Subglacial conditions under the last ice sheet in northwest Germany: ice-bed separation and enhanced basal sliding?

Theoretical reconstruction of ice thickness during the Weichselian Glaciati on in a portion of northwest Germany yields maximum thicknesses of about 28 0 m some 50 km into the ice sheet from the margin. This corresponds to aver aged driving stress at the glacier bed in the order of 17 kPa. The increase in ice thickness away from the margin was non-uniform, caused by undulatio ns in pre-existing topography. Steady-state subglacial water pressure heads were high up in the ice sheet, reaching the flotation point in places. Hyd raulic lifting of the ice sheet probably occurred, causing non-deforming be d conditions where transformation of basal shear stresses to soft subglacia l sediments was prevented by a water film. Thin stringers of undeformed sor ted sediments occurring in tills are attributed to basal meltwater washing during phases of ice-bed separation. The basal water film was over 5 mm thi ck. Using a quantitative approach coupled with field data we demonstrate th at neither internal ice deformation nor bed deformation contributed much to the ice motion. Instead, the rapid movement of the Weichselian Ice Sheet w as caused by basal sliding with ploughing. The ice sheet was probably chara cterized by an unstable behaviour. (C) 1999 Elsevier Science Ltd. All right s reserved.