Studying the Yosemite Problem 



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deposits of sediment from the continent to the east, the sedi- 

 ments that later became the surface rocks. To decipher the 

 record of these multifarious events one must patiently study 

 the different rock species of the Sierra and work out the rela- 

 tions they bear to one another. 



The evolution of the Yosemite Valley, on the other hand, 

 is from the geologist's point of view a relatively modern and 

 brief episode. The valley is strictly an erosional feature 

 (Whitney's fault block hypothesis has been disposed of), 

 and like all the canons and valleys about us belongs to the 

 landscape of the present, that is, of the last of many time 

 divisions of the earth's geologic history — the one commonly 

 referred to as the Quaternary. The Sierra Nevada itself did 

 not attain its present height until the close of Tertiary time 

 (at the end of widespread crustal upwarpings and subsidences, 

 it remained standing in the form of a huge crust block, up- 

 heaved at the eastern edge and tilted to westward) : the series 

 of deep canons carved in its long western slope are therefore 

 to be regarded as dating essentially from the time of these up- 

 heavals. The canon of the Merced River, of which the Yo- 

 semite forms a section, is one member of this series of canons. 



The study of the evolution of the Yosemite Valley, it will 

 be clear from this, amounts virtually to a study of the erosional 

 events of Quaternary time and of the successive changes in 

 the landscape which these events brought about. Now the 

 general character of these events is already fairly well known 

 through research in other parts of the world. It is definitely 

 established, for one thing, that it was during Quarternary 

 times that occurred the great ice invasions which affected the 

 northern portion of this continent and of Europe. In those 

 days the Sierra Nevada was also covered with extensive ice 

 fields, as has been recognized by many observers; in fact, its 

 upper portions constituted a separate center of snow accumu- 

 lation whence long tongue-like glaciers flowed down through 

 the principal canons and valleys. Whether the ice reached 

 the Yosemite Valley or not, it will be remembered, was once 

 a hotly debated question. Unfortunately the exact extent of 

 the glaciers had not been traced out and the debaters had 

 to rely for the most part on the more obvious evidences of 



