Spatial variability

There has been a tendency to see glacial erosion as a uniform process that can be compared, for example, with fluvial erosion. One of the realizations from the use of remote sensing over the beds of former ice sheets is just how spatially variable glacial erosion is. There are now numerous accounts of surfaces, commonly marked by river valley networks, tors and blockfields, which have survived inundation by an ice sheet without significant modification, apparently as a result of cold-based glacier ice that is unable to slide at the ice-rock interface. The distribution of such preserved land surfaces in the Transantarctic Mountains, northern Canada, east and northern Greenland and northern Scandinavia points to the importance of cold continental climatic conditions in creating conditions suitable for ice accumulation at temperatures below the pressure melting point (e.g. Kleman & Hattesrand, 1999; Sugden, 1978). Such a link with cold-based ice is reinforced by more local patterns whereby the subaerially weathered landscapes are preferentially preserved on uplands where the presence of thinner, diverging ice favours cold-based ice. This is particularly well displayed on Baffin Island, Arctic Canada, and in Marie Byrd Land in West Antarctica where erratics of the last glaciation overlie bedrock surfaces that originated much earlier and actually lie in relict weathering pits and on the surface of tors (Briner et al., 2003; Sugden et al., 2005). In these cases the use of cosmogenic isotope analysis reveals the contrasting age of erratics and bedrock.

Such protected rock surfaces are often cheek by jowl with land-forms that reflect glacial moulding. The latter are most common at lower elevations, reflecting thicker, converging ice and are assumed to be a result of glacial erosion beneath warm-based ice that can slide at the ice-rock interface. The most extreme version of this landscape contrast is the landscape of selective glacial erosion that characterizes the plateau scenery of eastern Baffin Island and Labrador, eastern Greenland and the higher eastern hills of the former Scottish and Scandinavian ice sheets. In some situations the contrast is so sharp that one can sit on a relict surface and dangle one's feet over the glacially eroded trough.

In many low-lying shield areas of former ice sheets and in the warmer maritime sectors the landscape is worn extensively by glacier action. Such landscapes of areal scouring are common in western and central Scandinavia, western Scotland and in much of Canada and western Greenland. The blend of bare rock outcrops, moulded into streamlined hills or roches moutonnées, and bedrock basins is the result of glacier sliding, favoured by ice at the pressure melting point that permits sliding and erosion at the ice-rock interface. There has been a long debate as to how much erosion is achieved under such conditions. In support of relatively limited rates of erosion, one can point to the preservation of pre-glacial river valleys and patches of weathered bedrock, for example in the shield areas of Arctic Canada and Scandinavia. Indeed, old forestry maps of the river routes used to float logs in a glacially modified part of eastern Sweden clearly show the survival of the pre-existing river network, as well as locally disrupted areas where glacial erosion has achieved more of a transformation. On the other hand, analysis of the volume of offshore sediments of glacial erosion points to a greater depth of erosion by ice sheets (Andrews, this volume, Chapter 40).

One of the exciting developments in formerly glaciated areas resulting from the analysis of satellite imagery at the scale of an ice sheet is the recognition of contrasts in regional patterns of depositional landforms. Such studies in Arctic Canada and Scandinavia have revealed assemblages of streamlined landforms that mark the presence of former ice streams and they reveal remarkably abrupt margins (Stokes & Clark, 2001; Boulton et al., 2003). By looking at cross-cutting relationships between different assemblages of features and their relationship to preserved landscapes, it is possible to reconstruct the flow dynamics of an ice sheet and some insight into basal thermal regime during build up and decay. As this line of enquiry is developed and combined with surface exposure dating, there is the prospect of a record of change over time that will be invaluable for constraining models of former ice sheet behaviour. Another exciting step is to link the terrestrial record of ice streams with side-scan radar surveys of their offshore continuations. The offshore evidence of glacial streamlining marking the presence of former ice streams is remarkable for its clarity and resolution (Canals et al., 2000).

There is a stepped change in our potential to exploit the spatial variability of glacier forms and processes by engaging in extraterrestrial geomorphology. Studies of Mars have already established a theoretical flow model of current ice sheets (Nye, 2000), and high-resolution imaging shows rock glaciers and a variety of meltwater landforms (Baker, 2001; Lucchitta, 2001). Indeed there is a remarkable similarity between the rock glaciers of the Dry Valleys, Antarctica and those on Mars. New vistas open up with every new mission that deploys a new instrument and these present clear opportunities to apply and refine our terrestrial expertise.

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