﻿170 J. LeConte — Elevation of the Sierra Nevada. 



Application of these Principles. 



1. Plateau Region. 



This remarkable region, as is well known, is intersected by 

 profound canons which are among the wonders of the world. 

 The reason is, that it has been rising in comparatively recent 

 times and faster than the rivers could cut. But that the rising 

 has not been continuous and at uniform rate, is proved by the 

 structure of the Grand Canon as shown in the figure (fig. 1). 



k 



Ideal section across Grand Canon. After Dutton. 



It consists, as seen, of two canons, one within the other. The 

 outer canon is 5-6 miles wide, the inner only about f of a 

 mile wide, or about as wide as deep. This inner gorge is much 

 the more recent and therefore has not yet had time to widen 

 out. 



The history of the plateau region, as recorded in the Grand 

 Canon according to Dutton, is as follows : At the end of the 

 Eocene this region was first laid bare by the draining of the 

 great Eocene lakes, and the present river system was established. 

 During the whole Miocene there was a continual arching of the 

 earth-crust in the region, with consequent rising of the country 

 and proportionate erosion over the whole area. This however 

 is recorded rather in the erosion cliffs which characterize this 

 region, than in the canons proper. At the beginning of the 

 Pliocene a more rapid rising commenced and the outer canon 

 began to form and continued to deepen as the land rose. Then 

 the rise ceasing the river gradually reached its base level, the 

 canon ceased to deepen but continued to widen until the outer 

 canon reached nearly its present condition. Then later, viz : at 

 the beginning of the Quaternary, the land began again to rise, 

 and the river again to cut rapidly and has since that time cut 

 the inner gorge. It is probable that the rising, and certain that 

 the deepening, still continues. 



Outside of the Grand Canon northward there is a succession 

 of regular tables 10-20 miles wide, terminated by irregular E. 

 and W. cliffs 1000-2000 feet high (fig. 2). All these cliffs are 

 now receding by erosion. If the arching strata be restored 

 the amount of erosion is seen to be enormous. According to 

 Powell the average thickness removed on the whole area is 

 6000-8000 feet, and the extreme thickness on the highest part 



