Ploughing block on the slopes above Clach Bun Rudhtair, looking down the rear furrow

regolith  solifluction patterned ground

nivation hollows  weathering pits

Ploughing blocks

Definition: boulders that move down slope through the surrounding soil, leaving a vegetated furrow behind and forming a turf-covered rampart ahead

Ploughing blocks occur at numerous locations on the plateau, notably southwest of Cairn Gorm. The blocks are large, up to 3.5 m long, and the furrow can be of equivalent length (King, 1968). Haynes et. al. (1998) describe a typical location as “towards the edge of medium-lie snow patches on moderately steep, usually concave slopes (5-30°)”.

The freshness of the furrow and rampart leave little doubt that these are active periglacial forms. Shaw (1977) recorded rates of movement of 0.3-8.7 mm/yr on Lochnagar, rates that are directly comparable to those observed on the Drummochter Hills (Chattopadhyay, 1986). Movement is generally faster on steeper slopes (Chattopadhyay, 1983; Ballantyne and Harris, 1994) and mostly takes places during the spring thaw.

How do these large blocks move? One view is that the greater ability of the blocks to conduct heat leads to the development in winter of ice lenses in the regolith beneath them. During the spring thaw, melt of these ice bodies, together with saturation of the surrounding soils by snow melt, allows to regolith to behave as a viscous liquid and the block to move a short distance down slope. This liquefaction would appear to account for the bow-wave of soil in front of the block (Haynes et. al., 1996).