THE PLIOCENE PERIOD 313 



recent studies indicate that even the outer and broader part of the 

 valley, the esplanade, is younger 1 than was formerly thought, per- 

 haps post-Sierran, and raise a question as to whether the inner gorge 

 is not the topographic result of rock structure, rather than of a dis- 

 tinct 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 ungraded and 

 precipitous, and independent of stratigraphy and drainage. 3 



In the basin region, faulting and deformation were in progress, 4 

 and gave rise to two basins, one at the west base of the Wasatch 

 mountains, and the other at the east base of the Sierras. These depres- 

 sions prepared 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 at least the movement seems to have been along fault planes 

 established before the Pliocene period. Some idea of the great erosion 

 which has affected the Uinta mountain region, since the Eocene at 

 least, is gained from Figs. 462 and 463 



Fig. 462. — Section across the Uinta mountains. Pru, Uinta group Proterozoic (?); 

 Crv), Lodore and Red Wall formations, the former probably Cambrian, the lat- 

 ter Mississippian ; Cla, Lower Aubrey, and Cua, Upper Aubrey, are Carboniferous 

 (Mississippian and Pennsylvanian) ; T and J, Triassic and Jurassic formations 

 (Flaming Forge, White Cliff, Vermilion Cliff and Shinarump formations); Ksl 

 (Sulphur Creek and Henry's Fork formations), Ksw (Salt-wells formation), and 

 Kpr (Point of Rocks formation), Cretaceous; Ebc (Bitter Creek group) and Ebp 

 (Brown's Park group), Eocene. (After Powell.) 



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

 was still more marked. 5 The earliest Sierran folding of which the 

 history is well known, was at the end of the Jurassic period. 



1 Huntington and Goldthwaite, Bull. Mus. Comp. Zool. Geol. Ser., Vol. VI, p. 252. 

 While these authors do not state the time of the beginning of the canyon, they say 

 that " the canyon cycle (of erosion) must include at least the later part of the glacial 

 epoch." 



2 Davis, The Grand Canyon of the Colorado, Bull. Mus. Comp. Zool., Vol. XXXVIII. 



3 Huntington and Goldthwaite, p. 248. 



4 King, U. S. Geol. Expl of the 40th Parallel, Vol. I, p. 542. 

 'LeConte, op. cit., and Diller, 14th Ann. Rept. U. S. Geol. Surv. 



