A dry-based polar glacier in the Dry Valleys, Antarctica. Picture of the right margin of Suess Glacier, Taylor Valley with Lake Poppelwell in the foreground (Image by B. Hooker, 1996)
Definition: glaciers where the basal layers of ice are below pressure melting point and so frozen to the glacier bed
Glaciers are generally thought as powerful agents of erosion. The classic landscapes of glacial erosion, with deep glacial valleys and cirques or ice-moulded hills, are undoubted evidence of major denudation and the ability of glaciers to change the landscape. In the Cairngorms, however, glacial erosion has been highly selective, with, on the one hand, deepening of the major valleys by several hundred metres and excavation of large corries but also little apparent modification of the intervening plateaux. This contrast is typical of landscapes of selective linear erosion and is generally attributed to contrasts in the basal thermal regime of ice flowing through the valleys and across the plateaux.
In polar regions there are many signs that glaciers can protect rather than erode their beds. On plateau in northern Norway, periglacial patterned ground has been observed emerging from beneath retreating margins of ice caps (Gellatly et al., 1988). Older sediments may also be preserved. In the Cairngorms. it is the widespread survival of tors that is the most striking legacy of non-erosive ice covers.
There is also an absence of evidence that the glaciers were sliding across their beds. Not only are delicate landforms and sediments preserved, there is also an absence of striae and ice moulding. If there was no sliding then the ice must have been frozen to its bed - hence the terms cold- or dry-based glaciers.
This velocity profile from the University of Aberystwyth shows a section through a polar glacier. At all times of the year the base of the ice is below 0º C. Even if allowance is made for a great thickness of ice, the basal ice will not reach its pressure melting point. Movement is by internal deformation and creep and it is slow - the low ice temperatures passed on from low air temperatures through snowfall suppress both processes.
Cold-based glaciers are generally non-erosive because there is little movement across the ice-bedrock interface. In the Cairngorms the plateau ice cover was generally cold-based because air temperatures during the cold stages of the Quaternary were low. The ice over was also thin and so reached pressure melting point only in valleys (Hall and Glasser, 2003).
The situation on the ground is a little more complex. Many
tors show signs of subtle glacial modification.
Glacier flow by internal deformation may well be capable of toppling tor blocks.
Some parts of the plateau also do show evidence of glacial erosion. The basal
thermal regimes of the former glaciers covering the high tops of the Cairngorms
have clearly varied in time and space.