VOL. XVII. (i) SOME GLACIAL FEATURES IN WALES 35 



But what are of more immediate concern for the present 

 purpose are such telHng sketches of Prof. Davis's as figs. 14 and 

 21 (loc. cit., pp. 313 and 317), which depict ordinary valleys in a 

 subdued mountam-mass before and — according to his view — 

 after they had been occupied by valley-glaciers. According to his 

 view, the valley -heads of stream-genesis became filled with snow, 

 which was pressed down by additional falls to feed the valley- 

 glacier below. In the bergschrund (or great gaping crevasse 

 which so generally parallels the cwm-head walls, occurring be- 

 tween them and the ice-wall) the freezing and thawing of the 

 water gradually initiated the steep topmost portion of the now 

 precipitous head-walls, and the general downsHding of the 

 granular ice and debris had the effect, by " frictional plucking 

 and dragging," of scooping out the same cauldron-like hollows. 

 Where the heads of two contiguous stream-valleys became 

 neve-reservoirs, the intervening and originally-rounded spur 

 between them usually became sharpened in its upper portion 

 into a sharp, steep-sided, serrated ridge or "arete," as it is 

 technically called. 



Such in brief are the views of Prof. Davis with regard to 

 the formation of the cwms of Snowdonia and the intervening 

 aretes. The last products of the receding valley-glaciers in 

 waning Glacial times may be pointed to in the small moraines 

 that so frequently half-inclose the little moraine-lakes that are 

 backed by the steep precipitous head-walls of the cwms. 



A later writer than Prof. Davis, namely, Prof. \^^ H. 

 Hobbs, of Michigan, thinks, however, that while 



" the low-level sculpturing expressed by these sketches of Prof. W. 

 M. Davis is, in the opinion of the writer, [i.e., Prof. Hobbs], ad- 

 mirable, and a true rendering of nature. It is the failure to recog- 

 nise any additional process of erosion in higher altitudes whicli 

 destroys the value of the high-level sculpturing displayed. 

 It is the operation of an additional denuding process of the first 

 importance, head-wall erosion, that differentiates all types of 

 mountain-glaciers from continental ones. This distinguishing pro- 

 cess is responsible for the development of the cirque. . . ."I 



Personally, I think Prof. Davis set forth his views on how 

 a cwm was formed fairly fully ; but Prof. Hobbs writes that 



"The discovery of the method by which the glacier excavates 

 its amphitheatre must be credited to a keen American topographer- 

 geologist, Mr Millard D. Johnson, of the United States Geological 

 Survey." 



1 Geogr. Jour., 1910, p. 148. 



