Ice in glaciers and ice sheets is not homogeneous. In particular, there are distinctive ice facies found in significant depths in the basal zones that form as a result of the interaction of the glacier with its bed. These layers have a range of characteristics associated with the process of their formation, the net effect of which is that they deform more readily than overlying englacial ice. The cause of this enhanced deformation is hard to isolate, because there are complex and causal interdependencies between variables. The presence of solid impurities per se does not soften the ice, but the associated soluble impurities and enhanced water content seem to be causally associated with lower viscosity. There may also be differential thermal effects on different ice facies, so that temperature-sensitivity becomes more complex where different ice types are present. Despite the presence of distinctive basal layers, modellers examining response behaviour often make the simplifying assumption that viscosity is uniform. More research is needed to establish and isolate the causal variables in ice facies rheological variability, to establish the nature and extent of variability, and to more fully elucidate the effects of ice heterogeneity on glacier response behaviour.

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