Twenty-one Mio-Pliocene floras distributed from sea level in west-central California, across the Sierra Nevada and into western Nevada, are preserved in volcanic sediments occurring either (1) at or near the base of the andesitic Mehrten formation and its correlatives east and west of the Sierra Nevada, or (2) in the underlying rhyolitic Valley Springs formation and its close equivalents in western Nevada and coastal California. Since the floras are related to living communities whose topographic and climatic relations can be measured, they provide a basis for estimating the altitude of the Sierra Nevada and of west-central Nevada at the inception of vulcanism during Mio-Pliocene time. Local, provincial, and regional comparisons indicate that there were then only moderate east-west differences in vegetation, climate, and topography across this area where environmental diversity is marked today.

A number of paleogeographic inferences are made: Geologic—Just before late Tertiary vulcanism the Sierra Nevada was a broad ridge with its summit near 3000 feet. It was incised by 2- to 4–mile wide, 1000-foot-deep valleys, some of which drained western Nevada. The lowlands of western Nevada had an average elevation of 2000–2500 feet. Two major components of late Cenozoic diastrophism are recognized in the Sierra Nevada. Faulting accounted for an average maximum displacement of 3500 feet in this section of the range, and regional warping ranged from 1800 feet at Donner Pass to 3000 feet at Carson Pass 50 miles south. Climatic—Building up the Sierran ridge from 6000 to 8000 feet by vulcanism and diastrophism in the later Cenozoic (1) reduced temperature along the summit so that arctic-alpine and subalpine forest environments came into existence, and (2) lowered rainfall sufficiently over the lowlands to leeward to account for the present steppe (sage) and desert regions. Biologic—Plants (and animals) from contiguous milder environments gradually were invading these new zones during the later Cenozoic and have continued to evolve there.

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