EVOLUTION OF iMODERX SURFACE FEATURES 41 



Great Basin area, and form the older volcanic sequence of that area, 

 now much disturbed and mineralized. The northwestern corner of 

 the Great Basin was overlapped by basalt flows related to the 

 Miocene Columbia River basalt. Most of the volcanics farther south- 

 east have also been ascribed to the Miocene, but a tuff member in 

 the Alta formation of the Virginia City district contains middle 

 Oligocene plants. At some places in the eastern and southern parts 

 of the Great Basin, ash-rich sand, mud, and gravel were deposited 

 in basins that recently had been outlined by faulting. 



Available floras indicate that the western part of the Great 

 Basin stood at an altitude of about 2,000 feet above sea level in 

 middle Tertiary time, with the Sierra Nevada to the west projecting, 

 at most, only a thousand feet higher. Apparently neither the low 

 Sierra Nevada ridge nor the downfaulting of incipient basins inter- 

 fered materially with drainage westward to the Pacific. 



During one or more episodes before late Miocene time, and per- 

 haps mainly in the middle Miocene, the older Tertiary volcanic and 

 sedimentaiy rocks in many parts of the Great Basin were faulted 

 and tilted, then widely eroded (Van Houten, 1956, p. 2820). These 

 movements, premonitions of which we have seen in the middle 

 Tertiary basin deposits, are the first notable disturbance of the 

 region since the orogenic phase, and mark the beginning of develop- 

 ment of Basin and Range structure and topography. 



Late Tertiary {Late Miocene and Pliocene) Environments. During 

 late Miocene and early Pliocene time volcanics and sediments were 

 spread widely over the Great Basin and Sierra Nevada, covering an 

 eroded terrain that had been more or less deformed by the preceding 

 disturbances. As these deposits contain mammals and plants at 

 many places, and occasional invertebrates and other fossils, they 

 form not only a useful stratigraphic datum, but also an index of the 

 environments of the time (Van Houten, 1956, p. 2802; Axelrod, 

 1957, pp. 23-28). 



Much of the central and northern parts of the Sierra Nevada were 

 covered by several thousand feet of andesitic lava flows, remnants of 

 which are still preserved on stream divides and mountain tops. 

 Principal centers of eruption were near the modern crest of the 

 mountains, along an axis which continued northward into the 

 Cascade Range, but the flows also spread eastward into the Great 

 Basin. Andesitic debris was transported widely westward and 



