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  

COLFAX QUADRANGLE. 143 under the lava. Two long tunnels, now inaccessible, were driven some time ago. They are said to have shown the existence of two channels at considerably different elevations. The reports do not agree as to whether they would pay for drifting. BLUE TENT. We now return to the western edge of the quadrangle in order to trace the southern branch of the main stream. At Blue Tent, on the south side of the South Fork of Yuba River, the gravel appears extensively below the lava, filling a deep trough in the bedrock, the deepest part having the same elevation (2,483 feet) as Grizzly Hill, across the canyon. The bottom gravels are 25 feet thick, coarse and cemented, and is covered by over 300 feet of light-gray sand and clay mixed with fine quartz gravel. The sand is particularly abundant and nearly barren. About 15,000,000 cubic yards has been removed and’ some 90,000,000 yards remain, much of which is barren clay and sand. The lower gravel averaged 15 cents or more to the cubic yard, but the sandy top gravel contained only 2} cents. It is stated that the hydraulic operations were not remunerative. The bottom of the channel is reported to be 1,000 feet wide and the gravels of low grade. Even the 5 feet of gravel next to the bedrock did not contain more than 50 cents a ton, it is reported. Nevertheless, it is probable that some attempt will be made to open the channel from Blue Tent to Scotts Flat. QUAKER HILL AND SCOTTS FLAT. On the ridge northeast of Nevada City a small but rich channel has been drifted from the East and West Harmony inclines. The gravel, which is partly subangular, is taken out to a depth of 4 feet. In Rock Creek lie large masses of clay and sand similar to the deposits at Blue Tent. Still larger accumulations are exposed at Scotts Flat and Quaker Hill. The gravel, which is covered with rhyolitic tuff and andesite, fills a deep trough well exposed by Deer Creek and Greenhorn River. Along the principal channel the gravels are nearly 600 feet deep; the bench gravels surrounding the deepest trough are about 300 feet in depth. At Hunts Hill the deepest channel is exposed by mining operations at about the level of the tailings in the river at an elevation of 2,620 feet. North of this point it is not visible until exposed again at Blue Tent. The geologic evidence clearly shows that the deep channel is continuous from Hunts Hill to Blue Tent. A shaft has been sunk in the old diggings at Quaker Hill and a bedrock was found at an elevation of about 2,650 feet. A shaft sunk in the creek at Scotts Flat struck bedrock at an elevation of about 2,770 feet, the lowest bedrock not being found. At Quaker Hill the width of the channel said to pay for drifting is about 130 feet, and the depth of pay gravel is from 4 to 16 feet. As usual, the gravel is coarse and cemented in the deep trough, and the bench gravels, several hundred feet thick, consist chiefly of fine quartz gravel mixed with sand. The yield of the top gravel rarely exceeds 6 cents a cubic yard in fine gold, the size of a pinhead or less, but the bottom gravel may be very rich. It is estimated that near Scotts Flat 12,000,000 cubic yards has been removed and that 35,000,000 yards has been worked at Quaker Hill, where the gravel banks reach a thickness of 250 feet. A vast amount of workable gravel, estimated at 140,000,000 yards, remains at Quaker Hill. At both Quaker Hill and Scotts Flat it is difficult, if not impossible, to obtain dumping ground and sufficient grade for sluices. Deep gravels fringe the rhyolite for 3 miles east of Quaker Hill and represent without much doubt a tributary crossing the ridge near Central House (Galbraith). South of this place there is about 100 feet of clay underlain by some gravel. Here some drifting has been done on both the north and the south side. Heavy clay masses are exposed at Burrington Hill, where some hydraulic work was done long ago. The gravel of this tributary has also been hydraulicked on the north and south sides of the Quaker Hill ridge. High bedrock appears on the ridge 3 miles northeast of Quaker Hill. East of this are exposed the small Red Diamond channel on the north side of the ridge and other channels