Rock-slope failure scars on sandstone mountains in NW Scotland

• Published in: Proceedings of the Geologists' Association
• Main Author: Ballantyne, Colin K.
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2024
• Subjects: Debris-free failure scars; Cirques; Glacial erosion; Rock avalanches; Rock-slope failure; Torridonian sandstone; Ice Sheet
• Online Access: https://research-portal.st-andrews.ac.uk/en/publications/69941602-2a51-4e84-9e09-d54a3d14d556; https://doi.org/10.1016/j.pgeola.2024.05.003

Amongst the most neglected landforms in formerly glaciated mountain environments are the scars of major rock-slope failures where the displaced rock or runout debris has been removed by glacial erosion. The characteristics of 33 such debris-free failure scars (DFFSs) on Torridonian sandstone terrain were analysed. These range from 0.011 to 0.183 km2 in ground area, and scar volume implies removal of ~0.4–9.9 Mt of rock. Failure planes are planar or stepped at gradients of 37–50°, implying failure as ‘rough’ translational slides; most probably represent former rock avalanches or fragmented rockslides where shear through bedding planes resulted in runout fragmentation. Extension of 85 % to the slope crest is consistent with coseismic triggering during periods of glacio-isostatic crustal uplift. At least ten DFFSs demonstrably pre-date expansion of the last ice sheet at ~35 ka, and at least five probably represent failure that occurred in the Lateglacial interval of ~16–11.7 ka. Fourteen are located within cirques, confirming the importance of RSFs in cirque extension during ice-free interglacials or interstades; others have contributed to trough widening and the formation of arêtes. Older upper-slope cavities in Torridonian mountains are probably also the results of (possibly recurrent) rock-slope failures. It is concluded that the large-scale morphology of the Torridonian sandstone mountains reflects a long-term synergic relationship between alternating episodes of glacial erosion and rock-slope failure: reduction of rock-slope stability during each glacial-deglacial cycle has predisposed slopes to eventual failure, with subsequent glacial removal of displaced blocks and runout debris.