THE TERTIARY PERIOD 441 



The renewal of the uplifts late in the Tertiary and con- 

 tinuing into the next period brought on the conditions 

 which we now think of as characteristic of the region. It is 

 to these later movements that the present elevation of our 



FIG. 458. Stereogram of a part of the Columbia River lava plateau, 

 showing flows with interbedded layers of sand and gravel. 



high ranges is due ; and, as has been said, in some of them 

 the growth is still in progress. During the uplifts, the 

 streams sank their valleys deeper and deeper into the lands, 

 so that the West is now characterized, not only by moun- 

 tain ranges, but by high plateaus deeply cut by canons, 



FIG. 459. Young fault block mountains in southern Oregon. (Modified 



after Davis.) 

 Why are the depressed spaces between the blocks 'flat? 



such as those of the Colorado and the Snake rivers. The 

 growth of the mountains also deprived the winds from the 

 Pacific of a large part of their moisture, and thus condemned 

 the interior basins and the Great Plains to a much drier 

 climate than they had before. 



