210 Adventures in Scenery 



Sit different times. In the Yosemite Valley as in few places in 

 the world can be seen in cross section so remarkable a complex 

 of intrusive igneous rocks. They show the astonishing details 

 of a portion of the earth's crust that once lay miles below the 

 surface, beneath the roots of a former mountain system, and 

 that was disturbed by repeated upwellings of molten rock. 



The batholith of the Sierra Nevada is made up of a number 

 of distinct bodies of igneous rock, each differing somewhat from 

 its neighbors in mineral composition and representing a separate 

 upflow of molten material. Many of these igneous bodies are 

 of great extent, their areas at the surface being measured in 

 hundreds of square miles. The uplands flanking the Yosemite 

 Valley are made up almost wholly of such large intrusive bodies. 

 The Valley itself, however, crosses an area where many small 

 intrusive bodies and narrow projections from the larger ones 

 intersect each other in an intricate manner. It is to the great 

 diversity in character and the complexity of the rock forma- 

 tions that make up the walls and floor of what is called the 

 Yosemite Valley that the unique character of the Valley is due. 



The Yosemite upland north and northeast of the head of 

 the Yosemite Valley is in the main massive (mostly un jointed) 

 granitic rock the eroded surface of the Sierra batholith. The 

 Merced River, as has been stated, starts from near Mount Lyell, 

 amongst the crests and peaks of the High Sierra. Over the hard 

 massive granite passes the river, with comparatively slight ero- 

 sion. Massive granite, that is, not broken by joints or cracks, 

 erodes very slowly. In these places where the rock is broken 

 by joints or cracks erosion goes on much more rapidly. Blocks 

 and slabs are loosened by frost and other weathering agencies, 

 and moving ice is far more effective in quarrying and eroding 

 in jointed rocks. 



Yosemite Chasm Eroded in Jointed 

 Granitic Rocks 



The course of the Merced River, like the other streams of 



