PAST GEOGRAPHICAL MUTATIONS 9 



which, it may be remarked, is the ascertained limit of 

 glaciation, the area below that boundary, embracing all 

 the English shires south of the Bristol Channel, and 

 the valley of the Thames, having for the most part 

 endured from the early Pliocene Period. It is also 

 significant that no drift containing marine shells has 

 been found at high levels in Scotland. 



Whatever changes may have taken place elsewhere 

 in British Europe during the Glacial Epoch, it seems 

 pretty certain that the land which now lies buried under 

 portions of the Bay of Biscay down to the borders of 

 the French Landes, beneath the English Channel, the 

 Bristol Channel, St. Georges Channel, and to the south 

 of Ireland, endured through most if not all of that 

 period, and formed the bridge by means of which our 

 area was invaded by the large mammalia and other 

 forms of animal and vegetable life during the successive 

 mild inter-glacial periods. We shall yet see the even 

 greater importance of this southern land mass. It is, 

 however, suggested that during the third glacial period 

 the sea advanced up the English Channel — the high 

 temperature of its waters probably checking the 

 southern advance of the ice-sheets beyond the valley 

 of the Thames. Whether the depression extended up 

 the Bristol Channel or the Irish Sea area need not con- 

 cern us, for we have evidence to suggest that that 

 region was entirely glaciated or snow-clad. It is pre- 

 sumed that during this era the Selsey beds and the 

 various raised beaches which occur from Cornwall to 

 Brighton on the English coast, and from Brittany to 

 St. Valery sur Somme on the French coast were formed. 

 During this glacial period the North Sea is said once 



