Model of tor formation
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.
Unweathered granite, with variable density of vertical joints and slope-parallel
sheet joints becoming closer spaced closer to the ground surface
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.
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.
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.
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.