In the view across the trough, we see clearly the contrast between the deep erosion of Glen Avon and the relatively minor modification of the dome of Cairn Gorm (a) and the broad open valley of Coire Raibert (b). Yet in the middle ground lies the large roche moutonnée of Stacan Dubha (c) and the foreground shows an area of granite slabs (d), all that remains of a former tor.
Streetmap view of Loch Avon
Landscapes of selective linear glacial erosion
The Scottish Highlands shows a wide range of types of glacial scenery. In the northwest Highlands (Zone IV), especially on the Lewisian Gneiss, the terrain is one of areal scouring. Ice has been everywhere sliding across its bed and the resultant erosion has picked out structural weaknesses. The resultant ' knock and lochan' topography (Linton 1950) shows streamlined hills, roches moutonnées and rock basins from sea level to the ridge tops. In sharp counterpoint stand the lowlands of Buchan in NE Scotland (Zone 0). Here remnants of preglacial weathering and sediments occur widely and classic glacial landforms are generally absent. In this zone, basal sliding was absent as the ice was below its pressure melting point.
In the Cairngorms, glacial erosion has been selective and linear because it is most obvious and pervasive in valleys. Sugden (1968) recognised that this selectivity reflected glaciological controls on erosion. In the valleys, the ice was of sufficient thickness to induce pressure melting at the base of the glaciers. Sliding was possible and abrasion, plucking and others processes of glacial erosion became active. In contrast, ice over the plateau was thin and remained frozen to its bed. The cold-based ice moved slowly by internal deformation and creep but there was little movement over the ice-bed interface. In consequence, there was little erosion of delicate features such as tors and weathered rock on the plateau. The model has proved to be robust and it has been applied in a range of other glaciated landscapes, including Baffin Island and southern Greenland.
Recent work indicates that the situation in the Cairngorm is a little more complicated than this. Widespread modification of tors, even on summits implies disturbance by moving ice. The occurrence of localized ice-moulding on the plateau implies former periods of warm-based ice cover. The presence of deep weathering pits on some glacially-disrupted surfaces suggests that the period or periods of warm-based flow across parts of the plateau predated the last ice sheet.