BEFORE AND AFTER 81 



seems to me the most probable explanation. It appears 

 to me probable that in the Ice Age, south of the ice sheet, 

 the ground must have been both broken up by frosts, and 

 also held together by being frozen hard to some depth. 

 Then when thaws came in the short but warm summers, 

 or when an intermission of the severe cold took place, 

 great floods would flow down the valleys in the country 

 south of the ice sheet, and masses of ice with frozen earth 

 and stones would be borne along in a sort of semi-liquid 

 flow. In this way Mr. Clement Reid explains the mass 

 of broken-up chalk with large stones found on the heads 

 of cliffs on the South coast, and known by the name of 

 " combe-rock " or " head." 



The Ice Age was not one simple period, and it is still 

 difficult to fit together the history we read in different 

 places, and in particular to correlate the gravels of the 

 south of England with the boulder clays of the glaciated 

 area. There were certainly breaks in the period, during 

 which the climate became much milder, or even warm ; 

 and these were long enough for southern species of 

 animals and plants to migrate northward, and occupy the 

 lands where an arctic climate had prevailed. There were 

 moreover considerable variations in the relative level of 

 land and sea. So that we have a very complex history, 

 which is gradually coming into clearer light. 



That the gravels of the south of England belong largely 

 to the age of ice, is shown by remains of the mammoth 

 contained in many. These, however, are found in later 

 gravels than those we have considered so far, gravels laid 

 down after the land had been cut down to much lower 

 levels. These lower gravels are known as Valley gravels, 

 because they lie along the course of existing valleys, the 

 Plateau gravels having been laid down before the present 

 valleys came into existence. Teeth of the mammoth 

 are found in the Thames valley, and on the shores of 

 Southampton Water, in gravels about 50 to 70 feet above 



