Talus accumulation and Rockwall retreat, Trotternish, isle of Skye, Scotland

Abstract

Calculations based on the volume of talus accumulations at the foot of a basalt cliff indicate that 4.3–7.8 m (mean 5.6 m) of rockwall retreat has occurred since deglaciation at 17.5 Cal ka BP, implying average rockwall retreat rates of 0.25–0.45 m ka^‐1 (mean 0.32 m ka"') over the last 17,500 years. The composition of the talus sediments indicates that approximately 70 % of total cliff retreat reflects rockfall, and that the remainder has been due to granular weathering of the face. However, radiocarbon‐dated soil horizons within the talus suggest that rockfall has been very limited since the end of the Loch Lomond Stade at c. 11.5 Cal ka BP, implying that about 80% of total rockwall retreat occurred between 17.5 Cal ka BP and 11.5 Cal ka BP. Rockwall retreat rate during this period is estimated to have averaged c. 0.75 m ka"1. This relatively high rate of cliff recession probably reflects stress release following deglaciation and/or frost shattering under periglacial conditions. Current rates of cliff retreat due to granular weathering are estimated to average 0.10 m ka"1, and probably exceed those due to rockfall.

Key words:

• Rockfall
• Granular disintegration
• Rockwall retreat
• Talus
• Stress release
• Frost shattering