1 Examination of the literature suggests that there is confusion concerning the treatment of debris-rich basal ice and ice-rich subglacial sediment. What is described as subglacial permafrost by some researchers is treated as debris-rich basal ice by others.

2 Observations of subglacial conditions in cold-based glaciers in the McMurdo dry valleys demonstrate that in several cases there is no single clear boundary that separates basal ice from a frozen substrate. There is abundant evidence to suggest that deformation of the glacier substrate has resulted in both en masse entrainment of the glacier substrate and mixing of material from the glacier substrate with basal ice.

3 Deformation measurements made at the base of glaciers in the McMurdo dry valleys demonstrate that the strain fields close to glacier beds are much more complicated than would be expected from Glen's flow law. Such measurements, together with the close examination of the structure of basal zones, suggests that treatment of a glacier bed as a simple ice-substrate boundary is flawed.

4 In order to advance our understanding of the behaviour of subglacial materials we need to move away from the persistent separation of basal ice and the glacier substrate. Instead we need to recognize a continuum of material properties and deformation processes that is variable in time and space.

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