Deeply weathered psammite between Strath Ullie and Craig Scalabdale. Image courtesy of Colin Ballantyne
Definition: breakdown of rock to depths of many metres induced by the chemical alteration of rock minerals by groundwater
Deep weathering of bedrock occurs widely in southern Caithness and in adjacent areas of Sutherland. First noted by Godard (1965) and Omand (1975), its distribution and characteristics have yet to be mapped systematically. There are few reports of weathered rock from the parts of the plain of Caithness which were overrun by ice moving out of the Moray Firth. Instead, the weathering only becomes common in the area covered by ice moving from inland, a reflection of its much reduced capacity for erosion. This probably links to the former basal thermal regime of the glaciers, with much of the ice over inland areas being cold-based and protective.
The most impressive sections in deeply weathered rock occur just over the county boundary along the margin of the Helmsdale Granite at Suisgill. Here low-grade hydrothermal alteration has encouraged the disaggregration of the granite down to depths of many metres. The streams flowing into the Glutt, Berriedale and Langwell Waters show many good sections in weathered granite and schist. Typically, the degree of weathering is modest. Rock structures, including quartz veins, foliation and joints, remain clearly visible. Only the most vulnerable primary minerals have broken down so that the saprolite is a formed of blocks, granules and grains of broken rock with a low percentage of finer particles. Clay minerals are weakly developed but there is only a single investigation of their characteristics (Zauyah, 1976). This kind of weathering has been classified as the gruss weathering type and probably developed under temperate conditions similar to or a little warmer than today.
The age of weathering is unclear. Many saprolites are overlain by glacial deposits so must predate at least the latest phases of glaciation. Saprolite production by weathering appears to be rapid in geological terms, with formation of weathering profile 10 m deep possible in around a million years. The saprolites may represent the truncated portions of much deeper weathering mantles which existed before the Pleistocene ice age. In that case, the weathering probably developed over the last 10 million years, when warm to temperate climates prevailed in Scotland.