GEOLOGICAL SETTING OF NEW MEXICO 



235 



Desert ranges rising abruptly out of the plains about impart char- 

 acteristic form to the enisled landscape. The girdled mountain 

 attests the vigor of natural sand-blast action; and its maximum 

 effectiveness is at the plains line. Plateau plains of the desert 

 manifestly represent former levels of the general plains surface. 

 The notable absence of foothills around the mountain bases appears 

 to be an idiosyncrasy of arid lands. 



Arid planatation takes place uphill as well as down; anti- 

 gravitational gradation is unknown where streams erode. High 

 gradients of the intermontane plains and strong pitch of valley 



_ 10000 A.T 



Fig. 2.— Sierra de los Caballos: origin of enisled relief 



axes which are displayed on every hand are not possible in regions 

 where water action is directly the reverse of plains forming. Of 

 minor features attributable to wind abrasion in the lands of little 

 rain, there are a multitude that have been ascribed to normal 

 water corrasion but that are now known never to have been touched 

 by stream. Upon all of these the wind marks when once pointed 

 out are unmistakable. 



It so happens that the broad arid tract of southwestern United 

 States has within its borders abundant traces of former base-level 

 plains, one of which peneplains is now raised more than two miles 

 above sea-level. The attainment of its present position is regarded 

 as having taken place in late Tertiary times. This great peneplain 

 no doubt once extended over all of this desert region, at a level 



