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  

140 TERTIARY GRAVELS OF THE SIERRA NEVADA OF CALIFORNIA. yards has been excavated, and 130,000,000 cubic yards is said to remain. The same amount may be available in the vicinity of Lake City. Mining operations from the Derbec shaft have proved the existence of a deep channel extending for several thousand feet eastward. The Derbec channel, which has a steep grade, has been mined upstream from the shaft for a distance of 7,000 feet, following the curves; the width of pay gravel was from 150 to 600 feet and the height 8 to 16 feet from the bedrock. The gravel is coarse, with many bowlders, some of which are of granite. The average value per ton is $2.47. The mine was in operation from 1877 to 1893, and the production in some years reached $200,000. There can be little doubt that the Derbec channel continues toward Relief. At Relief erosion has exposed a deep trough in the old bedrock and about 200 acres of auriferous gravels. The oldest gravels, as usual coarser and containing less quartz, are 60 feet deep and are covered by 100 to 200 feet of alternating sand, fine quartz gravel, and clay. Some hydraulic work was done long ago at the southern and eastern rims of the channel, but for many years drifting operations only have been carried on. The Union tunnel, about 2,500 feet long, has been driven from the southwest side of the gravel area, and amounts up to $30,000 and $40,000 a year have been produced for a number of years. Drifting has also been done from the Blue Lead and the Waukesha tunnel, started from the northeast side of the deposit. Plate XX explains the Derbec and North Bloomfield operations and is based on a map kindly placed at the disposition of the Geological Survey by Mr. A. D. Gassaway. In 1901 the Union Blue Gravel Co., under the management of Mr. Gassaway, started a tunnel in lumbug Canyon above North Bloomfield, and on its completion began the mining of the Derbec channel upstream from the point where the Derbec Co. left it. The operations were very successful until an area of granitic rocks was encountered underneath the lava; the channel spread out at this point, and the gold values consequently became less concentrated. The first three bedrock figures to the left on Plate XX mark the North Bloomfield channel, as exposed by hydraulic work or drifting. The grade here is 100 feet to the mile; the direction southwest. From the’point marked “2979” to the deep ground found in the Last Chance incline, south of North Bloomfield, the direction is northwest and the grade only 67 feet to the mile. The channel was next found at the Derbec shaft, where the bedrock elevation is 3,349 feet, or about 300 feet higher than at Last Chance, the distance being about 1} miles and the stream flowing south. It appears now that the Derbec and North Bloomfield channels must be identical. In a former publication ! the writer has expressed doubt concerning this, but the bedrock relations are such that no other connection seems possible. The diflicult question is how to account for this abnormal grade between the two points in which the channel has been opened. It has been suggested that a fault exists here underneath the covering formation, and this will probably be found to be true. The Derbec channel has been mined for 1} miles in a general west-southwest direction, and found to have a grade of 131 feet to the mile. On the slope toward Bloody Run, north of Backbone House, there is an inlet of a large channel similar in its general characteristics to the North Bloomfield channel; the bedrock elevation of this inlet is not established with certainty on account of landslides which obscure the relations, but in the publication just cited it is assumed that this is the real upstream continuation of the North Bloomfield channel, and that from this point it curved out and has been eroded over the present river canyon to reappear at Woolsey Flat and Moores Flat. The grade from the Derbec shaft to this inlet would be about 50 feet to the mile, the direction being south-southeast. This view is opposed by Mr. Gassaway and others, who point out that a deep depression has been found at the Watts shaft, between Derbec and Moores Flat. The collar of the vertical Watts shaft has an approximate elevation of 4,262 feet; the shaft is 417 feet deep; a crosscut was run due west from it at the bottom level for 1,260 feet, and two winzes were sunk about 50 feet deep to bedrock. At the bottom of the shaft and at the breast this crosscut was in bedrock, and the presence of a deep trough was thus established. The gravel is reported to be too 1 Lindgren, Waldemar, Two Neocene rivers of California: Bull. Geol. Soc. America, vol. 4, 1893, p. 273.