122 RE. Clements 



The general trend toward desiccation in climate and climax 

 may well have found its chief expression at the time of greatest 

 elevation of the Sierra Nevada and Cross Ranges in the Pleisto- 

 cene, to be succeeded by the four or five glacial cycles already 

 mentioned. If the interglacial phases were long, it is plausible 

 that desert conditions similar to those of today intervened more 

 than once, though the secular trend of rainfall as well as the grass 

 relicts still found in the desert, particularly in Death Valley, sug- 

 gest that today constitutes at least one maximum. This view is 

 supported by the presence of two horses, two camels, a large 

 antelope, and a probable elephant from Lake Manix, indicative 

 of at least a fairly continuous grass-cover as late as this deposit. 



As to the particular times of mountain-making that produced 

 the desiccation of the grassland climate which finally terminated 

 in the desert climate and climax, Hershey places the structural 

 differentiation of the Sierra Madre-San Bernardino chain at the 

 opening of the Quaternary, concluding that the great bajadas 

 were finished by the Illinoian or lowan stage. Baker (1911) 

 thought it probable that deformation immediately preceded or 

 coincided with the formation of the Rosamond (Barstow) de- 

 posits of the Upper Miocene, and that an arid climate like that 

 of today prevailed during this time. An interval of diastrophism 

 then intervened, followed by a long period of erosion which was 

 succeeded by a time of mountain-making that produced the pres- 

 ent ranges of the desert. Knopf (1916) concludes that the latest 

 major uplift of the Sierra Nevada occurred early in the Pleisto- 

 cene, but Blake (1914) regards the end of the Pliocene as the 

 time of upUft in the Colorado region. Merriam (1919) considers 

 the Ricardo beds to be derived from a Sierra range that, in late 

 Tertiary, rose several thousand feet above the floor of the Mohave 

 Desert. Recendy, Blackwelder (1931) has studied the eastern 



