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University of California Publications in Geology [Vol. 8 



the present range slopes under the valleys at steeper angles than the 

 overlying upper Miocene beds, indicating that the present lines of 

 relief, of general north-south trend, had been initiated before the 

 deposition of the upper Miocene beds. 



The next event was the development of basins of interior drainage, 

 in which sedimentary beds containing upper Miocene mammalian 

 remains and other fossils were deposited. The formation of these 

 basins of deposition may have occurred in the later course of the 

 deformation which had previously tilted the lavas, or a period of 

 quiescence and erosion may have intervened between the first lava- 

 tilting period and the basin-forming movements. In these basins there 

 were deposited sandstones, shales, limestones, cherts, volcanic ash and 

 other sediments, in a large lake or lakes. The climate in upper Miocene 

 time was apparently somewhat more humid than at present. To judge 

 from the distribution of laeustral beds, the relief was considerably 

 less than it is today. After the deposition of the Esmeralda beds in 

 upper Miocene time further deformation occurred along the axes on 

 which the folding previous to laeustral time had occurred. It resulted 

 in the uplift of the present ranges relative to the valleys and in the 

 gentle folding of the laeustral beds into narrow anticlines and syn- 

 clines for the most part parallel to the ranges. Parts of Stewart and 

 lone valleys were relatively uplifted with respect to other parts of 

 the region. It appears that a period of prolonged erosion ensued, 

 with deposition in neighboring areas. Besides smoothly truncating 

 the folds of the laeustral series, the erosion of this time developed 

 an early-old-age topography on the harder rocks of Cedar Mountain 

 and to a less extent probably on those of the surrounding mountains. 

 This erosion period probably occupied much of Pliocene time. Two 

 sets of lavas were extruded during this erosion period. The first, 

 consisting mainly of porphyritic andesites, may have followed closely 

 upon the deposition of the laeustral beds. The other set consists of 

 basalt ; its extrusion probably occurred much later and perhaps after 

 the andesite had been considerably eroded. In late Pliocene or Pleis- 

 tocene time further differential uplift occurred. Cedar Mountain, 

 and presumably also the neighboring ranges, were further uplifted 

 relatively, as were also some parts of the valleys with respect to other 

 parts. This resulted in the partial dissection of the relatively smooth 

 surface cut in the Pliocene. On the relatively unresistant laeustral 

 series the Pliocene surface has probably been entirely destroyed, and 

 later mesa surfaces have been cut on them. 



