5±3 



CENOZOIC ERA— AGE OF MAMMALS. 



various kinds and ages of country rock are simply clotted to indicate 

 the presence of this surface-material. In sections, of course, it is easily 

 represented, as in Fig. 927. 



The Bowlders. — The most casual examination of the great bowlders 

 is sufficient in many cases to show that they do not belong to the coun- 

 try where they now lie, for they are of entirely different material from 

 the country rock. For example, blocks of granite are found where 

 there is no granite within many miles, blocks of sandstone on a country 

 rock of limestone, or vice versa. In many cases it is easy to find the 



<si* 



Fig. 928.— Bed-rock scored with glacial marks, near Amherst, Ohio. (From a photograph by 



Chamberlin.) 



parent ledge from which these great fragments were torn, and thus to 

 trace the directioyi of their transportation. From many observations 

 of this kind it has been determined that in New England the bowlders 

 have come usually from the northwest, in Ohio from the north, and in 

 Iowa from the northeast. In other words, from the highlands of 

 Canada and a ridge running thence northwestward (Archaean area), the 

 general direction of travel has been southeast, south, and southwest. 

 North of the Archaean areas the travel was probably in some cases even 

 northward. The distance carried may be only a few miles, or may be 



