CHAP. XIII.] PLEISTOCENE EPOCH. 289 



are, I believe, calculated to satisfy these requirements. 

 As the land sank lower and lower, such deposits were 

 carried farther and farther over its surface, and their accu- 

 mulation only ceased when Britain was reduced to a group 

 of islands, the high-level shelly gravels being probably 

 nearly the last deposits formed during the great subsidence, 

 and possibly these were formed after it had reached its 

 maximum extent. 



Mr. Mackintosh thinks that the water-worn character of 

 the pebbles in the high-level gravels of Wales, as con- 

 trasted with the angular character of the blocks at lower 

 levels, indicates a decrease in the rate of submergence, and 

 says, " at this period the district was probably in the 

 condition of a littoral zone, which may have lasted for a 

 time sufficient to enable the waves to round the stones, and 

 to allow the mollusca to multiply in the littoral and sub- 

 littoral zones." l The occurrence of such shelly gravels at 

 about the same level in Ireland, Wales, and at Maccles- 

 field Forest confirms this idea of Britain having remained 

 stationary for some time when the sea was at this level. 

 It must be remembered, too, that the submergence of the 

 land would partially ameliorate the severity of the climate 

 and enable some mollusca of less Arctic habit to live in the 

 sea which then covered our islands. 



From the icy sea by which it was submerged Britain 

 gradually rose again, and as the mountains rose higher and 

 higher above the water they were again covered by ice and 

 snow, but these did not accumulate to anything like their 

 previous thickness, because the conditions which had caused 

 the Glacial episode in the northern hemisphere were now 

 passing away. Snowfields, however, formed on the hills 

 of Scotland, Ireland, Cumberland, and Wales, generating 

 local glaciers, which have left small moraines to mark the 



1 <; Quart. Journ. Geol. Soc.," vol. xxxvii. p. 362. 

 U 



