Yol. 65.] GLACIAL EROSION IN NORTH WALES. 339 



reasonable that the ice from Cwm Glas, high up on the south side 

 of the valley, should not have descended into it, and have given 

 rise to rock-steps at successive positions of the glacier-end, com- 

 parable with those of neighbouring valleys. Until this difficulty 

 is cleared away, the Llanberis Valley must be regarded as beyond 

 ■explanation by the theory of ice-protection. Under the theory 

 of ice-erosion, that valley is an example of energetic work of ice- 

 channel deepening, carried forward to a fair degree of maturity in 

 its down-stream part, but still young and ragged near its head. 

 'To as great a height as the valley was filled with ice, the cliffs are 

 due to glacial scour ; at greater heights, the cliffs appear to result 

 from weathering and sapping. There is reason to think that the 

 •col at the head of this valley in pre-Glacial time was farther west 

 and decidedly higher than the present col ; and that we have here 

 a case of strong erosion by an ice-stream that overran a divide : 

 of which, more below. 



Hanging lateral valleys. — The valley-systems in the 

 immediate neighbourhood of Snowdon are hardly large enough 

 to exhibit hanging lateral valleys in their perfection ; but several 

 : good examples are seen of short cwm-like lateral valleys, distinctly 

 hanging over their wide-open main valley-floors. It is singular 

 that more was not made of these very curious features in the 

 •descriptions of Wales years ago ; but the hanging cwms and valleys 

 around Snowdon do not seem often to have called for special remark 

 as distinctly abnormal features. This can hardly be interpreted 

 •otherwise, than as meaning that no sufficient scheme of normal 

 land-sculpture was in the mind of the earlier students of this 

 ■district. 



The short hanging valleys on the south side of Nantlle Valley 

 will be first mentioned. Account of the fine cwms at their heads 

 lias already been given. Not less striking is the steep main-valley 

 side, down which the cwm-torrents cascade 300 or 400 feet, with 

 only a small beginning of entrenchment, as in fig. 26 (p. 325). One 

 (or two) similar but less conspicuous hanging valleys occur on 

 the north side of Nantlle Valley. The trough-like evenness of the 

 main valley-sides, which truncate spurs and hanging valleys in- 

 differently, is also a striking and significant feature. 



The valley of Afon Gwarfai is joined near its head by the valley of 

 the western cwm of Snowdon, with a pronounced break of 500 feet 

 •between the two, roughly shown in fig. 7 (p. 308). An exceptional 

 feature is here to be noted : the cwm-valley would to all appear- 

 ance be the source of the chief glacier for the valley of Afon 

 Gwarfai ; hence the path of this glacier ought to be the main 

 glacial channel, and over this channel all the branch valleys should 

 stand at discordant levels. The reverse is true ; for the broad- 

 floored path of the western cwm-glacier stands high above the 

 valley-floor of the upper part of Afon Gwarfai. I have interpreted 

 this as meaning that the upper Gwarfai Valley received much ice 

 from across the flat pass that separates it from the Afon Colwyn 



