6o6 THE PLIOCENE PERIOD 



12,000, leaving it 8,000 feet above sea-level. How much of this is 

 assignable to the close of the Pliocene is uncertain. It was Button's 

 view that the Colorado plateau was so elevated at this time as to 

 rejuvenate the Colorado River, and that the cutting of its inner 

 gorge some 3,000 feet (maximum) below the outer (Fig. 73), was the 

 work of later times. More recent studies indicate that even the 

 outer and broader part of the valley is younger than formerly was 

 thought, and raise a question as to whether the inner gorge is not the 

 result of rock structure, rather than of a distinct and later uplift. 1 

 If the whole of the canyon is post-Pliocene, the elevation of the 

 region since the close of the Tertiary must have been several thou- 

 sand feet. The later elevations in this region, largely by blocks, 

 were so recent that many of the fault scarps are distinct, and in- 

 dependent of stratigraphy and drainage. 



In the basin region, faulting and deformation 2 gave rise to de- 

 pressions between the Sierra Nevada and the Wasatch Mountains, 

 preparing the way for two great Pleistocene lakes (Bonneville and 

 Lahontan). It is probable that many other faults between the 

 Rockies and Sierras were developed at the same time, and in many 

 cases the movement seems to have been along fault planes estab- 

 lished earlier. 



In the Sierra region, the post-Tertiary (or late Tertiary?) up- 

 lift was still more marked. 3 Not only the deep canyons of these 

 mountains, but all the scenery of the high Sierras is post-Tertiary. 4 



Still nearer the Pacific, notable changes marked the transition 

 to the Pleistocene. In some parts of southern California (Los 

 Angeles County) marine Pliocene beds are said to occur up to alti- 

 tudes of 6,000 feet, and in others (San Luis Obispo), there was fold- 

 ing (Fig. 492) and faulting, while the shore-line was pushed out 

 toward the edge of the continental shelf. There are submerged 

 valleys along the Pacific coast, as along the Atlantic, but their 

 excavation has been referred to a time earlier than the close of the 

 Tertiary. 



In Washington, present knowledge points to the early Pliocene 

 as a time of prolonged erosion. The crests of the Cascade Moun- 



1 Huntington and Goldthwaite, Bull. Mus. Comp. Zool. Geol. Ser., Vol. VI, 

 p. 252; and Davis, ibid., Vol. XXXVIII. 



2 King, U. S. Geol. Expl. of the 4oth Parallel, Vol. I, p. 542. 



3 LeConte, op. cit., and Diller, i4th Ann. Rept., U. S. Geol. Surv. 



4 The beginning of the re-elevation of the Sierras, after peneplanation, was 

 mid-Miocene. 



