Rock-slope failure at Baosbheinn, Wester Ross, NW Scotland: age and interpretation

Abstract

SynopsisA massive arcuate ridge composed of large tabular boulders below a failure scar at the NW extremity of Baosbheinn has previously been interpreted as a protalus rampart or talus rock glacier of Loch Lomond Stadial (c. 12.9–11.5 ka) age. The ridge occurs downslope of a rockslide failure zone flanked by steep rock buttresses, and is here reinterpreted as consisting of rockslide debris produced by failure of 0.5–0.6 Mt of rock from the backing slope. The runout debris is inferred to have lost momentum as it impacted the basal break of slope and consequently accumulated as a steep-fronted ridge with a gentle backing depression, partly burying a lateral moraine attributed to the Wester Ross Readvance (WRR) of c. 14.0–13.5 ka. Cosmogenic 10Be exposure ages for large boulders on the rockslide runout debris indicate that the rockslide occurred between 14.0 ± 1.6 ka and 13.3 ± 1.2 ka under cool temperate conditions during the Lateglacial Interstade. Failure probably occurred within a few centuries after deposition of the over-ridden WRR moraine, and is attributed to rock-slope weakening due to deglacial unloading and consequent paraglacial stress release, possibly enhanced by thaw of ice within bedrock joints.