Subglacial processes and drumlin formation in a confined bedrock valley, northwest Ireland

Abstract

Knight, J. 2010: Subglacial processes and drumlin formation in a confined bedrock valley, northwest Ireland. Boreas, 10.1111/j.1502-3885.2010.00182.x. ISSN 0300-9483. Subglacial processes beneath the Late Weichselian ice sheet in northwest Ireland are deduced from sediments and structures within drumlins in a bedrock valley at Loughros Beg, County Donegal. Here, a glacially smoothed bedrock surface underlies the drumlins, which are composed on their up-ice side of stacked, angular rafts of local bedrock. Overlying and down-ice from these rafts are down-ice-dipping beds of massive to bedded diamicton that contain sand and gravel interbeds. In a down-ice direction the diamicton matrix coarsens and the beds become laterally transitional to water-sorted gravels. The down-ice end of one drumlin shows a concentrically bedded stratified gravel core aligned parallel to ice flow and resembling the internal structure of an esker. With distance away from this core, the gravels become more poorly sorted with an increase in matrix content, and are transitional to massive to stratified diamicton. A four-stage model describes the formation of drumlins in this sediment-poor setting. The sediments that are located directly above the bedrock represent deposition in a semi-enclosed subglacial cavity. A trigger for this process was the formation of subglacial relief by the thrusting up of bedrock rafts, which created the leeside cavity. Subsequent sediment deposition into this cavity represents a form of feedback (self-regulation), which may be a typical characteristic of subglacial processes in sediment-poor settings.