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  

CHAPTER 18. THE CARSON QUADRANGLE. GENERAL GEOLOGY. The Carson quadrangle adjoins the Truckee quadrangle on the east and the Markleeville quadrangle on the north. It lies entirely within the State of Nevada, the State line forming the western limit. A part of Lake Tahoe occupies a narrow strip in the southwest corner, and alongside the lake, delimited by fault scarps and continuing toward the northern boundary line, not far from Reno, extends the most easterly block of the Sierra Nevada. Along the center of the quadrangle lies a series of deep depressions at an elevation of about 5,000 feet, sinking to 4,500 feet near the northern edge. These depressions, from south to north, are Carson Valley, Eagle Valley, Washoe Valley, and Truckee Meadows; they lie from 1,200 to 1,700 feet below the level of Lake Tahoe and separate the Sierra Nevada from the desert ranges of the Great Basin, the first of which occupies the whole eastern part of the quadrangle. In the southeast corner of the quadrangle lies the north end of the Pine Nut Mountains. About the middle of the quadrangle Carson River cuts its way through a lower part of that range in a steep canyon similarly to the Truckee Canyon farther north, and then continues eastward into the gradually widening basin of the Carson Sink. North of Carson River rise the Washoe Mountains and the Flowery Range, and in this part of the quadrangle the celebrated Comstock mines are situated. These desert ranges attain maximum elevations between 7,000 and 8,000 feet and are composed mainly of volcanic rocks. Among these rocks andesites predominate; many of them are apparently of late Tertiary age, similar to those of the Sierra Nevada, but others are decidedly older and in places show a transition to holocrystalline forms entirely unknown in the andesites of the Sierra Nevada. There are also smaller areas of granite and patches of older sediments which, from some fossils found near Dayton, are believed to be of Triassic age. The elevations along the Sierra are decidedly higher and reach 9,000 and 10,000 feet, or more, the culminating point being Mount Rose (10,800 feet), north of Lake Tahoe. The main part of the narrow buttress separating Lake Tahoe from the central valleys consists of a granodiorite which in places is dioritic. Embedded in this rock are two bodies of older sedimentary rocks, at Carson and at Genoa. In both areas they consist of slates with some nonfossiliferous limestone and a considerable quantity of amphibolitic rocks, probably altered andesites or basalt; in many places the latter are rich in epidote. In the southern part of the quadrangle scattered areas of andesitic rocks cover the granodiorite. In Little Valley the andesite is underlain by some rhyolite and auriferous gravels. At the north end flows of andesite, extending within a few miles of Reno, almost completely cover the underlying rocks. The thickness of these flows, which culminate in Mount Rose, probably amounts to 3,000 or 4,000 feet. In contrast to conditions farther west, massive flows prevail. At several places the rocks have been altered by hydrothermal processes and their outcrops now assume white and yellow colors. The summits north of Mount Rose are veneered with thin flows of basalt of a late date of eruption. They were extruded near the summit, but their flows, which follow the present slopes, descended on the west to Truckee River in fiery cascades. A smaller basalt area covers the granite at Steamboat Springs, where a group of hills in the central valley form an outlier of the Washoe Mountains. Lake beds of late Tertiary age are exposed at Verdi, a little north of the extreme northwest corner of the quadrangle, at the foot of the slope, where the andesites reach Truckee River. Some Tertiary lake beds are shown in the vicinity of Carson, especially at the State prison. The beds at this place are of small area and consist of coarse sandstone in which were found ! Spurr, J. E., Descriptive geology of Nevada south of the fortieth parallel and adjacent portions of Cali-ornia: Bull. U. 8. Geol. Survey No. 208, 1903. p. 123. 192