34 PROCEEDINGS COTTESWOLD CLUB 1910 



adequate in many cases to bring about the formation of 

 mountain-glaciers.' One of the principal features of mountain- 

 glaciation is the formation of steep head-walled cwms.^ In 

 Snowdonia there are excellent examples of such cwms, and 

 abundant evidence for the occurrence of a Glacial Period. Very 

 similar cwms are to be seen in the Brecon Beacons ; and those 

 in the Brecon Beacons remind one of such great amphitheatral 

 hollows as that of Charlton Common, and suggest that the ex- 

 cavation of this hollow — and of certain other similar ones 

 in the Cotteswold Hills — may be in part due to somewhat 

 similar erosive agents. 



Two months after my return from Snowdonia, and the 

 country around Bala, there appeared the masterly and sugges- 

 tive paper on " Glacial Erosion in North Wales," by Prof. W. 

 M. Davis, in the Quarterly Journal of the Geological Society of 

 London.^ In it, the origin of the cwms of Snowdonia receives 

 considerable attention. Prof. Davis is a firm believer in the 

 erosive power of moving ice, and fortifies his position with the 

 aid of telling sketches of actual features. 



His restoration of pre-Glacial Snowdon is particularly 

 fascinating, and concerning it he writes that it was probably a 



" subdued mountain-form with dome-like central summit, large 

 rounded spurs and smooth waste-covered slopes, and with mature 

 valleys drained by steady flowing streams which branched delicately 

 hcadwards, with steepened slope, and which joined each other 

 northwards in the accordant fasliion that so systematically charac- 

 terises the drainage of all normal subdued mountain-masses." 



During Glacial times, however, he says this full-bodied 

 mass was transformed 



" chiefly by the glacial e-vcavation of valley-head cwms and by the 

 glacial widening and deepening of the valleys themselves, into a 

 central peak which gives forth acutely-serrated ridges between 

 amphitheatres : the serrated ridges changing into broad-spreading 

 spurs as they are followed outwards ; the wide amphitheatres, 

 backed by high rocky cliffs, opening by great rock-steps to irregu- 

 larly deepened trough-like valleys, with oversteepened, undissected 

 sides, sometimes smooth, sometimes of a peculiarly roughened 

 slope ; the smaller lateral valleys hanging in strikingly discordant 

 fashion over the floors of the larger valleys ; and the streams, 

 far from following graded courses in steady flow, frequently halting 

 in lakes or hastening in rapids and cascades." 



I Prof. Hobbs rightly remarks that not sufficient distinction has been drawn between the types 

 of land-form that result from the glaciation (i) of districts of strong relief that were not entirely 

 submerged beneath snow and ice, but were sculptured by mountain-glaciers, etc. ; and (2) from those 

 that were sculptured beneath a glacier of continental dimensions. 



3 Or cirque : Ger. , cirkits : Scotland, corric (uplands), coirc (lowlands) : Scandinavia, bolii: and 

 Bavaria-Austria, kahr. 



3 Loc. oil., igog, pp. 281-344. 



