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  

DOWNIEVILLE QUADRANGLE. 111 There are a considerable number of remnants of gravel deposits besides those already mentioned, and Turner states that many of those which are described in the following paragraphs probably belong to the epoch of intervolcanic channels. On the north side of the Middle Fork of Feather River, about half a mile west of Nelson Point, is a gravel deposit that was formerly mined by the hydraulic method. Andesitic breccia occurs on the slope to the north and presumably at one time covered the deposit. The occurrence is remarkable as being only about 200 feet above the present Feather River, at an elevation of about 4,000 feet. On the south slope of Clermont Hill, near the summit, at an elevation of about 6,400 feet, 4 miles west-northwest of Nelson Point, an English company exploited a gravel channel that is covered by andesitic breccia. The gravel seen by Turner at the mouth of the tunnel was chiefly of quartz and other siliceous rocks and was largely subangular in character, indicating a small watercourse. The gravel is said to have contained a good deal of gold in spots. The New Nelson placer mine is situated 3 miles northeast of Nelson Point, on the slope of the high ridge overlooking Feather River, the tunnel having an elevation of about 4,500 feet. The channel opened by this tunnel has been followed under the volcanic cover for some distance in a northeasterly direction, and it is believed that it may connect with the gravels at Spring Garden, 3 miles farther to the northeast, or with the channel found in the tunnel of the Western Pacific Railroad near by. As the elevation of the gravels at Spring Garden is at least 500 feet lower than that of the placer mine on the Feather River side, it is probable that a fault intervenes, and such a fault is indicated on Turner’s map of the Downieville quadrangle. It would represent the northward continuation of the Mohawk Valley line of dislocation and involve a considerable downthrow on the northeast side. In their present depressed position ’ the Neocene gravels at Spring Garden can hardly be consistently connected with any known channel except as above stated. On the steep east slope of the ridge, 44 miles northeast of Johnsville, are some prevolcanic gravels. At Miller’s tunnel, the altitude of which is about 6,500 feet, a gravel deposit has been found under the andesite capping of the ridge. The material seen is subangular and contains small fragments of blackened wood. The bottom of the Miller gravel channel is about 400 feet vertically under the top of the ridge. It evidently represents a small deposit of gulch gravel. On the ridge north of Indian Valley and east of Canyon Creek are a number of gravel deposits at the edge of the large area of andesitic breccia capping the ridge. These may be parts of a single subordinate channel. The southernmost occurrence is at the Rocky Peak drift mine, about 2 miles north of Indian Village; the next is on the west side of the volcanic cap at Bunker Hill; another area is at the Sailor Boy diggings, and the most northern is just west of McMahons. The gravel at all these points is more or less similar, the pebbles being of dark quartzite, siliceous schist, and Tertiary lavas. At McMahons the lowest gravel consists chiefly of white quartz, the pebbles of which are usually 3 inches or more in length, and there are in addition the same pebbles as noted above. The elevation at McMahons is less than 5,000 feet, and the other masses noted lie at a successively lower level toward the south, so that at the Rocky Peak mine the elevation is only about 4,000 feet. The course of the channel is therefore probably southward. There is no likelihood of any connection of the gravel at McMahons with the large channel at Eureka, as higher bedrock intervenes, but it is not impossible that the McMahons channel may have joined that at Scales, although the character of the gravel indicates that it is part of the Sailor Boy channel. Gravels occur under the fragmental andesite on the high ridge south of Downieville. They were mined by a shaft at the Pliocene mine and by a tunnel at the Ruby mine, north of Table Mountain, in a ravine draining into Rock Creek. Two distinct channels have been found. One, the older, extends toward the Bald Mountain Extension mine channel but is not certainly connected with it. The younger channel lies 116 feet lower and connects with the old Rock Creek diggings and with those at the City of Six. At the head of Slug Canyon is the City of Six gravel deposit. The material exposed is 500 feet wide and one-third of a mile long. The pebbles are of quartz and of the older metamorphic and igneous rocks. This channel was tunneled