Subglacial Shearing and Crushing, and the Role of Water Pressures in Tills from South-East Iceland

Abstract

Till exposed at the retreating margin of Breidamerkurjökull has a two-layer structure; an upper horizon of high void ratio and with a high concentration of fines, and an underlying, denser, less fine-grained horizon with a platy structure. It is suggested that the upper horizon has undergone continuous subglacial shearing in which dilation and interparticle crushing have occurred, the latter having produced the high concentration of fines, and that much grain crushing generally attributed to glacial transport may well be post-depositional in origin. The lower horizon is thought to have lain below the level at which shear stresses imposed by the overriding glacier were attenuated to values below the strength of the till, and thus to have undergone compaction only.Under normal circumstances, the frictional strength of the till will rise under increasing ice loads whereas the basal shear stress will remain constant, so that till deformation will be largely a marginal phenomenon. However the effective pressure in the till is reduced by the existence of a subglacial water pressure.Equations are developed which show how the effective pressure in subglacial beds depends not only on the shape and size of the glacier but also on the compressibility of the beds and their hydraulic transmissibility at zero effective pressure. A low hydraulic transmissibility gives a low effective pressure, which is conducive to deformation of subglacial sediments.Implications for glacier surging, drumlin formation and till fabric orientation are discussed.