Vol. 65.] 



GLACIAL EROSION IN NORTH WALES. 



327 



sides are strongly marked, in contrast with the gracefully rounded 

 form of the mountain as a whole. But the cwm is of so moderate 

 a size that the great mass of the mountain still remains above and 

 alongside of it. Now looking southwards across Nan tile Valley to 

 the ridge on its southern side, we find several cwms, one of which is 

 shown in fig. 26 (below). This one is separated from another (Cwm 



Fig. 26. — Sketch of a cwm on the south side of Nantlle Valley, 

 looking southwards. (The beacon is on Mynydd Tal-y-Mignedd.) 



Dwyfawr) on the farther (south) side of the mountain by a narrow, 

 grass- covered ridge, slightly breached at one point; the ridge is 

 sketched in fig. 27, as seen from the west. Evidently the col is 

 just on the verge of becoming a crib or arete : here the cwms 



occupy a large part 



Fig. 27. — Sketch of the head ridge of jig. 26, of the original 



looking eastwards. mountain - mass. 



Next farther west 

 two cwms (on the 

 northern slope of 

 Craig Cwm Silin, 

 beyond the limits 

 of the outline - 

 map, PI. XIV) 

 have partly con- 

 sumed the spur 

 between them, so 

 that they open 

 laterally into each 

 other. Thus 

 various stages of 

 cwm-development and of mountain-consumption, are easily dis- 

 tinguished. The same is true on Snowdon, as is indicated at the 

 opening of this article. The Ordnance Survey map (Snowdon 

 sheet) is very expressive in showing the narrow ridges between 



z 2 



