Basal ice can be defined as a relatively thin layer of ice that forms through a combination of thermal and mechanical processes which link a glacier to its substrate. These thermal and mechanical processes result in several characteristics, including layers and/or lenses of debris entrained from the bed, deformation structures, relatively high solute concentrations and unusual gas composition, that collectively distinguish it from ice that has formed solely by the firnification of snow at the glacier surface (Hubbard & Sharp, 1989; Knight, 1997). Basal ice is thought to be an important component in the rheological behaviour of glaciers.

Most models of glacier behaviour are founded on the assumptions of homogeneous deformation behaviour that is described by Glen's flow law and on the presence of a sharp ice-substrate interface. These assumptions are increasingly at odds with field and laboratory observations that demonstrate that basal ice is characterized by compositional and mechanical properties that distinguish it from glacier ice formed by precipitation alone. These observations suggest that unconsolidated deposits beneath glaciers are common and that these deposits may deform in response to shear stresses imposed by the flowing ice. The separation between modelling efforts and field and laboratory observations is compounded by a tendency to conceptualize the behaviour of basal ice and the glacier substrate as independent entities when such a separation may not be supported by field observations.

Despite a call for enhancement of our understanding of the behaviour of ice at glacier beds by Theakstone in 1967, and restatement of the same imperative in a landmark review of basal ice formation and deformation by Hubbard & Sharp (1986) we have not yet achieved a complete understanding of the formation and deformation of basal ice. Until such comprehension is achieved it seems likely that basal ice will continue to be treated as an isotropic material. The purpose of this chapter is to examine recent developments in the understanding of the deformation at the base of glaciers in the light of field observations and experiments conducted beneath cold-based glaciers in the McMurdo dry valleys.

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