Vol. 65.J 



GLACIAL EROSION IN NORTH WALES. 



317 



may be associated with, if not included under the general term, 

 glacial erosion. 



It must be understood that the work of an eroding glacier, like 

 that of an eroding river, is accomplished gradually. In a com- 

 paratively early stage of glacial erosion the transformation of a pre- 

 Glacial mature valley into a glacial channel will be characterized 

 by the development of many moderate inequalities of slope, which 

 may produce ice-rapids, as illustrated in fig. 17 (p. 316) ; but, with 

 progressive advance in the cycle of glacial erosion, these inequalities 

 will be worn down, and a smoother and smoother trough will be 

 carved out, as illustrated in figs. 18 and 19. Duration of glaciation 

 is therefore a very important factor in our problem. Brief glaciation 

 would produce rough-floored troughs of a very unfinished appear- 

 ance; long-lasting glaciation would produce smoothly carved 

 troughs suggestive of well-accomplished work. 



Naturally enough, results of the destructive action of the glaciers 

 will not be fully seen until the post-Glacial milder climate is estab- 

 lished and the glaciers have disappeared, as in fig. 21. Then the 



Fig. 20. — Diagram showing the 

 forms revealed by the disap- 

 pearance of a protective glacier. 



Fig. 21. — Diagram showing the 

 forms revealed by the disap- 

 pearance of a mature eroding 

 glacier. 



rock- cliffs of the empty neve-amphitheatres or cwms may be traced 

 down to the well scoured cwm-floors ; the empty glacial channels 

 will be rather ragged in wall and bed where the rocks are strong, 

 but will be well reduced to open troughs of curved cross-profile 

 where the rocks are weaker ; the empty channels will appear so 

 large, in comparison with the streams that drain them, that they 

 will be spoken of as ' valleys.' 



