Palaeonunataks and the altitude of the last ice sheet in the SW Lake District, England

Abstract

The mountains of the SW Lake District support a high-level weathering limit that separates an upper zone of shattered bedrock, blockfields and tors from a lower zone of glacially-moulded bedrock. This weathering limit declines from c. 870 m in the east of the area to 800—830 m in the NW. Analyses of the depths of stress-release joints and clay fraction mineralogy indicate more advanced weathering of bedrock and soils above the weathering limit, and thus imply that it marks the upper limit of Late Devensian glacial erosion. The weathering limit is therefore interpreted as a periglacial trimline cut around palaeonunataks, and thus as representing the approximate altitude of the surface of the last ice sheet at its maximum thickness. High-level striae and roches moutonnees confirm that lower peaks and cols in the area were over-run by ice moving westwards or southwestwards. consistent with radial movement of ice away from an independent ice dome located over the central Lake District. The inferred ice altitude and direction of ice movement are reasonably consistent with ice-sheet models proposed by Lambeck (1993, Geophysics Journal International , 15 , 960–990; 1995, Journal of the Geological Society, London , 152 , 437–448), but not with earlier models.