HILLS AND MOUNTAINS 



415 



feet above the Colorado Plateau and steep cliffs bound 

 it on both its east and west sides. These fault cliffs, 

 as they are called, are found at several other places in this 

 region, showing that the whole area was much broken when 

 it was uplifted. The Kaibab Plateau itself is so much 

 higher than the plateaus on either side that it intercepts 

 sufficient rainfall to support forests, whereas the plateaus 

 about it are almost 

 barren of trees. 



In the walls of the 

 Colorado Canon some 

 of these great breakage 

 lines can be traced and 

 the same strata seen to 

 be thousands of feet 

 higher on one side of 

 the line than on the 

 other. In front of these 

 breakage cliffs or fault 

 cliffs, accumulations of 

 debris extend along the 

 entire distance, show- 

 ing that since the uplift there has been time for much 

 erosion even in this dry region. The Colorado River 

 passes over these great faults regardless of their existence. 

 The canons in the region seem not to have been influ- 

 enced by the faulting. Probably it took place too slowly. 



194. Hills and Mountains* Irregular elevations of the 

 earth's surface are called hills, or mountains when they are 

 of considerable height. In the general use of these terms 

 there is no exact line of separation. Elevations which in 

 mountain regions would be called hills would in a flat region 

 be called mountains. As a rule, elevations are not termed 

 mountains unless they are at least 2000 feet high. But if 



