Glaciers of North Wales. 405 



Cumberland were mountains at least before Old Red 

 Sandstone times, and the last great movement of the 

 rocks of Wales is certainly older than the Permian epoch, 

 and, probably, like the mountains of Cumberland, very 

 much older. 



There was therefore plenty of time, in what is now 

 Wales, long before the beginning of the great Glacial 

 episode, for the more ordinary agents of denudation to 

 have formed deep valleys, down which, when that episode 

 began, the growing glaciers might gravitate, deepening 

 their channels as they pressed forward, and mammillating 

 and striating the rocks over which they slid ; for the 

 great original valleys of the mountains were by no 

 means entirely scooped out, but merely modified by the 

 glaciers. 



Thus, for example, it happens in Wales that all the 

 striations in the valley of Dolgelly and the estuary of 

 the Mawddach, in Merionethshire, follow the south- 

 westerly trend of the valley, the glacier that filled it 

 when at its greatest being fed by the snows of the slopes 

 of Cader Idris and Aran Mowddwy, and those of the 

 tributary valleys of Afon Eden and the Mawddach that 

 joined it from the north ; while from a central low 

 watershed, near the sources of the Wnion, another branch 

 pressed north-easterly, into and far beyond the region 

 now occupied by Bala lake. 



The striated rocks exposed among the sands at low 

 tide in the ectuary of the Mawddach, and the islet-like 

 heathy bosses of rock that stand out amid the marshy 

 moss opposite Barmouth, are merely roches mou- 

 tonnees, once buried deep beneath the glacier that 

 pressed forward to join the great northern glacier that 

 then filled Cardigan Bay. 



In like manner all the western valleys of the 



