Anomalous terrain at Dove Crags ‘cirqueform’ and Gasgale Gill asymmetric valley, English Lake District, attributed to large-scale rock slope failure of pre-LGM origins

2016-06-21T11:55:51Z (GMT) by David Jarman Peter Wilson
<p>An unusually bold, cirque-like cavity, above a small, apparently fluvial side valley rich in landscape anomalies, is evaluated as a large-scale (<em>c</em>. 1 km<sup>2</sup>) rock slope failure (RSF) with possible pre-Last Glacial Maximum (LGM) origins. Although the failed mass is much subdued, field evidence includes dry gullies and a tarnless hollow; a sharp flank scarp and a high cutbank toe suggest reactivation. Indirect support is provided by a pioneering (in Britain) schematic reconstruction of a whole-valley pre-RSF landscape, employing a ‘clone-stamping’ procedure, which resolves the main terrain anomalies. The RSF interpretation is satisfactorily tested by: (1) compatible proxy cavity and slipmass volumes of 35 and 25 million m<sup>3</sup> respectively, with the deficit attributed to trimming by an LGM valley glacier; (2) comparisons with other RSFs by area, cavity parameters, and a total extant volume (derived from an inferred balanced long section) of <em>c</em>. 50 million m<sup>3</sup>; and (3) a proposed ‘zone of crush’ model that allows a large failed mass, <em>c</em>. 100–200 m thick, to descend incrementally by <em>c</em>. 300 m without disintegrating. A landscape paradox is identified whereby the valley now has an asymmetric, quasi-fluvial profile, with till covering the slope below the cirqueform cavity, yet is inferred to have had a pre-RSF glacial form and an LGM glacier. This is resolved by envisaging: (a) that a pre-LGM RSF created a cavity that was exploited by cirque glaciers on at least two occasions; (b) that the RSF was pared back by an LGM valley glacier; and (c) that it subsequently re-descended to the valley floor after the LGM. A pre-LGM origin for the RSF would account for the till-covered slipmass. It also allows time for excavation of the present cirque headwall by 50–100 m, and for the opposite valleyside to erode back by <em>c</em>. 50 m in response to a progressively displacing river and valley glacier. If this proposition is correct, Dove Crags would be one of the first substantial pre-LGM RSFs to be recognized in the glaciated mountains of Britain and Ireland, implying that others ought to be present. </p>