266 WELSH GLACIAL DRIFT. chap. xiv. 



covered with glaciers, which radiated from the central heights 

 through the seven principal valleys of that chain, where stria) 

 and flutings are seen on the polished rocks directed towards 

 as many different points of the compass. He also described 

 the "moraines" of the ancient glaciers, and the rounded 

 masses of polished rock, called in Switzerland "roches mou- 

 tonnees." His views respecting the old extinct glaciers of 

 North Wales were subsequently confirmed by Mr. Darwin, 

 who attributed the transport of man.y of the larger erratic 

 blocks to floating ice. Much of the Welsh glacial drift had 

 already been shown by Mr. Trimmer to have had a sub- 

 marine origin, and Mr. Darwin maintained that when the 

 land rose again to nearly its present height, glaciers filled the 

 valleys, and "swept them clean of all the rubbish left by the 

 sea."* 



Professor Eamsay, in a paper read to the Geological Society 

 ill 1851, and in a later Avork on the glaciation of North Wales, 

 described three successive glacial periods, during the first of 

 which the land was much higher than it now is, and the 

 quantity of ice excessive; secondly, a period of submerg- 

 ence wiien the land was 2300 feet lower than at present, and 

 when the higher mountain-tops only stood out of the sea as 

 a cluster of low islands, which nevertheless were covered 

 with snow; and lastly, a third period when the marine boulder 

 drift formed in the middle period was ploughed out of the 

 larger valleys by a second set of glaciers, smaller than those 

 of the first period. This last stage of glaciation may have 

 coincided with that of the parallel roads of Glen Eoy, spoken 

 of in the last chapter. In Wales it was certainly preceded 

 b}'- submergence, and the rocks had been exposed to glacial 

 polishing and friction before they sank. 



Fortunately the evidence of the sojourn of the Welsh 



* Philosophical Magazine, ser. 3, vol. x.xi. p. 180. 



