The Selectivity of Glacial Erosion in the Cairngorm Mountains, Scotland

Abstract

THE CAIRNGORMS are a distinctive mountain group lying just north of the Dee headwaters in the north-eastern Grampian mountains. The granite summits include the second highest mountain in the British Isles, Ben Macdui, I3I0 m (4300 ft), while four others exceed an altitude of I220 m (4000 ft). Indeed, as the Kings Majesties water-poet, John Taylor, marvelled on his money-less perambulation in I6I8, 'Highgate Hill, Hampsted hill, Birdlip hill or Malverne hilles are but Mole-hilles in comparison'. The morphology of the mountains is marked above all by two contrasting landscape elements (Plate I). On the one hand, there are smooth gently rolling slopes which were well described by John Hill Burton in 1864: 'Here there are five summits springing from one root.... The circumference of the whole group is as that of one mountain. One might imagine it to have been a huge, wide rounded hill, Ben Muick Dhui being the highest point, and the whole as smooth and gentle of ascent as some of the Ural range .... The scene is one of solidity and firmness. .. .' On the other hand, the 'rounded hill' is deeply dissected by a network of narrow and often precipitous valleys. In the west, Glen Einich with cliffed sides nearly 600 m (2000 ft) high bites deeply into the massif, while in the centre, the massif is actually breached from north to south in two places. Most remarkable is the Lairig Ghru which cuts through the summit plateau between Ben Macdui and Braeriach as a narrow, cliffed gash almost 300 m (Iooo ft) deep. In addition, some forty corries (cirques) scallop the mountain flanks and inner valleys with crescentic cliffs. The two groups of features have long been noted, but in recent years attempts to understand their significance have led to contradictory interpretations of the evolution of the mountains. On the one hand, there are those who appreciate the importance of valley glaciers and corrie glaciers in cutting the troughs and corries, and yet recognize in the intervening plateau areas relicts of a former, more subdued landscape which has escaped glacierization. D. L. Linton in a series of papers (I949c, I950, 1952, I9S5) has drawn attention to the occurrence of tors, rotted rock and deep regolith on such slopes of gentle form. He puts forward convincing arguments as to why tors can be regarded as relict forms, once surrounded by deeply-rotted rock in situ but now exposed, and why the surfaces on which they stand may therefore be regarded as inherited from a warmer and more humid environment, probably in late Tertiary times. Considering that it was unlikely that tors could survive the passage of even feebly moving ice, Linton wrote in I955 that 'it seems necessary to regard parts of these uplands, with their tors and other topographic features which may be regarded as pre-glacial or interglacial survivals as essentially unglaciated during the last Ice Age, though doubtless deeply covered by neve' (p. J479). Recent widespread discoveries of deeply-rotted rock exposures in Scotland which, it is argued, date from pre-glacial times (E. A. Fitzpatrick, 1963; A. Godard, 1965), have supported the view that the topographical features described by Linton date from