328 



PROF. W. MORRIS BAVIS ON 



[Aug. 1909, 



the encroaching cwms, and the fractional remnants of the mountain- 

 dome still unconsumed. 



The maps of the Snowdon district indicate a greater frequency of 

 cwms in valley-heads which open northwards or north-east- 

 wards than in those which open in the opposite direction. This is 

 a feature that has been noted in various other glaciated mountains, 

 and is evidently to be associated with the protection from sunshine- 

 that is afforded in valleys of northern aspect. It would be inter- 

 esting to learn whether the cirques of mountains in the Southern 

 Hemisphere are more commonly open to the south and south-east. 

 A correlated feature is found in the unsymmetrical slope on the 

 opposed sides of many ridges, as already noted : the north-eastern 

 slope may be a precipitous cliff, while the south-western slope is of 

 much more moderate declivity. This rule appears to hold in the 

 south-eastern (tig. 23, p. 324) and north-western (fig. 28) spurs of" 



Fig. 28. — Sketch of the north-western spur of Snowdon, looking 

 south-eastwards from Moel y Cynghorion. Cwm Dur-arddu is- 

 on the left ; Civm Clogwyn on the right. 



Snowdon ; also in the ridge (fig. 26, p. 327) between the two cwms- 

 that open into Nan tile Valley next west of Y Garn, and in various 

 other cases. It is presumably to be explained by unlike rates of 

 weathering on the two slopes, as has been suggested by Dr. G. K. 

 Gilbert in his discussion of the unsymmetrical inter-cirque ridges 

 3f the Sierra Nevada (California). 



The depth of glacial erosion indicated by the cwms of Snowdon 

 and its neighbours may be roughly inferred by reconstructing the 

 whole moel from so mucli of it as remains ; but, besides the depth 

 thus shown, something must be added for the loss of the moel 

 itself by general weathering during the Glacial Period. I am un- 

 able to offer any satisfying estimate of the latter quantity. Of 

 the former, the construction of several curves gives measurements 

 ranging from 200 or 300 to 500, 600, or even 700 feet. 



