DIASTROPHICS OF NORTHERN MEXICO 83 



moderate flexing nor the profound faulting are found to be so recent 

 as to retain their impress in full force upon the present relief. 

 These major crustal movements are mainly quite remote. Begin- 

 ning at the close of Paleozoic time they continue without interrup- 

 tion until the present day. The effects which their supposed 

 dominancy produces in the existing desert ranges are now known 

 to be due entirely to other causes. On the whole the desert ranges 

 seem to owe their physiognomy to vigorous wind erosion under 

 the stimulus of aridity rather than to recent deformation. Under 

 these abnormal conditions old structures are brought out into 

 strong relief by simple differential erosion of weak and resistant 

 rock belts. This desert degradation so vigorous at the present 

 day may have been of long duration, having gone on since the 

 beginning of Tertiary times. 



The post-Cretaceous wrinkling of the Rocky Mountains is 

 reflected in the areal distribution of the rock formations far beyond 

 the southern terminus of that cordillera, reaching many miles into 

 the Mexican tableland. 



