Protalus rampart development and the limits of former glaciers in the vicinity of Baosbheinn, Wester Ross

Abstract

Synopsis J. B. Sissons’ interpretation of a massive debris accumulation at the NW end of the Baosbheinn ridge in terms of two stages of protalus rampart development is shown to be inconsistent with the field evidence, and palaeoclimatic inferences and rockwall retreat rate calculations based on this interpretation are consequently invalid. Part of the debris accumulation is reinterpreted as two lateral moraine fragments that were deposited at the southern limit of westwards-moving ice from the Loch Maree trough at the maximum of the Wester Ross Readvance, some time before c. 13,000 B.P. During the Loch Lomond Stadial of c. 11,000–10,000 B.P. two valley glaciers advanced northwards on either side of Baosbheinn into the area formerly occupied by Wester Ross Readvance ice and truncated the earlier readvance limit, thereby isolating the lateral moraine fragments on the flanks of Baosbheinn. The upper part of the debris accumulation is apparently a giant protalus rampart of Loch Lomond Stadial age, but various characteristics of this feature suggest that unlike other Lateglacial ramparts it was not formed by slow rockfall accumulation. Instead, it probably formed when a massive rockslide or series of rockslides involving failure of c. 200,000 m 3 of rock over-rode a former snowbed and accumulated as a boulder ridge that partly buried the higher lateral moraine fragment.