Geomorphic and Modeling Evidence for a Late Pleistocene Cirque Glacier in Southwest England

Abstract:

Several small ice caps have been identified outside of the margins of the last (late Devensian) British ice sheet, but few sites in the uplands of southwest England show evidence for late Devensian niche/cirque glaciers and/or perennial snowbanks. Here we present evidence for the southernmost site yet discovered, in west Penwith, Cornwall. Here, a NE-facing hollow developed on granite bedrock is surrounded by tors with deep bedrock fractures and displaced angular blocks. Bedrock is exposed at the base of the hollow but sediments form a subdued ridge, interpreted as a terminal moraine, at the front of the hollow. The sediments comprise gravelly diamicton containing facetted clasts that are occasionally striated, interbedded with sand lenses. Sediments are deformed by periglacial involutions. We used a 2-D glacier energy balance and iceflow model (Plummer and Phillips, 2003, Quat Sci Rev) to discover if the climatic conditions during the late Devensian could have sustained a cirque glacier at this site. Met Office regional climate data (1981-2010) were used to define mean monthly air temperatures, lapse rate, precipitation, cloudiness and relative humidity, and surface energy fluxes were calculated based on solar position during the present day and last glacial maximum (LGM). We ran a range of different glacier model simulations to test an envelope of late Devensian paleoclimate conditions inferred from other climate proxies. Model results suggest that a cirque glacier could have formed at this site under conditions equivalent to a change in temperature from present-day values of –8.5°C accompanied by a 20% decrease in precipitation, which is consistent with the regional LGM. Response times for these simulations are greater than 500 years, indicating that it is unlikely that such cirque glaciers maintained steady state and were instead short-lived features.