Collection: Books and Periodicals

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Title: The Tertiary Gravels of the Sierra Nevada of California by Waldemar Lindgren (1911) Collection Type: Books and Periodicals Source(s): Google Books # of Pages: 301 Download PDF: User Login Required Download OCR Text: User Login Required

Date Scanned: 2024-07-14 06:26:22 Item Sourced From: PDF First Created: 2025-03-17 05:53:20 Last Updated: 2025-03-17 05:53:20

Original Held At: Searls Historical Library Location: 161 Nevada City Hwy, Nevada City, CA 95959

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184 TERTIARY GRAVELS OF THE SIERRA NEVADA OF OALIFORNIA. Before the andesitic eruption the surface of the rhyolite suffered considerable erosion, so that its thickness differs much in different places. The heaviest masses are not found near the place of eruption, but near the western boundary of the quadrangle. A maximum thickness of 400 feet is found on Plum Creek, but ordinarily the thickness does not exceed 300 feet and locally it is much less. A tendency to form steep bluffs distinguishes the rock in many places. It is commonly massive, tuffs occurring only near the western boundary of the quadrangle. The normal rhyolite is a white, gray, or pink fine-grained rock, somewhat porous and easily dressed with the hammer. ANDESITE. The andesitic flows were the latest of the Neocene series of eruptions and cover large areas in the southern part and the northwest corner of the quadrangle; the northeastern part is — remarkably free from them. In general, the andesitic rocks now form the tops of the ridges, but the contact line with the underlying granitic or schistose series is far from being as regular and even as it is at many places lower on the slope of the Sierra; indeed, proofs are everywhere abundant that the surface upon which the andesitic lavas flowed out was an irregular one of considerable relief. The present canyons, however, have been cut considerably below the Neocene surface, and during this process a great part, perhaps half, of the original volume of the lava flows has been removed. It is evident that the flows once covered continuously almost the whole southern half of this quadrangle, and that only a few higher bedrock points near Round Top, Mokelumne Peak, and possibly Leek Spring Hill projected above the volcanic plateau. On the other hand, it is also evident that the larger part of the northern, higher half has never been submerged in a similar manner. In many places in the deeper parts of the old channels the andesite rests on rhyolite, but over the larger part of the area it lies directly on granitic or schistose rocks. These appear, in the few good exposures, to be soft and crumbling, but no evidence of any notable accumulations of débris has ever been found except in some of the channels, as stated above. The thickness of the flows is considerable. In the northwest corner of the quadrangle it reaches 1,000 feet; in the southwestern part it ranges from a few hundred up to 1,000 feet along the deeper drainage channels. The greatest thickness is found on the northeast side of Silver Lake, where it reaches 2,000 feet. In the eastern glaciated part the exposures are very much better; in numberless places the beautifully bedded appearance resulting from the superimposing of numerous flows of slightly differing structure is brought out. These long slopes, of a somber dark-gray or reddish-gray color, covered by scanty herbage or scattered trees, alternate with precipitous walls strongly resembling fortifications with scarps, parapets, and buttresses. In places where erosion has carried its work still further, as in the vicinity of Thimble Peak, peaks and pinnacles of the most fantastic form result. The andesitic flows consist almost entirely of tuffs and tuffaceous breccias in an indefinite number of sheets, differing in hardness as well as in size and abundance of the andesite bowlders, which range up to several feet in diameter. They all consist of angular andesite fragments bound in a cement of finer andesitic detritus; very little nonandesitic material is present, though granitic bowlders may occur here and there. The andesite is a dark, rough, and porous rock, containing porphyritic crystals of plagioclase and almost invariably pyroxene, principally augite but also hypersthene; hornblende is less abundant, but also common; the groundmass varies from microcrystalline to glassy. Flows of massive andesite occur rarely, but in many places near the volcanic centers the tuffs and breccias contain necks of massive hornblende andesite, as on Old Round Top, north of Twin Lakes. TERTIARY TOPOGRAPHY. As there are, within this region, no evidences of Neocene or post-Neocene faulting, nor evidences which would lead to the belief that any strongly marked deformations of the surface have occurred, it follows that a study of the numerous contact lines of the Neocene eruptive rocks with the underlying ‘‘ Bedrock series”’ may give a correct idea of the detailed topography