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

Page: of 301  

SMARTSVILLE QUADRANGLE. 129 increases rapidly as the high ridge of siliceous argillite is approached. It has been held by many that the Harmony channel continues for an indefinite distance up the ridge. This is impossible, as only a few miles eastward the deep Yuba channel, from Scotts Flat to Blue Tent, crosses the ridge. The Harmony channel is well up toward the headwaters and under Harmony Ridge divides into several branches. The subangular character of its gravel and the steep grades prove that the divide is not far distant. Its richness is due not to its being a main and important channel but to its crossing a system of rich quartz veins. It is barely possible that a deep gorge cuts through the ridge of siliceous rock and extends as far east as the Fountain: Head mine, but this must be characterized as highly improbable. The area in the vicinity of the Fountain Head mine probably drained eastward toward the main Yuba River. There is, of course, no reason why auriferous channels should not be found on the east as well as on the west side of the divide. There is also a difference in elevation between the surfaces of the rhyolitic flows of 200 to 300 feet between the vicinity of Cold Spring and Fountain Head. So great a difference would scarcely exist if there had been a way of communication between the two localities. Considering the subsequent tilting, the Tertiary bedrock surface must originally have had a less sharp westward slope than at present. Banner Hill, instead of rising 1,250 feet above the Manzanita channel, as now, was only 1,050 feet above the channel. DEPOSITION OF THE AURIFEROUS GRAVELS. The surface on which the Tertiary deposits rested having thus been examined, it remains to outline briefly the events which caused its burial under Neocene sedimentary and igneous deposits. At a period immediately preceding the volcanic eruptions of rhyolite and andesite the accumulations of gravel were not deep in any part of this area located well up on the ridges dividing the main drainage lines. Along these main rivers, and principally along the great longitudinal valley of the Yuba from You Bet up to North Columbia, masses of gravel several hundred feet in depth had accumulated. One of the principal causes (though not the only one) of this exceptionally heavy gravel mass is to be found in the fact that Yuba River, flowing on the middle slopes in a broad and open valley, had to turn and force its way through the foothill range of Jurassic lavas in a relatively deep and narrow canyon, almost as deep as that of to-day and well shown by the present relations at Smartsville. This foothill range acted as a barrier, restraining the gravel masses in the open valleys of the middle slopes. In ‘the Nevada City area the prevoleanic gravels reached the greatest depths along the Manzanita channel, and it is doubtful whether they have at any place exceeded a thickness of 40 feet. Rhyolitic fragments are found at that elevation above the bedrock, and even lower. It is doubtful whether the gravels, 60 feet thick, of the hydraulic pits northwest of Nevada City are antevolcanic; the gravel is different from that generally found in the deepest parts of the Neocene channels and has more the appearance of the extremely well washed ‘‘black gravel” which occurs at the higher elevation and which belongs in the rhyolitic period. Outside of the main drainage channel there was only a few feet of gravel on the bedrock along the streams, and in by far the greatest number of exposures the andesite or rhyolite rests directly on the bedrock. There is no reason to believe that the antevolcanic gravel in this vicinity antedates the Neocene period. THE VOLCANIC FLOWS. Such were the conditions when eruptions of enormous masses of rhyolitic tuffs began on the headwaters of the Tertiary Yuba River. Their general character has been referred to above, It is probable that they were erupted as mud flows, emerging from the crater mingled with much water, and that there was not only one but a long series of flows, in the intervals between which the older flows were to some extent worked over by the running water and interstratified with clay, sand, and gravels of local origin. These rhyolitic flows, 200 to 300 feet thick, are well exposed at Alta, on the Central Pacific Railroad, and at Chalk Bluff,.near You Bet, both in the 88337°—No. 73—l11——-9