A fossil valley-wall rock glacier in the Cairngorm mountains

Abstract

Fossil rock glaciers in Scotland have previously been described in Wester Ross (Sissons 1975), in the Isle of Jura (Dawson 1977) and in the Cairngorms (Sissons 1979). The most distinctive rock glacier in the last area occurs at the foot of a talus slope that descends from the N to NE facing wall of Coire Beanaidh to the north of the Braeriach Plateau [NH 953 013]. Its maximum length and width are 480 m and 240 m respectively. At the frontal margin it rises up to 65 m above the adjacent ground. This note presents a study of this rock glacier, comprising mapping and analysis of the component material. The main mass of the Cairngorm mountains is formed of medium to coarse grained granite. The main granite, composed chiefly of oligoclase and quartz, is the bedrock in the area. Many of the Cairngorm corries were occupied or partly occupied by glaciers during the Loch Lomand Stadial but some, including Coire Beanaidh, lacked glaciers (Sissons 1979). Fossil rock glaciers normally indicate the former existence of permafrost and associated large-scale frost shattering. The Coire Beanaidh is typical of valley-wall rock glaciers identified in the Colorado Front Range by Outcalt and Benedict (1975), for it occurs at the base of a talus slope and extends onto a valley floor to terminate with a tongue-shaped margin. The above authors, however, stated that this type “… is related to avalanching rather than glacial activity”. A detailed contour map of the rock glacier was made with . . .