CHAP. XIII.] PLEISTOCENE EPOCH. 285 



would require to become deeper than now by 1,400 or 1,500 

 feet at the least." l Now if we add the latter depth to the 

 height at which marine shells occur at Chapelhall (526 

 feet) we obtain a total of over 2,000 feet, which is just 

 the extreme amount of subsidence indicated by the Welsh 

 Drifts. 



This consideration appears also capable of explaining 

 what Professor J. Geikie calls an "insuperable objection '* 

 to Mr. Mackintosh's views regarding the dispersal of 

 erratics over England. Many of these erratics have come 

 from Criffel in Galloway, and Professor Geikie thinks it 

 " very strange that there is not a vestige or trace of any 

 such submergence either in the neighbourhood of Criffel 

 itself or in the region to the north of it ; " 2 but if the ice 

 had anything like the thickness he supposes, it is clear that 

 it would never loose hold of Criffel during the subsidence 

 unless it was melted off by a rise of temperature, and the 

 absence of marine shells consequently ceases to be smv 

 prising, if indeed it ever was very much to be won- 

 dered at. 



Admitting, however, that in the present state of know- 

 ledge and opinion it is unsafe to attempt any connected 

 account of the physical history of the Great Ice Age in 

 Britain, there are, nevertheless, certain well-ascertained 

 facts upon the interpretation of which nearly everyone is 

 agreed, and these enable us to indicate with tolerable cer^ 

 tainty some of the geographical changes which took place 

 during the prevalence of Glacial conditions ; changes which 

 were largely concerned in the development of the present, 

 geographical outlines and physical features of the British 

 Islands. 



It was stated on p. 246 that the slight submergence with 



1 " The Great Ice Age," second edition, p. 186. 



2 " The Intercrossing of Erratics in Glacial Deposits " (" Scottish, 

 Naturalist," 1882). 



