320 SUMMARY OF GEOGRAPHICAL EVOLUTION. [CHAP. XIV, 



Leaving these problems for future solution, we must 

 pass to the time when the rigour of the Ice Age had mode- 

 rated, and when the British region had emerged from the 

 Glacial sea ; when the land-ice had retreated to the moun- 

 tain-valleys, and had left the lowlands free of ice and 

 snow, though covered with the thick mantle of Boulder- 

 clay and gravel which was the gift of the Glacial period. 

 The movement of upheaval continued till Britain again, 

 stood high above the sea and was united to the Continent 

 across the valley of the English Channel and the plains of 

 the North Sea. 



This phase of Pleistocene geography is represented in 

 Plate XIV., and on comparing it with that of the Pliocene 

 in Plate XHI. it will be observed that the chief difference 

 is in the relative level of the floor of the North Sea. In 

 Pliocene times this area was below the sea-level, though the 

 general level of the land was higher then than in the later 

 period ; but the cause of this difference is probably to be 

 found in the large quantities of detritus which must have 

 been carried into this sea by the Scottish and Scandinavian 

 ice, and which must have buried the Pliocene sea-bed 

 under several hundred feet of ice-borne material. A recent 

 boring at Utrecht has proved that the surface of the Plio- 

 cene deposits is more than 500 feet below that part of 

 Holland, and, as the Pliocene sea probably deepened north- 

 ward, it is therefore hardly too much to assume that 

 the same surface lies some 100 fathoms below the pre- 

 sent bed of the North Sea between Norway and Britain, 

 and consequently that the upheaval necessary to convert 

 this sea into land after the Glacial Period was 100 fathoms 

 less than would have been required to effect the same 

 result in later Pliocene time. 



The sea-beds on the western side of England and Scot- 

 land were doubtless shallowed in a similar manner, and 

 even on higher ground many of the Pliocene valleys were 



