80 Adventures in Scenery 



along the eastern side of the Sierra region. From these volcanoes 

 lavas poured down the valleys. A long interval of erosion en- 

 sued during which lavas which had been poured out, and much 

 rock waste, were carried away by streams. During late Miocene 

 time volcanic activity and earth movements were renewed on a 

 vast scale both in the region of the Coast Ranges and the Sierras. 

 The earth movement at this time increased the altitude of the 

 Sierra region by several thousand feet. Lavas were poured forth 

 filling valleys and covering all but the highest mountain peaks. 

 Rivers were choked and forced to seek new courses. Faulting 

 or breaking of the earth's crust occurred on a large scale along 

 the eastern border of the Sierra region. The great depression 

 in which Lake Tahoe is situated was formed at this time by a 

 sinking of a block of the earth's crust. A long time of compara- 

 tive quiet from volcanic or earthquake activity followed, which 

 lasted mainly throughout the Pliocene epoch. In the northern 

 Sierras where the valleys had been filled by lavas rivers sought 

 new courses. The grade of the western slope had been increased 

 by uplift, and most of the deep canyons of the Range the 

 Yosemite, the Tuolumne, and the Mokelumne were established 

 at this time. Sequoias, ancestors of the big trees of the present 

 time, flourished in Pliocene time. 



Final Uplift of Sierra Nevada in 

 Quaternary Time 



The end of Tertiary time was marked by events of stu- 

 pendous importance. The Quaternary period was ushered in 

 with a grand climax of mountain building. It was at this time 

 that the Sierra Nevada acquired its present great altitude. Up- 

 lifting and tilting of a great block of the earth's crust brought 

 the summit peaks to almost double their former height. Mt. 

 Lyell, which in Eocene time probably had an altitude of 7,000 

 feet, now was elevated to 13,000 feet. At the same time frac- 

 turing and faulting of the rocks took place on an enormous 

 scale. Thus the Sierra Nevada came to stand out in its present 



