A. C. RAMSAY ON THE ISLAND OF ANGLESEY. 117 



of the whole of Anglesey consist chiefly of glacial detritus, with here 

 and there small alluvial meadows by the sides of the streams. 



When freshly stripped of glacial debris, or even of a mere thin 

 turfy soil, the underlying rocks are often found to be ice-smoothed 

 and marked with glacial striae, running generally from about 3(J° to 

 40° west of south. The larger valleys of MaUdraeth Marsh and 

 the Mcnai Straits (with others of minor note) ruu in hollows in 

 the same general direction. 



This memoir is not the place in which to discuss the theory of 

 the Glacial epoch ; but for my general argument it is necessary to 

 touch on one or two points connected therewith. 



It is well known that in all mountain regions where glaciers 

 exist, or have in past times existed, the disturbances of the earth's 

 crust that produced the elevation of the mountains go back to 

 periods long antecedent to the last great Glacial epoch. Thus the 

 first great upheaval of the Alps is cf prse-Miocene age, and the 

 mountains of Scotland and Cumberland were mountains before Old- 

 lied-Sandstone times, while 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 ordi- 

 nary 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 mamillating 

 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 tstuary of the Mawddach, in Merio- 

 nethshire, 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 beyond the 

 region now occupied by Bala lake. 



The striated rocks exposed at low tide in the estuary of the 

 Mawddach, and the islet-like heathy bosses of rock that stand out 

 amid the marshy moss opposite Barmouth, are merely roehes mou- 

 tonnees, once buried deep beneath the glacier that pressed forward 

 into Cardigan Bay. 



In like manner all the western valleys of the Cambrian mountains 

 of Merionethshire, from Barmouth to Diphwys west of Trawsfynydd, 

 such as those of Afon Atro and Ardudwy, are marked by deep 

 grooves and striations pointing more or less westward, according to 

 the trend of the valleys. The broad flats and roughly hilly, but not 

 mountainous, country of Cors-goch, Afon Eden, Trawsfynydd, and the 

 ground bounded by the amphitheatre of scarped mountains formed 

 by the Arenigs, the Manods, the Moelwyns, and the Cambrian steeps 



