75 



ISOTHERMAL MAP, SHOWING THE PROBABLE MEAN ANNUAL TEMPERATURE 

 (FAHRENHEIT) DURING THE GREATEST SEVERITY OF THE GLACIAL EPOCH. 



Note. — No attempt has been made to restore the ancient topography ; 



as far as yet known there was little change in the boundaries 

 of sea and land during the glacial epoch. 



The soil and subsoil during such a time must have been 

 deeply frozen all the winter. When the spring- thaw set in, 

 the rush of water over the still-frozen and impervious sand or 

 chalk caused the surface everywhere to be torn up and channelled 

 in a way that we now never see in the south of England. For 

 such rapid erosion we have to go to the frozen lands of Siberia 

 and Canada. It is under conditions such as these that the dry 

 valleys in the plateaus about Bournemouth were formed, as well as 

 the dry coombes in the chalk of Salisbury Plain and the South 

 Downs. 



During this period of intense cold the English Channel was 

 to a large extent frozen over, and drift ice breaking away in the 

 spring carried erratic blocks far and wide, transporting them 

 across the Channel with the greatest ease. 



It may seem that I am getting somewhat far away from my 

 subject ; but it is not so. In order to understand the formation 

 of the valleys of Bournemouth, the climatic vicissitudes that we 

 have passed through must be realized. We will, therefore, 

 examine a little more of the abundant evidence for such vicissi- 

 tudes in the south of England. 



