'Yosemite National Park 209 



that had been deposited long ages ago but not reaching the sur- 

 face. The molten rocks surged up under the confining roof 

 of slate, quartzite, marble, and volcanic rocks, whose buckled 

 and folded beds formed the ancestral Sierra ridges of Creta- 

 ceous time. The granitic rocks that are now exposed to view 

 over large areas are now at the surface due to the fact that in 

 the course of periods probably aggregating 100,000,000 years 

 the roof rocks were removed by the slow wearing action of 

 streams and other eroding agents. 



The flow of the Merced was accelerated by the uplifting and 

 tilting of the great Sierra block. Its course was determined by 

 the slope and its channel established in the comparatively soft 

 rocks of the sedimentary formations that formerly lay over the 

 great batholith. As the softer sedimentary formations were 

 removed the river became entrenched in its course. It cut 

 down into the hard granitic rocks of the batholith, and having 

 become entrenched it was not able to leave its established chan- 

 nel and so has continued to move across obstructions of hard 

 granitic rocks encountered as it deepened its channel into the 

 batholith. 



Complex Character of Rocks Explains 

 Cutting of Gorge 



The reason for the remarkable canyon cut by the Merced 

 River lies in the complex character of the rocks encountered 

 in its course. At first glance the walls and domes of the 

 Yosemite region appear to be formed of one and the same kind 

 of rock. In reality there are present in the Yosemite region 

 about a dozen distinct types of rock, all granitic in character 

 and ranging in color from nearly black to nearly white. Tech- 

 nically the species of rock range from nearly black horneblende 

 gabbro through successively lighter-colored diorite, grano- 

 diorite, quartz monzonite, and biotite granite to nearly white 

 alaskite. All exhibit a strong family resemblance, yet show by 

 their differences that they were upwelled in the great batholith 



