1914] 



Buwalda: Tertiary Mammal Beds in West-Central Nevada 



359 



PHYSICAL CONDITIONS DURING UPPER MIOCENE TIME 



The character of the Esmeralda beds and the contained fossil 

 material indicate certain of the conditions of topography, drainage, 

 volcanism, and climate existing in the Cedar Mountain region in 

 upper Miocene time. 



The large areas occupied by the lacustral beds, and the fact that 

 much of the present relief of the region has been brought about by 

 relative uplift of the ranges and by lava extrusions in post-lacustral 

 time, indicates that the relief in upper Miocene time was probably 

 considerably less bold than it is today. That the region was not a 

 flat country, however, is shown by the abutting of the strata at some 

 localities against sloping surfaces of the older rocks, and by the usual 

 absence of old soils where the lacustral beds rest on older rocks. Some 

 relief is further attested to by the presence in the lacustral series 

 of strata of coarse materials. The abutting of the lacustral series 

 against the older rocks along the present mountain flanks demonstrates 

 that the elevated areas in upper Miocene time were approximately on 

 the sites of the present ranges. Unless the lake was of great depth 

 it is quite certain that islands of older rocks dotted its surface, for 

 the lake-beds completely surround, and abut against, masses of the 

 older rocks. Cedar Mountain itself is probably completely surrounded 

 by the lacustral sediments and was probably an island during the 

 greater part of lacustral time. 



From the fact that the upper Miocene strata are apparently not 

 underlain by older Tertiary or Cretaceous sediments, but that they 

 rest instead on pre-Cretaceous metamorphics or intrusives and Ter- 

 tiary lavas, it is inferred that the Cedar Mountain region was under- 

 going erosion during practically all of Cretaceous and pre-lacustral 

 Tertiary time. The drainage during this time was probably to the 

 sea, as early Tertiary as well as Cretaceous sedimentary deposits are 

 up to the present time unknown in the whole western Nevada region. 



The Esmeralda formation, in large part of lacustral origin, indi- 

 cates that during upper Miocene time the drainage waters of the 

 region were ponded. Not enough is known of the upper Miocene 

 geography of western Nevada to make it possible to say with certainty 

 whether the lake extended along the foot of the Sierra Nevada of 

 upper Miocene time or received waters through rivers from the ele- 

 vated region known to have occupied the position of the present Sierra 



