Yosemite National Park. 211 



the Sierra slope which descend through great gorges, flows down 

 the slope essentially in disregard of rock structures. (It is a 

 "superimposed" river.) At the head of what is known as the 

 Yosemite Valley the river enters upon a zone of rocks of great 

 complexity, all granites to be sure, but varying in texture and 

 composition, and variously, and in places very closely, jointed. 

 To this change in the character of the rocks is due this most 

 remarkable canyon. 



Photo by F. C. Calkins, U. S. Geol. Survey 



FIG. 65. Crumpled and contorted chert. Above the mouth of Ned 

 Gulch, lower Merced River. 



The Yosemite Valley extends down the course of the Mer- 

 ced a distance of 7 miles. The head of the valley is marked by 

 a wall of hard granitic rock (Half Dome quartz monzonite) 

 1,000 feet in height. The lower end of the valley terminates 

 in a rising sloping wall of hard massive granite (El Capitan) . 

 The river leaves the valley through a narrow canyon cut in this 

 hard rock known as the Merced Gorge. The gorge, which is 

 U-shaped and ice-polished, continues through The Gateway 

 (which marks the transition to another type of rock, grano- 

 diorite) , a distance of 8 miles, to the vicinity of El Portal. 



