296 THE EARTH'S SURFACE AND PLANT LIFE 



or why is unknown. This period when the ice was of great 

 extent is called the Glacial Period. Probably during the 

 earth's history there have been several of these periods, but 



AREA IN NORTH AMERICA COVERED BY THE ICE OF THE GLACIAL. 

 PERIOD 



to the last is due the great change wrought upon the present 

 surface of the country and upon plant and animal life. 



The greatest ice invasion during this period extended 

 from northern Canada across New England into the sea, 



