208 matured 



the cause of these deposits. The line is called 

 the " terminal moraine." By examining a 

 map of North America and tracing the line of 

 the moraine as we have described it, it will be 

 seen that about two-thirds of North America 

 was at one time covered with ice to a greater 

 or less depth. How deep, is simply a matter 

 of conjecture, but in the central portions of 

 the great glacier, where was the bulk of snow- 

 fall, it must have reached a depth of several 

 miles to account for the enormous pressure 

 that would be required to carry the ice so far 

 southward. 



But let us go back and define what is meant 

 by a moraine. A moraine is a name given to 

 the deposits that are of stone, gravel, and 

 earth that have been carried along by the 

 movement of the glaciers and deposited at 

 their margins, sometimes piled up to great 

 depths. The composition of these moraines is 

 determined of course by the nature of the 

 country over which the stream of ice is flow- 

 ing. Bowlders of enormous size have been 

 carried for hundreds of miles, and the experi- 

 enced geologist is able to examine any one of 

 them and tell us where its home was before the 

 glacial period. Moraines are divided into dif- 

 ferent classes according to their position and 

 constitution. The moraine found at the ex- 

 treme limit of ice-flow is called the "terminal" 

 moraine, as before mentioned. Those that are 



