Deeply weathered psammite, Gaick plateau
Definition: breakdown of rock to depths of many metres induced by the chemical alteration of rock minerals by groundwater
Chemical weathering is the breakdown of rock induced by reactions between rock minerals and groundwater. If surface erosion is limited then weathering over thousands and hundreds of thousands of years reduces solid rock to a thick mantle of disintegrated, rotten rock.
In the lowlands of north-east Scotland, deep weathering is widespread (Hall, 1985). Its ubiquity reflects the low intensity of glacial erosion across this area, outside the main valleys. The low relief also reduced erosion rates prior to glaciation and allowed deep weathering profiles to develop.
In the Cairngorms, chemical weathering of the granite is
restricted in its extent and depth. The exposures provided by glacial cliffs demonstrate
that the granite is generally covered only by a thin regolith. Narrow
alteration zones occur in places where subaerial weathering has exploited
lines of weakness provided by hydrothermal alteration - rock alteration
induced by hot circulating waters during or after the cooling of the granite.
Nonetheless chemical weathering has been a significant
process in the formation of regolith on the Cairngorm plateau.
Weathered granite occurs locally in two main situations in the Cairngorms: