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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  

176 TERTIARY GRAVELS OF THE SIERRA NEVADA OF CALIFORNIA. 26 feet in vertical depth. The total tunnel grade is 7 feet, consequently the elevation of the channel at the end of the tunnel is 1,981 feet. Goodyear states that in the Cedar Spring tunnel the channel is at first 300 feet wide but soon expands to between 500 and 600 feet. It is very crooked, and the bedrock rises steeply on each side 40 to 60 feet. The pay gravel is from 4 to 6 feet thick. Connection has been made through the hill with the Green Mountain tunnel, the portal of which is situated on the south side of the ridge. The elevation of the deep channel at the Green Mountain tunnel is 1,948 feet, or 17 feet above the tunnel. From the Green Mountain tunnel the same channel has been mined southward for several hundred feet, going below Pascoe’s mill and as far as the divide south of Chile Ravine. From the Pascoe tunnel, which starts in bedrock at an elevation of 2,000 feet, one-fourth mile northeast of the Green Mountain tunnel, bench gravel without rhyolite cobbles has been mined. Such cobbles appear, however, farther in the hill in the same tunnel, on a bench 75 feet above the deep channel. The deep channel has a grade of about 50 feet in a little less than half a mile, or about 110 feet to the mile, the general direction being from north to south. The swinging benches have a much smaller grade. In 1901 the deep channel had not been mined under the ridge south of Chile Ravine, but it extends through to some point not yet exactly determined on Webber Hill. Ina letter of 1911 Mr. G. W. Kimble states that the Green Mountain channel is now practically worked out. A later andesitic channel of no great value appears to run from east to west along this south front of the hill. SPANISH HILL. The flat at the head of Cedar Creek, northeast of the Linden tunnel, is covered with ‘white lava” or rhyolite tuff; one point of bedrock is exposed which probably is on the rim dividing the Green Mountain and Linden channels. North of the Cedar Spring tunnel rises Big Spanish Hill, the bedrock of which is 25 feet higher than that at the inlet of the Cedar Springs channel. High bedrock is found on the rim above Little Spanish Hill, with elevations up to 2,308 feet. This high rim descends abruptly to the bedrock of the pit, which has an elevation of 2,170 feet. The hydraulic bank shows 60 feet of white rhyolite tuff covered by andesite tuff. There is no deep channel on Big Spanish Hill corresponding to the deep Green Mountain channel, and the gravels probably lie on a bench that is considerably higher than the deep channel and rises to Little Spanish Hill. Three deep crevices rich in gold traversed the bedrock in Big Spanish Hill. It is uncertain whether they were cracks or fissures or water channels. The stream outlined by Spanish Hill, Cedar Springs, and Green Mountain formed a welldefined tributary to the main channel in Webber Creek, separated from Coon Hollow on the west and from the Deep Blue lead on the east by high bedrock. The absence of andesite gravel and of all late andesite channels is noteworthy. There is a great thickness of rhyolite tuff. The main character of the channel is the same as that of the Deep Blue lead—a deep rhyolite channel and broad, swinging benches. DEEP BLUE LEAD AT WHITE ROCK CANYON. The first point where the Deep Blue lead appears is at Georgia ITIill, overlooking the South ’ Fork of American River on the east side of White Rock Canyon. Here a fraction of the channel is preserved, swinging off again on the canyon side of White Rock Point. The bedrock at Georgia Hill has an elevation of 2,320 feet (2,340 feet, according to Goodyear),and the section Mastrated . in figure 16 is shown. At White Rock Point there is andesite gravel above 30 to 40 feet of rhyolite tuff and thin gravel and the deposit forms a bench 25 to 90 feet above the Georgia Hill deep channel. Having swung around White Rock Point the channel crosses White Rock Canyon and enters squarely into the lava hill southwest of the canyon. The elevation of the bottom of the channel is 2,218 feet.