252 



THE ICE AGE IN NORTH AMERICA. 



of the glacier. This brings the plums in the pudding to the 

 surface, and mixes them with the medial moraine blocks which 



Pig. 74. — Glaciated pebble from southern Indiana. Natural size. This was one of the 

 graving tools of the glacier. The striae are parallel with the longer axis. 



have ridden down from the forks of the valley. A constant 

 concentration of the debris of the whole glacier thus goes on 

 at its surface in spite of the occasional loss of some of the stuff 

 by dropping into crevasses, the blocks thus temporarily lost 

 rejoining their fellows at the surface, if the glacier be long 



