A view from the plateau edge down into Loch Avon along a major vertical joint with signs of hydrothermal alteration


hydrothermal alteration

How the granite pluton has shaped the relief

Feith Buidhe

Close to the granite margin in Strath Nethy

Garbh Uisge Mor and its waterfall at the ice-moulded head of Glen Avon. Note the influence of at least 3 major joint sets: (i) the curved sheet joints control  the shape of the slabs, (ii) the gently curved vertical joints control the alignment of the rock steps and (iii) the perpendicular set of vertical joints that have been exploited to form the canyon of the waterfall.

Cairngorm Granite

The Cairngorm Granite is one of the largest plutons in the United Kingdom. The extensive Main Granite is a coarse, pink biotite granite. It shows both porphyritic and non-porphyritic facies. The smaller Glen Avon granite is a porphyritic white biotite granite. The Beinn Bhreac granite is a medium pink biotite poor granite and the Carn Ban Mor granite is fine-grained and porphyritic (Harrison, 1986).

The British Geological Survey has mapped the structure of the granite in detail (Thomas et al, 2004). It is clear that different structural elements influence relief forms at different scales. The parallelism of the main glens hints at regional fracture patterns. NE-SW fractures, following the Caledonian trend found throughout NE Scotland, control the orientation of Strathspey and, perhaps, upper Glen Avon. The main N-S breaches may be aligned along linear zones of hydrothermal alteration or micro-fracturing, judging from the frequency of reddened and disintegrated granite adjacent to their floors.

Many summits have dome-like forms and this is a reflection of the dominance of curved sheet or unloading joints on the relief. The detail of the summit terrain is controlled by the density of vertical joints. Tors occur is areas of widely-spaced joints, with avenues within and between tor groups where the joint density is higher. The streams also tend to follow linear courses aligned with joints or with alteration zones.