ABSTRACT. Eight relict rock-slope failures (RSFs) on. Skiddaw Group terrain in the Lake District, northwest. England, are described. Five of the failures are rock-.
GEOMORPHOLOGICAL CHARACTERISTICS AND SIGNIFICANCE OF LATE QUATERNARY PARAGLACIAL ROCK-
GEOMORPHOLOGICAL CHARACTERISTICS AND SIGNIFICANCE OF LATE QUATERNARY PARAGLACIAL ROCK-SLOPE FAILURES ON SKIDDAW GROUP TERRAIN, LAKE DISTRICT, NORTHWEST ENGLAND BY PETER WILSON1 AND ALAN SMITH 2 1 School
of Environmental Sciences, University of Ulster, Coleraine, UK 2 Rigg Side, Grange Park, Keswick, Cumbria, UK
Wilson, P. and Smith, A., 2006: Geomorphological characteristics and significance of Late Quaternary paraglacial rock-slope failures on Skiddaw Group terrain, Lake District, northwest England. Geogr. Ann., 88 A (3): 237–252.
ABSTRACT. Eight relict rock-slope failures (RSFs) on Skiddaw Group terrain in the Lake District, northwest England, are described. Five of the failures are rockslides, one is a product of slope deformation, and two are compound features with evidence for sliding and deformation in different sectors. As none appears to have been overrun and modified by glacier ice it is concluded that they all post-date the Last Glacial Maximum (LGM; c.21 ± 3 cal. ka BP). Slope stress readjustments resulting from glacial and deglacial influences are considered to have weakened the slopes, and application of the term paraglacial is appropriate. Permafrost aggradation and degradation, seismic activity and fluvial erosion are among processes that may have contributed to failure at certain sites. The failures are significant as potential debris sources during future ice advances, contributing to valley widening and cirque enlargement and, possibly, for acting as sites of cirque initiation. Previously, Skiddaw Group rocks have been regarded as homogeneous and of limited resistance to the weathering and erosion associated with Quaternary glacial, periglacial and fluvial processes. These characteristics and processes have been used to explain the steep smooth slopes and rounded hills that dominate Skiddaw Group terrain. Rock-slope failure has also helped shape this terrain and should be incorporated in future interpretations of landscape development. Key words: rock-slope failure, Skiddaw Group terrain, paraglacial, Last Glacial Maximum, Late Quaternary, landscape evolution, Lake District.
Introduction The Lake District is an upland region (60°. Below the ridges, slope irregularities contin© The authors 2006 Journal compilation © 2006 Swedish Society for Anthropology and Geography
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Fig. 8. The Latrigg rock-slope failures (W, western; E, eastern). Contour interval is 50 m and marginal grid interval is 500 m. See Fig. 2 for explanation of symbols (© Crown copyright Ordnance Survey. All rights reserved).
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ue downslope towards the River Greta but this area has not been mapped in detail because of the woodland cover. Clough Head Clough Head (726 m a.s.l.; Fig. 1) is at the northern end of a ridge of high ground that runs from near Ambleside in the south to near Keswick in the north. Two RSFs have been identified. The western RSF (Fig. 9a) has a prominent arcuate head scarp 30 m in height and consists of talus and small areas of exposed bedrock. This scarp and the conspicuous flank scarps, which decline in height downslope, define a distinct basin-like failure cavity. The cavity floor is stepped, and levee-like vegetated ridges trend obliquely downslope from the head and flank scarps towards the longitudinal axis of the failure track. Below the cavity, hummocky displaced material extends down to c. 270 m a.s.l., and streams rise at 300 m and 250 m a.s.l. Surface morphology is less irregular farther downslope. The toe of the failure extends onto the valley floor. The northern RSF (Fig. 9b) is characterised by 245
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Fig. 9. (a) The Clough Head (West) rock-slope failure. Contour interval is 50 m and marginal grid interval is 500 m. (b) The Clough Head (North) rock-slope failure. Contour interval is 50 m and marginal grid interval is 200 m. See Fig. 2 for explanation of symbols (© Crown copyright Ordnance Survey. All rights reserved).
an array of low antiscarps and narrow benches. Antiscarps are especially prominent in the central part of the failure where they form sinuous ridges up to 340 m in length (Figs 9b and 10). Backslope depressions have a maximum depth of 2 m. Above and below the antiscarps there are several benches, the longest of which is broadly arcuate in plan and 350 m in length (Fig. 9b). Bench surfaces are up to 5 m wide and in places they pass laterally into low (
