Abstract

Reconstructed stream profiles and paleobotanical evidence suggest that in the Yosemite region altitude increased approximately 4000 feet following the Broad Valley stage. Since this was preceded by a 2500- to 3000-foot uplift of the Late Pliocene Boreal (“Eocene”) surface during the Plio-Pleistocene transition, total post-Pliocene uplift is about 6500–7000 feet.

With similar methods, divergent results have been reported for the Lake Tahoe sector. The presence in the summit region of Miocene deciduous hardwood forests typical of mild-temperate climate, the absence there of montane conifers, the spatial relations of the Miocene forest zones, and the regional pattern of climate they suggest all imply altitudes near 2000 feet. Reconstructed stream profiles indicate an altitude of 5000–7000 feet. These results are tested for plausibility by utilizing (1) sea-surface temperatures to calculate temperature and, hence, climate, at any altitude postulated for the Sierran summit and (2) sequences of later Tertiary floras on opposite sides of the barrier to determine the magnitude of the rain shadow. Both lines of evidence suggest only low altitude

The postulate of a high Tertiary barrier, and of only 4000 (Yosemite sector) to 2000 feet (Lake Tahoe sector) uplift in the Pleistocene raises problems: the Tertiary section has defied erosion; fine-grained sediments were deposited well up in the range and at its summit; formations were deposited across the crest; Tertiary rivers and basalt flowed across the summit; faulting and warping of dated sections and erosion surfaces show post-Pliocene uplift of 4000–9000 feet; and the scarps have resisted erosion. These “problems” disappear if the range originated as a major post-Pliocene topographic barrier.

Two basic weaknesses to the method of determining Tertiary altitude from ancient stream profiles are: (1) The assumption that certain well-graded stretches of the ancient streams had pretilt slopes equal to those of well-graded parts of the modern rivers does not agree with the nature of the Tertiary sediments in the range. (2) Recurrent tectonism during the Tertiary tilted the basement and Tertiary river channels, giving them gradients more nearly like those of present-day streams, and, hence, the illusion that the range has not been uplifted appreciably since Eocene.

First Page Preview

First page PDF preview
You do not currently have access to this article.