460 MR T. J. JEHU ON 



In Wales, as in other glaciated countries, erosion must have been carried on most 

 rapidly along the chief lines of ice-flow. At certain stages of the Glacial Epoch the 

 glaciers in the main valleys must have been engaged in deepening and widening the 

 channels along which they moved, while the tributary valleys and cwms were pro- 

 tected underneath a mantle of neve and snow. One is therefore not surprised to find 

 that the more important valleys are at places over-deepened as compared with the lateral 

 valleys, and now have a trough-like form with a flat bottom and steep cliff-like walls, 

 giving a U-shaped figure in cross-section. Features of this kind are well seen in Nant 

 Francon, where the valley from a little above Ogwen Bank right up to its head at Ogwen 

 Falls is remarkably flat and deep, and at the sides the walls rise steeply from the broad 

 floor to heights of over 1000 feet. On the western side six tributary cwms mouth at 

 a height of several hundred feet above the valley floor, reminding one of the hanging 

 valleys of the Ticino district, described by Davis. A study of the ground shows that 

 the valley, at the time of maximum glaciation, was filled up to the level of these cwms 

 with ice. In his little book on The Old Glaciers of Switzerland and North Wales, 

 Eamsay writes thus : " Taking Nant Francon in connection with its branching valleys, 

 an attentive consideration of all the circumstances has led me to think that it was so 

 filled with ice that the mouths of the minor valleys, to a height of from 700 to 1000 

 feet above the river, were over-ridden by the main stream of ice, which flowed across 

 the lower end of the spurs that branch from the crested ridge on the west." A photo- 

 graph of Nant Francon is given (Plate II. fig. 2), showing a view of this straight 

 flat-bottomed valley as seen from the heights above Llyn Ogwen. It has already been 

 stated that the bottom of this valley was for a time probably occupied by a lake, which 

 has now disappeared by becoming drained at the exit. 



At Pen-y-Gwrhyd, a low water-shed separates the Vale of Gwrhyd from the Vale of 

 Gwynant. It is interesting to compare these two valleys. The Vale of Gwrhyd has a 

 comparatively gentle slope from Pen-y-Gwrhyd to Capel Curig ; but the descent from 

 the watershed to the bottom of the Vale of Gwynant, near its head, is very steep. 

 This is accounted for by the fact that the mass of ice moving down the Vale of Gwynant 

 was much greater than that moving eastwards towards Capel Curig. For not only did 

 much of the snow which gathered on the slopes to the south of Gorphwysfa (at the 

 head of the Pass of Llanberis) pass down the Vale of Gwynant, but the huge glacier 

 coming down from the great upland valley of Cwm Dyli entered the Vale of Gwynant 

 near its head, and so greatly increased the thickness of the ice in that region. The 

 vallev above Llyn Gwynant is broad and flat and deep, with walls rising sharply 

 at the sides. We have here again an approach to that U-shaped form which is so 

 characteristic of a deepened valley. The level of the valley floor in pre-glacial times 

 is possibly indicated by the shelf of Penmaen Brith, which passes above Llyn 

 Gwynant on the right side. Lower down, Cw T m-y-Llan opens out at some distance 

 above the present valley floor. No cwms of any importance open out along the course 

 of Nant-y-Gwrhyd. 





