THE SURFACE OF THE EARTH 



163 



duces lines of weakness where folding will occur. This 

 process goes on very slowly. To produce mountains 

 like the Rockies has taken untold ages of time. In 

 every mountain there is a period of birth, growth, 

 maturity, decay, and death. The Appalachian moun- 

 tains are now in the period of decay ; at one time they 

 were almost as high as the Rockies. 



Permission of U. S. Geological Survey. 

 FIG. 60. An anticlinal fold in the rock at Levis Terrane, Quebec. 



Plateaus are highlands of considerable area. Not all 

 plateaus are the same height; the Piedmont Plateau 

 between the Appalachian Mountains and the Atlantic 

 Ocean is rather low; while the Plateau of Tibet is 

 about 15,000 feet high. 



Plains are the lowlands of the earth. They have 

 great economic importance, for here is located $ 



