Geol.— Vol. I.] TURNER— ORIGIN OF YOSEMFfE VALLEY. 293 



6. Cause of the Glacial Period. 



Without endeavoring to elucidate the various theories 

 that have been advanced to account for the Glacial Period, 

 it may still be in place to call attention to certain general 

 facts concerning it. 



The immediate cause of the Glacial Period in the Sierra 

 Nevada, as elsewhere, is undoubtedly to be ascribed to 

 heavy precipitation combined with a sufficient degree of 

 cold to cause the water to fall as snow. These conditions 

 could be brought about by a greater elevation of the land or 

 by a general change of climate. In the Sierra Nevada 

 there is excellent evidence that the range stood higher in 

 Pleistocene than in Middle Tertiary time, but there is no 

 good evidence, so far as I can see, that the range is not 

 approximately as high now as during the time when the ice 

 last occupied the canyons ; for while the general altitude of 

 the range has been decreased by erosion, it may likewise 

 have undergone some elevation, as seems to be suggested by 

 late displacements along the old fault zones. Moreover, the 

 most vigorous erosion in Pleistocene time was certainly that 

 of the Sierran Period, which, as we have seen, in the main 

 preceded the Glacial Period. 



A general review of the glacial histor^^ of the northern 

 hemisphere appears to suggest that glaciation was a general 

 phenomenon and occurred at the same time at all points. 

 If this be true, the cause of glaciation was evidently cosmic 

 and cannot be ascribed to elevation alone, as it seems im- 

 probable that all of the northern hemisphere experienced a 

 general elevation at the same time. 



The yearly precipitation at Summit Station on the Central 

 Pacific Railroad (altitude 6, 750 feet) varies from twenty-three 

 to eighty-eight inches,^ and if it is as great as this along the 

 crest of the range in general, a slight average decrease of 

 temperature would cause the pigmy glaciers of the Sierra 

 Nevada of to-day to again advance. In the winter of 1889 (?) 



* Physical data of Statistics of California, State Engineering Department, Sacramento , 



1889. 



( 3 ) November 9, 1900. 



