562 C. R. KEYES PROFILES AND STRUCTURES IN DESERT RANGES 



The fourth stage may be considered as beginning with the peneplain 

 upraised high above the sealevel, the introduction of arid climatic condi- 

 tions, and the full play of eolic erosional influences producing after a 

 long time the present day facial expression of the country (line d, fig- 

 ure 16). 



With these purely deductive conclusions the facts observed seem strictly 

 to accord. A concrete, and at the same time most typical, example is 

 the great San Andreas ridge in southern New Mexico, directly north of 

 El Paso. Its crest is 4,000 to 5,000 feet above the plains and 9,000 to 

 10,000 feet above sealevel. Of all the desert ranges, this one would seem 

 to present most clearly a tilted fault-block rising abruptly out of a vast 

 sea of earth. For a distance of 100 miles the steep eastern face of the 

 range appears to be as true a faultscarp as is to be found in any ideal 

 Basin range. 



When the critical evidences are sought of recent upraising and tilting 

 of this long mountain block, and, indeed, of other similar ranges, they 

 appear to be lacking in a surprising manner. The fault-line is found to 

 be located several miles out on the plains, far beyond the base of the 

 mountain. A transverse profile of the range shows it to be rather sym- 

 metrically developed, with the crest-line nearly midway between the 

 fault-line and the base of the backslope. The plains on either side have 

 rock-floors. The long, narrow belt of remnantal mountain rock is bor- 

 dered by broad areas of weak shales. Very inadequate explanations at 

 once appear in the fault hypothesis of the rearing of the mountain range. 

 One must turn to some other suggestion for the great mountain-forming 

 agency. Nor does ordinary erosion suffice. In view of the well known 

 deficiencies of rainfall, with the still more marked absence of running 

 waters, one is tempted to seek some erosive process other than stream 

 action. It is in the arid region that the eolian influences flnd their maxi- 

 mum activities as erosional processes. 



Under conditions of aridity, the differential effects of wind-scour, or 

 deflation, upon rock belts of contrasted induration are very different 

 from what they are in a normal humid climate. The inequalities of sur- 

 face relief are in consequence very much more rapidly accomplished than 

 when stream-action is the chief eroding process. In general, it may be 

 stated that in the case of hard rock-masses in an arid land eolic erosion 

 is probably less than one-tenth as efficient as in a normal humid country 

 water action would' be, while in the case of weak rocks it is more than 10 

 times greater. This is, no doubt, the principal reason why to most ob- 

 servers in the desert regions such manifest evidences of enormous erosion 



