Paleoelevation Estimated from Tertiary Floras

One method of estimating the elevation of Tertiary flora involves the difference in elevation indicated by a flora in the uplands that is similar to vegetation in the lowlands, shown by examples from Tibet-Himalayas and the Andes. A second method relies on the difference in estimated paleotemperature between a paleoflora now in the uplands and that of similar vegetation in the lowlands, assuming a normal lapse rate in most cases. The former method provides only a general indication of the amount of uplift. The latter can suggest not only the amount of uplift, but the decrease in length of the growing season (Warmth: days), the frequency of freezing (% hrs/yr), and the degree of temperateness as illustrated by examples from the Sierra Nevada. The latter method also suggests that in the Sierra Nevada, the condition for treeline (W 10°C, 0 days warmer) increased 100 m in 10 m.y. between two superposed floras. Following 7 to 5 Ma, the range added 637 m to reach its present level of 2775 m. Regionally, Miocene floras from the central Great Basin represent deciduous hardwood forests that indicate only moderate elevation over the lowlands into the late Tertiary (～5 Ma), following which much of the present relief developed. Eocene floras from the Rocky Mountains, where major volcanism was widespread, show that forests above lowland broadleafed evergreen forests (i.e., Wasatch, Kisinger, etc. floras) were zoned with respect to decreasing Warmth at higher elevation, from W 15°C (227 days) to W 11.3°C (96 days), and from nearly frostless in the Eocene lowlands to fully 15% hrs/yr subfreezing in the uplands at treeline (W 10°C, 0 days), then at 2365 m (6600 ft.).