Rock slope failure in the Lake District, NW England: an overview

Abstract

ABSTRACT An inventory of 84 Rock Slope Failures (RSFs) (mean size 0.17 km2; total area 14.5 km2) is presented for the Lake District and Howgill Fells, northwest England. Most are developed on Ordovician and Silurian metavolcanics and metasediments, with a minority on Ordovician igneous intrusives. The RSFs are predominantly paraglacial in nature, a few are parafluvial. Rock slope deformations, rockslides, and rock avalanches occur in the proportions 46%/36%/18%. Some RSFs, or components thereof, pre-date the LGM, some are probably of Lateglacial Interstadial age, some may date from the YDS, and others are demonstrably of Holocene age. However, numerical ages are not available. A few RSF deposits have previously been mis-interpreted as lateral moraine, and either ice-cored (glacial) or protalus/talus-foot (periglacial) rock glaciers; some RSF cavities have been mis-interpreted as cirques, although they may be evolving into them. Spatial incidence of RSF is generally sparse; several areas lack evidence, but two clusters account for 56% of the population and 58% of the RSF area. Geological factors have greater influence over mode of failure than over spatial incidence; seismicity is unlikely to have been a prime cause. A Concentrated Erosion of Bedrock model could account for RSF clustering around glacially-breached cols and enlarging trough-heads, if petro-isostatic rebound is locally augmenting generic glacio-isostatic rebound stresses. RSF incidence in the Lake District can be seen as a microcosm of the Scottish Highlands pattern. The contribution of RSFs to landscape evolution and geodiversity in the area has been underplayed: some cases display bold impacts amenable to geo-interpretation.