Yol. 65.] 



GLACIAL EROSION IN NORTH WALES. 



30: 



pre-Glacial form and comparison of non-glaciated and glaciated 

 districts — have already been presented .in some detail, with the 

 conclusion that the abnormal forms of glaciated mountains are in 

 some way associated with glaciation. It remains, therefore, to 

 consider whether the abnormal forms of such districts, and 

 especially of the Snowdon district, are of a kind that glaciers would 

 produce if they were eroding agents ; and, as a safeguard against 

 error, the possibility of explaining the abnormal forms by the 

 opposed hypothesis that glaciers are protective agents, will also be 

 discussed. Lack of space will prevent due consideration here of 

 certain alternative explanations ; but it may be noted that all such 

 explanations, so far as they have come to the present writer's 

 attention, have been carefully examined, with the result of finding 

 no explanation so competent to account for the abnormal features 

 of a glaciated mountain-group as the one which associates them 

 closely with glaciation. 



Fig. 6. — Diagram of the pre- Glacial features of the Snowdon mass. 



Si 



It is evidently desirable, in testing the two hypotheses of the 

 erosive and the protective action of glaciers, to begin with a 

 reasonable conception of the land upon which glaciation takes place. 

 It is largely on this account that the earlier pages of this essay 

 have been devoted to the pre-Glacial history of the Snowdon district, 

 the inferred form of which, at the beginning of the Glacial Period, 

 is now given closer definition by means of a type-diagram (fig. 6). 

 It is also desirable, while pursuing the discussion, to bear in mind 

 a definite conception of the existing features of the Snowdon dis- 

 trict : hence a type-diagram (fig. 7, p. 308) is here submitted, in which 

 the abnormal features, previously shown separately, are compactly 

 generalized and presented in their natural association. The problem 

 before us is, therefore, to inquire as impartially as possible whether 

 the transformation of a normally subdued mountain-group typified 

 in fig. 6, to the abnormally eroded mountain-group typified in fig. 7, 

 can be better accounted for by the supposition that glaciers are 



