CHAPTER XI 

 MOUNTAINS AND PLATEAUS 



The term mountain is used very loosely to indicate a conspicuous 

 height of land. In flat regions such as southern New Jersey and 

 the plains of Texas, heights rising more than ioo to 200 feet are 

 dignified by the name mountain, while in mountainous regions ele- 

 vations of 1000 or 2000 feet are often called hills. It is evident that 

 the term is a relative one, since on plateaus a mile or two above 

 the sea a conspicuous elevation must be still higher, and a mountain 

 in such a situation would be at least 6000 feet above sea level. A 

 mountain ridge or range is usually long, with a narrow crest; when 

 numerous ranges are associated, they constitute a mountain chain. 

 In ancient paintings and in old geographies, the slope of mountains 

 was usually depicted as very steep, an angle of 6o° from the hori' 

 zontal not being uncommon, but such slopes seldom occur in nature, 

 and angles as high as 35 ° are rare. 



Mountains of Accumulation. — Volcanoes are typical of this class, 

 as they are built up by the accumulation of ash, or lava, or both. 

 They sometimes occur singly, sometimes they are arranged along 

 fracture lines (p. 267), and sometimes no definite order can be recog- 

 nized. 



Since sand dunes (p. 52) occasionally attain a height of 600 

 feet and moraines (p. 159) a height of 1000 feet, they are sometimes 

 called mountains in regions where other elevations are inconspicuous 

 and must therefore be included under the head of accumulation 

 mountains. 



Residual Mountains. — These are formed when a plateau has been 

 extensively dissected by rivers, and the ridges and pyramids, 

 the remnants of the plateau, which have escaped erosion, stand 

 so high above the valleys as to constitute mountains. The many 

 " temples " in the Grand Canyon of the Colorado in Arizona (Fig. 

 340) show at a glance how such mountains are formed, and the 

 Cat skills of New York furnish an excellent example of residual 



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