160 Professor J. W. Gregory — 



valleys vary in direction they do so at sharp angles. Thus the 

 Nant Gwynant runs north-eastward from Beddgelert to Penygwryd, 

 where it bends east-north-east to Capel Curig ; and on the continua- 

 tion of its original direction occurs the valley of Llyn Cawlyd. The 

 Nant Gwynant -Llyn Cawlyd valley is approximately parallel to 

 the remarkably straight course of the 500 ft. contour along the 

 north-western and western slopes of Snowdonia. 



These rectilinear valleys have cut across some of the older valleys. 

 Thus the formation of the Llyn Cwellyn valley beheaded the Afon 

 Drws-y-coed (the upper part of the NantUe valley) and captured 

 its original head streams in the Cwm-y-Clogwyn, which, both from 

 direction and gradient, appears the natural continuation of the 

 Afon Drws-y-coed. 



The course of this network of valleys is not what would be 

 anticipated if they had been excavated by ice. If the valleys were 

 glacial in origin they would have been expected to radiate from 

 Snowdon, whereas the mountain is an oblong block enclosed in a 

 rectilinear moat. The block would be almost an exact oblong but 

 for the sharp bend in the south-western valley, which adds a 

 triangular area to the southern corner. 



A second resemblance to Arran is that Snowdon, as was clearly 

 pointed out by Sir Andrew Ramsay (1881, pp. 271-3), was never 

 completely overridden by extraneous ice. It was a centre of local 

 glaciation ; and as Ramsay remarked of its north-western glaciers 

 (1881, p. 272), " on escaping from the high-bounding-walls of the 

 Tipper parts of their valleys, they partly spread out in the shape of 

 broad fans on the north-western slopes of the minor hills that 

 now overlook the Straights." According to Ramsay, therefore, 

 the valleys were pre-Glacial. The question is merely how far the 

 glaciers enlarged and deepened these pre-existing valleys. The 

 amount depends upon the condition of Snowdonia at the beginning 

 of Glacial times ; and, like the mountains of Arran, Snowdon was 

 surrounded by a high platform trenched by pre-Glacial valleys. 



Snowdon, at one part of the Pliocene, was apparently a mountain 

 about 2,000 feet high surrounded by a low, widespread plain. The 

 district was raised by a Pliocene uplift, and the plain formed the 

 platform that now surrounds Snowdon at the height of from 1,250 

 to 1,400 feet. This platform occupies most of the area between the 

 1,250 and 1,500 ft. contours on the map (Fig. 6). It is shown in 

 Ramsay's section through Yr Aran (1881, p. 158, fig. 23) ; the 

 platform is at the level of from 1,200 to 1,400 feet to the north- 

 west of Yr Aran, and somewhat lower to the south-east of it. 



The uplift of Snowdonia led to the erosion of canyons along the 

 floors of the old valleys, the width of which indicates that they had 

 been worn down to base-level and their walls cut backward before the 

 Glacial period. As the climate became more severe the gullies on the 

 upper sides of these gorges were occupied by snow, and corries were 

 excavated about them by the combined action of frost and streams. 



