530 



CHAPTEK XXXI. 



RELATION OF EIVER VALLEYS AND GRAVELS TO THE GLACIAL 

 DRIFTS RIVER TERRACES BONES OF EXTINCT MAMMALS 

 AND HUMAN REMAINS FOUND IN THEM RAISED BEACHES, 

 ETC, 



IT is certain that by far the greater number of the river 

 valleys of Britain, north of Bristol Channel and the 

 Thames, have been very much modified, and some of 

 them deepened during the Glacial period, a fact indeed 

 sufficiently proved by the Glacial excavation of all the 

 lakes that lie in rock-bound basins. Some valleys in 

 England have been greatly modified since the Glacial 

 period came to an end. 



It may, however, be safely said that before the 

 Grlacial period the larger features of the river systems 

 of Britain were much the same as now. When, before 

 and during partial submergence, Boulder-clay over- 

 spread great part of the country, the river channels of 

 the lower lands often got filled with that clay entirely, 

 or in part. When the land emerged and surface 

 drainage was restored, most of the rivers followed their 

 old channels. In some cases they nearly scooped the 

 Boulder-clay entirely out of them from end to end, but 

 in others, as with the Tyne and the Wear, accidents 

 partly turned the rivers aside, and having disposed of a 

 thin covering of Boulder-clay, they proceeded to exca- 

 vate deep and winding valleys in the Sandstone rocks 



