Geol.— Vol.. I.] TURNER— OK JGIN OF YOSEMITE VALLEY. 269 



III. Pleistocene Periods. 



1 . The Sierran Period. — A period of extensive erosion 

 and canyon cutting, probably occupying the larger part of 

 the early Pleistocene, and greater than all the remainder of 

 post-Tertiary time. This may be called, following Le 

 Conte, the Sierran Period. 



2 . The Glacial Period. 



3. The Recent or Post-Glacial Period} — This may also 

 be called the human period, since whatever the time of the 

 appearance of man on the earth, he first reached an 

 advanced status at this time. 



/. The Sierran Period. 



It has long been recognized" that the period of canyon 

 cutting of the Sierra was a very long one. Recently Her- 

 shey^ has proposed to designate this time of erosion as the 

 Ozarkian Period. Professor Joseph Le Conte ^ has laid 

 stress on the long duration of the Ozarkian Period in the 

 Sierra Nevada, and suggests that inasmuch as the term 

 " Ozarkian " has been used by Broadhead for a Lower Si- 

 lurian series in the Ozark region, the term " Sierran," in 

 place of "Ozarkian," be used for this early Pleistocene 

 period of erosion. Undoubtedly a large part of the materi- 

 als filling the Great Valley of California and those forming 

 the older alluvial fans of the Great Basin were deposited at 

 this time, and the enormous amount of these detrital masses, 

 as well as the tremendous canyons of the Sierra, make it 

 probable that the Sierran Period comprises the larger part 

 ^ of post-Tertiary time. 



2. The Glacial Period. 



That a large portion of the higher Sierra was formerly 

 covered by a vast field of ice and snow is patent to every 



1 Many geologists do not include the Recent Period iu the Pleistocene. It has been 

 the custom, however, of the U. S.' Geological Survey to relegate all time from the close 

 of the Pliocene to the present or historic period to the Pleistocene. 



2 See " Description of Gold Belt" in the folios of the Sierra Nevada, U. S. Geol. Surv. ; 

 also Journ. Geol., Vol. IV, 1896, p. 900. 



3 Science, Vol. Ill, 1896, p. 620. 

 ''Jouru. Geol. Vol. VII, 1899, p. 525. 



