Box 69 Landforms Of Erosion And Deposition Associated With Coldbased Glaciers

Cold-based glaciers do not produce large quantities of subglacial meltwater, so it is commonly assumed that they neither slide nor abrade their beds. As a result, it is commonly assumed that they do not achieve much glacial erosion. Atkins et al. (2002) challenged this assumption, describing glacial geomorpho-logical features associated with erosion and deposition by a cold glacier in the Allan Hills, Antarctica. They mapped and described a range of landforms, bedrock features and sediments including abrasion marks and striae, subglacial deposits, glaciotectonically deformed substrate, isolated blocks, ice-cored debris mounds and boulder trains. The photograph below, for example, shows a series of striations formed on a bedrock surface beneath a cold-based glacier in the Allan Hills, Antarctica. All of these features were inferred to have formed beneath a cold-based glacier as it advanced and receded. This study is important because it provides evidence that cold-based glaciers are capable of eroding, transporting and depositing material subglacially. The study also sets out some of the criteria for identifying the presence of former cold-based glaciers in the geological record.

Source: Atkins, C.B., Barrett, P.J. and Hicock, S.R. (2002) Cold glaciers erode and deposit: evidence from Allan Hills, Antarctica. Geology, 30, 659-62. [Photograph: C. Atkins]

Ice margin

6.4 Landscapes of Glacial Erosion 177 Ice divide

Ice-surface profile

Ice-surface profile

High connectivity

Valley patterns

Low connectivity

Figure 6.18 Schematic cross-section through a warm-based ice sheet showing the possible distribution of the landforms of glacial erosion.

where velocity is low and the ice may remain cold-based. Roches moutonnees develop where the ice is fast flowing and relatively thin, conditions that are ideal for cavity formation, whereas whalebacks occur only under thicker ice. Valley modification is greatest in the zone of fast-flowing ice close to the equilibrium line. One of the greatest advances in studies of glacial erosion in recent years is the use of cosmogenic isotope ('exposure age') dating, which can be used to test conceptual models such as this concerning the age of glacial landscapes, their evolution through time and their relationship to former ice dynamics and basal thermal regime (Box 6.10).

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