The Sierra Nevada Range 175 



Sierra Range Young, Yet Dates 

 Far Back. 



The history of the uplifting of the Sierra Nevada Range 

 embraces a long span of time. In terms of geologic periods the 

 present Sierra Nevada Range is young. In the genealogy of 

 the mountain systems of the world it may be said to belong to 

 the latest generation. It was "finished" as a mountain range in 

 the geologic period just preceding the present. Indeed it is not 

 known but that it may still be in the process of being uplifted. 



While the range is geologically young yet its birth or begin- 

 ning dates back requiring nine figures to express the time in 

 years. The earlier of the two ancestral mountain systems dates 

 back to Palaeozoic time, estimated to have been 200,000,000 

 years. The folded, crumpled and metamorphosed rocks that 

 now lie on the western slope of the range properly belong in 

 the ancestral lineage of the Sierra Range. 



Toward the end of the Eocene (early Tertiary) epoch 

 about 40,000,000 years ago there commenced a period of vol- 

 canic outbreaks. Volcanoes on the eastern border of the Sierra 

 region poured out streams of (rhyolite) lava and mud. These 

 flowed down the valleys burying the river channels. During 

 this time, by gradual uplifts the Sierra region was raised and 

 tilted to the southwest. The disturbances finally died out, and 

 during a long interval of relative quiet most of the rhyolite and 

 much other rock waste were removed by erosion. Then dur- 

 ing the Miocene epoch (middle Tertiary) about 12,000,000 

 years ago volcanic activity and earth movements began again 

 on a vast scale. Eruptions of lava of a different type (ande- 

 site) flowed down and filled the valleys. These lava flows 

 accumulated to thicknesses of thousands of feet. The crustal 

 movements of the earth increased the height of the Sierra region 

 thousands of feet. Strong faulting occurred along some parts 

 of the eastern border, and the great depression in which Lake 

 Tahoe is situated was formed by subsidence of its basin. Dur- 



