Henry Dewey — Land-forins in Caernai'vonsldre. 155 



Cegin, the Afon Seiont, and the Afon Cadnant have each cut valleys 

 several hundreds of feet deep in the plateau. 



Some agency has truncated all the land at a 'common level, and 

 inspection of the Ordnance map shovi^sthat level to be 430 feet above 

 that of the sea. There are, however, a few hills that rise above this 

 plain, and on almost every one of them a hill-fort consisting of 

 circular earthworks is preserved. To mention some examples, there 

 are the two camps situated respectively on the west and the east of 

 the Padarn Yalley near Cwm-y-glo ; . the fine hill-fort at Pen-y- 

 ddinas by Llanddeiniolen, the Castell near llhiwlas, the Camp by 

 Tregarth, and another at Rhiw Gocli. Similarly on Anglesey ^ the 

 earthworks are placed on the few hills that rise above 400 feet and 

 are tliere described as various Mynydds. 



But this plain does not extend far to the east of the River Ogwen, 

 as the mountains run out to the coastline near Aber. Its margin is 

 i-endered obvious on the map by the crowding together of the 

 contour-lines above 400 feet, but it is still more conspicuous in 

 Nature. Pig. 15, p. 150, is drawn to the natural scale, and shows 

 the abrupt change of slope at the base of the mountains. 



It is difficult to follow the edge of the plain across country, because 

 there is no road running parallel with it, but the feature is distinctly 

 seen even from a distance. Nevertheless, when the margin is 

 reached the ground is usually boggy and often covered with saturated 

 peat, with small streams soaking through it. Tregarth village is 

 built in part on the plain, and here the rise to the adjacent 

 mountain is marked by several boggy meadows. But perhaps the 

 feature is best seen in the country lying between Llanddeiniolen and 

 Moel PJiiwen, especially near Waen, where hillocks composed of 

 boulder beds rise above the general plain to form dry patches of 

 arable land in a region generally wet. 



The feature cuts straight across the mouth of the Llanberis Valley 

 and does not run up into that valley, a fact that implies the formation 

 of the valley subsequently to that of the plain. It then extends in 

 a general south-westerly direction near Llanrug, where Garth is 

 situated on an isolated hill rising out of the plain. Thence by 

 Oroeslon and Pen-y-groes it spreads toward the Lleyn Peninsula, 

 but I have not traced it in detail much beyond the valley of the 

 Seiont. 



Such a sudden change of topography suggests a dijfferent degree of 

 hardness of the rocks underlying the two areas, but reference to the 

 geological map (Sheet 78) shows that in both areas similar rocks 

 occur. These consist of slates, grits, limestones, and shales with 

 bedded and intrusive igneous rocks. In the one area all of them 

 have been planed down to a common level ; in the other differential 

 hardness has led to variety of sculpture. 



Of late it has been the fashion to adopt American terminology in 

 describing upland plateaux and also to reject the sea as the agent 

 which has produced these features. In some cases the term 

 "peneplain" may be applicable, but it is difficult to imagine why 

 subaerial agencies should cease operating along a purely arbitrary 



^ See Mr. Greenly's forthcoming Memoir on Anglesey (Mem. Geol. Surv.). 



