The Beinn Alligin rock avalanche, NW Scotland:: cosmogenic 10Be dating, interpretation and significance

A tongue of very coarse rockslide debris that extends 1.25 km downvalley below Beinn Alligin in NW Scotland has been variously interpreted as a glacier-cored rock glacier, landslide debris redistributed by glacier ice or in excess-runout landslide. Exposure dating with cosmogenic Be-10 demonstrates that the the debris mass was emplaced at 3950 +/- 320 yr BP, and therefore was not associated with glacier ice. Calculations based on frictional considerations imply that the feature is an excess-runout rock avalanche (sturzstrom) deposit. The morphological characteristics of the deposit appear consistent with movement by grainflow or fragmental flow. Failure is inferred to reflect time-dependent paraglacial stress release and consequent propagation of an internal joint network. but may have been triggered by seismic activity. The late-Holocene age of failure implies persistence of the effects of paraglacial stress release over a time-scale of several millennia.