254 NEOZOIC TIME. [CHAP. xii. 



This general elevation was followed by a local and partial 

 subsidence, limited apparently to what is now the North 

 Sea and its borders. This subsidence allowed the waters 

 of an eastern sea once more to invade English territory, 

 and eventually opened up a communication between this 

 sea and the Arctic Ocean. 



It is highly probable that it was the movement that 

 led to the formation of the North Sea which gave the 

 easterly dip to the Eocene and Cretaceous rocks of eastern 

 England and an easterly tilt to the whole London Basin ; 

 it is certain, at any rate, that any slight easterly dip which 

 they may previously have had was at this time greatly in- 

 creased. We have seen that the geography of the older 

 Pliocene epoch bore little resemblance to that of modern 

 England, and though some of our Midland hills and valleys 

 may date from Miocene times, yet it is probable that the 

 development of the Cretaceous escarpments and of the corre- 

 sponding longitudinal valleys, as well as the trenching of 

 these escarpments by certain of our rivers, was accomplished 

 during the period we are now discussing. 



That the close of the Pliocene epoch found the main 

 physical features of England fully developed, and the 

 Mesozoic escarpments occupying their present positions, we 

 know from the relations of the Pleistocene (Grlacial) de- 

 posits to these features. We may, therefore, fairly infer 

 that, though the rivers which flow eastward and cut through 

 the great Chalk escarpment may have first established their 

 channels at an earlier epoch, yet it was the easterly tilting 

 of the country at this time which gave them the power of 

 excavating their present valleys in such a comparatively 

 short space of time, and which enabled their tributaries to 

 remove such large areas of Eocene and Chalk. We must 

 remember, also, that this process of erosion and denudation 

 would proceed contemporaneously with the formation and 

 recession of this Chalk escarpment. I am, therefore, in- 



