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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

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TOPOGRAPHY AND GENERAL GEOLOGY. 49 trary, that it washes the very base of the mountains and occupies the position that would result from orographic movement of the nature indicated by the deformation of the beach lines. The eccentric position of Lake Mono in reference to what would be its normal position had the basin remained undisturbed is in all respects similar to the abnormal position of Great Salt Lake and is due to a similar tilting of its basin. The shifting of the load from the mountains to the valley would tend to produce such a movement as has been observed; it may be in part the cause of the recent movement, but the commencement of the faulting must have occurred before any considerable transfer of load could have taken place and is probably due to other causes. The variations of surface temperatures, weight of water in ancient lake basins, transfer of load from the heaved to the thrown side of a fault, etc., by which the movements have been explained by various writers, it seems to me are secondary results of some great slow-working and wide-reaching series of forces which have made themselves felt * * * throughout the Great Basin. SEDIMENTATION AND EROSION. To those who study the larger movements of the earth’s crust and their relation to sedimentation and erosion the Sierra Nevada presents a particularly important problem. A mountain range since earliest Cretaceous time, subject continuously to erosion and intermittently to orogenic movements, it faced the western sea for a long period during the Cretaceous and early Tertiary and its streams discharged into this sea enormous masses of sediments. The problem, in so far as it relates to subsidence of the Great Valley caused by the weight of the sediments from the erosion of the Sierra, has been effectively treated by Ransome.! Some students of the geology of the Pacific slope, particularly G. F. Becker,? have thought that the uplift of the Sierra Nevada is due to the isostatic subsidence of the sediment-laden valley floor, which would produce a viscous flow of material underneath the rising mountain mass. Ransome rejects this explanation, showing by a diagram the decided incompetency of the sedimentary mass of the valley, even if liberally measured, to produce an elevation of the range. According to the results presented in the present report the incompetency is even greater than was indicated by Ransome, because the uplift involved not only the Sierra but the adjacent mass of the Great Basin. Ransome justly points out that the detrital masses from the enormous early Cretaceous erosion of the Sierra Nevada were deposited mainly on the site of the present Coast Ranges and only to a smaller degree in the Great Valleyitself. As shown in this report, the first breaks along the eastern fault system were outlined during the Cretaceous period, but at a distance from the main deposits that would seem to preclude any direct connection between the sedimentation and the faulting. Erosion continued in the Sierra Nevada during the whole of the Tertiary, but at so slow a rate that no sediments of excessive thickness could have been deposited in the Great Valley. The orogenic movement took place at the end of the Tertiary, and here, again, the incompetency of the supposed cause is apparent. Indeed, in the last part of the Tertiary the range itself became loaded by volcanic material flowing down over its slope from volcanoes at or east of the summit, to an extent which probably much more than balanced the amount removed by erosion during the whole of the Tertiary. The epoch of erosion that began immediately after the close of the volcanic flows unquestionably removed an enormous load from the range and deposited it in the Great Valley. The valley gradually subsided, at least in that part adjacent to the territory here considered, as shown by the absence of débris fans from the Sacramento Valley, but that the subsidence is due to the weight of the sediments is a hypothesis pure and simple. No corresponding elevation of the Sierra Nevada has taken place in Quaternary time. The numerous and great oscillations of the shore line along the valley border (see pp. 21-28) have also taken place independently of loading or unloading and constitute a strong argument against isostatic movements by erosion or sedimentation. 1 Ransome, F. L., The Great Valley of California. a criticism of the theory of isostasy: Bull. Dept. Geology, Univ. California, vol. 1, No. 14, 1896, pp. 371-428. . 3 Structure of a portion of the Sierra Nevada: Bull. Geol. Soc. America, vol. 2, 1891, pp. 49-74. 88337°—No. 73—11——4