Henry Dewey — Land-forms in Caernarvonshire. 153 



aretes or cribs occur below that level, although they commence 

 immediately above. 



These cwms are shown in PI. YII, Fig. 2, and their bases are all very 

 closely at the same altitude. This also corresponds with a plateau 

 feature above Bethesda and lying betAveen Ifant Pfraiicon and 

 Clegyr. It is marked on the old map as a Turbary plain, but a lake 

 used as a reservoir now occupies most of the area formerly filled with 

 peat. This broad moor is covered with drift and extends into the 

 valley of Marchlyn-mawr, while remnants of it are seen on the 

 opposite side of the Ogwen Valley near Afon Berthan, the Llafar, and 

 Afon Caseg Valleys. The feature is consj)icuous at Moel Ehiwen 

 and near Douglas Hill. 



In Nant Ffrancon the foothills of the mountains are deeply 

 striated, the striae pointing down stream ; but these do not extend 

 above the level of the lips of the cwms. 



Now all these features are attributed by the two schools of 

 glacialists respectively to the protective or the erosive function of 

 glaciers. The views, however, held by the one school are not 

 entirely contradictory to those of the other, but rather are supple- 

 mentary, i.e. those who ascribe to glaciers a protective function do 

 not exclude thereby erosive action of ice nor the backward stoping 

 of Bergschrund. 



Thus Garwood admits the power of ice to erode, but also insists 

 on its efficacy under certain circumstances to act much as a bed of 

 clay would in protecting underlying rock from disintegration due to 

 the expansion and contraction of freezing water. In cwms the 

 gently -sloping valley heads are thus protected, while the higher 

 slopes are exposed to sun-heat and frost, especially in such cases 

 where the main glacier has retreated up its valley leaving tributaries 

 above the ice-line, in cwms receiving only small amounts of sun-heat 

 on account of their north-easterly or easterly prospect. The almost 

 invariable rule of the cwms facing these and the absence of cwms 

 facing other directions strongly supports Garwood's contention. 



The Lowek Valley of the Ogwen. 



Between Bethesda and Bangor the Ogwen flows rapidly through 

 a deep and well-defined valley, which everywhere bears record of 

 glacial activity. The stronglj' moutonneed rocks and deeply incised 

 striae are well preserved on the Bethesda slates near the village and 

 at the mouth of Nant Ffrancon, and at first the wide valley appears 

 to be free from drift deposits. Closer examination, however, proves 

 that this supposition is incorrect. To take two or three instances 

 only among many others, there is first the pit near Felin hen Station, 

 where upwards of 20 feet of boulder beds are exposed in a low hill- 

 side rising gently from a plain; while near the Halfway House the 

 lower slopes of a hill have been cut into, and the open pit exposes 

 more than 30 feet of similar boulder material. Elsewhere in the 

 area described low hills rising above the general plain level are seen 

 to consist of boulder-clay or sand, and these occur at various hei^'hts 

 above sea-level and down to and below low-tide mark, asnear Penrhyn 

 Castle and at Beaumaris on Anglesey. Old valleys are partially 



