X.] ICE AND ITS WORK. 165 



have been scooped out by the action of huge masses of 

 moving ice. Nor is it only the effects of land-ice which 

 the glacialist sees marked upon the rocks of Britain. During 

 part of the Glacial Period, the land must have been sub- 

 merged beneath the waters of an icy sea ; and icebergs, 

 drifting from the north, scattered their freight upon the 

 rocky floor which has since been upheaved as dry land. 

 Even in the very neighbourhood of London, as at Finchley, 

 deposits of gravel and clay may be found crowded with ice- 

 borne boulders, which still retain their glacial scratches. 

 This drift, as it is termed, not to cite other evidence, suffi- 

 ciently shows that there must have been a time when ice 

 played its part, as an agent of denudation, within the basin 

 of the Thames. 



