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 2. TOPOGRAPHY AND GENERAL GEOLOGY. TOPOGRAPHY. This report deals with the northern and larger part of the Sierra Nevada, lying between parallels 37° 30’ and 40° 30’ and extending from the Great Valley of California on the west to the escarpment facing the Great Basin on the east. From the plains of the Sacramento Valley the first foothills of the Sierra rise rather abruptly. Except along the Central Pacific Railroad, where an easily eroded mass of granodiorite extends to the plains, the valley is bordered by a series of ridges parallel to the crest of the range and rapidly attaining elevations of 1,000 to 2,000 feet. The slope of the foothills is decidedly stronger than that of the range as a whole. At an average elevation of about 2,500 feet the main plateau or middle slopes begin, characterized by the absence of longitudinal ridges and by a gentler undulating surface, in many places reduced to an even table-land with uniform and slight westerly slope. Above the general surface of this plateau rise groups of rugged hills of more resistant material, like Sand Mountain and Slate Mountain in Eldorado County, and the Blue Mountains in Calaveras County. An average elevation of about 6,000 feet marks the western boundary of the high Sierra, a region where the plateau-like character of the middle slopes becomes obscured and finally almost completely lost. High ridges and peaks, in places longitudinally arranged, here rise above the snow line. Through all three of these divisions the torrential streams have trenched deep canyons, V-shaped and extremely abrupt in the lower two divisions, but usually more U-shaped and wider in the high Sierra. Many of the canyons have been cut to a depth of 3,000 and even 4,000 feet. In the northern and southern parts of the area discussed the Pacific drainage reaches back to the most easterly summits of the Sierra Nevada, but in the central part two rivers of the Great Basin—the Truckee and the Carson—break through the eastern escarpment and drain considerable areas within the higher portion of the range. At the southeast corner of the area here treated the range breaks off in a magnificent slope of 6,000 feet from Mount Dana to Mono Lake, and this escarpment probably continues through the northern part of Mono County to Topaz, where it faces West Walker River, but ceases a short distance farther north. Another escarpment forming the eastern slope of the Genoa Ridge and having a height of about 5,000 feet, begins some 20 miles to the northwest of Topaz and continues due north for about 30 miles to a point a few miles southwest of Reno; another offset of a few miles to the west follows and is succeeded north of Truckee River by 8 somewhat lower escarpment, which is practically continuous to the north end of the Sierra, at Susanville, in Lassen County. The eastern front of the range is thus marked by four escarpments, arranged en échelon, each offset a few miles toward the west. Mr. G. K. Gilbert states that this arrangement continues south of West Walker River, at least as far as Bishop. A more westerly eastward-facing escarpment, or crest line, is less steep. It begins a short distance south of Lake Tahoe and continues north-northwestward for about 60 miles, to an area beyond Mohawk Valley in Plumas County, where it gradually becomes effaced. Between these two crest lines lie a series of deep depressions. The southernmost is that of Lake Tahoe, which is about 20 miles long and 10 miles wide. North of Lake Tahoe a bridge of high volcanic mountains connects the two crest lines. The next depression is Truckee Valley, and north of this another bridge of volcanic ridges connects the two divides. The third depression is Sierra Valley, a deep circular basin surrounded by andesite flows and filled with alluvium. Truckee Valley and Lake Tahoe are both drained by Truckee River, which cuts through the eastern ridges of the Sierra between Reno and Truckee and ultimately discharges into Pyramid 14