GEOLOGIC EVOLUTION OP DESERT RANGES 



561 



latter. It is, in fact, about as far from the fault-line to the crest of the 

 range as it is from the latter to the base of the backslope. As a rule, 

 the ranges are bilaterally symmetrical. They would hardly be so if the}' 

 were recently reared and tilted fault-blocks. 



For simplicity, four distinct stages of Desert-range development may 

 be premised. First is the period of completed sedimentation, represented 

 by the general geologic column. Of this we have ample evidence in the 

 sequence of geologic formations represented in the different parts of the 

 region. Above the great thickness of hard Paleozoic limestones come 

 Qon-resistant beds consisting of 3,000 feet of red-beds (1,000 feet of 

 which are Mid-Carbonic in age, 1,000 feet of Late Carbonic, and 1,000 

 feet of Triassic age), a varying thickness of soft Jurassic sandstones, 

 7,000 feet of Cretacic shales and sandstones, and, finally, 5,000 feet of 

 Tertiary beds. 



Figure 16. — Stages of Development of Desert Ranges 



In point of time, according to our best information, the major flexing 

 of the region took place in Jurassic times, the major faulting in early 

 Tertiary times. Unaffected by erosion, the dislocations would give re- 

 sults similar to those shown in the diagram below (figure 16, a). De- 

 formation and erosion no doubt went on simultaneously, giving a more or 

 less subdued profile, represented by the line h in figure 16. As erosion 

 progressed the country must have been reduced practically to the condi- 

 tion of a peneplain (line c, figure 16), with possibly here and there some 

 of the older and harder rock-masses protruding above its even surface. 

 The remnants of this great plain beveling the substructure ever3rvvhere 

 appears to exist in that high-lying mass constituting the Eaton range in 

 northeastern New Mexico, the flat summit of which is known as the Mesa 

 de Maya.^*^ Indications of general peneplanation are also shown in many 

 of the other plateau plains of the region. 



" Proceedings of the Iowa Academy of Sciences, vol, xiii, 1908, p. 221. 



