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  

186 TERTIARY GRAVELS OF THE SIERRA NEVADA OF CALIFORNIA. The basin of the Tertiary Mokelumne River in this region coincides, roughly speaking, with its present basin, but also takes in the headwaters of the present Cosumnes. The old channel of the Mokelumne is exposed near Fort Grizzly, whence it continues southwest below the andesite ridge into the Jackson quadrangle. It can be traced upward, crossing Tiger Creek at Tarrs Saw Mill and Panther Creek near Dutch Henry. It probably crossed the southern boundary near Westmoreland, and is again found in the Big Trees quadrangle south of the present river. South of this channel line the andesite contact rises several hundred feet, but the great Mokelumne Canyon has eroded the larger part of the Neocene valley slope. Northeast of Dutch Henry the Tertiary surface rose 1,700 feet in 2 miles, to the level of the plateau of Leek Spring Hill. The modern canyon of the Mokelumne is in this vicinity no less than 1,200 feet below the Neocene river. An important tributary, which will be referred to as Dogtown Creek, joined the Mokelumne at Fort Grizzly and extended northward to Camp Creek. With its several branches it occupies the rather wide Neocene valley lying between the Leek Spring Hill plateau and another high plateau in the adjoining Placerville quadrangle of which Baltic Peak is the remnant, rising to an elevation of 5,100 feet. Along the main Tertiary valleys of the American and the Mokelumne there is evidence of the existence of two channels, the later one being eroded in the interval between the rhyolitic and the andesitic flows. This intervolcanic erosion produced an irregular surface of the rhyolite, and in many places the new channel cut through the rhyolite and trenched the bedrock surface below that rock. This is shown near the bend of Silver Fork, northwest of Bullion Bend, near Morgan, and on Sopiago Creek, while along Plum Creek it is evident that the rhyolite flows, which here are very deep, had not been cut through. Nowhere does the later channel lie more than 100 feet below the earlier one, and the general character of the surface was not affected by this erosion. GRADES OF THE TERTIARY STREAMS. The Tertiary American River, as explained above, followed closely the present canyon of the South Fork, from Bullion Bend (elevation 3,600 feet) to Johnsons Pass (elevation 7,500 feet). In a distance of about 28 miles, following the probable river curves of the old stream, there is a grade of 139 feet to the mile. The grade of the lower half varies from 100 to 133 feet to the mile; the upper part, from Georgetwon Junction to Johnsons Pass, had a grade of 160 feet to the mile. The direction of the river is throughout a few degrees south of west. From Johnsons Pass to Luthers Pass the direction of the former channel is northwest and the grade is only 50 feet to the mile, which seems to indicate that the fault lines on each side of Lake Tahoe have not appreciably disturbed the rocks 10 miles south of it. The grade of the tributary which joined the main river at Bullion Bend and headed near Round Top is 160 feet to the mile, the direction being a few degrees north of west, but the grade increases rapidly from 170 feet to the mile in its lower course to 220 feet to the mile near the headwaters, a short distance north of Round Top. The tributary to the Tertiary Mokelumne River which joined it near Fort Grizzly after a southward course of about 12 miles has a grade of only about 100 feet to the mile.