LINEAMENTS OF THE DESERT 



27 



down the land. The intermont plains of the arid region of western 

 America have beveled rock-substructures, but their surfaces are 5,000 

 to 8,000 feet above sea-level. The beveled character of the strata is 

 almost conclusive evidence that these plains are not areas of extensive 

 aggradation. In the absence of adequate water-action their origin 

 must rest mainly in long-continued and effective deflation. 



Critical examination of the substructure of the plains, in favorable 

 places, shows clearly that the rock-floor itself is a plain. Although 

 not always apparent at first glance this rock-floor is commonly only 

 thinly mantled by wash debris and soil. Hence the rock-floor and the 

 present plains-surface are not very different in their detailed relief 

 characters. 



Sierra Organo. Striking example of nbsence of rock-weathering in arid country : 

 the peaks rise 5,000 feet above the plain. 



A phenomenal feature of the desert plains is the plateau-plain. 

 Mesas they are called in southwestern United States and Mexico. 

 These mesas, as their Spanish name signifies, are extensive, fiat-topped, 

 table-like areas rising abruptly from the general plain to heights of 

 from one or two hundred feet to a thousand feet or more. The great 

 Mesa de Maya, in northeastern New Mexico, is 3,500 feet above the 

 next lower plain. 



The surface of the plateau-plain is usually found to be composed 

 of some hard rock layer, as in the case of the vast Llano Estacado, or 

 " walled plains," or staked plains as it is called by the Texans ; or is 

 made up of an extensive lava flow, as, for example, the Mesa de Maya, 

 the Ocate mesa and the majority of the plains of this kind. The sur- 

 face beneath the lava flows of the mesas is itself a plain worn oiit on 



