Subglacial erosion, deposition and deformation associated with deformable beds

Abstract

There is an inter-relationship among subglacial deformation, deposition and erosion associated with a glacier moving over a deformable bed. It is suggested that both subglacial deposition and erosion are produced by a number of processes at the 1) icesheet/subglacial deforming layer interface; 2) within the subglacial deforming layer; and 3) at the base of the deforming layer. These processes may result in two styles of net subglacial deforming bed deposition: 1) constructional deformation where the deforming layer moves upwards through the sequence; or 2) excavational deformation where the deforming layer moves down through the sequence. It is suggested that the best evidence for net subglacial deforming bed erosional conditions are drumlins. Changes in subglacial hydrology are also discussed, and the relationship of eskers and tunnel valleys to deforming bed conditions. Sedimentary techniques to identify homogeneous deforming bed tills in the field are also suggested. Criteria include a wide range of fabric strengths depending on the thickness of the deforming layer, rotated clasts, miniflutes, flow around clasts, predictable basal till units (constructional or excavational) and possibly boulder pavements. Finally the extent and style of deforming bed conditions that existed during the glaciations of Britain and Ireland are discussed.