Vol. 65.] GLACIAL EROSION IN NORTH WALES. 303 



pattern. The smoothly rounded forms of normally subdued moun- 

 tains are not inviting to painters, tourists, or photographers, who 

 have naturally preferred the abundant detail shown in the rugged 

 outlines, the broken slopes, the openly visible cascades and the 

 picturesque lakes of glaciated mountains. Just as a sharp-cut 

 gorge and not a nearly featureless peneplain is commonly, but very 

 inappropriately, offered in illustration of the magnitude of the work 

 of erosion, so the rugged forms of glaciated mountains are too 

 often selected instead of the tame slopes of subdued mountains, 

 when illustrations of the verity of mountain-sculpture are sought 

 for, normal sculpture being tacitly implied. But, unfamiliar as the 

 nearly featureless forms of subdued mountains may be, it is precisely 

 such forms that must be carefully and consciously reconstructed for 

 the Snowdon district, if one attempts to estimate by the method 

 here suggested the amount of erosion that the Welsh mountains 

 have suffered in their transformation from their normal pre-Glacial 

 forms to their present highly abnormal forms. 



XVI. Association of Abnormal Features with Glaciation. 



The abnormal features of Snowdon and its neighbours appear to 

 be in some close way associated with the glaciation. The cwm- 

 floors, the valley side-slopes, the valley floor-steps are scored and 

 striated. Similar abnormal features occur in other glaciated 

 districts, all the world over, varying in intensity rather than in kind ; 

 but they are practically unknown in non-glaciated mountains. So 

 persistent an association has naturally led many observers to look 

 upon the abnormal forms as the result of glaciation ; but it has 

 been urged by other observers that the hanging attitude of lateral 

 valleys over their main valleys, which so strikingly characterizes 

 glaciated mountains, may be produced under normal conditions. 

 The explanation thus offered is in essence as follows : — If a nor- 

 mally sculptured mountainous area, drained by mature streams 

 with accordant junctions, be uplifted, and especially if the uplift be 

 so disposed as to increase the slope of the chief drainage-lines, then 

 the main rivers will be impelled quickly to deepen the main valleys, 

 and thus the lateral valleys will be left hanging over the newly 

 deepened floors of the main valleys. There can be no question that 

 hanging valleys of this kind may be normally produced in the early 

 stages of a new cycle introduced by regional uplift, particularly 

 where the main river is much larger than its branches, and where 

 the uplift is rapid. But such hanging valleys can endure only so 

 long as the main river has a narrow gorge-like valley ; the lateral 

 valleys must be worn down to accordant junctions by the time the 

 main valley-floor has become open by the lateral erosion of its 

 graded river and by the wasting away of its walls. Hence the 

 suggestion, that lateral valleys may remain hanging over well-open 

 main valleys which have been deepened as a result of land-tilting, 

 is not acceptable. 



