The central chamber of Clach Bhàn
Weathered granite and the base of a tor, West Meur Gorm Craig
Origins of the Cairngorm tors
There is general agreement that the tors reflect long-term differential weathering and erosion. The zones of widely-spaced joints weather more slowly than their surroundings and the removal of the weathered material leads to the emergence of the tor.
What is less certain is the nature of the processes of weathering and removal. In the classic models of tor evolution in SW England, weathering is considered to be a result of
whilst removal of weathered material is attributed to solifluction under periglacial conditions during the Ice Age.
Neither model appears to fit closely the evidence for the origins of the Cairngorm tors. Firstly, deep weathering appears to be absent from the plateau, as shown in the long, continuous sections in the plateau edge provided by the cliffs of corries and troughs. Such deep weathering may well have existed in the Tertiary but if no pockets remain then it is unlikely that the present tors are related to former deep weathering. Equally, although there is abundant evidence of frost-riving and block fall around small tors, many large tors have few blocks at their base (Ballantyne, 1994. This indicates that large scale frost shattering has been unable to make an impression on monolithic tor blocks.
The emergence of small tors is more likely to reflect the cyclic development and stripping of relatively thin regolith from the plateau during the Quaternary. The illustration above shows a small tor with an undercut base. This basal notch is quite common on tors in the Cairngorms and it seems to reflect the increased rate of weathering within former soil layers, as compared with bare rock surfaces. The availability of water and organic acids in the regolith promotes granular disintegration due to chemical attack, whilst also allowing micro-scale frost action to continue. The protruding rock surface remains relatively dry and weathers more slowly. Repeated development of regolith and stripping by solifluction and wash, as well as glacial erosion, may then allow the tor to grow in height.
Basal notches and granular disintegration also occur around the base of larger tors, such as Clach Bun Rudhtair. Yet the size of these tors, up to 15 m high, indicates that many cycles of regolith development and stripping probably occurred and that these tors therefore have a long history. Recent cosmogenic exposure ages indicates that the largest tors have summits which have been exposed for >500,000 years (Phillips, et al., 2006). Emergence rates are estimated at 11-31 mm/ka, allowing small tors to emerge quite quickly.