io6 



ELIOT BLACKW ELDER 



surface which was finally produced just before the deposition of the 

 Eocene strata is, however, preserved in the sub-Eocene unconform- 

 ity. Profiles of its forms can be observed along the canyon walls 

 in several of the ranges. At some points near the west end of the 

 Owl Creek Mountains the old land surface has recently been 

 exhumed by the denudation of the soft Eocene clays and yet not 

 seriously disfigured during the process (Fig. 5). This surface was 

 post-maturely hilly upon the harder rocks, and locally retained 

 a relief of over 1,000 feet; but, as would be expected from 

 their unresisting character, the softer Cretaceous and Jurassic 

 beds were generally worn down to plains. In many places the 



Fig. s 



Eocene now crosses the axes of the anticlines and thus shows that 

 the ranges had been largely worn low. 



Deposition of the Eocene and Oligocene formations. — Before the 

 hilly and even mountainous parts of the early Eocene surface could 

 be reduced to base-level, there set in more or less abruptly a 

 change which caused the erosive processes to be supplanted in the 

 depressions by deposition. The nature of the change is not 

 obvious, but it may have been either a warping, which unbalanced 

 the river systems, or a desiccation, which substituted torrential 

 or seasonal erosion and alluviation for the slow continuous degrada- 

 tion of a moister epoch. Since the effects seem to have been nearly 

 alike from Montana to Mexico, a widespread and rather uniform 

 change is suggested. The climatic change answers this require- 

 ment, but there is uncertainty as to whether so great a thickness of 

 sediment can be accounted for in this way. There is no question 

 as to the competency of warping to bring about the erosion of some 



