Unconsolidated continental deposits constitute the principal source of ground water in the San Joaquin Valley. On the west side of the valley, fresh water is effectively separated into two aquifers by the Corcoran Clay member of the Tulare Formation, which is encountered at an average depth of about 600 feet below land surface. The upper aquifer on the west side contains high dissolved solids (about 3000 ppm) in the upper 300 feet, similar to present recharge sources; below this depth, there is a gradation downward into water containing about 1500 ppm immediately above the clay; low concentration waters (about 800 ppm) are found in the lower aquifer. It is concluded that a marked dessication of the climate that occurred within the last 600,000 years, possibly initiated or accentuated (or both) by uplift of the Diablo Range and other elements of the Coast Ranges, was primarily responsible for the observed changes in concentration within the upper and lower west side aquifers.

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