12 THE ICE AGE IN NORTH AMERICA. 



bowlders bearing marks of the peculiar attrition to which 

 they have been subjected in their motion underneath the 

 ice. The rocks brought along upon the surface of the gla- 

 cier of course are not thus striated, and ordinarily the mate- 



Fig. 11.— Glacial scorings (after Agassiz). 



rial of the kames has been so much rolled by water that if 

 the pebbles ever were scratched, the marks have been erased. 

 With this brief account of the physical characteristics of 

 ice, and of the effects produced by its movement in a gla- 

 cier, we are prepared to enter more understandingly upon a 

 survey of the actual facts relating to the past and present 

 extent of the ice-fields over the northern part of North 

 America. Reserving the discussion of theories concerning 

 the cause and date of the glacial period to the latter part of 

 the treatise, we will first consider the facts concerning the 

 glaciers still existing in America, and then briefly, by way of 

 comparison, those concerning glaciers in other portions of 

 the world ; after which we will present in considerable 

 detail the more recent discoveries concerning the extension 

 and work of the great American ice-sheet during the so- 

 called Glacial period. 



