GEOTECTONICS OF ESTANCIA PLAINS 



439 



it the beds are very much contorted. The inclination of this thrust- 

 plane is now rather steep, but this is due partly to the fact that the 

 present position of this structure is not the original one. Since the 

 time of its formation the thrust-plane also has been tilted to a marked 

 degree. In point of time the formation of the thrust-plane long ante- 

 dates the uprising of the present mountain block. 



There are in the Sierra de los Caballos at least three distinct periods 

 of faulting. The first was before the formation of the present moun- 

 tain ridge; the second was coeval with its formation; and the third 

 was long subsequent to its uprising. 



Sea.le 



Fig. 5. — Unconformity of Cretaceous on Carboniferous, near Dios Springs. 



Were it not for the exceptionally clear evidence to the contrary, 

 casual examination could very easily lead to the conclusion that the 

 Caballos mountain range had been produced by sharp folding, and 

 that the crest of the asymmetric fold had been removed through ero- 

 sion. The deduction is a natural one especially when in a view from 

 the summit of the range there are plainly shown the strata dipping 

 eastward to form a broad syncline, and coming up again with westerly 

 dips in the great San Andreas block, 30 miles away. 



It so happens that in the region under consideration the general 

 sequence of geologic events is sufficiently well known to us to give 

 a good insight into some of the actual conditions that have existed.' 



I American Journal of Science (4), Vol. XVIII, pp. 356-58, 1904. 



