CHAPTER VIII 

 THE GREAT RELIEF FEATURES OF THE LAND 



MOUNTAINS, plateaus, and plains have been referred to 

 frequently in previous pages. The more important points 

 concerning their origin and life history are summarized in 

 the present Chapter. Most of the geological processes and 

 agents are concerned in their formation and development. 

 The present characteristics of any given relief feature are 

 determined chiefly by (1) the method of its formation, (2) its 

 original altitude and its distance from the sea, (3) the char- 

 acter and structure of its rocks, (4) the character of the 

 agents of erosion that are slowly wearing it down and the 

 conditions which govern their work (especially the nature 

 of the climate), and (5) the stage in their wofk which the 

 erosive agents have reached. 



MOUNTAINS 



A mountain is an elevation which rises prominently above 

 the surrounding country, and which has a comparatively 

 small area on top (Fig. 287). On a low, flat plain, a moun- 

 tain may have a height of only a few hundred feet ; in more 

 rugged surroundings, a much higher elevation may be called 

 a hill. The matter is therefore a relative one, and no fixed 

 height is necessary in order that an elevation may be classed 

 as a mountain. Certain plains for example, the western 

 portion of the Great Plains are higher than many moun- 

 tains, for instance, the northern Appalachians (Figs. 43 and 

 44, pp. 62 and 63). Most mountains, however, are higher 

 than most plateaus and plains. A few of them reach eleva- 

 tions above sea level of nearly 30,000 feet, or about 5| miles. 



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