The P re-historic Rhine. 543 



elevation of the land and sea-bottom, was re- united to 

 the Continent, chiefly by a broad plain of Boulder-clay. 

 Through this plain I think that the Rhine must have 

 wandered in pre-historic times to what is now a 

 northern part of the North Sea, and all the eastern 

 rivers of England the Thames, the rivers of the Wash 

 and the Humber, the Tyne and possibly some of the 

 rivers of Scotland, were its tributaries. 



This Boulder-clay, from the manner in which it was 

 formed had a very irregular surface, enclosing lakes 

 and pools, some of which may still be seen on the 

 plains of Holderness. I have said that after the 

 deposition of the Boulder-clay, Britain was re-united to 

 the Continent, but it is well known that various oscilla- 

 tions of the relative level of the land to the sea took 

 place during the Glacial epoch, and under these circum- 

 stances it may, not improbably, have been partly joined 

 to the mainland during inter-Glacial episodes, or again, 

 when glacier ice covered broad tracts of country. 



At such times the present mouths of many British 

 rivers could have had no immediate relation to their 

 ancient mouths, for the places of their present mouths 

 then lay far inland. Under such circumstances it 

 seems not unlikely that alluvial gravels, such as those 

 of Bedford Level, may have been deposited in lakes 

 dammed up by some old Boulder -clay that formed part 

 of the plain through which the rivers flowed. The wide 

 gravel plain within the circuit of the great moraine of 

 the Dora Baltea in Piedmont forms a sort of case in 

 point, for, according to Gastaldi, an old lake-hollow 

 has there been entirely filled with gravel borne by the 

 river from the Val d'Aosta. 



It is often difficult to account for the great 

 thickness of these lowlying gravels on any other hypo- 



