Main image above: Suisgill Burn

Definition: slow breakdown of rock to depths of many metres induced by the chemical alteration of rock minerals by groundwater

Deeply weathered psammite between Strath Ullie and Craig Scalabdale

Image courtesy of Colin Ballantyne

Deep weathering is interesting partly because its distribution tells us something about the effectiveness of glacial erosion in the Ice Age. 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. In contrast, 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 lower capacity for erosion. This links the survival of deep weathering to the former basal thermal regime of the glaciers. Much of the glacier ice over inland areas was frozen to its bed – the ice moved internally by creep, without sliding.

Deep weathering is also interesting because it is old and tell us about former climates. The most impressive sections in deeply weathered rock in Caithness occur just over the county boundary along the margin of the Helmsdale Granite at Suisgill. Here low-grade alteration by geothermally heated groundwater in the distant past facilitated disaggregation of the granite down close to the present surface to depths of many metres. Low grade gold mineralisation was weathered out of the granite and Au was concentrated by wash and gravity sorting on stream beds. The resultant placer gold led to the Great Sutherland Gold Rush of 1869.

The remote headwater streams flowing into the Glutt, Berriedale and Langwell Waters show many good sections in weathered granite and schist. Typically, the degree of chemical alteration of the rock minerals 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 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 sandy weathering developed under temperate conditions similar to or a little warmer than today.

The age of weathering is unclear and may vary between localities. Many saprolites are overlain by glacial deposits so must predate at least the latest ice sheets. 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 most likely developed over the last 10 million years, when warm to temperate climates prevailed across Scotland.

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