Debrisrich basal ice or icerich basal sediment

Early observations of the nature of glacier beds demonstrated that dirty basal ice can rest above frozen sediment and ice. For example Hansen & Langway (1966) and Herron & Langway (1979) described several metres of an ice-laden till-like subice material at the Camp Century core site and Koerner & Fisher (1979) observed a glacier bed that consisted of loose angular rock fragments with 31% rock fragments by weight.

More recently, several studies have sought to understand the distribution of strain at the base of glaciers that have such unconsolidated substrates. For example Truffer et al. (2000, 2001) describe a subglacial till layer between 4.5 and 7.5 m thick that was 'clearly separated' from overlying basal ice containing some dirt layers at the base of Black Rapids glacier in Alaska. A sample of the subglacial sediment retrieved from the drill hole yielded a water content of 29% when melted suggesting that the change in material properties at the glacier bed is one of dirty basal ice to ice-rich debris.

In contrast, Echelmeyer & Wang (1987) encountered bands of frozen subglacial till beneath the Urmqui No. 1 Glacier in China and concluded that 10 to 25% of motion occurred within these bands. They reported a sharp contact between the clear glacier ice, which had a debris content of about 1%, and material that they termed the glacier bed, which consisted of 'active ice-laden drift several metres thick'. Samples of the ice-laden drift contained 21-39% debris by volume. These descriptions of the material properties of the glacier substrate overlap with the material properties of what other workers refer to as basal ice.

Examination of the basal zones of glaciers in the McMurdo dry valleys, where most glaciers are cold-based, has demonstrated a wide variety of types of basal ice (Holdsworth & Bull, 1970;

Englacial faciès Amber faciès

Cavities Solid faciès

Basal stratified faciès

Englacial faciès Amber faciès

Cavities Solid faciès

Basal stratified faciès

Bed : frozen sand and gravel

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