Direct observations of deforming sedimentary layers beneath glaciers come from tunnels and cavities. Boulton and Hindmarsh (1987) excavated a series of tunnels within the basal ice of Brei9amerkurjokull in Iceland, from which a series of strain markers (small cylinders) and other instruments were inserted through drill holes into the subglacial till. The experiment lasted for 136 hours, during which time glacier velocity and the subglacial water pressure were monitored. At the end of the experiment water was pumped from the subglacial bed beneath the ice tunnels, and a section was dug through the bed to study the strain markers. The results are shown in the diagram below. Displacement of the strain markers clearly illustrates the deformation of an upper horizon of till over a relatively undeformed lower layer. It was estimated that approximately 90% of the forward movement of the glacier was due to this subglacial deformation. These observations were used to derive a mathematical flow law to describe the behaviour of glaciers overlying deformable sediments. This flow law has subsequently been used to model and predict the behaviour of deforming sediments beneath glaciers and is critical to theories about the formation of subglacial landforms. More recently, Iverson et al. (2007) conducted experiments on a large prism of sediment (1.8m x 1.6 m x 0.45 m) emplaced beneath 213 m of ice in a tunnel under Engabreen, Norway (see Boxes 5.1 and 5.2). Their experiments lasted between 7 and 12 days, during which time the glacier froze downward into the prism to depths of 50-80 mm, adding sediment to the glacier sole. They also pumped water into the sediment and found that the behaviour of the subglacial sediment was greatly determined by pore-water pressure. At near zero pore-water pressure the glacier slipped across the bed surface; pumping water into the prism increased pore-water pressure, which weakened the sediment and caused it to deform.
Sources: Boulton, G.S. and Hindmarsh, R.C.A. (1987) Sediment deformation beneath glaciers: rheology and geological consequences. Journal of Geophysical Research, 92, 9059-82. Iverson, N.R., Hooyer, T.S., Fischer, et al. (2007). Soft-bed experiments beneath Engabreen, Norway: regelation infiltration, basal slip, and bed deformation. Journal of Glaciology, 53, 323-40. [Modified from: Boulton and Hindmarsh (1987) Journal of Geophysical Research, 92, figure 2, p. 9062].
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