84 T. BELT ON THE DRIFT OP DEVON AND CORNWALL. 



sides of the valleys have been left there during their excavation ; for 

 as we have seen, the configuration of the country dates back at least 

 to early Tertiary times. 



Water must have been present to form the gravels of Dartmoor 

 up to about 1200 feet above the sea, and also to allow ice to trans- 

 port the erratic blocks. Was the water that of the ocean, or of a 

 great freshwater lake ? AVith respect to the area in question, there 

 is this insuperable objection to the theory of marine submergence, 

 that over the whole district no marine beaches, and not a single 

 marine organism has been found, excepting within a few feet of the 

 present sea-level. Nor can sea-shells have once existed in the de- 

 posits, and been since destroyed ; for mammalian remains and land 

 and freshwater shells are preserved ; and any agency that would have 

 obliterated the traces of the one, would not have spared the others. 



After much study of this question, not only with regard to the 

 part of the country now under consideration, but with reference to the 

 erratic blocks and the stratified and terraced gravels of nearly the 

 whole of Northern Europe, the only theory I can find that meets 

 all the facts of the case and deals with all its difficulties, is : — 

 (1) that at the height of the Glacial Period the bed of the Atlantic 

 was occupied by ice flowing from the north-west, from the direction 

 of Greenland, reaching the coast of Europe either at Brest or 

 some other point further south, and damming back the whole of the 

 drainage of northern Europe into an immense lake of fresh water, 

 which, at its greatest extension, reached to a height of at least 1200 

 feet above the present level of the sea ; (2) that this lake was gra- 

 dually lowered for some hundreds of feet, probably by the deepening 

 of a channel of outlet, and then suddenly and completely drained 

 by the breaking away of the ice- dam causing an enormous flood or 

 debacle ; (3) that the complete denudation of the detritus from 

 the low grounds was prevented, and its outspread in great sheets 

 caused, by the outflow of the waters being checked by the immense 

 area drained through the single outlet of the British Channel. 



In some Alpine lakes caused by glaciers, the waters periodically 

 break away, and are again pent up by the readvance of the ice ; and 

 so it is possible that a series of floods might be caused by the great 

 Atlantic glacier. But I can find no evidence of more than one 

 debacle in the southern counties, and I am inclined to agree fully 

 with the conclusion that Professor Sedgwick arrived at fifty years 

 ago — that " diluvial torrents have swept over the land, driving be- 

 fore them enormous masses of gravel," and that " all the diluvial 

 detritus originated in the same system of causes, which, having pro- 

 duced their effects, were never repeated " *. If, however, I were 

 discussing the origin of the glacial beds north of the Thames, I 

 should have to show that although there is evidence of only one 

 sudden and tumultuous discharge of the waters, yet, after the 

 debacle so caused had occurred, they rose again, but were this time 

 more quietly lowered. 



Completely as this theory seems to explain the production and 

 * Annals of Philosophy, vol. ix. p. 248. 



