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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  

150 TERTIARY GRAVELS OF THE SIERRA NEVADA OF CALIFORNIA. GEORGIA HILL, YANKEE JIM, AND SMITHS POINT. At Georgia Hill, opposite Yankee Jim, a thickness of 100 feet of gravel is exposed below the lava, and a few acres has been washed off along the edge. At Yankee Jim there is a larger area of gravel, from 40 to 100 feet thick, which toward the east disappears under the lava. The gravel is fairly coarse, being composed of metamorphic rocks, with some quartz. The bedrock elevation, 2,595 feet, is about the same as at Georgia Hill, and the main channel seems to have had a northeast-southwest direction, though a somewhat higher channel extended eastward and probably connected with the Smiths Point bench gravel 14 miles distant, on the South Fork of Brushy Creek. The gravel at Smiths Point is 50 feet thick, interstratified with sand. It is estimated that 8,630,000 cubic yards have been removed from Georgia Hill, Yankee Jim, and Smiths Point, and that yield has been about $5,000,000. The amount remaining available for hydraulic work is undoubtedly less than that removed, for the voleanic cap would soon make hydraulic work impossible. A quarter of a mile east of Georgia Hill the Anthony Clark tunnel has recently been run in a southerly direction for 550 feet, and is reported to have shown the existence of a large channel with much granitic detritus. The tunnel was found to be too high, striking the channel above bedrock. It is believed that the Yankee Jim channel extended northiw ard toward Wisconsin Hill via Kings Hill. It is also believed that it connects, below the lava, with the Dardanelles channel, though the later intervoleanic channels may have removed much of the earlier accumulations and in some places destroyed the older channel. DARDANELLES, MAYFLOWER, AND BATH. At Dardanelles and Forest Hill the canyon slope has exposed, below the lava, a long, low trough filled with gravel and rhyolitic tuff (Pls. V, p. 30; XXIV, A). The gravel is moderately coarse, is composed of quartz and metamorphic rocks, and is well cemented near the bedrock. Above it rests rhyolitic tuff intercalated with some gravel, clay, and sand. The thickness of these two formations varies exceedingly. At the New Jersey claim the gravel is only 8 feet thick and is overlain by rhyolitic tuff. At Dardanelles it has a maximum thickness of 70 feet. In the region about Mayflower are extensive bodies of rhyolitic tuffs, with intercalated gravels, as well as clays and sands of more doubtful origin. The depth of these accumulations at Mayflower, over the deep channel, is 350 feet. In the intercalated gravels granitic and rhyolitic cobbles are common. At the Adams tunnel 178 feet of rhyolitic clays are exposed, with two smaller gravel bodies. At the Blackhawk, Wasson, and Westchester claims similar bodies are exposed. At Bath the same channel is exposed with about 250 feet of overlying gravels and white tuffs. The lower part is a trough 500 feet wide and 100 feet deep, filled in the bottom with washed and rounded bedrock bowlders, composed chiefly of serpentine and greenstone. Above this comes a thick stratum of the usual coarse quartz gravel, and above this a series of rhyolitic tuffs with intercalated gravels, which have a maximum thickness of 30 feet and contain granite and rhyolite bowlders. The rhyolitic series varies from 100 to 250 feet in depth, and it is in turn covered by 270 to 300 feet of andesitic tuff-breccia. The main prevolcanic channel enters the ridge at Bath (bedrock elevation 2,900 feet) and runs northward for a mile with very slight grade, then curves west and south, assumes a grade of 60 feet to the mile, and passes below Mayflower (bedrock elevation 2,800 feet) and Forest Hill to Dardanelles (bedrock elevation 2,670 feet), where it curves northwest again toward Yankee Jim without leaving the ridge. The mining operations in this vicinity have been very extensive. (See Pl. XXV.) Hydraulic work has mainly ceased, though a considerable amount of ground is still available at Dardanelles and around the head of Brushy Canyon. At Dardanelles (Pl. V, B, p. 30) and at Forest Hill 4,850,000 cubic yards has been excavated; at the head of Brushy Canyon probably 7,350,000 cubic yards. The main old channel has been drifted at Dardanelles for 2,500 feet in a northwesterly direction, to a point where it was cut out by a deeper, intervoleanic channel. The gravel,