Studies in the Sierra 



197 



Fig. 4 is a section across the lower portion of the valley of 

 Illilouette south of Mount Starr King. In this case the bottom 

 is naked, and the dotted reconstructed portions of the huge 

 granite folds A B C D have evidently been eroded.* Even the 

 smoothly curved trough of two rock-waves which afford sec- 

 tions like Fig. 5 can not be regarded as a valley originating in a 



Fig. 5. 



fold of the surface, for we have shown in the first paper of this 

 series that domes or extended waves, with a concentric struc- 

 ture like A C, may exist as concretionary or crystalline masses 

 beneath the surface of granite possessing an entirely different 

 structure or no determinate structure whatever, as in B. 



The chief valley- eroding agents are water and ice. Each has 

 been vaguely considered the more influential by different ob- 

 servers, although the phenomena to which they give rise are im- 

 mensely different. These workmen are known by their chips, 

 and only glacier chips form moraines which correspond in kind 

 and quantity to the size of the valleys and condition of their 

 surfaces. Also their structure unfolds the secret of their 

 origin. The constant and inseparable relations of trend, size, 

 and form which these Sierra valleys sustain to the ice-fountains 

 in which they all head, as well as their grooved and broken 

 sides, proclaim the eroding force to be ice. We have shown in 

 the second paperf that the trend of Yosemite valleys is always a 

 direct resultant of the forces of their ancient glaciers, modified 



* Water never erodes a wide U-shaped valley in granite, but always a narrow 

 gorge like E F, in Fig. 4. 



t Reprinted in Sierra Club Bulletin, Vol. X, No. i, January 1916. 



