194 Adventures, in Scenery 



and slowly moving glacier ice have been the factors that have 

 determined these great canyons. 



Three Stages of Glaciation 



The details of the glacial history of the Yosemite region 

 would make a somewhat complicated and long story. It may 

 serve our purpose to state briefly that at least two definite and 

 distinct invasions of glacier ice have occurred in the Yosemite 

 region. Indeed there is evidence of three glacial epochs or 

 invasions of ice, separated by long interglacial intervals. 

 Whether there have been more than three glacial invasions is 

 still open to question. That two, and probably three, epochs 

 of glaciation have occurred on the Sierra Range is beyond 

 question, and some observations have been made that indicate 

 more than three. It is not known whether the glacial epochs 

 in the Sierra region occurred simultaneously with the several 

 epochs of glaciation on the continent of North America. There 

 were five glacial epochs on the continent separated by long 

 inter-glacial intervals during what is known as the Glacial 

 Period of the Quaternary Era, but correlation of these epochs 

 with Sierran glaciation has not been worked out. 



The cause or causes of the intervals between glacial epochs 

 is not known. That the intervals between glacial epochs were 

 long is evident from what can be seen in the field. What may 

 have been the causes of greater warmth, or what may have 

 caused periods of greater or less precipitation, is not known. 

 The epochs of ice invasion, and the intervening periods when 

 the ice was not present, are facts established by observation of 

 conditions that now exist. There were two, or three, epochs 

 of ice invasion, and corresponding interglacial epochs. The 

 facts are established, but our knowledge of the causes is largely 

 lacking. 



That ice moved; that it acted with tremendous power; that 

 it disrupted rocks and carried them long distances; there can be 

 no question. That rock fragments were torn from parent 



