Deeply weathered granite, Caol Ghleann
Deep weathering profiles
Definition: vertical changes in the characteristics of weathered rock between the surface and unaltered rock
Deep weathering of the Cairngorm Granite is restricted in its extent. Profiles up to 10 m deep are found close to valley floors. At these sites, there is little change in the characteristics of the weathered granite with depth - it is a granular grit throughout, with a low fines content. Clay minerals include kaolinite and gibbsite, together with illite and chlorite. These latter clays probably reflect inheritance from hydrothermal alteration. Rock breakdown can be seen as a combination of weakening by hydrothermal alteration during cooling of the granite, opening of micro-fractures due to stress release as the granite approaches the ground surface in response to erosion and a final phase of subaerial weathering.
Deep weathering of sandy metamorphic rocks is widespread on the Moine Mhor and its preservation reflects the former existence of cold-based ice over the western part of this plateau. The deepest weathering profiles, however, occur in the Gaick Forest. Here stream sections show up to 15 m of weathered psammite, often overlain by till and sand and gravel (Hall and Mellor, 1988). The Gaick area supported an independent and probably cold-based ice dome during the Quaternary (Sissons, 1974).