276 THE EARTH'S CRUST 



The ocean floor near continents slopes off gradually until 

 it reaches a depth of about 600 feet, when it suddenly changes 

 to a sheer depth of two or three miles. This gradually 

 deepening border is called the continental shelf. Upon 

 such shelves lie the great continental islands and fishing 

 banks. The upward movement of these continental shelves 

 gives us our coastal plains and has greatly increased the 

 habitable land of the earth. The depression of continental 

 borders has given us our estuaries, deep harbors, and con- 

 veniently navigable coasts. 



Mountains are the result of folding, pushing up, crumpling, 

 and other distortions of 'the earth's crust that have occurred 

 during ages of change. Mountains are usually found in 

 ranges and the peaks are the results of erosion. Large 

 areas of horizontal rock that have been elevated high above 

 the sea level are called plateaus. If subject to great erosion, 

 plateaus eventually become dissected and finally worn down 

 to a comparatively low level, with only occasional mesas 

 and buttes rising here and there. The Great Plains are a 

 vast sloping surface that was probably tilted upward when 

 the crust of the earth was folded along the great continental 

 divide. 



QUESTIONS 



What changes have taken place in the earth's condition? 



To what great classes do the rocks in your neighborhood belong ? 



For what would you look if endeavoring to determine whether 

 a coast had been elevated or depressed. 



What advantages does an elevated coast furnish its inhabitants ? 

 A depressed coast? 



To what is the height of mountains due ? 



Describe the characteristics of a young plateau. 



Why do not dissected plateaus attract a dense population? 



What are the characteristic features of an old plateau ? 



