard the concept
of fast glacier flow as a relative one in the context of regional
palaeo-ice flow indicators, whereby bundles of long streamlined forms (megaflutings to mega-lineations) juxtaposed with
areas devoid of such features represent relatively fast and slow
former ice flow, respectively. We also use Swinthinbank’s
(1954) and Bentley’s (1987) widely adopted definition of an ice
stream as ‘part of an inland ice sheet in which the ice flows
more rapidly than . . . the surrounding ice’. Additionally, we
acknowledge that the margins of terrestrial ice streams will
terminate as lobate snouts (Jennings, 2006) and therefore
Copyright ß 2008 John Wiley & Sons, Ltd.
produce many of the characteristic landform–sediment assemblages observed at smaller-scale modern outlet glaciers.
Study area and previous work
The landscape of Alberta in western Canada displays the
prominent imprints of Late Wisconsinan glaciation by ice
moving eastwards from the Rocky Mountain Cordilleran Ice
Sheet and westwards from the Laurentide Ice Sheet (see
Klassen, 1989, for review). Coalescence of these two ice masses
in the high plains of the province resulted in the deflection of
ice flow in a south-southeasterly direction and the deposition of
the Foothills Erratics Train along the suture zone (Stalker, 1956;
Jackson et al., 1997; Rains et al., 1999). Advance and recession
of Laurentide ice throughout the Quaternary always resulted in
the development of large proglacial lakes and their connecting
spillways, because westerly ice flow dammed the natural
drainage of the region (St Onge, 1972; Evans and Campbell,
1992, 1995; Evans, 2000). The systematic mapping of
Quaternary geology and glacial landforms in Alberta, as
summarised by the maps of Prest et al. (1968), Shetsen (1987,
1990) and Fulton (1995), has facilitated the identification of
generalised patterns of former ice flow and major moraine
systems relating to the southwest margin of the Laurentide Ice
Sheet (Dyke and Prest, 1987). Coupled with till lithology,
glacial geomorphology has been employed in the reconstruction of individual ice lobes or streams based on traditional
concepts of subglacial streamlining by ice and concomitant till
deposition (Shetsen, 1984; Andriashek and Fenton, 1989; Evans
et al., 1999; Evans, 2000, 2003). In contrast, the glacial
geomorphology of southern and central Alberta (Fig. 1) has
been critical to the erection of a subglacial megaflood
explanation of regional glacial geomorphology since the initial
work of Shaw (1983). This theme has developed largely
because of the availability of digital elevation models (DEMs)
from which interpretations of landform genesis have been made
based on form analogy, prompting its proponents to suggest that
subglacial sediments and bedrock are truncated by an erosional
surface cut during the megaflood. Such interpretations have
been based on localised case studies (e.g. Shaw and Kvill,
1984; Shaw et al., 1996, 2000; Munro and Shaw, 1997; Beaney
and Shaw, 2000; Munro-Stasiuk and Shaw, 2002) which have
been used to support proposals for regional subglacial
megaflood pathways/corridors through Alberta (Shaw et al.,
1989; Rains et al., 1993).
The glacial geomorphological and sedimentological evidence presented in this paper is entirely consistent with the
concept of subglacial streamlining by glacier ice. Corridors of
smoothed and fluted terrain terminating at moraines and
stacked sequences of till and glacitectonised materials are
predicted by the conceptual models of fast glacier flow and
palaeo-ice streaming (e.g. Boulton, 1996a, 1996b; Stokes and
Clark, 1999, 2006; Canals et al., 2000; Clark and Stokes, 2001,
2003; Ó Cofaigh et al., 2002, 2005; Jansson and Glasser, 2005;
Dowdeswell et al., 2006; Jennings, 2006; Wellner et al., 2006)
which are, in turn, informed by observations on modern ice
sheets (e.g. Alley et al., 1986; Bindschadler et al., 1987;
Blankenship et al., 1987; Shabtaie et al., 1987; Tulaczyk et al.,
1998, 2000). In the absence of a marine environment in which
ice stream margins calve and deposit substantial glacimarine
depocentres like trough-mouth fans (Vorren and Laberg, 1997),
an ice stream is likely to fan out to produce a lobate snout (Clark
and Stokes, 2003), where subglacial sediments are deposited as
tills in down-ice thickening wedges (cf. Boulton, 1996a, 1996b;
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LAURENTIDE ICE SHEET FAST GLACIER FLOW
251
actions of deformation and ploughing due to fast glacier flow,
and to acknowledge the localised erosional imprint of
subglacial and proglacial meltwater.
Methods
Figure 1 Location maps of the study area, showing the province of
Alberta, Canada, and the coverage of the study area within Alberta. The
area outlined by a dotted line is the area mapped in this study and
depicted in Fig. 3
Jennings, 2006). The recession of the lobate margins of
fast-flowing trunk ice is documented by suites of subglacial
and marginal landform–sediment assemblages (landsystems)
that are identical to those being produced by active temperate
glaciers in modern glacierised catchments (e.g. Hart, 1999;
Evans and Twigg, 2002; Evans, 2003; Evans and Hiemstra,
2005). Although evidence for substantial subglacial meltwater
discharges is widespread in the form of tunnel valleys (e.g.
Sjogren and Rains, 1995; Evans and Campbell, 1995; Evans,
2000), it has been clearly demonstrated that there is no need to
invoke megafloods to explain the glacial landsystems in
southern Alberta (Evans et al., 1999; Clarke et al., 2005; Benn
and Evans, 2006). For these reasons we consider it appropriate
to interpret the subglacial landform and sediment record of the
region as the imprint of substrate streamlining by the combined
Copyright ß 2008 John Wiley & Sons, Ltd.
Regional-scale geomorphology of the southern and east central
part of Alberta was assessed utilising a shaded-rendition DEM
of the province, produced from the Canadian Digital Elevation
Dataset. Although this source is of relatively low resolution it is
suitable for broad-scale, reconnaissance style mapping of
regional patterns of glacial lineations, moraines and meltwater
channels. For the areas of suspected fast flow, digital versions of
the DEM were processed to allow the production of
topographic profiles.
The availability of a DEM for the whole province (Fig. 2) has
allowed us to map the distribution of glacial lineaments and to
identify areas of smoothed terrain within which glacial flutings
are widespread. These ice flow parallel features contrast with
ice flow transverse, multiple ridged topography that displays
markedly less linearity than the flutings and drumlins but in
some cases appear to have been partially streamlined or
superimposed by flutings (Fig. 3). This regional-scale mapping
must be regarded as a preliminary assessment of glacial
landform distribution; the spatial and temporal relationships
require testing by future localised studies. For example, in order
to assess and portray the finer details of particular landform
assemblages we have mapped glacial lineaments on 1:63
360-scale aerial photograph mosaics covering critical areas
(see boxes on Fig. 3).
The sedimentary record of glaciation is represented in
stratigraphic logs and cross-sections from critical locations,
derived from the literature and from new field research. This
enables the comparison of ice flow directions recorded in till
fabrics with those represented by glacially streamlined landforms; we interpret flutings and drumlins as subglacially
moulded and streamlined features rather than meltwater scour
features as implied by the megaflood theory, because landforms
often have cross-cutting and/or misaligned relationships and
can be directly compared to active glacier ice-moulded
features in modern glacial environments where glacier forelands uncovered by historical ice recession have never been
affected by subglacial megafloods (e.g. Benn and Evans, 2006).
We now present details of the landforms and sediments from
southern Alberta and provide interpretations of their genesis in
the context of former subglacial processes.
Descriptions of landforms and sediments
Megageomorphology and lineations (flutings)
Lineament mapping highlights two prominent corridors of
streamlining (Fig. 3), hereafter referred to as the west and
central corridors so that comparison can be made with previous
work that has referred to the corridors as ‘west’ and ‘central
lobes’ of glacier ice (Shetsen, 1984; Evans, 2000). These are
>500 km long swaths of smoothed topography with subtle,
often patchy lineations stretching from the Athabasca basin in
the north to the Milk River Ridge in the south. Other, smaller
swaths of streamlined terrain also occur and are cross-cut and/
or misaligned, indicating temporal separation. The streamlining
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Figure 2 Solar-shaded DEM of topography, compiled from Canadian Digitial Elevation Data (CDED) at a spatial resolution of 55 m: (a) an overview of
the streamlined terrain and its relationship to general topography; (b) central and southern Alberta (produced by the University of Alberta) showing the
corridors of streamlined terrain (named ‘west’ and ‘central’ in the text for comparison with previous work), outlined by solid black lines and containing
smoothed and fluted topography. The largest moraine complexes previously recognised in the literature are also identified and are referred to in this
paper as ‘interlobate (inter-corridor) terrain and hummocky terrain’ in order to convey both our landscape definitions and previous descriptions of such
areas in the literature
is visible as corridors of smoothed topography within which
streamlined landforms (flutings, fluted ridges) display high
levels of spatial coherency and fan out in southern Alberta to
terminate at series of minor transverse ridges; the latter are
organised as inset concentric ridges arranged en echelon and
with their crests aligned at right angles to the flutings (see
section on minor transverse ridges, below).
The flutings located in the corridors of smoothed topography
are often widely spaced and weakly developed, although some
pockets of densely spaced examples do occur. Significantly,
larger flutings emanate from smoothed ridges that lie transverse
to the trend of the streamlining (see section on transverse ridges,
below). A remarkable 65 km long megafluting complex has
been described previously by Evans (1996, 2000; Fig. 4). This
feature has developed down flow from an overridden transverse
ridge and based on sediment exposures appears to have a
pre-existing esker network at its core. The megafluting complex
fans out and terminates at a prominent arc of thrust bedrock
Copyright ß 2008 John Wiley & Sons, Ltd.
blocks south of the Red Deer River (Evans, 1996, 2000).
Although most of these thrust blocks have been streamlined and
were therefore interpreted as glacially overridden by ice when
it terminated further south in the vicinity of the Alberta/
Montana border, the innermost thrust blocks in the arc are
sharp-crested and demarcate the southern limit of the
megafluting complex. South of the sharp-crested thrust ridges
lies a complex braided esker network, interpreted by Evans
(1996, 2000) as the continuation of the streamlined esker to the
north.
Smaller-scale flutings are evident at the aerial photograph
scale and have been mapped for several representative areas in
association with minor transverse ridges (see minor transverse
ridges below; cf. Evans, 2003; Evans et al., 2006a). When
viewed over an area typically covered by one or more aerial
photograph mosaics the flutings and end/push moraines form
misaligned and cross-cutting assemblages, manifest either as
superimposed flutings or adjacent fluting fields whose
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Figure 3 Regional map of glacially streamlined, ice flow parallel landforms and ice flow transverse ridges, major meltwater channels and eskers
based on mapping from DEM. Also included are the locations of the main axes of the buried valley thalwegs in the province based on maps produced
by the Alberta Research Council. Boxes demarcate the areas used in this study for localised mapping with aerial photographs. Abbreviations and
numbers in black dots locate figures and section logs
Copyright ß 2008 John Wiley & Sons, Ltd.
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Figure 4 Satellite image of the megafluting complex in the Red Deer River drainage basin (Evans, 1996, 2000). Note the initiation of the flutings at the
overridden thrust moraines to the north of the image (very prominent in Fig. 2(b) lower). The overridden moraines at this location are situated on a col
between two preglacial drainage basins and comprise glacitectonised bedrock. The megafluting complex terminates at a broad arc of thrust bedrock
ridges, south of which an esker network continues around and into Lake Newell (flat area at bottom centre of image). This figure is available in colour
online at www.interscience.wiley.com/journal/jqs
orientations are significantly different. It has been explained by
previous researchers (e.g. Benn and Evans, 2006; Evans et al.,
2006a) that such relationships cannot be explained by
contemporaneous ice flow deviations or by substrate erosion
by subglacial sheetfloods.
ridges are predominantly composed of glacitectonically folded
and thrust bedrock and sediment (Fig. 7). Tsui et al. (1989) have
previously reported that thrust bedrock appears to be most
common at the margins of the buried valleys or on the uplands
between them. This explains many specific locations of
streamlined transverse ridges in the corridors of smoothed
topography (e.g. Figs 4 and 8).
Large transverse ridges
Clearly visible on the DEM are numerous large-scale ridges,
which are aligned transverse to the predominant orientations of
the lineations/flutings. These features are either streamlined
(i.e. smoothed and often adorned with flutings; Figs 4 and 5) or
non-streamlined, the latter manifesting as sharp-crested ridges
(Fig. 6). They also mostly occur as multiple-crested features and
are concave in plan form, with their limbs pointing up-ice, as
defined by various ice flow indicators (i.e. ice flow in the region
is from the north or northeast). Large transverse ridges
predominantly occur on the down-ice side of preglacial or
buried valleys (Fig. 3). Exposures reveal that the transverse
Copyright ß 2008 John Wiley & Sons, Ltd.
Minor transverse ridges (push moraines)
Densely spaced arcuate ridges of only a few metres relief occur
over large areas of southern Alberta. Individual ridges are
characterised by multiple lobate plan-forms and intricate
crenulations and saw-tooth morphologies, typical of modern
push moraines (Price, 1970; Matthews et al., 1979; Evans and
Twigg, 2002; Evans, 2003). Fluting fields both within the
smoothed corridors and on terrain between corridors fan out so
that individual flutings meet the minor transverse ridges at right
angles. Exposures through the ridges display massive diamicton
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Figure 5 Satellite image showing details of overridden moraine located immediately south of Calling Lake, north of Athabasca, north-central Alberta
(located on Fig. 19(a)). The moraines likely represent material displaced from the lake, thereby constituting a hill–hole pair. Flutings continue from the
moraine crests in a down-ice direction
or folded/thrust stratified sediments (Fig. 9). Due to these
characteristics, the minor transverse ridges have previously
been interpreted as minor push moraines indicative of active
temperate glacier recession (Westgate, 1968; Evans et al.,
1999; Evans, 2000, 2003).
Remarkably well-preserved sequences of minor transverse
ridges occur at the southern limits of the smoothed corridors
and often drape the surfaces of the intervening terrain or
interlobate/inter-corridor terrain (see section on ‘interlobate’
(inter-corridor) terrain and hummocky moraine belts, below;
Fig. 10). For example, the southern portion of the ‘west corridor’
is characterised by an impressive sequence of inset arcuate
minor ridges and associated flutings, particularly well illustrated by the area around Frank Lake and Granum (Evans et al.,
1999; Evans, 2003; Fig. 10(c)). Similarly, the southern limit of
the ‘central corridor’ is demarcated by densely spaced minor
transverse ridges that display crenulated and bifurcating
patterns in plan view (Figs 3 and 10(a), (b) and (d)), a suite
of landforms previously termed the ‘Lethbridge Moraine’ by
Stalker (1977; cf. Horberg, 1952; Stalker, 1962). The western
lateral margin of the ‘central corridor’ is also demarcated by a
prominent ridge complex (Figs 3 and 11), comprising a 470 km
long belt of hummocks that display numerous discontinuous
and closely spaced ridges. This feature abuts the eastern
boundary of the McGregor interlobate ‘moraine’ belt (see
below) and forms a continuation of the Lethbridge Moraine to
the south. Localised mapping from aerial photographs of the
area around McGregor Lake identifies discrete fields of
Figure 6 Extract from an aerial photograph mosaic of the Neutral Hills, east-central Alberta, showing the clearly delineated multiple ridges of a
proglacially thrust bedrock mass. This moraine and many similar examples in eastern Alberta record readvances by ice lobes/streams flowing into
Alberta from the northeast during overall ice sheet recession
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Figure 7 Sketch of glacitectonic disturbance structures in bedrock at Smith Coulée, west of Lake Pakowki, southern Alberta. The structures include
low-angle thrusts, overfolds and liquefaction features associated with bedrock shearing. Former ice flow direction was to the southeast, away from the
viewer. This is the largest of many exposures in southeastern Alberta, where till forms only a thin veneer on thrust and fluted bedrock. This figure is
available in colour online at www.interscience.wiley.com/journal/jqs
glacially streamlined features (flutings) terminating at series of
inset transverse ridges (moraines) organised in broad arcuate
bands (Fig. 12; Benn and Evans, 2006; Evans et al., 2006a).
These moraine arcs are locally lobate in plan form and are
clearly superimposed in some areas. Also evident are
misaligned and cross-cutting flutings, represented either by
superimposed features or adjacent fluting fields whose
orientations are significantly different.
‘Interlobate’(inter-corridor) terrain and
hummocky terrain
The large areas that lie between corridors of smoothed/
streamlined terrain in Alberta contain a range of glacial
depositional landforms with no consistent orientation (Fig. 2).
Previous research on the glacial geomorphology of the southern
part of the province has referred to these areas as assemblages
Figure 8 Examples of glacitectonised bedrock located on the high terrain that separates the preglacial drainage basins of southern and central Alberta
and often forming the cores of transverse ridges: (a) Lowden Lake (from Tsui, 1987); (b) northwestern Sullivan Lake (from Tsui, 1987), where the thrust
bedrock blocks have been modified by subglacial streamlining to the extent that the local land surface is characterised by flutings; (c) Chin Coulée,
where a glacitectonically stacked succession lies on an upland ridge and has been draped by recessional push moraines. See Fig. 3 for locations
Copyright ß 2008 John Wiley & Sons, Ltd.
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LAURENTIDE ICE SHEET FAST GLACIER FLOW
Figure 9 Exposure through a minor push moraine near Lethbridge
showing contorted laminae indicative of glacier pushing. This figure is
available in colour online at www.interscience.wiley.com/journal/jqs
of ‘hummocky moraine’ separating corridors of thinner, fluted
till cover, for example the Viking/Suffield Moraine and the
McGregor/Buffalo Lake Moraine (Bretz, 1943; Shetsen, 1987,
1990; Evans, 2000). These have traditionally been regarded as
either the end moraines of the ice sheet in Alberta (e.g. Johnston
and Wickenden, 1931; Bretz, 1943) or ‘interlobate’ moraines
(e.g. Shetsen, 1984; Evans, 2000). An alternative view is that the
‘moraine’ is erosional in origin and was produced by subglacial
megafloods, with the corollary that the ‘moraines’ are not
moraines sensu stricto (Munro and Shaw, 1997; Munro-Stasiuk
and Sjogren, 2006). This view reflects a general dissatisfaction
with the use of the term ‘hummocky moraine’ in glacial
research, because it is associated with ice stagnation. We
therefore adopt the more descriptive term ‘hummocky terrain’
proposed for these non-streamlined tracts by Munro and Shaw
(1997) and Munro-Stasiuk and Sjogren (2006), but the
recognition of fast ice streamlining of corridors between the
hummocky terrain tracts renders them interlobate or intercorridor in nature. Although this hummocky terrain is often
mapped as till, there are considerable outcrops of folded and
thrust bedrock in most ridges that are observable on the DEM
(Fig. 13). The orientations of these transverse ridges are variable
but individual clusters tend to lie at right angles to the most
adjacent flutings that run up to the ‘interlobate’ terrain. Many
areas of such minor flutings are visible on aerial photographs
and they lie sub-parallel or orthogonal to the larger lineations
within neighbouring corridors of smoothed terrain. In some
cases, particularly at the southern end of the ‘McGregor
Moraine’, the minor flutings terminate at inset sequences of
minor push moraines whose plan forms demarcate the former
lobate margins of glacier snouts receding from the ‘interlobate’
terrain (Evans et al., 2006a).
Subglacial drainage pathways
Large numbers of eskers occur throughout southern and central
Alberta (Shetsen, 1987, 1990) but they are difficult to identify
on satellite imagery and DEMs unless they are large, complex
networks. Consequently, only the largest esker networks are
depicted in Fig. 3 but smaller examples are mapped at a larger
scale (Figs 10 and 12). Large braided esker networks occur
along the centres of the west and central corridors in southern
Alberta (Fig. 3), documenting the concentration of subglacial
drainage along the centrelines of these smoothed tracks.
Numerous glacial meltwater channels in the region document
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257
the erosional impact of large volumes of meltwater (Shetsen,
1987, 1990) but the differentiation of subglacial vs. proglacial
flood tracks still needs to be resolved before subglacial drainage
networks can be reconstructed (e.g. Evans, 1991, 2000; Evans
and Campbell, 1995).
In addition to the features identified above, the smoothed
corridors of terrain in Alberta have been alternatively
interpreted as subglacial flood pathways by Shaw et al.
(1989) and Rains et al. (1993). This interpretation is contentious
and fundamentally questions the traditional view that flutings
and other subglacially streamlined forms are moulded by the
passage of glacier ice. It is also an interpretation that has been
widely rejected based on its inability to provide the least
number of assumptions (cf. Clarke et al., 2005; Benn and Evans,
2006; Evans et al., 2006a). Moreover, it is unclear as to why the
megaflood proponents prefer a meltwater erosional interpretation for the streamlined corridors when such landscapes have
been unequivocally explained by glacier ice streamlining in
modern glacier systems (e.g. Hoppe and Schytt, 1953; Benn,
1994; Evans and Twigg, 2002; Evans and Hiemstra, 2005).
However, the smoothed corridors are cross-cut by unequivocal
subglacial meltwater channels, the best-documented examples
being in the Coronation-Spondin area on the bed of the central
corridor (Sjogren and Rains, 1995; 528 N, 1118 300 W on Fig. 3).
Till stratigraphy
In addition to the occurrence of transverse ridges immediately
down-ice of buried valleys, our assessments of till stratigraphy
and clast A/B plane macrofabrics identify localised stacking
and thickening of tills and plucked bedrock in such settings
(Figs 14 and 15(a)). The thickest and most complex sequences
of tills, glacitectonites and bedrock mega-rafts in southern
Alberta crop out in the vicinity of the Lethbridge moraine,
specifically around the cities of Lethbridge and Medicine Hat,
as depicted by our vertical profile logs for Evilsmelling Bluff,
Pavan Park, Fort Whoop Up, Cameron Ranch and Milk River
and Little Sandhill Creek (Fig. 15(a, A–F); see locations on
Fig. 3). Evilsmelling Bluff is a particularly important location
because it contains maximum Late Wisconsinan ages for the
complex tills of the area, demonstrating that discrete packages
of subglacial sediment have been delivered to the area during
the last glacial cycle. The thick till sequences in this part of
southern Alberta are in marked contrast to tills at the centres of
the smoothed corridors (Fig. 15(b)). For example, in the vicinity
of Carolside Dam/Berry Creek at the centre of the central
corridor (Fig. 15(b, C); CDBC on Fig. 3) only a thin veneer of till
overlies bedrock or preglacial sediment. Similarly, relatively
thin tills occur in the west streamlined corridor at Red Deer city
and at Big Bend in Edmonton (Fig. 15(b, A and B); RDC and BB
on Fig. 3). In other parts of Alberta, the thickening of
stratigraphically older till units also appears to be related to
the location of buried valleys but additionally coincides with
overridden transverse ridges and glacitectonised stacked
sequences in ‘interlobate/inter-corridor terrain’. For example,
in the Cooking Lake area, east of Edmonton, hummocky terrain
is associated with folded and stacked tills and associated
sediments; radiocarbon-dated lacustrine sediments provide
maximum ages for the upper Late Wisconsinan tills (Fig. 15(c, A
and B); Jennings, 1984). Similarly, at Morrin Bridge, in the
centre of the ‘McGregor/Buffalo Lake Moraine’ or inter-corridor
terrain, tills and glacitectonised bedrock are stacked at the
centre of large transverse ridges (Fig. 15(c, C); MB on Fig. 3).
Deciphering multiple ice flow phases and their associated
glacier margins is aided considerably in areas where extensive
information on till stratigraphy is available. This is illustrated by
J. Quaternary Sci., Vol. 23(3) 249–272 (2008)
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258
JOURNAL OF QUATERNARY SCIENCE
Figure 10 Details of glacial landforms taken from aerial photograph mosaics of selected areas of southern Alberta. Maps show recessional push
moraines (crenulated black lines), minor flutings (straight black lines or dot–dash outlined areas on 10(c), major meltwater channels of possible
subglacial and proglacial origin (barbed broken lines) and eskers (en-echelon arrow heads). Water bodies are coloured black and where these are
closely spaced they record localised ice stagnation: (a) the Milk River area of southern Alberta. The general pattern of moraine and fluting distribution
records the recession of a lobate ice sheet margin towards the northwest. Ice stagnation topography is particularly noticeable along the north-facing
scarp of the Milk River Ridge and Del Bonita Tertiary monadnocks, where compressive ice flow presumably stacked up debris-rich basal ice sequences
so that supraglacial reworking was significant in moraine construction; (b) the area immediately west of Pakowki Lake, southern Alberta (the southern
arm of the lake is visible on the eastern extremity of the map). The general pattern of moraine and fluting distribution records the recession of a lobate
ice sheet margin towards the northwest; (c) the Frank Lake area, south of Calgary (after Evans et al., 1999). Moraines record the northward recession of
the centre of the HPIS back up the valley of the Little Bow River; (d) aerial photograph of the recessional push moraines around the southern arm of Lake
Pakowki (Energy, Mines and Resources, Canada). Note the moraine ridge bifurcations that document partial moraine overprinting during ice recession
the case study of Andriashek and Fenton (1989) from the Cold
Lake area of east-central Alberta, wherein multiple tills of
different provenance are seen to thicken towards possible
former margins. The macrofabric assessments of these tills
(summarised by principal flow vectors) also allow their
correlation with cross-cutting lineations (Fig. 16 upper). The
mega-scale glacial lineations mapped in the Cold Lake area
using DEM (Fig. 3) reveal a complex cross-cutting pattern that
documents the flow of competing ice streams. This complexity
of ice flow phases is recorded in the clast macrofabrics and till
lithology in the area (Andriashek and Fenton, 1989). We have
reproduced the till stratigraphy along one of many borehole
transects reported by Andriashek and Fenton (1989) in order to
Copyright ß 2008 John Wiley & Sons, Ltd.
demonstrate the relationships between till provenance and
thickness and subglacial streamlining (Fig. 16 lower).
Interpretations of landforms and sediments
Megageomorphology (streamlined corridors)
and megaflutings
We interpret the corridors of smoothed topography as zones of
former subglacial streamlining produced by fast glacier flow
J. Quaternary Sci., Vol. 23(3) 249–272 (2008)
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LAURENTIDE ICE SHEET FAST GLACIER FLOW
259
palaeo-ice streaming as defined by Swithinbank’s (1954)
classification of ice streams. We therefore hereafter refer to
the west and central corridors as the palaeo-fast-flow tracks of
the High Plains Ice Stream (HPIS) and Central Alberta Ice
Stream (CAIS), respectively. In the north of the study area,
around Lac La Biche/Cold Lake, the switching of flow directions
by palaeo-ice streams is recorded by cross-cutting lineations
and superimposed till sheets (Andriashek and Fenton, 1989; see
below).
Large transverse ridges
Figure 11 Part of an aerial photograph mosaic showing the hummocky moraine tract damming Little Fish Lake and Handhills Lake and
forming the western margin of the CAIS where it abutted the higher
terrain of the Hand Hills Tertiary monadnock. Note that faint flutings
trending NW–SE lie outside the moraine to the west, documenting an
earlier flow phase by ice from the west. Little Fish Lake in the southwest
corner is 6 km long
within the southwest margin of the Laurentide Ice Sheet. The
corridors of terrain smoothing and lineation development are
similar to those cited as evidence of palaeo-ice stream activity
in offshore surveys (e.g. Canals et al., 2000; Ó Cofaigh et al.,
2005) and in terrestrial landform associations (e.g. Dyke and
Morris, 1988; Clark and Stokes, 2001, 2003; Stokes and Clark,
1999, 2001, 2003). The occurrence of long flutings or
mega-lineations is also indicative of fast glacier flow (Bluemle
et al., 1993; Stokes and Clark, 2002; Clark and Stokes, 2003),
because they are found in locations where former fast glacier
flow and ice stream flow is unequivocal (Canals et al., 2000;
Anderson et al., 2002; Evans and Rea, 2003; Ó Cofaigh et al.,
2005; Wellner et al., 2006). Moreover, the emergence of fluted
till surfaces from beneath the receding margins of fast-flowing
temperate glacier margins that are known to deform and
slide over their beds, clearly demonstrates that substrate
moulding and ploughing are crucial to the development
of subglacially streamlined landforms (e.g. Benn, 1994;
Boulton et al., 2001; Tulaczyk et al., 2001; Evans and Twigg,
2002; Clark et al., 2003; Evans, 2003). The subglacial
landforms in the smoothed corridors probably represent the
glacier bed during a single flow phase, thereby constituting a
‘rubber stamp’ imprint of palaeo-ice stream activity (Clark and
Stokes, 2003).
The areas of fast glacier flow delineated by the subglacially
streamlined corridors were previously identified by till pebble
lithology by Shetsen (1984), who referred to two major ice
lobes (‘west lobe’ and ‘central lobe’). These ‘lobes’ were
differentiated from an eastern ice lobe or lobes flowing in to
Alberta from Saskatchewan. Our mapping has confirmed the
existence of the two separate fast-flow ice tracks represented by
the west and central corridors. Because ice was clearly flowing
faster in these streamlined corridors than over the intervening
‘interlobate/inter-corridor terrain’, they constitute evidence for
Copyright ß 2008 John Wiley & Sons, Ltd.
Large transverse ridges are interpreted as thrust block moraines
or composite ridges and hill–hole pairs (sensu Aber et al.,
1989), an origin that is consistent with their multiple crests and
internal glacitectonic disturbance. They were constructed
either during initial ice advance and then overridden, as
evidenced by their appearance as streamlined/smoothed
features or cupola hills (sensu Aber et al., 1989) or during
ice recession (sharp-crested features). An unequivocal proglacial construction is evident in the recessional transverse
moraine ridges (Fig. 6), because glacitectonic structures clearly
cross-cut all sedimentary and bedrock structures. Landsystems
associated with recessional thrust ridges in the Lloydminster
area of central Alberta (LL on Fig. 3) indicate a surge by an ice
lobe flowing into Alberta from the northeast (Evans et al., 1999).
Flutings emanating from smoothed transverse ridges appear
to be the product of grooving of the bed (Fig. 5). This is best
explained by the subglacial process of ice keel ploughing
(Tulaczyk et al., 2001; Clark et al., 2003), whereby the
transverse ridge, which likely originated as a moraine during
ice advance, increased local ice stream bed roughness once it
was overridden. This then initiated differential pressure melting
and the scoring of the ice sole. The fast-flowing ice then
grooved the substrate downflow of the overridden moraine.
To the east of the CAIS track, the lobate margins of palaeo-ice
streams were responsible for the construction of large thrust
block moraines during their recession from eastern Alberta, as
documented by the sharp-crested composite ridges in Cretaceous bedrock such as the Neutral Hills (Fig. 6). These
impressive proglacial thrust structures lie at the termini of
narrow corridors of densely spaced flutings.
Based on the interpretations presented so far, we can provide
an explanation of the landform associations in that part of the
CAIS track that lies in the Red Deer River drainage basin and
comment on their significance for palaeo-ice stream activity.
Specifically, we are referring here to the megafluting complex
and associated transverse ridges and eskers, as described
previously by Evans (1996, 2000; Fig. 4). Streamlining of the
esker complex must have occurred during a substantial
readvance of the CAIS. This interpretation explains the
occurrence of the esker complex in a non-streamlined form
to the south and in a streamlined form to the north of the
sharp-crested thrust blocks in the vicinity of the Red Deer River
(Fig. 4). It also accounts for the juxtaposition of streamlined and
non-streamlined thrust bedrock blocks in locations conducive
to substrate compression (i.e. at the margins of buried valleys;
e.g. Evans and Campbell, 1995; Evans, 2000; see buried valley
thalwegs in Fig. 3). For example, the streamlined thrust blocks
located to the south of the Red Deer River were overridden by
ice when it terminated further south in the vicinity of the
Alberta/Montana border, whereas the sharp-crested thrust
blocks in the same area demarcate the southern limit of the
megafluting complex and lie to the north of a deglacial esker
complex. The sharp-crested thrust blocks are therefore
J. Quaternary Sci., Vol. 23(3) 249–272 (2008)
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260
JOURNAL OF QUATERNARY SCIENCE
Figure 12 Sequences of push moraines and flutings mapped from aerial photographs of the area around McGregor Lake, south-central Alberta, an
area previously mapped from DEMs by Munro-Stasiuk and Shaw (1997). Note the overprinting of end/push moraines with slightly offset alignments and
the localised lobation that coincides with topographic hollows. To the north of Queenstown it appears that a re-entrant was produced in the ice margin
during its recession along the McGregor preglacial valley, likely in response to accelerated calving in a proglacial lake (from Evans et al., 2006a)
recessional features constructed by proglacial thrusting at the
same time that the megafluting complex was being produced
by subglacial streamlining. Prior to this readvance, the esker
network was being constructed beneath the CAIS as it
downwasted/receded from the Lethbridge Moraine to a
location north of the Red Deer River.
Minor transverse ridges (push moraines) and
minor flutings
We reaffirm previous interpretations of the inset sequences of
minor transverse ridges, at which flutings terminate, as end
moraines pushed up by the receding lobate margins of
Figure 13 Exposure through glacitectonically folded bedrock in the Bittern Lake hill–hole pair, located in the interlobate moraine belt south of
Edmonton. This figure is available in colour online at www.interscience.wiley.com/journal/jqs
Copyright ß 2008 John Wiley & Sons, Ltd.
J. Quaternary Sci., Vol. 23(3) 249–272 (2008)
DOI: 10.1002/jqs
LAURENTIDE ICE SHEET FAST GLACIER FLOW
Figure 14 Section through the multiple tills and bedrock rafts at Fort
Whoop Up, Lethbridge (see also Fig. 15(a)). Lower grey till with light
grey Cretaceous bedrock rafts is overlain by a brown till and lightbrown glacilacustrine sediments recording final deglaciation. This
figure is available in colour online at www.interscience.wiley.com/
journal/jqs
fast-flowing ice streams. Significant here are the occurrences of
folded and thrust sediments in many ridge exposures, especially
in the suite of inset push moraines referred to previously as the
‘Lethbridge Moraine’. Given the large number of individual
moraine ridges in the area and their largely uninterrupted
extension across southern Alberta, together with the fact that
the Late Wisconsinan terminal moraine lies to the south in
Montana (Fullerton and Colton, 1986; Fullerton et al.,
2004a,b), we suggest that the term ‘Lethbridge Moraine’ is
inappropriate and misleading, especially as it has been
associated with the ‘classical’ Late Wisconsinan glacial limit
(Stalker, 1977).
The association of closely spaced push moraines and flutings
at the margins of contemporary fast-flowing temperate glaciers
has been used by Evans (2003), Benn and Evans (2006) and
Evans et al. (2006a) as a landsystems analogue for the glacial
geomorphology of large areas of southern Alberta, and in the
absence of any alternative, more viable interpretation of the
fluting and end moraine landform association in the region we
adopt their process-form model here.
Areas of misaligned fluting fields and associated end
moraines are routinely interpreted as glacier flow sets (Clark,
1997) and the superimposition of flow sets is often identifiable
in cross-cutting flutings (Dyke and Morris, 1988; Boulton and
Clark, 1990; Clark, 1993; Kleman et al., 2006; Evans et al.,
2006a). Cross-cutting flow sets and their associated misaligned
end moraines have been interpreted as the subglacial imprint of
receding lobate glacier margins (e.g. Clark, 1997) and this
interpretation satisfactorily explains similar landform assemblages evident in southern Alberta while being fully compatible
with observations from modern glacier margins (Krüger, 1994,
1995; Evans et al., 2006a).
‘Interlobate’ (inter-corridor) terrain
Evans (2000) previously highlighted the fact that the fast-flow
tracks of the HPIS and CAIS are separated by large moraine
belts (cf. Johnston and Wickenden, 1931; Bretz, 1943; Fig. 17)
referred to in this paper as ‘interlobate’ or inter-corridor terrain
and containing glacial landforms described as ‘hummocky
terrain’. An ‘interlobate’ or inter-ice stream origin for the major
hummocky terrain belts of southern Alberta can be upheld. The
Copyright ß 2008 John Wiley & Sons, Ltd.
261
orientations of the large transverse ridges in the interlobate
terrain lead us to conclude that they were constructed by the
lobate margins of neighbouring (competing) ice streams as they
advanced into southern Alberta (Evans, 2000). During full
glacial conditions, when the southwest Laurentide Ice Sheet
margin occupied Montana, fast flow within the ice sheet was
concentrated in the HPIS and CAIS, resulting in the larger and
longer flutings in the smoothed corridors compared to the
minor flutings associated with recessional push moraines over
the interlobate terrain. Although we have mapped most of the
large transverse ridges in the interlobate moraine as initial
advance/overridden features, many are classified as such
merely because they are not of obvious recessional age and
could have been constructed throughout the period of ice
occupancy.
The minor flutings visible on aerial photographs document
ice flow into the interlobate belts at the margins of the fast-flow
zones during overall recession. Many of these flutings terminate
at inset push moraines (Evans et al., 2006a), thereby
constituting a landsystem typical of active recession by the
uncoupling ice lobes. The nature of the overprinting and
misalignment of flutings and minor push moraines precludes a
subglacial megaflood origin.
Subglacial drainage pathways
Although we disagree with the interpretation of the smoothed
corridors as former subglacial megaflood pathways, as
proposed by Rains et al. (1993) and Shaw et al. (1996), among
others, we acknowledge the subglacial origin of many channels
on the beds of the palaeo-ice stream tracks, as depicted on
Fig. 3. Additionally, towards the southern end of the fast glacier
flow tracks in Alberta, esker networks visible on the DEM and
associated glacifluvially eroded bedrock record drainage that
was driven by the subglacial hydraulic gradient. The
association of these glaciofluvial landforms with recessional
push moraines and their integrated fluting fields is typical of
deglaciated forelands at the margins of active, temperate
glaciers (Evans and Twigg, 2002).
In contrast, the alignment of the Coronation-Spondin
channels (Sjogren and Rains, 1995) transverse to former ice
flow in the CAIS is difficult to explain assuming a normal
subglacial hydraulic gradient. Significantly, a large portion of
the fluted bed in this vicinity is characterised by a thin till cover,
indicating that basal sliding rather than till deformation was
driving fast ice flow (cf. Stokes and Clark, 2003). Therefore,
ice-bed decoupling may have been driven by increased
subglacial meltwater activity. Misaligned drainage routes
(i.e. oblique to ice stream flow) have been found elsewhere
in the context of ice streams, usually ascribed to post-streaming
changes in ice configuration. For example, Clark and Stokes
(2001) found eskers at oblique angles on the M’Clintock
Channel Ice Stream that related to a later phase of activity once
the ice stream had closed down. Significant with respect to the
deglaciation of Alberta is the production of numerous, large
ice-dammed lakes (St Onge, 1972; Evans, 2000), whose
decanting waters would have been flowing along the ice sheet
margin and possibly penetrating the bed in some locations. It is
conceivable that subglacial flood tracks were produced
submarginally by proglacial lake water where one lake dumped
into another via a subglacial pathway, similar to the drainage
pathway of the Graenalon floods beneath Skeidararjökull in
Iceland (Bjornsson, 1992; Tweed and Russell, 1999).
Other prominent meltwater erosional features are demonstrably ice-marginal. For example, decanting proglacial lake
J. Quaternary Sci., Vol. 23(3) 249–272 (2008)
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JOURNAL OF QUATERNARY SCIENCE
(a)
C S S G D
Pavan Park (PP)
Evilsmelling Bluff (EB)
0
Fort Whoop Up (FW)
after Stalker & Wyder (1983) and Proudfoot (1985)
Sm / Fm
C S S G D
2
0
4
2
C
S
S
G
D
0
2
4
6
Dml (brown) + attenuated &
distorted sand lenses
6
8
4
Fl
8
10
N
10
12
EB2
12
14
6
Fl(d)
8
14
16
16
Dml (grey-brown) & sand / silt
lenses (banded)
18
10
2.0
FL(d)
18
Dmm (grey-brown)
12
20
20
2 .0
1. 0
3 .0
22
22
N
14
PP2
24
Dml (grey-brown)
+ sand lenses
24
2. 0
1. 0
16
N
26
N
26
28
28
32
EB1
Gm
Fl
2.0
1.5
20
Organics (Evilsmelling
Band = 24.5 - 28.6ka)
metres
38
PP1
4 .0
5 .0
Fl / Fm + involutions
EB2
22
2. 0
1. 5
1. 0
0.5
Dmm (brown)
36
1.0
2.
3 .0 0
34
36
0.5
N
Fl
1.0
2.0
26
6.
5. 0
4. 0 0
3. 0
2.0
48
Fl(d)
30
54
50
EB1
32
Gm lag
1 .0
52
1. 0
2. 0
3. 0
+ ++
+
+
34
PP3
56
0
58
Empress Group sediments
36
%
50
52
100
Sand
54
60
56
Grey-brown Dml
64
60
66
38
Clay
44
PP3
+
Bedrock raft
FWU1
Dmm + bedrock & sand rafts
0
68
70
100
Sand
Bedrock raft
+
Yellow Dml in greybrown Dmm
Silt
Clay
+
Dmm (grey-brown, banded) +
bedrock rafts
+
Lower grey-brown Dmm
Brown Dmm(s) / Dml
Zone of yellow Dml
++
+ ++
+
46
FWU1
66
+ +
+
+
+
+
+
+ +
%
50
Dmm(s) / Dml (brown) +
bedrock rafts
Empress Fm (diapirs at top)
72
2.0
2 .0
42
Grey Dmm (base)
70
64
6.0
4.0
FWU2
40
Brown Dmm (base)
68
62
+ +
+
+
+ +
+ +
+
+
+
+
+ +
+ +
Silt
Dmm (grey) +
bedrock rafts
62
58
10. 0
8.0
+
+
+
+
1.0
50
1.0
48
Dmm / Dml (grey-brown) + contorted
bedrock rafts & sand lenses
++
+ +
+
+
28
46
46
N
40
44
44
FWU2
+
+
PP1
40
42
1. 0
2. 0 0
3. 0
4. 0
5.
+ + +
+
+
24
38
42
N
+
+
Sand & gravel lens
34
metres
32
metres
18
Dml (grey-brown)
+shale stringers
PP2
30
30
74
76
48
+ + +
78
50
Empress Fm
72
52
B
A
C
N
Cameron Ranch (CR)
S
S
G
D
C S S G D
2
0
4
1
N
Little Sandhill Creek (LSC)
Milk River (MR)
0
C
1 .0
C
54
0
5.0
4.0
3.0
2.0
S
S
G
D
Fm
Gm
1. 0
2
1. 0 0
2.
3.
1. 0
4
Dmm (brown, banded)
2
6
Dmm (brown)
MR4
CR2
3
8
5. 0
3. 0
1. 0
6
4 .0
3.0
2.0
1.0
4
10
BL
Sm
Dml(p) (brown)
CR2
14
6
2.0
Fm
2 .0
1 .0
5
12
LSC3
N
8
metres
Dmm (brown)
10
5. 0 0
4.
Dmm + sand lenses
& intraclasts
N
MR4
0
LSC3
N
12
14
1.0
N
metres
MR3
Sm / Fm
metres
18
Sm
22
LSC2
Dmm (brown)
18
+
+
+
CR1
CR1
Bedrock rafts
Sr
Dmm + sand lenses
LSC1
11
24
4
0
26
%
50
MR2
BL / Dml(p)
12
100
6.0
5.0
4.0
3.0
2.0
1.0
20
MR3
10
22
+
LSC2
N
4.0
2. 0
9
3. 0
2. 0
1. 0
Dml (grey-brown, banded) +
disaggregated bedrock rafts
16
N
8
4.0
3.0
+ +
20
24
3. 0
2. 00
1.
7
16
2.0
MR2
+
+ +
1.0
LSC1
Sand
Brown Dmm
28
3
13
Silt
Clay
30
Dml (grey, banded) + attenuated
lenses and rafts
Inter-till Sm / Fm (top)
14
Grey-brown Dml
15
32
Dmm (grey)
Grey Dml (base)
0
%
50
38
2.0
1.0
100
Scour fill
Silt
4
Empress Fm
4.0
3.0
1
34
36
N
Dml (grey)
Dml (grey)
MR1
2
Clay
1.0
MR1
3
2
40
1
D
E
F
Figure 15 Examples of till sequences from southern and central Alberta: (a) from areas overlying, or immediately down-ice of preglacial valleys; (b)
from areas in fast ice flow trunks; (c) from proposed overridden glacier margins. Sites are located on Fig. 3. Facies codes are from Evans and Benn (2004)
and clast A/B plane macrofabrics are on Schmidt equal area stereoplots. In some situations clast fabrics do not dip in the direction of former ice flow but
in the opposite direction due to the influence of local topography (buried valleys) over which the ice traversed. Note the correlation between ice flow
directions indicated by macrofabrics and local fluting directions depicted in Figs 3 and 10(a)
waters excavated ice-marginal channels such as Kipp, Etzicom
and Seven Persons coulées (Evans, 2000). Together with
flutings, overridden moraines and push moraines, the glacifluvial features of southern Alberta constitute a landform
assemblage that is entirely consistent with deglaciation by
active temperate glacier lobes (Evans and Twigg, 2002; Evans,
2003).
Copyright ß 2008 John Wiley & Sons, Ltd.
Till stratigraphy
Glacitectonically stacked transverse ridges and locally
thickened tills and bedrock mega-rafts on the down-ice side
of buried valleys (Figs 14 and 15(a)) strongly suggest that glacier
ice was excavating pre-existing sediment sequences from
valley fills. This implies that the valley fills and the bedrock
J. Quaternary Sci., Vol. 23(3) 249–272 (2008)
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(b)
N
Red Deer City (RDC)
N
N
Carolside Dam / Berry Creek (CDBC)
Big Bend, Edmonton (BB)
C
S
S
G
S
S
G
D
0
0.5
D
C
Fm
0
2 .0
1 .0
2. 0
1.5
4
4
metres
N
metres
+
+
+
+
1. 0
2.0
1.5
1.0
0.5
0.5
Dmm (shale-rich) + BL
+ sand lenses
8
N
Sp (in scour fills)
10
RDC2
Sp
1. 5
1. 0
0 .5
12
Sr / Gm scour fills
Dmm (grey-brown)
BB2
Sr / Gm scour fills
Dmm (brown) + Sr lenses
Empress Fm. (involutions at top)
0.5
+
20
Empress Fm
3. 0
2. 0
1. 0
14
+
+
1 .0
BB2
18
1.0
BB3
Dmm (brown) + sand & gravel lenses
BB1
Gm (scour fill)
Dmm / Dml (brown)
Bp
Shear zone in Empress Fm
16
CDBC
Sp
+
Dmm + distorted sand lenses
Dml / Dmm (brown)
14
2. 5
2. 0
1. 5
D
CDBC
RDC1
RDC1
Dml / Dmm + sand lenses
8
12
G
0.5
6
BB3
6
10
2
Fl(d) + water escape structures
4
Fl (d)
S
Dmm
3.0
2
0.5
1.5
S
0
1.0
1. 0
2
metres
C
RDC2
N
40
30
20
10
22
10
20
A
BB1
B
C
Figure 15 (Continued)
cliffs of preglacial valleys are crucial to sediment generation at
the glacier bed. Although this thickening of subglacial material
occurs on the down-ice sides of most preglacial valleys, there is
also a net thickening of till units, glacitectonites and associated
bedrock mega-rafts around the southern margin of the CAIS.
Such large-scale patterns of till thickening are expected in
situations where fast-flowing trunk glaciers deliver subglacial
Copyright ß 2008 John Wiley & Sons, Ltd.
material to an ice sheet margin over a significant period of time
(Evans et al., 2006b). Although this ice-marginal thickening of
till is predicted by models of subglacial deformation (e.g.
Boulton, 1996a, 1996b), the net effect of all subglacial
processes is to advect material towards glacier margins where
till units become stacked (e.g. Evans and Hiemstra, 2005; Evans
et al., 2006b). In the Lethbridge area, the interbedding and
J. Quaternary Sci., Vol. 23(3) 249–272 (2008)
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Figure 16 Stratigraphy from the Sand River map area: (upper) vertical profile log of stratigraphic succession at Cold Lake showing the superimposed
tills and the palaeo-ice flow vectors as determined by clast macrofabrics; (lower) north–south transect through the Sand River map area showing the
Quaternary stratigraphy. Only the tills are shade coded and labelled. Broken lines indicate boundaries of till subunits (after Andriashek and Fenton,
1989). The transect is located on Fig. 3
Copyright ß 2008 John Wiley & Sons, Ltd.
J. Quaternary Sci., Vol. 23(3) 249–272 (2008)
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LAURENTIDE ICE SHEET FAST GLACIER FLOW
265
dictated by the localised ‘continuity’ of subglacial materials
(Piotrowski et al., 2004; Evans et al., 2006b).
The existing data on multiple tills and former ice flow
directions provide an excellent relative chronology of events in
the Cold Lake area. The earliest till, the Bronson Lake
Formation, is restricted to the buried valleys of the area, and
the ice flow responsible for its deposition is uncertain. A later
till, the Bonnyville Formation, was deposited by ice flowing
from the northeast and thickens towards the southern end of the
borehole transect. The overlying till of the Marie Creek
Formation was deposited by ice flowing from the northnorthwest and also thickens towards the south end of the
transect. If proposed models of submarginal thickening of
subglacial tills are accurate then these two till sheets may
record the locations of the ice margin for at least part of the
glacial phases responsible for their deposition. The most recent
glacigenic deposit in the area, the Grand Centre Formation,
comprises four tills whose ice-directional data appear to
coincide with the superimposed mega-scale glacial lineations
appearing on the DEM (see also Andriashek and Fenton, 1989,
for fluting maps based on aerial photographs). Ice flowing from
the northeast is recorded by the Hilda Lake and Reita Lake
Members and NE–SW trending flutings located east of the
transect. This is superimposed by a N–S ice flow recorded by
the Kehiwin Lake Member and a N–S trending fluting flow set.
The final ice flow phase in the area is clearly recorded by a
NNW–SSE trending flow set and the Vilna Member, not
recorded along the chosen borehole transect.
Discussion
Figure 17 Map of morainic landforms in southern and central Alberta
compiled from the maps of Shetsen (1987, 1990). Further examples of
thrust moraine have been identified during this study. The only moraine
included here for the area located north of 548 300 N is the Cree Lake
Moraine taken from the Surficial Materials of Canada map (Fulton,
1995). The ‘west’, ‘central’ and ‘east’ lobes of Shetsen (1984) and Evans
(2000) are marked W, C and E, respectively
capping of tills with thick sequences of laminated sediments
with dropstones (Fig. 15a, B and C); see also Stalker, 1958)
attests to the fluctuation of the margin of the CAIS in a
glacilacustrine environment. This most likely records the
presence of Glacial Lake Lethbridge (Shetsen, 1980; Evans,
2000) during ice advance and recession, as would be expected
in an area where regional drainage is dammed by ice flow from
the north and northeast (Evans, 2000).
In other parts of Alberta, the thickening of stratigraphically
older till units also appears to be related to the location of
buried valleys but can mark former, overridden glacier margins.
This is especially clear where till thickening coincides with
overridden transverse ridges, as for example occurs in the
Cooking Lake area, east of Edmonton (Fig. 15(c)). Thick,
complex till sequences and associated glacitectonic rafts likely
record ice-marginal moraine construction and subglacial till
thickening during ice sheet advance. The preservation of
intraclasts and rafts in the tills suggests that travel distances in
the subglacial traction layer were short, basal ice motion was
driven by sliding and lineation moulding was produced by ice
keel ploughing. The corollary is that the beds of fast-flowing
trunk ice can contain palimpsests of initial glacier advance
phases, and that late-stage streaming can mould and
redistribute pre-existing depocentres/moraines so that the
occurrence and pattern of bed deformation and sliding are
Copyright ß 2008 John Wiley & Sons, Ltd.
What controls the positioning of fast-flowing ice or ice streams
in an ice sheet is a matter of current debate. The most prominent
glacier flow tracks identified by our mapping (CAIS and HPIS)
reveal that fast ice flow was influenced by topography but the
general N–S flow direction was enforced on the Laurentide Ice
Sheet in the region by coalescence with Cordilleran ice in the
west (Dyke and Prest, 1987; Klassen, 1989; Rains et al., 1999).
The superimposition of the palaeo-ice stream tracks identified
in this study on the regional bedrock topography reveals that
the ice flowed obliquely to the contours of the substrate (Fig. 18)
but the CAIS also appears to have been influenced by localised
topography, both bedrock and morainic (Fig. 19). Specifically,
although we have not identified the extent and shape of the
onset zone, it is apparent that the upstream trunk of the CAIS
flow track was fixed in position by a major col. This is illustrated
by Fig. 19, in which Transect 1 indicates flat topography
upstream and Transect 2 shows a col that we presume
controlled the position of the fast-flowing ice.
The bedform record is less distinct than in many other
palaeo-ice streams or fast-flowing glaciers, but it is striking that
there is a notable change in roughness between a smooth
streamlined bed in the fast-flow tracks and rough terrain of the
‘interlobate’ (inter-stream) belts. The high topography located
north of the Red Deer River and clearly depicted on the long
profile of the CAIS track (Fig. 19) represents another col on
the glacier bed and coincides with a wide arc of glacially
streamlined transverse ridges, interpreted above as an overridden moraine belt. Although no natural exposures are
available in this feature, it is part of an arcuate belt of bedrock
upland covered by a till veneer (Shetsen, 1987), suggesting that
it was constructed by glacitectonic thrusting of bedrock by ice
as it flowed southwards and upslope over the local preglacial
drainage divide (see buried valley thalwegs in Fig. 3). This is a
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Figure 18 Bedrock topography of Alberta, from The Geological Atlas
of the Western Canada Sedimentary Basin (reproduced with permission
from Alberta Energy and Utilities Board/Alberta Geological Survey),
with the palaeo-ice stream tracks of this study superimposed. Contours
in metres above sea level. This figure is available in colour online at
www.interscience.wiley.com/journal/jqs
prime location for bedrock thrusting in the region (Tsui et al.,
1989).
The densely spaced push moraines (minor transverse ridges)
and associated flutings at the southern limits of the CAIS and
HPIS (Fig. 10(c)) are typical of landforms produced by actively
receding lobate ice margins. They document the sequential
marginal recession of the CAIS and HPIS (Figs 3 and 10(a), (b)
and (d)). The right lateral margin of the CAIS is demarcated by
the 470 km long belt of hummocks and discontinuous and
closely spaced ridges (Fig. 11) that abuts the eastern margin of
the McGregor interlobate moraine belt. This feature, which
continues into the Lethbridge Moraine to the south, marks the
boundary of fast-flowing ice in the CAIS and the more sluggish
ice that lay over the McGregor interlobate moraine belt.
The recession of the thinner marginal ice of the CAIS and
HPIS from the McGregor ‘interlobate’ terrain is recorded by the
discrete fields of flutings terminating at arcuate bands of minor
push moraines (Fig. 12). Localised moraine arcs record
topographically induced lobation of the ice margin during
recession. Minor readvances are documented by the localised
superimposition of minor push moraines and the misalignment
and cross-cutting nature of superimposed flutings (flow sets).
Subglacial drainage pathways beneath these lobate ice margins
are recorded by fragmented single and anabranched esker
networks and occasional elongate water-filled depressions
(Evans et al., 2006a).
Copyright ß 2008 John Wiley & Sons, Ltd.
The pattern and spatial relationships of the end/push
moraines and their associated flutings are difficult to explain
in terms of the subglacial fluvial erosional ripple mark genesis
suggested previously by Munro and Shaw (1997) for the
moraines around McGregor Lake (Benn and Evans, 2006).
Moreover, the full assemblage of landforms, including flutings,
eskers and end/push moraines, is similar in every respect to the
glacial landsystem reported by Evans and Twigg (2002) from
southern Iceland, characterised by inset sequences of integrated subglacial and ice-marginal landforms produced by
lobate marginal recession of relatively fast-flowing, active
temperate glaciers. Historical fluting fields such as those in
southern Iceland allow us to relate sediment and landform
characteristics to genetic processes with a high degree of
confidence (Evans and Twigg, 2002; Evans, 2003). The flutings
are aligned parallel to known former ice flow directions in
slightly misaligned flow sets that terminate at moraines. Eskers
mark the location of channelised meltwater in an integrated
subglacial drainage network. This landsystem provides us with
a clear process-form model and Benn and Evans (2006) have
demonstrated that it is a small logical step to assume that it can
be applied to ancient landform–sediment assemblages with a
wide range of closely similar characteristics.
Closely spaced push moraine sequences are usually
associated with actively receding temperate glacier margins
(Evans and Twigg, 2002) and therefore document climatically
driven, active recession. We therefore suggest that a similar
form of cyclical forcing of ice lobe marginal oscillations
occurred during the early deglaciation of southern Alberta and
that the streamlined corridors represent the beds of fast-flowing
trunk glaciers flanked by slowly moving ice (ice streams by
strict definition). In other words, the glacial geomorphology,
particularly at the southern ends of the palaeo-ice stream tracks,
suggests to us that the Alberta ice stream margins did not
behave in a similar way to the margins of the present-day
Antarctic ice streams where ice flow variability is controlled by
changes in longer-timescale basal water pressure and friction,
and marginal fluctuations are consequently not dominated by
seasonal signals (e.g. Bindschadler et al., 1987, 1996;
Engelhardt and Kamb, 1997, 1998; Anandakrishnan et al.,
2001; Kamb, 2001). The occurrence of subglacial meltwater
channels and esker networks in association with the push
moraine sequences indicates a well-drained glacier bed at the
margins of the Alberta ice streams. This suggests to us that the
subglacial meltwater driving the streaming further up-ice was
effectively bled off in the snout zone (Hooke and Jennings,
2006) where ice-marginal oscillations were climatically driven,
a similar scenario to that observed in modern fast-flowing outlet
glaciers like Breidamerkurjökull in Iceland.
The distribution of glacial landforms in southern and central
Alberta attests to a change in regional glacier dynamics during
deglaciation. In southern Alberta, the prominent push moraine
sequences record climatically driven recession of the ice
margins. Specifically, the margin of the CAIS deposited push
moraines during its recession over the Milk River Ridge back to
the location of the ‘Lethbridge Moraine’ assemblage. North of
this assemblage, the bed of the CAIS is devoid of minor push
moraines and dominated by streamlined landforms that
document fast flow to the ‘Lethbridge Moraine’. This fast-flow
event equates with a 14 000 14C yr BP regional readvance in
Alberta, Saskatchewan, NE Montana and northern North
Dakota, thought to have been initiated by changes in the
locations of ice divides over the McKenzie/Keewatin region;
the dominance in ice flow from the Plains ice divide during full
glacial conditions is replaced sometime prior to 13 000 14C yr
BP by flow from a NNE–SSW trending ice divide lying over
northeastern Alberta (Dyke and Prest, 1987; Fullerton et al.,
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LAURENTIDE ICE SHEET FAST GLACIER FLOW
267
Figure 19 Cross-profiles (left) and long profile (bottom) of the CAIS fast-flow track in central Alberta extracted from CDED elevation data at a
horizontal spatial resolution of 55 m. This figure is available in colour online at www.interscience.wiley.com/journal/jqs
Copyright ß 2008 John Wiley & Sons, Ltd.
J. Quaternary Sci., Vol. 23(3) 249–272 (2008)
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JOURNAL OF QUATERNARY SCIENCE
Figure 20 Landsystems associated with (a) isochronous, (b) time-transgressive and (c) superimposed time-transgressive bedform and ice-marginal
landform production by ice streams or fast-flowing glacier lobes (Clark and Stokes, 2003). Note that the situation depicted in (c) is based on the Albertan
case study presented in this paper and includes surge geomorphology such as thrust block moraines and associated crevasse-squeeze ridges; (see Evans
et al., 1999)
2004a,b). The push moraines of the HPIS occur further north in
the upper Bow River basin. To the north of these active
temperate landform assemblages, the glacial geomorphological record is largely devoid of minor push moraines and
dominated by cross-cutting corridors of fluted terrain terminating at recessional thrust block moraines. In the western half of
the study area particularly, these landform assemblages are
indicative of former surging glaciers (Evans et al., 1999; Evans
and Rea, 2003). Additionally, landform assemblages to the
north of Edmonton, particularly crevasse-squeeze ridges and
long flutings (Richard, 1985a,b, 1986, 1987), may also be
indicative of former surging by fast-flowing trunk glaciers
within a downwasting Laurentide Ice Sheet.
Conclusion
Preliminary regional mapping of lineations associated with the
glaciation of southern and central Alberta, Canada, has been
facilitated by the production of a DEM of the whole province.
We have identified corridors of smoothed topography and
streamlined landforms that display high levels of spatial
coherency. These corridors are interpreted as the beds of
former fast-flowing trunk glaciers (technically palaeo-ice
streams). Additionally, transverse lineaments are interpreted
as series of minor recessional push moraines, thrust block
moraines constructed during readvances/surges or overridden
moraines. Flutings that emanate from overridden ridges located
in fast-flow tracks appear to be the product of grooving, best
explained by ice keel ploughing (Tulaczyk et al., 2001; Clark
et al., 2003) after the ice sole has been scored by the rough bed.
Together these landforms demarcate the beds and margins of
former fast ice flow trunks within the southwest sector of the
Laurentide Ice Sheet. Localised cross-cutting and/or misalignment of flow sets indicates temporal separation and the
overprinting of the lobate margins of ice streams during the last
glacial cycle. The streamlined bedform record is less distinct
than in many other palaeo-ice streams or fast-flowing glaciers
(e.g. Stokes and Clark, 2003; J. Evans et al., 2004) but the
notable change in roughness between the smooth corridors and
the rough surrounding terrain allows us to differentiate between
Copyright ß 2008 John Wiley & Sons, Ltd.
areas of subglacial streamlining and interlobate or inter-stream
moraine construction and minor streamlining, the latter
affected by more sluggish ice.
The fast-flow tracks are separated by interlobate or interstream moraine belts which contain hummocky moraine,
overridden transverse ridges and overprinted flow sets
comprising minor flutings and recessional push moraines.
Overridden transverse ridges often occur down-ice from
preglacial valleys or on the uplands between them and are
predominantly composed of glacitectonically folded and thrust
bedrock and sediment. The orientations of the transverse ridges
suggest that the inter-stream moraine belts have been
constructed by the margins of neighbouring or competing
lobate ice streams as they advanced into and receded from
southern and central Alberta (Fig. 20). Minor flutings document
ice flow into the interlobate belts at the margins of the fast-flow
zones and where they comprise cross-cutting flow sets and are
associated with misaligned end moraines they are interpreted
as evidence for lobate margin recession (Clark, 1997). The
association of cross-cutting flow sets, push moraines displaying
crenulated and bifurcating patterns and single and anabranched esker networks constitutes a landsystem similar in
every respect to that produced by lobate marginal recession of
active temperate glaciers (Evans and Twigg, 2002; Evans,
2003). We therefore suggest that ice lobe marginal oscillations
that took place during the early deglaciation of southern Alberta
were in response to seasonal climatic forcing when subglacial
meltwater was effectively bled from the system through tunnel
valleys and esker networks.
Much of the till around the southern margins of the
Laurentide Ice Sheet has been widely interpreted as the
product of subglacial deformation (e.g. Alley, 1991; Evans and
Campbell, 1992, 1995) and because subglacial deformation
has been closely linked with fast glacier flow (e.g. Boulton and
Hindmarsh, 1987; Alley et al., 1986; Dowdeswell et al., 2004)
it follows that the Albertan tills played a significant role in the
fast ice flow identified in this study. However, widespread
sliding also appears to have been significant at the beds of some
palaeo-ice streams (e.g. Piotrowski et al., 2001, 2004; Stokes
and Clark, 2003) and the occurrence of large subglacial
meltwater channels in the fast ice flow corridors of Alberta
attests to the occurrence of substantial meltwater discharges,
albeit of uncertain contemporaneity with the fast-flow event.
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LAURENTIDE ICE SHEET FAST GLACIER FLOW
Additionally, the occurrence of thin tills at the centres of the
fast-flow corridors, in many places unconformably overlying
stratified sediments (e.g. Piotrowski et al., 2004), suggests that
widespread till deformation may have been subordinate to
basal sliding in driving fast ice flow. Nevertheless, the general
thickening of tills towards the ice stream/lobe margins is
consistent with the theory of subglacial deformation presented
by Boulton (1996a,b). Evidence of marginal thickening of
subglacial sediments is manifest in the thick sequences of tills,
glacitectonites and associated bedrock mega-rafts around the
southern limits of the CAIS for example, but the thickening of
stratigraphically older till units beneath streamlined ice flow
transverse landforms can also mark overridden glacier margins.
The construction of transverse ridges and an associated
thickening of tills and plucked bedrock immediately down-ice
of buried valleys strongly suggest that pre-existing sediment
sequences (valley fills) are crucial to sediment generation at the
glacier bed. It is possible that the preglacial sediment sequences
in the buried valleys acted as aquifers, locally draining
subglacial meltwater, increasing the basal shear stress and
thereby causing till thickening either by tectonic stacking and
folding or by freeze-on and later melt-out (e.g. Christoffersen
and Tulaczyk, 2003). This explains how the leakage of
subglacial meltwater through preglacial valley aquifers could
conceivably have caused ice stream deceleration. However,
the stacked sediments would act as a blockage to streaming ice
further up-flow which would have thickened and eventually
advanced over the stack, streamlining its surface. Therefore, the
beds of fast-flowing trunk ice can contain palimpsests of initial
glacier advance phases, and ice streaming can mould and
redistribute pre-existing materials so that a mosaic of bed
deformation and sliding is produced by the localised
‘continuity’ of subglacial materials (Evans et al., 2006b).
On the basis of the evidence described in this paper and its
links with the published literature, we regard the area of central
and southern Alberta to have been traversed by numerous ice
streams, the most prominent of which we here name the Central
Alberta Ice Stream (CAIS) and the High Plains Ice Stream (HPIS).
The detailed footprints and timing of these ice streams remain to
be elucidated and many questions remain unanswered. In this
area of relatively low relief we speculate that ice streaming was
highly dynamic and transitory, sometimes with ice streams
topographically fixed in their onset zones and with the terminus
migrating laterally, and at other times with entirely new tracks
being occupied (e.g. Lac la Biche surging ice stream; Evans
et al., 1999). We note significant similarities between the style
of deposition and behaviour of these ice streams with those
reported elsewhere along the southern margin of the Laurentide
Ice Sheet (Patterson, 1997, 1998; Jennings, 2006).
Finally, the regional patterns of glacial landform development during deglaciation are instructive with respect to the
changing dynamics of palaeo-ice streams. In southern Alberta
we have identified fast-flowing trunk zones terminating in inset
sequences of push moraines and flutings indicative of active
recession. This is in contrast to central Alberta, where glacial
landsystems indicate surging ice streams. The corollary is that
the ice stream lobate snouts were uncoupled from climatic
drivers, and ice throughput along the ice stream trunks became
intermittent as the ice sheet margin receded northwards. Two
factors are thought to be significant in explaining this trend.
First, thicker ice over the northern dispersal centres during the
early stages of deglaciation maintained the ice volume and
driving stresses required for persistent ice streaming, whereas
during later stages of deglaciation the progressive recession and
thinning of the ice sheet resulted in pulsed streaming in
‘binge–purge’ type behaviour (e.g. MacAyeal, 1993) as ice
thicknesses over dispersal centres took progressively more time
Copyright ß 2008 John Wiley & Sons, Ltd.
269
to build up the reservoirs of mass necessary to operate ice
streams. Second, recession of the southwest margin of the
Laurentide Ice Sheet from Alberta resulted in the damming of
progressively more extensive and deeper proglacial lakes
which decanted marginally and submarginally, the latter
documented by the localised development of subglacial flood
pathways that cross-cut the regional pattern of glacier flow.
Such lakes and their decanting water may have facilitated ice
stream surging through ice margin decoupling and drawdown
(e.g. Stokes and Clark, 2004).
Acknowledgements Fieldwork in Alberta was funded by the Carnegie
Trust for the Universities of Scotland, the Royal Society and the
Robertson Bequest from the University of Glasgow (DJAE). Satellite
imagery was purchased with financial aid provided by the Robertson
Bequest. Thanks to Rod Smith and Don Lemmen at the Geological
Survey of Canada for logistical support and to the late Ron WhistanceSmith (University of Alberta) for generous support with the access to and
usage of aerial photograph mosaics of Alberta. Mike Shand, Yvonne
Finlayson and Les Hill (University of Glasgow) and Chris Orton (University of Durham) produced the figures. Reviews by Carrie Jennings,
John Andrews and Mandy Munro-Stasiuk have helped us to clarify the
concepts presented in this paper. Thanks, as always, to Doug Benn, but
especially for his inspirational and timely quote: ‘That landscape looks
like exactly what you’d expect to see if you lifted Fjallsjökull off the
planet tomorrow!’
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