The Glaciological Setting

The East Antarctic Ice Sheet is believed to have been stable since Miocene times (Denton et al. 1993; Marchant et al. 1993; Sugden et al. 1993). Evidence from dated 40Ar/39Ar in situ volcanic ashes occurring in association with soils from unconsoli-dated tills in the Dry Valleys, from basaltic flows interbedded with widespread tills and from reworked clasts in moraine sequences, indicate that there has been no significant expansion of this ice sheet or landscape evolution at least since mid-Miocene times. The West Antarctic Ice Sheet has a different history. It rests on bedrock mostly below sea level, and is dramatically affected by sea-level changes. There is clear evidence that during low sea levels, the associated ice shelves grounded and expanded, causing ice to flow backwards into valleys along the

Fig. 2.3 a View looking east towards the coast along Wright Valley. Expansions of the Ross Ice Shelf deposited moraines in the valley mouth with earlier incursions extending far up the valley. The four alpine glaciers on the far right have moraine sequences dating to > 2.1 million years. Foreground surfaces have old Miocene aged tills and soils. Wright Valley, formerly a fjord, was probably carved by a through-flowing glacier in the Oligocene. b View looking northwards along the Transantarctic Mountains and across Wright Valley. Tills with patterned ground are alpine moraines with weathering stage 2 soils. An older landscape and soils occur on the rounded patterned ground-free terrain in the middle

Fig. 2.3 a View looking east towards the coast along Wright Valley. Expansions of the Ross Ice Shelf deposited moraines in the valley mouth with earlier incursions extending far up the valley. The four alpine glaciers on the far right have moraine sequences dating to > 2.1 million years. Foreground surfaces have old Miocene aged tills and soils. Wright Valley, formerly a fjord, was probably carved by a through-flowing glacier in the Oligocene. b View looking northwards along the Transantarctic Mountains and across Wright Valley. Tills with patterned ground are alpine moraines with weathering stage 2 soils. An older landscape and soils occur on the rounded patterned ground-free terrain in the middle

Transantarctic Mountains (Denton and Hughes 1981). The last expansion in the Late Last Glacial period (Ross Glaciation; Denton et al. 1971) resulted in widespread deposition of tills to more than 1,000 m elevation in valleys and coastal surfaces. Alpine glaciers are small and independent of the ice sheets, and comprise ice from snow accumulations in local névés, etc. These glaciers respond to changes in local conditions and, like the East Antarctic Ice Sheet, have moraine sequences which indicate that changes in their masses since the late Pliocene have been relatively small (Everett 1971).

Tills that are associated with the three ice sources, and in which the soils and permafrost occur, have broadly similar characteristics, usually diamictons which are predominantly bouldery sands or silty sands. Tills on older inland surfaces are mainly unconsolidated, often deeply weathered and sometimes include several layers separated by paleosols, which are indicative of multiple ice advances. Younger tills, especially those of the Ross Glaciation, are typically unweathered and firmly ice-cemented, while some tills are underlain by massive ice that is believed to be several million years old (Campbell and Claridge 1987; Sugden et al. 1999). Tills cover most of the exposed landscapes throughout Antarctica, but steep slopes, upland plateau and benched surfaces commonly have bedrock outcrops and felsenmeer that are estimated to make up 10-15% of all bare ground surfaces. Aeolian deposits and fluvial deposits are rarely found.

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