THE PLIOCENE PERIOD 837 



nental creep along the slope between the continental platforms and 

 the ocean basins may have lowered the valleys notably as it carried 

 them seaward. The earlier assumption that the land along the 

 Atlantic seaboard must have stood 2,000 to 3,000 feet, or perhaps 

 even '7,000 to 12,000 feet ' above its present level to allow the 

 excavation of these valleys, may be quite unnecessary, even 

 if the valleys in question are merely submerged river valleys. 

 (See p. 294, and Chapter XXXI). 



In the Mississippi basin there was also notable elevation at 

 this time, though probably less than has sometimes been estimated. 

 It seems possible, or perhaps even probable, that the evolution of 

 the principal physiographic features of the interior, so far as due 

 to erosion, began with the Ozarkian epoch, though the study of 

 the evolution of the topography of this region has not advanced 

 so far as to make this conclusion certain. 



In the west, too, there were notable closing-Tertiary movements. 

 The plateau region was in process of uplift, periodically, through- 

 out the Tertiary, during which it has been estimated to have under- 

 gone an elevation of 20,000 feet (Button), and a degradation of 

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

 assignable to the Sierran epoch 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 (p. 137), was the 

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

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

 thought, perhaps post-Sierran, 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. 2 If the whole of the canyon is post- 

 Sierran, the elevation of the region in the Sierran epoch (and later) 

 must have been several thousand feet. The later elevations, 

 largely by blocks, were so recent that the fault scarps are almost 

 always distinct, and independent of stratigraphy and drainage. 3 



1 LeConte, op. cit., and Spencer, Am. Jour. Sci., Vol. XIX, 1905. 



2 Huntington and Goldthwaite, Bull. Mus. Comp 

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



3 Huntington and Goldthwaite, op. cit., p. 248. 



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

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



