234 MR ALFRED HARKER ON 



tributary stream itself. Higher up, however, we find the striae emerging from the 

 corrie in the natural direction and then curving away into the direction of the Coruisk 

 valley. In this case it seems impossible to doubt that there has been a considerable 

 pre-Glacial valley, the lower half of which has been completely planed awa}?-. The 

 Coir' an Lochain ice was too powerful a body to be ponded back, and it accordingly 

 swept out to join the main stream ; but it did this at a high level, the lower part of 

 the valley thus ceasing to operate as a channel and being thereupon gradually ground 

 out of existence by the action of the Coruisk ice streaming directly across it. Only in 

 this way can we explain the situation of this and other high-level niches in the 

 Cuillins, though the amphitheatral form to which they so generally tend involves 

 another element not yet considered. 



What has just been described as illustrated by Coir' an Lochain has much in 

 common with what Davis,* following Gilbert, has termed ' hanging valleys.' 



These are simply tributary valleys which debouch at levels considerably above the 

 floor of the trunk valley, into which they therefore drain by cascades of some height. 

 They seem to be a characteristic feature of some glaciated districts, and are explained by 

 the greater amount of ice-erosion in the main valley as compared with its tributaries. If 

 the pre-Glacial stream of Coir' an Lochain had been larger, and its gradient more 

 moderate, something more closely comparable with the typical hanging valley might 

 have resulted ; but this and other examples which might be cited in the Cuillins are 

 better described as ' hanging ' cirques, or, as we have already called them, high-level 

 niches. Our small area does not comprise many tributaries of more than very moderate 

 dimensions. The best example is Tairneilear, which debouches some 250 or 300 feet 

 above the floor of Coir' a' Mhadaidh or Coire na Creiche, and may be taken as a fairly 

 typical hanging valley. 



Professor Davis lays stress especially upon the deepening of the main valley. 

 Either deepening or widening may conceivably bring about the result, though the 

 former at a greater cost of total erosion. In the middle and lower courses of valleys 

 like those of the Cuillins we have already seen reason to attach special importance to 

 glacial erosion in the lateral direction, but we shall see that in some circumstances there 

 has also been a considerable amount of erosion in the vertical sense. 



(vii.) Cirques: Character of Ridges. 



We come next to what is perhaps the most striking characteristic of the surface- 

 relief of the Cuillins, viz., the evidence of excessive erosion in the upper parts of all the 

 valleys. Quite apart from what has been described in other countries, we should be 

 led by general considerations to connect this peculiarity with glacial erosion. As we 



* W. M. Davis, "Glacial Erosion in the Valley of the Ticino," Appalachian vol. ix. jrp. 136-156, pi. xv., xvi., 

 1900 ; "Glacial Erosion in Fiance, Switzerland, and Norway," Proc. Bost. Soc. Nat. Hist., vol. xxix. pp. 273 322, pi. 

 1-3, 1900. 



