The study area includes about 5,000 square miles of the northern part of the San Joaquin Valley, a broad structural trough drained by the San Joaquin River. Fresh ground water is mostly in unconsolidated deposits derived from the Coast Ranges on the west and the Sierra Nevada on the east.. The interfingering of Coast Range and Sierran alluvium, together with the variability and lenticularity of each, causes variation in the hydrologic properties both vertically and horizontally in San Joaquin Valley deposits. A persistent confining stratum, the Corcoran Clay Member of the Tulare Formation, can be correlated through most of the study area. Informally, the deposits above the clay are termed the upper units the clay is termed the confining clay stratums and the deposits from the bottom of the clay to the base of fresh water, where known, are termed the lower unit. The upper unit is composed of beds, lenses, and tongues of gravel, sand, and clay ranging in thickness from about 100 feet in the north to 500 feet at the south end of the study area. A distinctive white clay, at or near the base of the upper unit, can be traced across the center of the valley. The confining clay stratum is the greenish to bluish gray lacustrine Corcoran Clay Member of the Tulare Formation; it ranges from 0 to 130 feet in thickness. No attempt to delineate the top and bottom of the Tulare Formation was made because of the thickness and similarity of deposits overlying and underlying the Corcoran. The lower unit is lithologically similar to the upper unit and ranges in thickness, where the base of fresh water has been mapped, from 90 to more than 1,400 feet.
