Selective Linear Erosion Research Articles

Morphology and genesis of the Yaselda glacial valleys in Belarus

Geological mapping revealed the oldest Yaselda (Don (on the territory of Russia), Sanian 1 (on the territory of Poland)) generation of valleys in the Quaternary strata. Valleys are widespread in marginal and inner zones of the Yaselda glaciation area. They are located in the pre-Yaselda lowlands, in the areas of deformable deposits that lie above the more hard rocks protrusion and in fault systems. The distribution of glacial valleys is depends on the radial-sectoral structure and dynamics of the Yaselda glacier, which predetermined both the common and regional features of the valley regularity and morphology. The common features are the location in lobed sectors, occurrence in heterochronous ice marginal complexes, the predominance of polygenetic valleys. The regional features include isolated position, submeridional and meridional orientation, smaller size and weak expression of valleys in the maximum phase relief, the predominance ofvalleys of glacial exaration and extrusion and tunnel valleys. Valleys of the younger (Novogrudok) phase are located in the marginal complexes of the Middle Neman, Naliboky and Middle Berezina ice lobes and tongues. Valleys are characterised by connection with the belt of marginal highlands, sublatitudinal large-festoon ordering on the area of the lobes and a fan structure in the tongues, large size and maximum depths, clear expression in relief. Here, mainly polygenetic basins and trough valleys, extrusion valleys and subglacial tunnel valleys have been established. Basins arose simultaneously with the advance of outlet glaciers and are the result of exaration and glaciotectonic overthrusts of substrate rocks in the territory of the Belarusian anteclise and lowlands enveloping from the north. Their formation became possible at the regressive stage, when the readvances of the ice lobes led to the extrusion of a large amount of bed material and its accumulation in the basin borders. Trough valleys arose due to the wedging of outlet glaciers into paleovalleys or selective linear erosion along zones of structural weakening. In areas of rock bed, they were formed by plucking and abrasion, and in areas of soft substrate – by plucking, removal of thrust blocks. Numerous extrusion valleys appeared due to the extrusion in the Meso-Cenozoic deposits from under the glacial tongues into the marginal zones during the glacial retreat phase. Tunnel valleys originated as a result of canal erosion of sediments during rapid outbursts of water from subglacial lakes established for the glacier reduction phase. Most of the large valleys are of polygenetic origin.

Topographical evolution and glaciation history of South Greenland constrained by paired 26Al/10Be nuclides

The pattern of cosmogenic nuclide inheritance within bedrock surfaces in previously glaciated regions can be used to assess the efficiency of glacial erosion and constrain topographic evolution. Here, we present fifty new in-situ cosmogenic 10Be and 26Al pairs in bedrock and boulder erratics from South Greenland. The data demonstrate rapid retreat of the South Greenland Ice Sheet in the early Holocene and a clear gradient in cosmogenic nuclide inheritance in bedrock with elevation. Sites <800m above sea level (m a.s.l.) generally show limited or no nuclide inheritance (exposure from before the last glaciation). In contrast, the nuclide inheritance in samples from sites at higher elevations varies from negligible to >90kyr of exposure before the last glaciation, depending on the topographic setting. Our results suggest that steering of ice into troughs led to substantial erosion (>2.6m) in the troughs over the last glacial cycle, even at elevations above 1500 m a.s.l., while bedrock on summit flats at similar elevations experienced much less erosion. Inverse Markov-Chain Monte Carlo (MCMC) modelling of the nuclide inventories further indicates that selective linear erosion developed >1Myr ago at the summit flat closest to the present ice margin, and effectively limited the subsequent erosion to <2.6m. Erosion of the Redekammen Ridge some 40 km from the present ice margin was slightly more efficient over the last 1 Myr, indicating that selective linear erosion is less pronounced, or developed later, here than at the summit flats. Long-term ice-cover histories are generally less well-constrained than the erosion histories. The results suggest that the summit flats were possibly ice-covered during large parts of the last 1 Myr and experienced minimal erosion during this interval, but it is also possible that they were only covered by ice during the coldest parts of the glacial periods and that a cover of regolith or overlying rock was stripped off during one of the latest major glaciations. Although inverse modelling cannot distinguish between these scenarios, we identify the more likely ice-cover scenarios for each site based on the geological setting and surface characteristics.

Erosion-driven uplift in the Gamburtsev Subglacial Mountains of East Antarctica

The relative roles of climate and tectonics in mountain building have been widely debated. Central to this debate is the process of flexural uplift in response to valley incision. Here we quantify this process in the Gamburtsev Subglacial Mountains, a paradoxical tectonic feature in cratonic East Antarctica. Previous studies indicate that rifting and strike-slip tectonics may have provided a key trigger for the initial uplift of the Gamburtsevs, but the contribution of more recent valley incision remains to be quantified. Inverse spectral (free-air admittance and Bouguer coherence) methods indicate that, unusually for continents, the coherence between free-air gravity anomalies and bedrock topography is high (>0.5) and that the elastic thickness of the lithosphere is anomalously low (<15 km), in contrast to previously reported values of up to ∼70 km. The isostatic effects of two different styles of erosion are quantified: dendritic fluvial incision overprinted by Alpine-style glacial erosion in the Gamburtsevs and outlet glacier-type selective linear erosion in the Lambert Rift, part of the East Antarctic Rift System. 3D flexural models indicate that valley incision has contributed ca. 500 m of peak uplift in the Gamburtsevs and up to 1.2 km in the Lambert Rift, which is consistent with the present-day elevation of Oligocene–Miocene glaciomarine sediments. Overall, we find that 17–25% of Gamburtsev peak uplift can be explained by erosional unloading. These relatively low values are typical of temperate mountain ranges, suggesting that most of the valley incision in the Gamburtsevs occurred prior to widespread glaciation at 34 Ma. The pre-incision topography of the Gamburtsevs lies at 2–2.5 km above sea-level, confirming that they were a key inception point for the development of the East Antarctic Ice Sheet. Tectonic and/or dynamic processes were therefore responsible for ca. 80% of the elevation of the modern Gamburtsev Subglacial Mountains.

Glacial landscape evolution in the Uummannaq region, West Greenland

The Uummannaq region is a mosaic of glacial landsystems, consistent with hypothesized landscape distribution resulting from variations in subglacial thermal regime. The region is dominated by selective linear erosion that has spatially and altitudinally partitioned the landscape. Low altitude areas are dominated by glacial scour and higher elevations are dominated by plateaux or mountain valley and cirque glaciers. The appearance and nature of each landscape type varies locally with altitude and latitude, as a function of bedrock geology and average glacial conditions. Selective linear erosion has been a primary control on landscape distribution throughout Uummannaq, leading to plateau formation and the growth of a coalescent fjord system in the Uummannaq region. This has allowed the development of the Uummannaq ice stream's (UIS) onset zone during glacial periods. Fjord development has been enhanced by a downstream change in geology to less‐resistant lithologies, increasing erosional efficiency and allowing a single glacial channel to develop, encouraging glacier convergence and the initiation of ice streaming. The landscape has been affected by several periods of regional uplift from 33 Ma to present, and has been subject to subsequent fluvial and glacial erosion. Uplift has removed surfaces from the impact of widespread warm‐based glaciation, leaving them as relict landsurfaces. The result of this is a regional altitude‐dependent continuum of glacial modification, with extreme differences in erosion between high and low elevation surfaces. This study indicates that processes of long‐term uplift, glacial erosion/protection and spatial variability in erosion intensity have produced a highly partitioned landscape.

Configuration of the Northern Antarctic Peninsula Ice Sheet at LGM based on a new synthesis of seabed imagery

Abstract. We present a new seafloor map for the northern Antarctic Peninsula (AP), including swath multibeam data sets from five national programs. Our map allows for the examination and interpretation of Last Glacial Maximum (LGM) paleo-ice-flow paths developed on the seafloor from the preservation of mega-scale glacial lineations, drumlinized features, and selective linear erosion. We combine this with terrestrial observations of flow direction to place constraints on ice divides and ice domes on the AP continental shelf during the LGM time interval. The results show a flow bifurcation as ice exits the Larsen B embayment. Flow emanating off the Seal Nunataks (including Robertson Island) is directed toward the southeast, then eastward as the flow transits toward the Robertson Trough. A second, stronger "streaming flow" is directed toward the southeast, then southward as ice overflowed the tip of the Jason Peninsula to reach the southern perimeter of the embayment. Our reconstruction also refines the extent of at least five other distinct paleo-ice-stream systems that, in turn, serve to delineate seven broad regions where contemporaneous ice domes must have been centered on the continental shelf at LGM. Our reconstruction is more detailed than other recent compilations because we followed specific ice-flow indicators and have kept tributary flow paths parallel.

Dating Disappearing Ice with Cosmogenic Nuclides

Cosmogenic nuclides are remarkably well suited to dating glacial landforms. Exposure dating of boulders on moraines and of glacially sculpted bedrock allows the determination of the ages of former ice margins, from which past glaciations can be temporally constrained. Where moraines are lacking or are poorly preserved, outwash is dated with depth-profile dating. Two-nuclide methods can be used to determine the ages of buried till. Multinuclide measurements of bedrock ages also provide insights into periods of non-erosive ice coverage and can be used to identify regions with selective linear erosion. Of particular interest is the use of cosmogenic nuclides to assess rates of glacier retreat and glacial erosion.

The evolution of the subglacial landscape of Antarctica

The aim is to investigate the evolution of the subglacial landscape of Antarctica using an ice sheet and erosion model. We identify different stages of continental glaciation and model the erosion processes associated with each stage. The model links erosion to the basal thermal regime and indicates that much of the Antarctic interior may have been subject to less than 200 m of erosion. The depth of erosion reflects the presence or absence of warm-based ice and the consistency of ice flow direction. This information, linked with knowledge about landscapes of glaciation in the northern hemisphere and some simple but robust assumptions about initial topography, is used to generate a map of the subglacial Antarctic landscape in which much of the lowland interior resembles the landscapes of areal scouring typical of the Laurentian and Scandinavian shields. Near the continental margins selective linear erosion has overdeepened pre-existing river valleys by as much as 2.8 km. High elevation plateaus adjacent to such drainage systems have survived largely unmodified under cold-based ice. High erosion rates result from steep thermal gradients in basal ice. Mountain regions such as the Gamburtsev Mountains, uplands in Dronning Maud Land and massifs in West Antarctica are likely to bear features of local alpine glaciation. Such landscapes may have been protected under cold-based ice for the last 34 Myrs or possibly longer.

Glacial erosion at the fjord onset zone and implications for the organization of ice flow on Baffin Island, Arctic Canada

Organized conduits of fast ice flow at the fringe of present and past ice sheets move the majority of ice from ice sheet interior to periphery. These relatively narrow corridors play a critical role in feedbacks between landscape evolution and ice sheet dynamics. Here we combine bedrock 10Be concentrations with distribution of lakes on Baffin Island, Arctic Canada, to constrain the pattern of glacial modification at a fjord onset zone — the region of increasing relief from the continental interior to the fjord head. High lake density and a lack of 10Be inheritance in the valley bottom indicate significant glacial scouring for some duration of the last glacial cycle. In contrast, 10Be inheritance and a lack of lakes at higher elevations indicate a lack of glacial scour and only slight glacial modification despite the entire region being covered by the Laurentide Ice Sheet during the last glaciation. This pattern of glacial erosion at the fjord onset zone is similar to what is found in the outer fjords ∼ 100 km toward the last glacial maximum ice margin, and indicates that ice flow becomes organized well inland from the fjorded coast. The relief-generating process of selective linear erosion occurs at the fjord onset zone and was responsible for evolving Baffin Island fjords even in the late Quaternary.

Glacial landscape evolution — Implications for glacial processes, patterns and reconstructions

This special issue presents a collection of papers that address a wide range of important challenges and exciting advances in the field of glacial landscape evolution. Primarily, these papers reflect persistent uncertainty that surrounds the mechanisms and timescales of glacial landscape evolution. For example, estimates of the duration of glacial occupancy required for the evolution of characteristic glacial valley forms from previously fluvial landscapes range from 100 kyrs for landscapes beneath large ice sheets (Jamieson et al.) to ~ 400–600 kyrs for glaciated alpine terrains (Brook et al.). Further, the mechanisms of glacial erosion are debated through analyses of the importance of ice thickness (Brocklehurst et al.; van der Beek and Bourbon), ice surface steepness (Vieira) and, in the case of large ice sheets, the co-evolution of ice sheet thermal regime, dynamics, and subglacial topography (Kleman et al.; Swift et al.). Debate concerning the potential climatic impacts of landscape evolution in alpine terrains is represented by van der Beek and Bourbon, who infer a significant increase in relief as a direct result of glacial erosion, and by Brocklehurst et al. and Heimsath and McGlynn, who demonstrate respectively that glacial relief production can be surprisingly modest and that rates of glacial erosion may be lower than those for fluvial incision. Further confirmation that valleys beneath large ice sheets evolve through selective linear erosion comes from studies that have combined geomorphological evidence with cosmogenic nuclide (Briner et al.) and apatite (U–Th)/He thermochronometry (Swift et al.), and the resulting style of landscape evolution is demonstrated by the antiquity of fjords in East Greenland (Swift et al.) and of deep erosion zones and thick drift covered zones in Fennoscandia (Kleman et al.), although the location of areal scouring zones may be subject to major alteration during single glacial events (Kleman et al.). Another set of papers shows that analyses of glacial lineation systems continue to provide important data on the dynamics of glacial landscape evolution, whether the lineations are formed underneath ice streams (Bradwell et al.; Andreassen et al.) or not (Jansson and Glasser), and whether they indicate intricate patterns of landscape modification (Andreassen et al.) or preservation (Jansson and Glasser). The final three papers address rarely-reported issues relating to landscapes of glacial deposition, including moraine degradation (Putkonen et al.), proglacial hydrogeology (Robinson et al.), and the evolution of hummocky-till topography (Clayton et al.).

Cosmogenic 10Be and 26Al exposure ages of tors and erratics, Cairngorm Mountains, Scotland: Timescales for the development of a classic landscape of selective linear glacial erosion

The occurrence of tors within glaciated regions has been widely cited as evidence for the preservation of relic pre-Quaternary landscapes beneath protective covers of non-erosive dry-based ice. Here, we test for the preservation of pre-Quaternary landscapes with cosmogenic surface exposure dating of tors. Numerous granite tors are present on summit plateaus in the Cairngorm Mountains of Scotland where they were covered by local ice caps many times during the Pleistocene. Cosmogenic 10Be and 26Al data together with geomorphic relationships reveal that these landforms are more dynamic and younger than previously suspected. Many Cairngorm tors have been bulldozed and toppled along horizontal joints by ice motion, leaving event surfaces on tor remnants and erratics that can be dated with cosmogenic nuclides. As the surfaces have been subject to episodic burial by ice, an exposure model based upon ice and marine sediment core proxies for local glacial cover is necessary to interpret the cosmogenic nuclide data. Exposure ages and weathering characteristics of tors are closely correlated. Glacially modified tors and boulder erratics with slightly weathered surfaces have 10Be exposure ages of about 15 to 43 ka. Nuclide inheritance is present in many of these surfaces. Correction for inheritance indicates that the eastern Cairngorms were deglaciated at 15.6 ± 0.9 ka. Glacially modified tors with moderate to advanced weathering features have 10Be exposure ages of 19 to 92 ka. These surfaces were only slightly modified during the last glacial cycle and gained much of their exposure during the interstadial of marine Oxygen Isotope Stage 5 or earlier. Tors lacking evidence of glacial modification and exhibiting advanced weathering have 10Be exposure ages between 52 and 297 ka. Nuclide concentrations in these surfaces are probably controlled by bedrock erosion rates instead of discrete glacial events. Maximum erosion rates estimated from 10Be range from 2.8 to 12.0 mm/ka, with an error weighted mean of 4.1 ± 0.2 mm/ka. Three of these surfaces yield model exposure-plus-burial ages of 295 − 71 + 84 , 520 − 141 + 178 , and 626 − 85 + 102 ka. A vertical cosmogenic nuclide profile across the oldest sampled tor indicates a long-term emergence rate of 31 ± 2 mm/ka. These findings show that dry-based ice caps are capable of substantially eroding tors by entraining blocks previously detached by weathering processes. Bedrock surfaces and erratic boulders in such settings are likely to have nuclide inheritance and may yield erroneous (too old) exposure ages. While many Cairngorm tors have survived multiple glacial cycles, rates of regolith stripping and bedrock erosion are too high to permit the widespread preservation of pre-Quaternary rock surfaces.

Reconstructing the basal thermal regime of an ice stream in a landscape of selective linear erosion: Glen Avon, Cairngorm Mountains, Scotland

The Cairngorm Mountain area of Scotland is a classic example of a landscape of selective linear glacial erosion, with sharp contrasts in the intensity of glacial erosion between the deeply incised troughs and valleys and the undulating high plateau. This article examines the Quaternary development of Glen Avon, a 200 m deep glacial trough set within the high plateau of the mountains. Evidence concerning the aggregate basal thermal regimes of the topographically controlled ice streams that formerly developed in this area is reconstructed from the geomorphological record, including bedforms indicative of wet‐based, sliding ice and of dry‐based ice frozen to its bed. This mapping indicates that basal sliding was not confined exclusively to the troughs but extended towards valley heads and on to parts of the plateau adjacent to troughs. The extent of basal sliding appears to have been greatest beneath pre‐Late Devensian ice sheets. Basal ice temperatures are modelled under steady‐state conditions for the last ice sheet at c. 18 ka BP. Basal thermal regimes are predicted using a reconstruction of the preglacial relief and for the current topography of the area. Convergent flow of ice through the preglacial valley system appears to have been sufficient to induce basal melting and therefore to initiate valley deepening. This effect is enhanced when the model is run across the present topography. Comparison of results of the geomorphological mapping and the modelling reveals significant differences between the actual and predicted extent of basal sliding outside the main ice stream. The overall conclusion is that many ice streams in mountainous terrain are inherited from the locations of preglacial valleys, which serve to accelerate ice flow and promote frictional heating beneath ice sheets.

Reconstructing the basal thermal regime of an ice stream in a landscape of selective linear erosion: Glen Avon, Cairngorm Mountains, Scotland

Reconstructing the basal thermal regime of an ice stream in a landscape of selective linear erosion: Glen Avon, Cairngorm Mountains, Scotland

The Cairngorms — a landscape of selective linear erosion

(1998). The Cairngorms — a landscape of selective linear erosion. Scottish Geographical Magazine: Vol. 114, No. 2, pp. 124-129.

Drumlins of the Puget Lowland, Washington State, USA

The last advance of the Puget Lobe, a piedmont glacier that extended southward down the axis of the fjord-like Puget Lowland from the main mass of the Cordilleran Ice Sheet, produced an extensive drumlin field approximately 170 km in length and nearly 100 km wide. The timing of both the advance and retreat of the lobe across the region is well constrained, and took no more than a total of 1500 years. In the southern portion of the field, drumlin axis orientations are more closely perpendicular to recessional ice margin positions than to the reconstructed ice-maximum surface contours. This suggests that the drumlin field was formed in a time-transgressive manner as the ice front retreated from south to north, and that each of the numerous recessional phases was characterised by a narrow drumlin-forming zone that extended no more than a few kilometres back from the margin. This would also indicate that only a few decades were required for the formation of individual drumlins in the Puget Lowland.The drumlin forms are composed of compact till that overlies and truncates a 100 m thick sand and gravel unit. The sand has been interpreted as proglacial outwash, originally formed as a continuous blanket deposited in front (south) of the advancing Puget Lobe. It forms the main component within the lower 10–50% of the drumlin till, while farther-travelled components appear in progressively greater proportion with increasing height above the contact. The drumlins are thus interpreted as being of constructional origin. Evidence of minor subglacial meltwater activity occurs in the form of scattered, discontinuous water-sorted lenses interbedded throughout the till.The entire drumlin field is subdivided into a series of sub-parallel regions separated by deep troughs that extend to depths as great as 850 m below the drumlin-capped upland surface. Regional relationships indicate that the drumlins and the troughs formed contemporaneously, but that both post-date the deposition of the outwash. Thus, drumlin formation (net subglacial deposition) beneath some portions of the Puget Lobe was accompanied by simultaneous selective linear erosion in adjacent regions. This intense erosion may have been due in part to excavation by concentrated subglacial water flow, in contrast to the minor effects of subglacial water beneath the drumlin-forming portions of the ice sheet. Such fluvially enhanced subglacial erosion would be consistent with previous estimates of high sliding velocity and resultant large volume of meltwater production for the short-lived Puget Lobe.

The Characteristics and Significance of Deep Weathering in the Gaick Area, Grampian Highlands, Scotland

The Gaick area is a landscape of selective linear glacial erosion. Deep weathering of Moine psammites occurs on preglacial slopes but is absent from glacially-overdeepened valleys. The weathering i...

The Characteristics and Significance of Deep Weathering in the Gaick Area, Grampian Highlands, Scotland

The Gaick area is a landscape of selective linear glacial erosion. Deep weathering of Moine psammites occurs on preglacial slopes but is absent from glacially-overdeepened valleys. The weathering i...

Glacial Erosion of a High Arctic Valley

AbstractA large valley, ideally suited for “selective linear erosion” by ice, extends from the Kreiger Mountains to Tanquary Fiord, north–central Ellesmere Island. During the last glaciation, the outlet glacier at the head of the valley advanced 18 km and was at least 250 m thick where it contacted the sea in the lower valley. Erosion of bedrock inside the last ice limit is recorded by an abraded diabase dike, and by crag–and–tail features developed in limestone. During deglaciation (7800 B.P.), melt–water streams along the ice margin incised a large alluvial fan that pre–dates the last glaciation. The fan shows little alteration by the over–riding ice and its final erosion by the melt–water streams incised, but did not remove, its original ice–wedge polygons.The preservation of the fan indicates that the glacier was locally non–erosive and that it probably advanced across the fan by over–riding a protective frontal ice apron. Although it is commonly assumed that such alluvial fans occupying glaciated valleys are of post–glacial age, this need not be the case in permafrost terrain. In fact, at this site, there has been a net increment of alluvium versus glacial erosion or deposition spanning the last glacial cycle. The paper discusses the processes of erosion associated with sub–polar glaciers and questions whether erosion by them or more pervasive ice is responsible for such High Arctic valleys and fiords.

Glacial Erosion of a High Arctic Valley

AbstractA large valley, ideally suited for “selective linear erosion” by ice, extends from the Kreiger Mountains to Tanquary Fiord, north–central Ellesmere Island. During the last glaciation, the outlet glacier at the head of the valley advanced 18 km and was at least 250 m thick where it contacted the sea in the lower valley. Erosion of bedrock inside the last ice limit is recorded by an abraded diabase dike, and by crag–and–tail features developed in limestone. During deglaciation (7800 B.P.), melt–water streams along the ice margin incised a large alluvial fan that pre–dates the last glaciation. The fan shows little alteration by the over–riding ice and its final erosion by the melt–water streams incised, but did not remove, its original ice–wedge polygons.The preservation of the fan indicates that the glacier was locally non–erosive and that it probably advanced across the fan by over–riding a protective frontal ice apron. Although it is commonly assumed that such alluvial fans occupying glaciated valleys are of post–glacial age, this need not be the case in permafrost terrain. In fact, at this site, there has been a net increment of alluvium versus glacial erosion or deposition spanning the last glacial cycle. The paper discusses the processes of erosion associated with sub–polar glaciers and questions whether erosion by them or more pervasive ice is responsible for such High Arctic valleys and fiords.

Glacial Erosion by the Laurentide Ice Sheet

Abstract The aim of the paper is to analyse landscapes of glacial erosion associated with the Laurentide ice sheet at its maximum and to relate them lo the three main variables affecting glacial erosion, namely former basal thermal regime of the ice sheet, the topography of the bed, and the geology of the bed. The key to the analysis is the comparison of the distribution of landscape types with the simulated pattern of the basal thermal regime of the former ice sheet. Landscapes of area scouring are found to be associated with zones of basal melting and occur beneath much of the former ice-sheet centre and in those places where the topography favoured converging ice flow. The landscape type may also have formed beneath cold-based ice when it was carrying debris inherited from an up-stream zone of regelation. Areas with little or no sign of glacial erosion occur primarily in the north in the Queen Elizabeth Islands but they also occur on uplands associated with diverging ice flow; they coincide with areas calculated to have been covered by cold-based ice devoid of debris. Landscapes of selective linear erosion are common on uplands near the eastern periphery of the ice sheet. In these situations, pre-existing valleys channelled ice flow and created a situation where there was warm-based ice over the valleys and cold-based protective ice over the intervening plateaux. Variations in the permeability of the bedrock base have modified the landscape pattern, mainly in those areas where there was a change from one basal thermal regime to another. In general, permeable rocks tend to have experienced less erosion than impermeable rocks. Using lake-basin density as an indication of the intensity of glacial erosion, a zone of maximum erosion is identified and this forms a ring between the centre of the former ice sheet and its periphery. This ring coincides with a zone where melt water from the ice-sheet centre is calculated to have frozen on to the bottom of the ice sheet. This regelation incorporated basal debris into the ice, forming a basal layer 20-50 m thick and afforded an efficient means of debris evacuation. A conceptual model is developed and hangs round the following postulates: (1) Landscapes of glacial erosion are related primarily to the basal thermal regime of the ice sheet. (2) Landscapes of glacial erosion are equilibrium forms related to maximum glacial conditions. This implies that at some stage in the Pleistocene the Laurentide ice sheet was in a stable maximum condition for a long period of time. (3) Mechanisms allowing evacuation of debris rather than those of abrasion or fracture may be the most important in influencing the amount of erosion achieved by an ice sheet. (4) Cold-based ice may accomplish erosion if it contains debris.

Glacial Erosion by the Laurentide Ice Sheet

AbstractThe aim of the paper is to analyse landscapes of glacial erosion associated with the Laurentide ice sheet at its maximum and to relate them lo the three main variables affecting glacial erosion, namely former basal thermal regime of the ice sheet, the topography of the bed, and the geology of the bed. The key to the analysis is the comparison of the distribution of landscape types with the simulated pattern of the basal thermal regime of the former ice sheet.Landscapes of areal scouring are found to be associated with zones of basal melting and occur beneath much of the former ice-sheet centre and in those places where the topography favoured converging ice flow. The landscape type may also have formed beneath cold-based ice when it was carrying debris inherited from an up-stream zone of regelation. Areas with little or no sign of glacial erosion occur primarily in the north in the Queen Elizabeth Islands but they also occur on uplands associated with diverging ice flow; they coincide with areas calculated to have been covered by cold-based ice devoid of debris. Landscapes of selective linear erosion are common on uplands near the eastern periphery of the ice sheet. In these situations, pre-existing valleys channelled ice flow and created a situation where there was warm-based ice over the valleys and cold-based protective ice over the intervening plateaux. Variations in the permeability of the bedrock base have modified the landscape pattern, mainly in those areas where there was a change from one basal thermal regime to another. In general, permeable rocks tend to have experienced less erosion than impermeable rocks.Using lake-basin density as an indication of the intensity of glacial erosion, a zone of maximum erosion is identified and this forms a ring between the centre of the former ice sheet and its periphery. This ring coincides with a zone where melt water from the ice-sheet centre is calculated to have frozen on to the bottom of the ice sheet. This regelation incorporated basal debris into the ice, forming a basal layer 20-50 m thick and afforded an efficient means of debris evacuation.A conceptual model is developed and hangs round the following postulates: (1)Landscapes of glacial erosion are related primarily to the basal thermal regime of the ice sheet.(2)Landscapes of glacial erosion are equilibrium forms related to maximum glacial conditions. This implies that at some stage in the Pleistocene the Laurentide ice sheet was in a stable maximum condition for a long period of time.(3)Mechanisms allowing evacuation of debris rather than those of abrasion or fracture may be the most important in influencing the amount of erosion achieved by an ice sheet.(4)Cold-based ice may accomplish erosion if it contains debris.