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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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TOPOGRAPHY AND GENERAL GEOLOGY. 29 depression which existed on the middle slopes between Forest Hill, in Placer County, and North Bloomfield, in Nevada County, the areas would not show prominently on a map of small scale such as Plate. I (in pocket). The andesitic and rhyolitic flows cover the largest parts of the gravels, The Tertiary deposits comprise several epochs which are distinguished in the following paragraphs. (See also fig. 2.) PREVOLCANIC DEPOSITS. Deep gravels—The deepest trough-shaped depressions (Pl. V, B) in the drainage basin of the Tertiary Yuba River are usually filled to a depth of 50 to 200 feet by coarse gravels which ordinarily have been cemented so that they can not be readily washed without previous crushing. In the main channels the pebbles are large and well rounded. They range in size up to cobblestones and even bowlders several feet in diameter (see Pl. XXI, B, p. 144), but all of them, unless subsequently decomposed, have a smooth or polished surface. They consist mainly of the rocks of the older series; quartz forms a part of the pebbles but rarely predominates. There is no clay and the cementing material. between the pebbles consists of coarse sand. The coarse and bowldery character of these lower gravels is especially emphasized in the smaller streams or in places where the large stream beds contract in passing through bars of hard rock. Conspicuous examples of such conditions are furnished by the Cherokee mine, in Butte County, and the Polar Star mine, in Placer County. The deposits evidently originated in a stream of fairly strong grade and large volume. In the southern Tertiary rivers—for instance, that finding its outlet from Vallecito to Valley Springs, in Calaveras County—the deep gravels are SUP ROILV EV OG VANS YB Son ha SoD GIy Vea BD Dine es ce 7 Spy eoreRe tr eee Z 3 O: cats ioe rye. NRT AS) fr, ceeatetehetatetstetatenets: aur < eet astoces SOHSOB IIIS EW Figure 2.—Schematic répresentation of the four principal epochs of Tertiary gravels in the Sierra Nevada. a, Deep gravels (Eocene); b, bench gravels (Miocene); c, rhyolitic tutfs and interrhyolitic channel; d, andesitic tuffs and intervolcanic channel. much thinner than along the Tertiary Yuba. In many places they are entirely absent. No fossils have been found in the deep gravels except at one place near Susanville, where an Eocene flora was discovered by Diller. It is likely that these gravels are of Eocene age, and some of them along the Tertiary Yuba River may even be Cretaceous. Bench gravels.—Covering the deep gravels and attaining a maximum thickness of 300 feet, the bench gravels are spread out, in places to a width of 1 or 2 miles, on the sloping shelves on both sides of the deepest troughs (Pl. V, B). These gravels usually contain much quartz and are much more admixed and interstratified with finer sediment than the deep gravels. The pebbles are also smaller and always, except close to the headwaters, well rounded and polished. (See Pl. XXIV, B, p. 150.) These gravels indicate an epoch when the streams became overloaded; the extensive deposition which resulted from the overloading and the lessening of grades created broad flood plains over which the rivers flowed in changing channels. The distinction between the deep gravels and the bench gravels is much more marked along the Tertiary Yuba River than along the streams to the south. Different conditions prevailed along Jura River, which flowed northward and found its outlet at a point west of Susanville. In the lower part of this stream, from the vicinity of Taylorsville to Mountain Meadows, the lowest . deposits consist of beds of sand, in some places with lignite; above this lie about 100 feet of coarse auriferous gravels. Near the old outlet the thickness of the prevolcanic deposits increases greatly. According to Diller 400 feet of sand is exposed, and this is covered by heavy prevolcanic gravels. As more fully stated in chapter 3, the age of the bench gravels is believed to be Miocene, the determination being based on large collections of fossil leaves.