Caithness Field club Bulletin
April 1975

Deep Weathering of Rock in Caithness
D. Omand

Deep weathering of rocks, thought to be due to the tropical or sub-tropical climate of the Tertiary, has been widely reported in Scotland (e.g. Sissons 1967). Caithness is no exception. Here, sites showing deep chemical weathering or pegmatite and conglomerate (Crampton et alia 1914) and of sandstone and granite (Fitzpatrick 1963, Smith 1968) have previously been reported. In the adjacent county of Sutherland pre-glacially weathered gneiss was recorded by Charlesworth (1955), and Godard (1965) found evidence of old Terra-Rossa type soils in fissures in the Cambrian limestone reminiscent of those found in Mediterranean lands.

Personal fieldwork in Caithness led to the recording of additional sites of deep weathering in sandstone, flagstone, mudstone, granite and mica-schist. At Altnabreac station(ND003457) the granite had weathered down to the base of the exposed section, viz. 5m (16ft). Granites and associated rock types appear to have decayed quite readily to a coarse arkose deposit and impart to the landscape a smooth outline such as is found in the Knockfin Heights and in the exhumed land surface of western Caithness where it borders Sutherland. The fact that the outcrop of Reay diorite occupies a basin is also suggestive of a history of pre-glacial deep weathering.

Deep weathering in such a diversity of rock types might indicate that a thick, comminuted regolith (soil mantle) covered the northern landscape at the outset of the Quaternary, although increasing stream incision due to the cooler conditions coupled with land uplift in later Tertiary times my have already begun the removal of some material.

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