3 6 ELEMENTARY SCIENCE 



determining the nature of the topography, especially in 

 northeastern United States. Not many thousands < 

 years ago the earth appears to have been a much cooler 



FIG. 14. Map showing how a great glacier once covered the north- 

 ern part of our country. After CHAMULKLIN. 



place than it is now. Picture the effect this would have 

 on the water. Even now billions of tons of water are 

 "locked up" in the form of ice in the polar regions, and only 

 a relatively small amount of this gets into circulation each 



