THE 



GEOLOGICAL MAGAZINE. 



' NEW SERIES. DECADE IV. VOL. VII. 



No. VL — JUNE, 1900. 



0:RXG-X15TJ^Xj .A-I^TICXjES. 



I.— Bala Lake and the Eiver System op North Wales. 



By Philip Lake, M.A., F.G.S. 



(With a folding Map, PLATE XI.) 



(Concluded from the May Number, p. 215.) 



Watersheds in other Valleys. 



IN North Wales there are several valleys which run parallel 

 or nearly parallel to the valley of the Bala fault, and, like it, 

 are crossed by watersheds which are quite inconspicuous. One of 

 these watersheds separates the head of Cwm Prysor from the source 

 of Afon Tryweryn ; a second parts the springs of the Conway 

 from those of Afon Dwyryd ; and a third divides the waters of 

 Nant Ffrancon from those of Afon Llugwy (a tributary of the 

 Conway). (PL XL) 



In the first two cases the valleys are not deep, but they are 

 sufficiently defined to form the lines chosen for the railway from 

 Bala to Ffestiniog, and for the road from Ffestiniog to Denbigh ; 

 and in the third case, the valley is at least as striking as that of 

 the Bala fault, and the watershed is even less conspicuous than 

 that at Pant-gwyn. 



All these watersheds lie upon one straight line — a line drawn 

 from Pant-gwyn to a point 1,200 yards east of Llyn Ogwen. This 

 may, indeed, be a mere coincidence, but the coincidence is remarkably 

 close, and it seems more probable that the positions of the watersheds 

 have been determined by some general cause. 



It can scarcely be supposed that ever since the formation of 

 these valleys (which are certainly pre-Glacial) the whole of Wales 

 has remained absolutely fixed and immovable. And if the region 

 has moved at all, there is little difficulty in supposing that the 

 motion was not uniform throughout. A very slight differential 

 movement, even now, would be sufficient to alter the positions of 

 these watersheds ; and it is quite possible, therefore, that their 

 present positions have been fixed by former movements. 



It may be remarked that this straight line crosses two other 

 important valleys in which it does not coincide with the watersheds 

 — the valley of Afon Lledr and Nant-y-gwryd. But this is easily 

 explained if we suppose that the heads of these valleys stood too 

 high to be affected by the differential movements. Moreover, iu 



DECADE IV. — VOL. VII. — NO. VI. 16 



