306 THE EARTH'S SURFACE AND PLANT LIFE 



How are glaciers formed ? How have they modified the surface 

 of the region where they are found? 



What was the extent of the North American ice sheet during the 

 Glacial Period? 



How has the Glacial Period affected the present agricultural and 

 industrial conditions -of the country over which the ice spread? 



In what ways does the process of decay affect the soil ? 



