4 1 o Glaciation of Menai Straits. 



narvonshire eoast, for three miles north of the town, 

 also overlying the limestone, there are soft shales of the 

 Coal-measures, sometimes red and marly, and contain- 

 ing thin seams of coal. 



In Anglesea, from three to four miles north-west of 

 the Straits, lies the valley of Malldraeth Marsh, the 

 rocks of which also consist of Carboniferous Limestone, 

 Millstone grit, soft Coal-measure shales, with a little 

 sandstone, beds of coal, and Permian strata ; and this 

 valley, nine miles in length, runs almost exactly parallel 

 to the valley of the Menai Straits. Many years ago, 

 at its north-eastern end, I saw deep glacial striations on 

 the Millstone Grit, running straight down the shallow 

 valley towards Caernarvon Bay. 



Considering that the south-westerly trend of each of 

 these valleys and of others of minor note, corresponds 

 with the general direction of the glacial striations of 

 Anglesea, and therefore with the onward course of the 

 great glacier that produced them, I have been led to 

 the conclusion that both of the shallow valleys were 

 scooped out in comparatively soft rocks, by the grinding 

 power of the vast glacier coming from the north-east, 

 and that when in the course of time the climate amelio- 

 rated, and the glacier disappeared, the sea flowed in 

 where part of the glacier had been, and thus it was that 

 Anglesea got separated from the mainland and first 

 became an island. The islets in the narrower and 

 shallower part of the Straits at the Menai and tubular 

 bridges are merely weathered roches moutonnees. once 

 overridden by the moving glacier, and Menai Strait is 

 merely a long and broad glacial groove, which was first 

 laid bare by the partial removal of the boulder-beds, 

 after the close of the Glacial epoch. 



