THE TERTIARY PERIOD 301 



there began a tremendous rejuvenation of the range, caused by 

 the development of profound faulting along the eastern side. The 

 vast earth-block was tilted westward with steep eastern front and 

 long gradual slope toward the west, with the crest of the block 

 forming the summit of the range. The maximum amount of dis- 

 placement along this fault zone is no less than 15,000 feet and, in 

 spite of subsequent erosion, the fault-scarp still stands out as a 

 topographic feature usually several thousand feet high. That the 

 faulting has not yet ceased is evidenced by the Inyo earthquake 

 of 1872, when a renewed displacement of 10 to 25 feet took place 

 along the fault zone for many miles. The mighty canyons (e.g. 

 Yosemite) and other relief features of the Sierras as we know them 

 today have been sculptured out of the great tilted earth-block by 

 weathering and erosion. 



The Cascade Mountains, too, appear to have approached the 

 peneplain condition by late Tertiary time, when a vigorous re- 

 juvenation took place by an arching or bowing of the surface 

 rather than by profound faulting. 



Great Basin and Colorado Plateau. — About the same time 

 (later Tertiary) the whole Great Basin region between the Sierras 

 and the Wasatch Mountains of Utah was also notably affected by 

 faulting. The steep western front of the Wasatch represents a 

 profound fault, while many of the north-south Basin Ranges of 

 Nevada are tilted earth-blocks. 



The Cenozoic history of the Colorado Plateau region still pre- 

 sents important problems for future studies. According to Dutton 

 the Plateau* was raised, more or less periodically, fully 20,000 feet 

 during Tertiary time, but its surface now shows an altitude of 

 only 7000 to 8000 feet, because of deep erosion during its uplift. 

 As a result of the rejuvenation of this region, the Colorado River 

 was very actively revived and has carved out the Grand Canyon 

 since the early Tertiary. Later investigations, however, seem to 

 show that the rejuvenation was much later, probably late Pliocene, 

 and that most, if not all of the Grand Canyon, is of post-Tertiary 

 age. 



Rocky Mountain and Western Interior Regions. — Late in the 

 Tertiary much of the Rocky Mountain region was also greatly 

 rejuvenated by an uplift or upwarp, unaccompanied by folding of 

 the strata. That this upwarp amounted to some thousand of feet 

 is distinctly proved by the fact that the Miocene beds east of the 



