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

Abstract

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.

Subglacial hydrology and drumlin sediments in connemara, western ireland

“…Deformation of LFA 1 therefore created subglacial relief and thus leeside accommodation space for the deposition of LFA 2, a situation commonly found in other Irish drumlins (Dardis et al . ; Hanvey ; McCabe and Dardis 1989b, ; McCabe ; Knight ). In detail, sediments comprising LFAs 2 and 3 are rhythmically bedded and formed as subaqueous gravity flows, mainly debris flows and grain flows, which correspond to repeated episodes of sediment loading on the developing drumlin apex and subsequent leeside failure.…”

Progressive formation of modern drumlins at Múlajökull, Iceland: stratigraphical and morphological evidence

“…Drumlins and drumlin fields are prominent features in the landscapes of Pleistocene ice sheets, such as in northern North America, Great Britain and Ireland, Scandinavia and the Baltic region (e.g. Menzies ; Hess & Briner ; Knight , ; Maclachlan & Eyles ; Stokes et al . ; Dowling et al .…”

Ductile and brittle styles of subglacial sediment deformation: An example from western Ireland

“…Bedrock strike, however, has not exerted a strong influence on ice flow direction, with striae and s-forms, where observed, commonly cross-cutting strike (Knight, submitted for publication). During the late Pleistocene glaciation, regional ice flow was dominantly westward through Clew Bay, similar to patterns elsewhere in western Ireland embayments (Knight and McCabe, 1997;Knight, 1999Knight, , 2011Knight, , 2012aKnight, , 2014. In Clew Bay this ice flow was in part directed by uplands to the north (Murrisk Mountains) and south (Croagh Patrick).…”