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A8 REGIONAL AQUIFER-SYSTEM ANALYSIS-CENTRAL VALLEY, CALIFORNIA 1986). Sedimentary deposits include both marine and continental deposits and range in age from Jurassic to Holocene. The valley is bordered on the east by the Sierra Nevada and on the west by the Coast Ranges (fig. 6). The Sierra Nevada is the source of most of the sediment that underlies the valley. GEOLOGIC SETTING Although there are differing viewpoints among geolo- gists as to the details of the origin of structural features we can view today in the Central Valley, there is general agreement on the larger aspects of the emergence of California as a landmass and the subsequent formation of the valley (King, 1977, p. 177). The formation of the Sierra Nevada and the Coast Ranges is important to understanding the deposition of aquifer material in the Central Valley as well as to understanding the distribu- tion and movement of ground water. To the east of the Central Valley is the Sierra Nevada. It is the largest single mountain range in the contermi- nous United States and is about 350 mi long and 55 to 80 mi wide about as long and wide as the Central Valley. The Sierra Nevada is composed primarily of granite and related plutonic rocks but includes metasedimentary and metavolcanic rocks that range in age from Late Jurassic to Ordovician or perhaps older. The Sierra Nevada batholith was emplaced after the Late Jurassic but prior to the Late Cretaceous (Shelton, 1966, p. 385). The Sierra Nevada, like the Central Valley, is asym- metric; the east side is considerably steeper than the west side (fig. 6), suggesting that the block was tilted upward toward the east. At the base of the east slope is evidence of recent faulting at several locales, while steep canyons have been cut into the gently sloping west side. Based on the steep canyons, Shelton (1966, p. 388) deduced that the huge granitic batholith was at a low elevation long enough to acquire a fairly flat erosional surface before it was tilted in successive stages. Wells drilled in the San Joaquin and Sacramento Valleys penetrated granitic rocks at increasing depths toward the west, indicating that the granite exposed in the Sierra Nevada is only a small part of the whole mass. Uplift of the granitic rocks resulted in increased precipitation in the Sierra Nevada, particularly near its crest, which in places exceeds 14,000 ft altitude. Warm, moist airmasses from the Pacific Ocean are forced aloft by the mountain range, causing the airmasses to cool and the moisture to condense, resulting in heavy precipitation that exceeds 80 in./yr in places (Rantz, 1969). Because the crest of the range is near the east side, much of the runoff is westward to the Central Valley and is a major source of water to the valley. The streams that debouch from the Sierra Nevada also have supplied a major part of the recent and older Cenozoic sediments that have been deposited in the valley. Bailey and others (1970) described the significance and processes of the overland thrusting of the Mesozoic marine sediments to the formation of the Coast Ranges and to the deformation of the Great Valley sequence. Two important hydrologic points can be made from Bailey's geologic discussion. First, the emergence of the Coast Range thrust and subsequent development of the Coast Ranges established an orographic barrier for moisture- laden onshore oceanic winds; as a result, the Central Valley effectively was put into a rain shadow since the formation of its west boundary an important factor in considering the source and distribution of ground-water recharge to the valley. A second factor that needs to be emphasized is the extensive deformation of the marine beds of the Great Valley sequence on the west side of the valley. In addition to establishing the asymmetric nature of the valley trough, these highly contorted beds may form a fault zone under the west side of the valley (Oakeshott, 1971, p. 289). The fault zone and the beds are barriers to ground-water flow. Variations in the chemical quality of ground water and in hydraulic heads are observed in closely spaced wells of similar construction drilled near the east boundary of the Coast Ranges (Hotchkiss and Balding, 1971). Although the ancestral Sacramento and San Joaquin Valleys were created by the emergence of many areas of the Coast Ranges by middle Cretaceous time, parts of the Central Valley remained inundated by the Pacific Ocean until late Pliocene time (about 2 to 3 million years before present). These inundated areas were continuously changing in size and shape as the Coast Ranges emerged. As a result, both marine and continental sediments were deposited. Marine deposition was dominant in the Cen- tral Valley from the Paleocene to the beginning of the Oligocene (fig. 7). During the early Oligocene, marine deposition was restricted to the southern part of the San Joaquin Valley; during the Miocene, marine deposits were laid down along the west flank of the San Joaquin Valley and throughout most of the southern San Joaquin Valley (fig. 7). The seas had retreated by the Pleistocene, and only continental sediments were deposited during the Pleistocene and Holocene. Marine deposits of Tertiary age, therefore, underlie large parts of the Central Valley; they crop out around Sutter Buttes, along the southwest flank of the Sacra- mento Valley, and along the west, southwest, south, and southeast flanks of the San Joaquin Valley. Because of many changes in the depositional environ- ment, the marine deposits differ greatly in sediment type, sorting, and thickness, and have been given nu-