tor destruction       pits paper

periglacial landscapes plateau

Clach Bhàn

Weathering pits

Definition: Depressions on a flat surface and varying in shape and ranging in size from a few centimetres to several metres in width. Weathering and erosion enlarge the pits; two or more pits may combine.

Weathering pits are a common feature of granite terrain. In the Cairngorms, weathering pits are found predominantly on flat granite surfaces away from valley and corrie floors.

Weathering pits vary in size. On surfaces that were heavily glaciated during the Late Devensian, deep pits are absent and post-glacial weathering has allowed only the development of shallow saucer-like depression a few cm deep. On the plateau, weathering pits occur in a variety of geomorphic settings. Large, tabular blocks within blockfields sometimes show pits up to 10 cm deep. Glacial slabs and roche moutonnées have pits up to 30 cm deep. The largest pits occur on tors, such as Clach Bhàn, where individual pits can up to 1 m deep. These pits occur in association with edge pits and deep grooves or lapies on the outer surfaces of the tors.

The weathering pits provide insights into the relative ages of various elements of the Cairngorm landscape. The absence of pits from surfaces eroded by ice in the Late Devensian indicates that the initiation of pits is a slow process. Once a depression has formed, however, it seems likely that pit deepening is relatively rapid and pits which survive beneath a cover of cold-based ice can be reoccupied again and again. As the Cairngorm tops were covered  by snow and ice for much of the last 80, 000 years, the formation of pits >10 cm deep probably dates from the warm phases of Oxygen Isotope Stage 5 (130-80 kyr). The deepest pits and sculpted tor surfaces probably reflect exposure during long periods of the Quaternary.