Parabolic glacial valley at The Sow of Atholl, Drumochter
Gairn breach X-section
The Lairig Ghru, a through valley which breaches the main preglacial watershed of the Cairngorms
Glen Tilt, a glacial breach viewed on Google Earth
Glacial valleys in the central Cairngorms.
Loch Avon in the centre, with the Lairig an Laoigh to the east and Strath Nethy to the west.
Image from Google Earth
Gleann Einich, with trough head and corries above Loch Einich
In cross-section, glacial valleys tend to have a parabolic shape that is efficient for the evacuation of varying volumes of ice (Sugden and John, 1976). At the close of phases of glaciation, the basic shape is modified by glacial and glacifluvial deposition to give a flat or terraced floor. Postglacial processes, such as debris flows and avalanches, carry further debris to the valley floor. Perhaps the most significant changes in cross-section are a result of major rock slope failures.
The long profile of a glacial valley is quite distinct to the graded profile of a river valley. The valley head is steep, falling abruptly away from the plateau to an over-deepened valley section sometimes occupied by a ribbon lake, of which Loch Avon is a good example. The over-deepening is a response to the thickening and rapid discharge of ice through this part of the valley.
The detailed form of the valleys in both cross-section and long profile is strongly controlled by geology. Cairngorm glacial valleys tend to be relatively steep-sided and narrow, a reflection of the high rock mass strength of the granite (Brook et al., 2004). RMS is a measure of the strength of a rock wall, incorporating measurements of its hardness, weathering and fracture density.
Linton (1963) introduced a useful classification scheme for glacial valleys:
Alpine valleys were cut by valley glaciers that were overlooked by high ground
Icelandic valleys form beneath ice sheets from existing valleys. The preglacial valley is deepened and straightened, often to leave a well-developed trough head at the point of accelerating erosion. Glens Avon and Einich have this form.
Through valleys are open at both ends. Usually this means that glaciers have breached the preglacial watershed. The Lairig Ghru is a superb example.
In the Cairngorms, although
valley glaciers were probably active for long periods of the Quaternary, the
phases of ice sheet cover mean that no true alpine valleys can be recognised.
This statement applies to Scotland as a whole, except perhaps for the Cuillin of
Skye. Fine examples of both troughs and
breaches do occur in the Cairngorms.