Synopsis

On the mountains of southern Caithness and east Sutherland, geomorphological evidence for the upper limit of glacial erosion implies that, at the last glacial maximum (LGM), warm-based erosive ice extended up to altitudes of 410–600 m. Above the upper limit of glacial erosion is a cover of frost-weathered detritus, with tors on conglomerate bedrock. Kaolinite and gibbsite in soils on such terrain imply that it escaped significant glacial erosion at the LGM. This is confirmed by cosmogenic 10Be exposure ages of (>) 146.6±15.1 ka and (>) 178.2 ± 18.1 ka obtained for summit tors; sites below the upper limit of glacial erosion yielded deglacial ages of 16.6 ±2.1 to 18.5 ± 2.4 ka. It is unlikely that glacially unmodified summits represent LGM nunataks, as the implied ice thickness is inconsistent with an ice sheet that extended across high ground on Orkney onto the Atlantic shelf; under conservative assumptions at least 70 m of ice must have covered the highest ground in southern Caithness at the LGM. We infer that the absence of evidence for glacial erosion on some summits and plateaux reflects cover by cold-based ice, frozen to the underlying substrate, that preserved the underlying tors and blockfields. This inference calls into question the use of similar evidence in NW Scotland to identify the maximum altitude of the last ice sheet.

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