Model of tor formation

The Cairngorms tors have formed by emergence after repeated formation and stripping of thin soils and regolith over the last million years. The impact of glaciation has been variable, with no modification of some summit tors, especially on Ben Avon and the complete removal of others to leave slabs and stumps.

Stage A
Unweathered granite, with variable density of vertical joints and slope-parallel sheet joints becoming closer spaced closer to the ground surface
 

Stage B
Emergence of resistant monoliths as small tors after differential weathering and erosion. Weathering under Quaternary cold climates by chemical disintegration and frost action produces a thin regolith. Wetness at the base of the tor leads to accelerated weathering and the formation of a notch. Weathering pits form on flat surfaces.

Stage C

Repeated formation and stripping of thin regolith leads to further tor emergence. The exposed blocks shed water to the surrounding regolith, further accelerating weathering. Weathering pits are gradually enlarged from small depressions on flat surfaces.



Stage D

Tower tor. The blocks have been rounded by surface disintegration and wind action. Joints are opened and weathering pits reach 1 m deep. By this stage the tor summits may have been exposed for >500,000 years.



Stage E

Glacial erosion. Removal and toppling of summit blocks to leave a more subdued form. Erratics may litter the surroundings of the tor. Regolith is wholly or partly removed. Movement of large tor blocks by ice is dated to at least the last three major phases of glaciation.