GLACIERS AND LAKES 247 



end of an old glacier. The glacier long ago retreated from 

 a former position and left moraine material where the ice had 

 been melting. This does not mean that the glacier moved 

 backward, but that the end melted faster than the glacier 

 advanced and thus each season the glacier receded farther up 

 the valley, nearer its source than the last season. Such will 

 be the history of a glacier if, because of change in climate or 



FIG. 131. GLACIATED ROCK 



This is hard trap rock exposed on the top of a hill. 1. Two sets of 

 lines are shown on the rock. Which were made by the glacier? How 

 do you judge? 2. If the thin layer of soil covering parts of the rock were 

 removed, do you think glacial marks would be found upon it? Why? 



elevation, there is a decrease in the supply of snow, whose 

 weight causes the glacier to move. Many glaciers in the 

 United States are known to have retreated to the summits 

 of the Rocky and Sierra Nevada mountains. There, melt- 

 ing during the summer, they now furnish a permanent source 



