Tor and linear alteration zone, Cnap a' Chleirich
Etching of joints, Stob an t' Sluichd
Tor with avenue and closely-spaced sheet joints, Beinn Mheadhoin
The spine of tors on the north ridge of Bynack Mòr
Tors and structure
"Not a touch is given, not a block is carved out, not a spire is isolated, not a boulder is detached, save in obedience to strict and ascertainable law. And the law is that the destruction of the granite is effected by means of its joints." Sir Archibald Geikie
The structure of the Cairngorm Granite comprises faults, linear zones of hydrothermal alteration and the ubiquitous joint systems. Joint densities and trends exert a fundamental control over the location and shape of features in all granite terrains. Waters' model developed for Dartmoor shows that the alignments of valleys and ridges often run parallel to joint trends and that the high and low points in any granite landscape are usually a reflection of joint spacing.
The tors represent compartments of granite that have proved to be relatively resistant to weathering and erosion. One important lineation that defines the location of bounding valleys in the Cairngorms is provided by zones of hydrothermal alteration, lines of weakness predisposed to rock breakdown (Thomas, et al, 2004).
Beinn Mheadhoin
Many Cairngorm summit tors cap low domes or curved ridges, notably at the eastern end of Ben Avon. The hill slopes are parallel to sheet joints in the granite and these are also an important control on the outline of the tor. Others form spines or cockscombs, as on Bynack Mor and Stob an t-Sluichd.
The detail of tor form is mainly due to the disposition of
joints. A number of large tors are developed essentially in monoliths, where
vertical joints may be almost 10 m apart. The vertical joints define the
tor faces and, in places, avenues are developed within tor groups where linear
zones of closer jointing occur. Horizontal and gently curving sheet joints
have been etched by weathering to produce a cyclopean masonry of immense
variety.