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Dartmoor's overlooked glacial legacy



Traditionally regarded as a relict permafrost and periglacial landscape that lay beyond the limits of Pleistocene glaciation, the granite uplands of northern Dartmoor in south-west England in fact contain geomorphological evidence for the former existence of a plateau ice cap, making the area the location of the southernmost independent glacier mass in the British Isles. In addition to weakly U-shaped valleys, the most prominent evidence comprises arcuate and linear bouldery ridges and hummocky valley floor drift, which are interpreted as latero-frontal moraines deposited by the outlet glacier lobes of a plateau ice cap. Inset sequences of these depositional landforms, in association with meltwater channels, demarcate the receding margins of the glacier lobes. A numerical model of ice cap development shows that a predominantly thin plateau icefield type glaciation is required in order to produce significant ice flow into surrounding valleys. The highest and most extensive plateau areas were occupied by ice for the longest cumulative period of time throughout the Pleistocene, thereby explaining: (1) the lack of tors in such areas as the product of ‘average’ glacial conditions preferentially removing tors or dampening their production rates, (2) the survival of high relief tors during glaciation if they occupied summits too narrow to develop thick and erosive glacier ice, and (3) the survival of subdued tors in areas glaciated less regularly during the Pleistocene.

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