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

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.

KEYWORDS:

• Rock slope failure
• paraglacial
• concentrated erosion of bedrock
• Last Glaciation
• Younger Dryas
• northwest England

Acknowledgements

Many friends and colleagues have been involved in discussions about and/or field visits to the sites listed. In particular we thank Tim Davies, Ian Evans, Wishart Mitchell, Eva Sahlin, Alan Smith and the late Richard (Dick) Clark, and participants in the 2015 Quaternary Research Association field excursion ably organized by Derek (Des) McDougall and Dave Evans. We are grateful for two supportive reviews which have prompted numerous clarifications, and again to Ian Evans for a meticulous critique of the revised paper, which has greatly helped clarify our views.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Notes on contributors

Peter Wilson

Peter Wilson is Emeritus Reader in Quaternary Environmental Change at Ulster University, Northern Ireland. His research interests are in the realms of glacial, periglacial and paraglacial geomorphology.

David Jarman

David Jarman is an independent researcher based in Scotland. He undertakes research into rock slope failures in the glaciated mountains areas of Britain and Europe, and explores the long-term evolution of upland landscapes.