Henry Dewey — Land-forms in Caernarvonshire. 151 



Rhaiadr Ogwen the gorge is not much deeper, and the amount of 

 post-Glacial cutting appears to be about 50 feet. Elsewhere in the 

 valley there is no evidence of a greater amount of post-Glacial 

 fluviatile incision, but another significant instance of a similar 

 nature is the gorge by the Salmon Pool near Pont-y-garreg. Here 

 the thick-bedded grits of the Lingula Flags are carried across the 

 valley as a barrier through which the Ogwen has cut a gorge with 

 vertical walls not 25 feet apart. At the same locality there are 

 three prominent terraces preserved, each about 30 feet above the 

 other and all smoothed and striated by ice. These Avere overwhelmed 

 by the great glacier; but as this retreated up-valley the barrier 

 must have held back the waters that formerly spread out as a lake 

 and filled IS'ant Ffrancon (PI. VII, Fig. 2). Subsequent erosion has 

 brought the base-level below that of the wide level tract of Nant 

 Ffrancon, and the Ogwen now rushes in a series of picturesque 

 cascades and torrents through this belt of country where the grits 

 occur. These grits strike straight across the valley and sweep 

 upwards to the west to form the precipices rising above Cwm 

 Graianog. They are intensely hard and coarse-grained rocks, and 

 under the microscope are seen to consist of rounded grains of quartz 

 with a subordinate amount of fresh plagioclase felspar and some 

 scales of white mica. The rock might be termed an arkose. 



A similar amount of post-Glacial erosion is indicated at many 

 localities over a wider area, especially in that little visited and 

 desolate tract of moorland that lies between the ridge forming 

 Carnedd Dafydd and Carnedd Llewellyn and the mountains near 

 Aber. - In this tract the rivers have here and there removed the 

 glacial detritus from their valleys, but the depth of the post-Glacial 

 valleys is seldom more than 45-50 feet. But the Afon Anafon 

 near Aber has cut a deep trough in debris and has led to a local 

 collapse of a huge scree below the mountain, while the great cli:ff 

 that remains appears to be a contradiction to the other evidence. 



Here, however, the effect of recent local denudation may be 

 noticed. In Cwm Coch near Blaen-y-nant Farm an enormous gash 

 has been rent through scree material by a cloud-burst. It is 

 upwards of 20 feet deep. Greenly^ describes the effect of a similar 

 cloud-burst on the scree beneath the lower slopes of Carnedd 

 Dafydd where the road was swept away by the torrential waters. 



The mode of occurrence of the glacial drift leaves little room for 

 doubt that the principal topographical features had been formed 

 before the coming of the ice. All tlie valleys examined indicate that 

 they are essentially pre-Glacial and that very little modification of 

 them has taken place in post-Glacial times. This subject will be 

 reverted to later and some evidence given in support of the views 

 expressed. In the meanwhile the glacial characteristics of the 

 Ogwen valley will be further considered. 



The Cwms and the Hanging Valleys. 

 On the western side of Nant Ffrancon there are many typical 

 cwms, and it is significant that all of them face either to the east 

 ^ Geol. Mag., 1901, pp. 68, 69. 



