268 THE EARTH'S CRUST 



Young Plateaus. Sometimes large areas of horizontal 

 rock are elevated high above the sea, forming lofty plains 

 whose surfaces are often irregular, owing to previous erosion. 

 Such areas are called plateam. The descent 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 crum- 

 bling 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 has 

 been eroded by a strong river gaining its water from a 

 distant region is that of the Colorado Canon Plateau. Here 

 is found the grandest example of erosion on the face of 

 the earth. The rocks are of various colors ; the gorge is 

 nearly a mile deep and in places some fifteen miles in width. 

 Words are inadequate to express the grandeur of the pan- 

 orama spread out before one who is permitted to see this 



