Rock avalanche scars in the geological record: an example from Little Loch Broom, NW Scotland

An embayment in the southern rock slopes of Little Loch Broom, NW Scotland, was surveyed using a combination of morphological mapping and engineering geology techniques. The evidence points to a large-scale rock slope failure (rock avalanche) mechanism of formation, occurring during glacial downwasting and retreat, perhaps initiated by debuttressing and stress release. Periglacial activity, such as macrogelivation or large-scale frost-wedging, may have exacerbated the internal joint network by expanding the aperture dimensions of pre-existing joints leading to an increase in stresses and a reduction in friction across joint surfaces. The rockwall cavity was completely evacuated by the rock debris, leaving the embayment with a mantle of sediments originating from the glacial diamicton that drapes the plateau above the backwall. The absence of any rock avalanche debris deposit has led to the conclusion that the debris run-out extended onto the surface of the glacier and was supraglacially transported and dispersed, which has implications for glacial dynamics and local glacial history for Little Loch Broom.