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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  

130 TERTIARY GRAVELS OF THE SIERRA NEVADA OF CALIFORNIA. Colfax quadrangle. At Chalk Bluff an extensive Neocene flora was collected by C. D. Voy and examined by Leo Lesquereux.! The rhyolitic flows of Nevada City are the exact stratigraphic equivalent of the rhyolite tuff of Chalk Bluff, and there can thus be no doubt about their age. Leaves similar to those of Chalk Bluff occur at many places in the vicinity of the Manzanita channel, and with some trouble it may be possible to obtain a good collection. The tuffs are well exposed at Quaker Hill and Scotts Flat, farther down the Neocene river, and again at the north side of the Washington ridge at Blue Tent, where they are several hundred feet thick, the top stratum attaining an elevation of 3,000 feet. Near the place where the upper North Bloomfield road crosses Rock Creek there was a low gap in the ridge between the main Yuba and the Nevada City basin; through this gap the rhyolitic tuffs poured into the granitic basin. The first flows found their way down into the Harmony and lower Manzanita channels, causing a damming of the latter, which, of course, produced an accumulation of sand and gravels in the upper part of the channel about Nevada City, and to this damming it is believed the heavy gravels of the Manzanita cut and Cement Hill are due. Subsequent flows found their way down to Round Mountain and Montezuma Hill, obstructing the channels to still greater extent. At last the whole of the lower part of the Nevada City basin became filled. The elevation of the top layers now ranges from 3,100 feet on the east side of the basin to 2,740 feet at the northwest corner of the Nevada City tract, a distance of 5 miles from east to west. It will be noticed that on the supposition of a tilting of 70 feet to the mile the surface would once, over this distance, have been approximately level, and about at the same elevation as the top stratum of the rhyolitic tuff at Blue Tent. The rhyolitic tuffs did not reach the southern and highest part of the Nevada City basin, nor did they overflow into the Town Talk or Grass Valley channel. To the east of the Neocene divide, rising along the eastern margin of the Grass Valley tract, the rhyolitic flows again appear, having reached that locality from the vicinity of You Bet. The divide was, however, just high enough to prevent their overflowing into the Alta channel. As may be seen by tracing the contacts of andesite and rhyolite, the surface was not even but was subjected to some eroding action in the interval between the two eruptions; the erosion, however, was not extensive enough to produce any marked change. In fact, intervolcanic channels, cutting far down into the rhyolite and even into the underlying bedrock, such as are so characteristic of the vicinity of Forest Hill, Placerville, and Mokelumne Hill, in the drainage of the Neocene American and Mokelumne rivers, are practically absent on the main Yuba, although they appear on the headwaters of the North Fork of the Yuba, near Forest City. This is evidently caused by a differing time interval between the two eruptions; in this vicinity the first andesitic flow from the Lola and Castle Peak volcanoes followed closely after the last eruption of rhyolitic tuff. During the later part of the rhyolitic period many divides were flooded and the drainage was partly changed. The great Neocene orogenic movement of the Sierra probably took place between the rhyolitic and the andesitic eruptions, as is indicated by the intensely eroding character of the “‘cement’”’ channels, or intervolcanic channels. A tilting took place, elevating the eastern part of the range most strongly and the western part but little. Flows of andesitic tuffs, emerging from the craters as a mud, poured down the flanks of the Sierra in rapid succession, obliterating the old drainage system and flooding many of the divides, so that Banner Hill and Osborne Hill alone emerged from the desolate lava plateau in this vicinity. On this inclined lava plain the rivers had to select new courses, in general differing considerably from the old ones. The present drainage system was developed, characterized more than the Neocene by a transverse direction of the rivers. 1 The exact locality seems a matter of some doubt. It is not now accessible, having been covered by hydraulic débris. Whitney states that the matrix is a rhyolite tuff, but in the few specimens I examined, by the courtesy of Prof. A. C. Lawson, of Berkeley, the rhyolitic character is not clearly apparent under the microscope. At the locality I was told that the leaves were found in clay just below the white tuff and at the top of the extensive bench gravels of You Bet, several hundred feet above the bottom of the deepest channel. .