CHAPTER XIII 

 THE HIGH AKEAS OF THE EAETH 



190. Young Plateaus. Sometimes large areas of hori- 

 zontal rock are elevated high above the sea, forming lofty 

 plains whose surfaces are often irregular, owing to pre- 

 vious erosion. Such areas are called plateaus. The de- 

 scent from a plateau to the lower land is usually steep. 

 Areas of this kind, where streams are present, suffer 

 rapid and deep erosion, since the grades of the streams 

 are steep because of the elevation. 



If there is not much rain there will be few streams, and 

 these will have deep and steep-sided troughs. Such troughs 

 render the area very difficult to cross. The valleys are too 

 narrow for habitation or for building roads, and the deep 

 troughs of the streams are too wide to bridge. Thus 

 the uplands are isolated. 



If these high areas are in a warm latitude, they are desir- 

 able for habitation on account of their cool climate, due to 

 the elevation ; but' if in temperate latitudes, their bleak 

 surfaces are too cold. 



As the river troughs wear back, the harder rocks stand 

 out like huge benches winding along the course of the 

 rivers. From the different benches slopes formed from 

 the crumbling of the softer strata slant backward. Thus 

 the general outline of the stream sides will be something 

 like that of a flight of stairs upon which a carpet has been 

 loosely laid. 



An excellent example of a region of this kind which 



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