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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  

200 TERTIARY GRAVELS OF THE SIERRA NEVADA OF CALIFORNIA. At the sharp bend near Douglas Flat and Vallecito (see Pl. XXVI), very heavy interrhyolitic gravels have accumulated in an old valley some 8 miles long and up to 2 miles wide, lying between Tertiary bedrock ridges rising about 1,000 feet above the deepest trough. The — prerhyolitic gravels are thin or absent, but the interrhyolitic series attains a thickness of 200 feet. Turner says in the Jackson folio that the gravels 2 miles northeast of Angels are very similar to the Neocene shore gravels about Valley Springs and that they are interbedded with rhyolite tuff and contain pebbles of rhyolite. They are normally overlain by an irregularly eroded sheet of rhyolite tuff which east of Vallecito is 200 feet thick. In places the interrhyolitic gravels gradually change into andesitic gravels which form a part of the cap of ande* sitic detrital material. The andesitic tuffs and gravels aggregate 700 feet in thickness, but are of course irregularly eroded. Few parts of the channel are accessible by tunnels. Shafts about 100 feet in depth are necessary. There is much water, and this, coupled with the fact that the gravels are not extraordinarily rich, has greatly delayed the exploitation of the channel. About 11 miles of main channel is comprised underneath these gravel areas and only a small portion of this distance has been drifted. It is not unlikely that this ground could be made to pay by operations on a large scale with adequate pumping machinery. The canyon of the Stanislaus has cut a great gash across the main channel 2 miles east of Douglas Flat and removed a large part of its eastern rim. The main channel is believed to enter the hill at the Eho mine, on the west side of the Stanislaus Canyon, 1,000 feet above its bottom, about 34 miles east of Douglas Flat. Near this place it was joined by a tributary, coming down in a southerly or southeasterly direction from the vicinity of Murphy. The main stream course should thence continue south-southwest for 3 miles to a point near Vallecito, where its bottom is probably exposed for a short distance at an elevation of about 1,800 feet. This part could no doubt be reached by tunnel from the canyon slope or by a shallow shaft from the Vallecito side. Much hydraulic work has been done near Vallecito on the upper gravels, but the bottom has been reached only in few places by shafts. The Vallecito Consolidated Mines control a considerable area between Douglas Flat and Vallecito and it was proposed in 1902 to open the ground by a 7,000-foot tunnel. At a point nearly east of Vallecito the channel has been proved through the Wild Goose shafts. The course of the main old channel is clearly demonstrated east and northeast of Vallecito in the Mitchell and Manitou shafts. It follows a westerly course with high rims to the northwest and southeast. From Vallecito the channel is practically continuous for 4 miles westward, and after a short gap, where cut by Angels Creek, it continues again under the lava for 4 miles in a northwest direction to the Jupiter mine. The Eho mine is situated about 34 miles east of Douglas Flat at an elevation of 2,137 feet, overlooking the junction of Stanislaus River and Rose Creek. Several stretches of the oldest channel have been located for an extent of nearly a mile to a point where it is cut off on the southwest. About a mile west of the Eho mine, at an elevation of 2,700 feet, a shaft is said to have been sunk 500 feet—possibly 800 feet—without striking bedrock. It is thought that the gravels at the Eho mine represent the main inlet of the old river and that from this point the deepest trough continues underneath the ridge in a south-southwest direction to Vallecito. In this distance the channel has not been exposed. A section about 550 feet in thickness, exposed in the slope between these two points, shows just above the bed of the main channel a bedrock rim bearing rhyolitic tuff overlain by gravel containing rhyolite, followed by gravel carrying pebbles of a rock that may be andesite, which is in turn succeeded by 175 feet of rhyolitic tuff, 100 feet of latite, a thin bed of gravel, and finally latite and andesitic conglomerate. In the rear of the Adams ranch a somewhat similar section is shown. Rhyolitic gravel on bedrock at an elevation of 2,025 feet is overlain by rhyolitic tuff, washed gravels, andesitic conglomerate, and a regular andesitic series.