Yosemite National Park 193 



of reducing back toward sea level the lands that had been ele- 

 vated to. the height of a great mountain range by upheaval. 

 In the long time since the beginning of the great upheaval 

 rock formations miles in thickness have been eroded and carried 

 away by streams from over the whole area of the Sierra Nevada 

 Range. Much of this material now rests in the Great Valley of 

 central California. Some of it indeed has been carried out 

 through the Golden Gate and deposited on the bottom of the 

 Pacific Ocean. 



Gorges Cut by Running Water and 

 Moving Ice 



The uplifting of the great mountain range caused the more 

 rapid movement of the waters of streams down the western 

 slope, and added greatly to their eroding power and their ability 

 to carry away rock waste. But further than this, the high ele- 

 vated regions came to have a colder climate, and thus resulted 

 in the development of an added agency of erosion, glacial ice. 

 Snows gathered on the high mountain lands of the Sierra, and 

 being compressed into hard ice moved by the pressure of the 

 weight of its own mass down the western slope. Valleys which 

 had been cut into the hard rocks became the paths down which 

 the ice from the high mountain crests moved. 



Ice, like running water, is a powerful agent of erosion. 

 How much of the great work of excavating the vast gorges 

 the many yosemites of the great Sierra slope has been due to 

 the eroding action of running water and how much to the 

 eroding power of ice, moving, as it is known to have done, in 

 vast viscous-acting streams, has been a question among geolo- 

 gists. Indeed there have been marked differences in judgment 

 of geologists as to the causes that have produced the great 

 canyons. Without troubling ourselves about the differing 

 opinions that have been expressed about the causes of these 

 great chasms, we may content ourselves with the conclusion 

 now generally accepted by geologists that both running water 



