snow patches

Regolith comprising small blocks in a matrix of pebbles, granules and grit between Beinn a' Bhuird and Stob an't Sluichd

Other types of regolith cover on the plateau

Blockfields lie at one end of a spectrum of regolith types on the Cairngorm Granite. Whilst a blockfield shows only blocks at the surface, other regolith covers show that the surface content of blocks and matrix material can vary. On parts of the plateaux of Braeriach and of Beinn a'Bhuird, a continuous and largely vegetated regolith is developed which lacks large blocks. The characteristics of the regolith have been studied in detail by Haynes et al., (1998).

Vegetated and stable regolith, comprising large blocks in a matrix of granular grit. Beinn a' Bhuird.

Structure is a major control on the character of the regolith. Parts of the Main Granite appear to be predisposed to granular disintegration, perhaps due to micro-fissures between minerals, and these form a thick regolith with few blocks. In blockfields, both openwork and matrix-rich, a modest degree of glacial disturbance shows that block size is related to the spacing of horizontal and vertical joints.

As little regolith has developed subsequently on surfaces stripped by ice during the Late Devensian, it is clear that deep regolith developed at an earlier stage. Frost riving has been an important process in detaching blocks from bedrock - the size of the blocks moved probably requires frost wedging and the former existence of permafrost. The growth of lenses of ground ice within joints would promote block detachment. Frost shattering has promoted granular disintegration and the formation of the sharp polymineralic grit in the matrix. Scanning electron micrographs show a dominance of mechanical breakage in the sand fraction (Paine, 1982). Some fine material may have been former in this manner but the importance of chemical alteration within the regolith should not be ignored. The contrast between the relatively smooth surfaces of exposed blocks and the roughened surfaces of buried blocks is probably a reflection of the higher moisture content of the regolith and the greater potential for chemical weathering. Mellor and Wilson (1989) recorded the clay mineral gibbsite from the C Horizons of soils at 12 sites on the Cairn Gorm and Ben Avon plateaux. Similarly, the high solute load of granite springs is indicative of significant modern rates of chemical weathering (Soulsby et al., 1989).

Contrasting formerly exposed and buried surfaces on a granite block, ski slopes adjacent to the top funicular station