Abstract
Video observations made in 16 boreholes drilled through a deforming valley glacier affirm that temperate glacier ice may be reasonably well represented as homogeneous in glacier flow models, but raise warnings about the complexities of basal boundary conditions and glacier sliding. Discrete englacial structures, including clear-ice layers, voids, and water conduits, compose a total of <3% of the ice mass. Planar features (clear-ice layers) are oriented near vertical and are not aligned with the sense of shear strain, meaning that the layers probably do not influence the homogeneity of the strain. Both direct observations of the ice and analysis of its light reflectance suggest an increase in crystal size and decrease in bubble content with depth. However, previous laboratory work indicates that such changes are unimportant in terms of the viscosity of the ice. Observations of the basal boundary or sliding surface indicate that there are areas of both “hard” bedrock and “soft” deformable till, which should cause spacial and temporal gradients in sliding rate.
