"Vol. 65.] GLACIAL EROSION IN NORTH WALES. 283 



Cambridge, to meet a number of geologists, British and foreign, 

 of broad experience in many fields, but of varying opinions as to 

 the efficiency of glacial erosion in mountain-sculpture, and to 

 discuss with them, greatly to my own edification, though perhaps 

 to their fatigue, the different phases of this interesting problem. 

 Following the celebration a few days were spent, partly with 

 Prof. Penck, of Berlin, in Devon and Cornwall for a purpose 

 which will appear below ; and then three field-days were given to 

 the Snowdon district again, with headquarters at Snowdon Ranger, 

 another isolated hotel, by Cwellyn Lake, in order to reconsider 

 the whole problem on the ground, previous to the presentation of a 

 general account of it to the Liverpool Geological Society the evening 

 before sailing, and to writing an outline of this article during the 

 homeward voyage to Boston, October 15th-25th, 1907. Such was 

 the delay in completing the manuscript that, before it was finally 

 ready for printing, the Snowdon district was again visited at the 

 end of August, 1908, in company with several members of the 

 Oxford Summer School of Geography; and a brief statement of 

 the discussion here presented was made before Section C of the 

 British Association at the Dublin meeting, early in September of 

 the same year. 



III. Welsh Terms, Local Place- Names, and Illustrations, 



Several Welsh geographical terms are used in this paper, because 

 they appear on the local sheets of the Ordnance Survey. One of 

 these is cwm (pron. cum) — essentially the equivalent of cirque 

 in the French Alps, of kar in the German Alps, of botn in 

 Scandinavia, of coire in the Scottish highlands (made over into 

 corrie for use in the lowlands), and of combe, or sometimes 

 cove, in the English Lake district — the name for the amphitheatral 

 form of valley-heads. Other Welsh terms are crib, a sharp, 

 steep-sided, serrated ridge, corresponding to the arete of the 

 French Alps; mo el, a smooth dome-like mountain, resembling 

 the balds of North Carolina ; afon, stream or river, with its equi- 

 valent in the Avoiis of England ; llyn, lake (the Welsh 11 having 

 the guttural ch and the 1 sounds combined) ; and nant, vale or 

 valley. 



For the convenience of the reader, the use of little-known place- 

 names as guides to the location of physical features is avoided as far 

 as possible. Nearly all the local names employed are given on the 

 accompanying outline-map (PI. XIV), from which it will appear that 

 the Snowdon mass, with a summit-altitude of 3560 feet (standing 

 about 10 miles south-east of the Menai Straits, the narrow sea-arm 

 by which Anglesey is isolated from North Wales), contains four 

 large cwms : — on the east, Cwm Dyli (pron. Duli) ; on the south, 

 Cwm Llan ; on the west, Cwm Clogwyn ; and on the north-west, 

 a cwm not named on the Snowdon sheet of the Ordnance Survey, 

 but which will here be called Cwm Dur-arddu, after a lake of that 



