242 Philip Lake — Bala Lake and River System. 



each case there is some evidence that near to the spot where the 

 line crosses, the rocky floor is higher than it is farther up the 

 valley. 



In the valley of Afon Lledr the line crosses a little way below 

 Koman Bridge station. Above, the river flows through an alluvial 

 flat : here, rocky spurs run down to the stream, and the stream 

 itself flows over rock in situ. 



Nant-y-gwryd is crossed near Dyffryn Mymbyr. The stream 

 flows over a drift-covered flat ; but at this point a line of rocky 

 knolls rises through the drift. 



These, then, are perhaps attempts at watersheds, frustrated by 

 the fact that the heads of the valleys lay at a higher level. I am 

 not, however, disposed to attach much importance to these unfinished 

 watersheds, until the ground has been more closely examined. The 

 evidence is somewhat doubtful, and near Eoman Bridge it is possible 

 that the river has forsaken its ancient channel. 



Development of the Eiver System of North Wales. 



Among the most striking features in the topography of North 

 Wales are the long deep valleys which run from north-east to 

 south-west, and which, where they enter the sea, form the wide 

 estuaries of the Mawddach, Dyfi, etc. Many of these valleys are 

 dry for a considerable part of their course, or are occupied only 

 by little streamlets out of all proportion to their own size. They 

 are crossed by the principal watersheds of the district, and even 

 at these points the floors of the valleys are comparatively low. 

 They are not perfectly straight, but generally form curves of long 

 radius, the convexity of which faces the south-east. (PI. XL) 



Sometimes, at least, the valleys coincide with lines of fault ; 

 and in one case, as I have shown, two faults form a trough for 

 a certain distance, and the floor of the valley coincides with the 

 bottom of the trough. 



My examination of the region not being by any means complete, 

 I do not attempt upon the accompanying map to trace these valleys 

 continuously across the district ; but have merely indicated by 

 heavy broken lines the principal streams, or portions of streams, 

 flowing in valleys which I believe to belong to this N.E.-S.W. 

 system. The continuity of the valleys is nevertheless sufficiently 

 evident, although not quite so clear as it is in the field. 



Among the most important of these valleys, or — to make less 

 of an assumption — of these lines of valleys, are the following : — 



1. The Menai Straits. 



2. The estuary Traeth-bach, Afon Dwyryd, Cwm-tegwel, Afon 

 Machno. 



3. Mawddach estuary, Afon Wnion, Afon Dyfrdwy below Pant- 

 gwyn, Bala Lake, Afon Meloch, Eiver Alwen from Maerdy to 

 Bettws Gwerfil Goch, and possibly a part of the Clwyd. 



The Mawddach estuary is not in a line with the general direction 

 of the Wnion. A valley in more direct continuation of the latter is 

 that in which lies the high road from Dolgelly to Towj'n. 



