| Summary: | Although theories of glacier movement generally assume that glaciers flow over rigid rock beds, there are many places where glaciers rest on beds of deformable sediment, and the great Pleistocene ice sheets which extended from time to time over much of Northern Europe and North America were largely underlain by such beds. Observations show that a large proportion of the forward move-ment of a glacier lying on such a bed may be contributed by deformation of the bed rather than the glacier. A theory is developed in which the glacier surface profile is related to the hydraulic and strength properties of potentially deformable bed materials. If these have a high hydraulic transmissibility, melt water is readily discharged sub-glacially, the bed is stable, and the profile is a normal parabolic one, governed by the rheological properties of ice. If bed transmiss-ibility is low, water pressures build up, the bed begins to deform, and a lower equilibrium profile will develop, so that in an extreme case the glacier approximates to a thin flat sheet, similar to an ice shelf.-from Authors
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