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GROUND WATER IN THE CENTRAL VALLEY, CALIFORNIA A9 merous names by petroleum geologists (Park and Wed- dle, 1959, pi. 3; Sacramento Petroleum Association, 1962, figs. 6, 7,10, 20, and 27, and table 2). In places the marine deposits provided the source material for the overlying continental deposits that form the freshwater aquifers of the valley. Generally, the marine deposits contain saline water, some of which has migrated into adjacent and overlying freshwater aquifers. In a few places in the San Joaquin Valley, the marine rocks and deposits have been flushed of saline water and contain freshwater, which they yield to wells. In the Sacramento Valley, no marine deposits were reported as yielding freshwater to wells, although Olmsted and Davis (1961, p. 134) reported that marine rocks were flushed of connate water locally. The marine rocks, then, provide very little freshwater in the Central Valley. Continental deposits of post-Eocene age overlie the marine deposits and contain most of the freshwater aquifers in the Central Valley. An important contribution to the quantification of storage capacity in the aquifer system was made when Page (1983) successfully mapped texture change in the continental deposits overlying predominantly marine rocks and deposits in about 1,000 mi2 of the San Joaquin Valley. In this report and in chapter D (Williamson and others, 1989), the base of the aquifer system is taken as coinci- dent with the base of the post-Eocene continental depos- its. This is not strictly true in the southeastern San Joaquin Valley where sandy marine beds underlying the continental deposits contain freshwater and are hydro- logically part of the aquifer system. The thickness of the aquifer system, based largely on a thickness map of post-Eocene continental deposits prepared by R. W. Page (U.S. Geological Survey, written commun., 1981; Page, 1974), is shown in figure 8. The thickness of the aquifer system averages about 2,400 ft and increases from north to south, with a maximum thickness of more than 9,000 ft near Bakersfield. However, the contact between conti- nental and the underlying marine deposits is not always certain because the two types of deposits interfinger in some places, particularly near the south end of the valley. DeLaveaga (1952, p. 102) suggested that the continental IT SHASTA " "~~~"~~ IT?__- ^T=^= ~ J PtOFILE WITHOUT VERTICAL EXAGGERATION \ 6. Generalized oblique view (northward) of part of Central Valley structural trough. Coast Ranges lie to the east; Sierra Nevada to the west. (From "Geology Illustrated" by John S. Shelton. Copyright 1966, W.H. Freeman and Company. Used with permission.)