THE FEATURES IN DESERT LANDSCAPES 



219 



. 



. 

 < 



The origin of the high plains which front 

 the Rocky Mountains. To the eastward of 

 the great backbone of the North American 

 continent stretches a vast plain gently in- 

 clined away from the range and generally 

 known as the High Plains region (plate 9). 

 The tourist who travels westward by train 

 ascends this slope so gradually that when he 

 has reached the mountain front it is difficult 

 to realize that he has climbed to an altitude 

 of five thousand feet above the level of the 

 sea. That he has also passed through several 

 climatic zones a humid, a semiarid, and 

 an arid and has now entered a semiarid 

 district, is more easily appreciated from study 

 of the vegetation (Fig. 234). The surface of 5* 

 the High Plains, where not cut into by rivers, S. 

 is remarkably even, so that it might be com- o* 

 pared to the quiet surface of a great lake. 2. 



The materials which compose the surface % 

 veneer of these plains are coarse conglomer- | 

 ates, gravels, and, sands, and the so-called g 

 " mortar beds," which are nothing but sands 

 cemented into sandstone by carbonate of lime. 

 The pebbles in all these deposits are far- %. 

 traveled and appear to have been derived | 

 from erosion of those crystalline rocks which ' 

 compose the eastern front of the Rocky 

 Mountains. These different materials are 

 not arranged in strictly parallel beds, as are f 

 the deposits of a lake or sea; but the beds st- 

 are made up of long threads of lenticular gj 

 cross section which are interlaced in the most 

 intricate fashion and which extend down the 

 slope, or outward from the mountain front 

 (Fig. 235). It is thus shown that the High 

 Plains are a bench or plain of alluviation 

 formed at the front of the Rocky Mountains 

 during an earlier series of pluvial periods, and that subsequent 



