The behaviour and possible instability of the West Antarctic ice sheet depend fundamentally on the dynamics of the large ice streams which drain it. Model calculations show that most ice-stream velocity arises at the bed 1,2, and radar sounding has shown the bed to be wet3, but the basal boundary condition is not well understood. Seismic evidence from the Upstream B camp (UpB) on the Siple Coast of West Antarctica4 shows that the ice stream there rests on a layer of unconsolidated sediment averaging 5 or 6 m thick, in which the water pressure is only ∼50 kPa less than the overburden pressure. Because this thin layer occurs well inland beneath an active ice sheet and rests on a surface showing flutes4 characteristic of glacial erosion5, we presume that it is glacial till. We propose here that deformation within the till is the primary mechanism by which the ice stream moves, and we discuss implications of this hypothesis.
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