Henry Dewey — Land-forms in Caernarvonshire. 149 



much deeper, but is gradually filling up by rock-falls from the 

 neighbouring heights. A mass of drift crosses the valley at the fooi 

 of the lake, and seems to be of sufficient depth to account for its 

 formation and disposes of the necessity for supposing it to be a rock- 

 basin. The configuration of the lake-bottom supports this view, for 

 there is no deep cup-shaped depression such as is found in other 

 lakes of North Wales, but an irregular floor with rocky knobs jutting 

 up here and there. Professor Jehu therefore concludes that the lake 

 is a barrier-basin "with a floor that may have been modified by 

 glacial action. 



The Valley-Steps at Rhaiadk Ogwen. 



Llyn Ogwen lies on a plain at 1,000 feet above sea-level. Llyn 

 Idwal lies on a plain at 1,250 feet above sea-level, while Nant 

 Pfrancon extends as a long wide flat for a distance of 3 miles at a 

 nearly uniform height of 700 feet above the level of the sea. There 

 are thus three plains, rising one above another in tiers; the rise, 

 however, between each is not gradual, but abrupt. These features 

 are shown on the profile section, p. 150 (Fig. la), drawn to natural 

 scale, and in the view of Nant Ffrancon (PI. VII, Fig. 2), where the 

 lower valley-step is seen across the top of the valley. 



Between Llyn Idwal and Llyn Ogwen. the step is steep, and the 

 river rushes down its face as a series of torrents, in places in shallow 

 gorges until it unites with the lliver Ogwen at Pont Pen-y-Benglog 

 to form the cascade known as Rhaiadr Ogwen. A fine view of the 

 chasm is obtained from near the bridge. After heavy rain the gorge 

 is choked with spray and the " monotonous roar that fills the ravine ". 



Various hypotheses have been advanced to account for the origin 

 of such "valley-steps" in glaciated countries. The two steps at 

 Hhaiadr Ogwen are certainly, in part at least, due to the harder 

 interbedded and intrusive igneous rocks that lie among the sediments, 

 and the absence of similar steps in Nant Ffrancon may be due to the 

 absence of igneous rocks in the rest of the valley. 



The gorges cut by the Rivers Idwal and Ogwen at llhaiadr Ogwen 

 have carried the drainage of the two upland plains into Nant 

 Ffrancon ; but the study of the topography of the locality indicates 

 an earlier drainage into the River Llugwy. This was suggested by 

 Brend, and the bathymetrieal survey of Professor Jehu at Llyn Ogwen 

 lends support to the hypothesis in that it proved the floor of the 

 lake to fall from west to east, i.e. in a direction opposite to the 

 flow of the water of the lake. This diversion of drainage was 

 brought about in glacial times when glaciers filled Cwm Idwal and 

 the plain at Llyn Ogwen. Sub-glacial streams then cut shallow 

 gorges in the valley, and these streams persisted when the ice 

 melted and carried off the drainage of the two upland plains into 

 Nant Ffrancon. 



Post-Glacial Erosion. 



The gorges thus initiated have since been deepened by the cascades 

 at Khaiadr Ogwen. Similarly, on the Avest of the valley a small 

 mountain torrent has ripped out a beautiful gorge near Blaen-y-nant 

 Farm ; it is cut iu bedded ash and is upwards of 40 feet deep. At 



