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Wz'S i S4, 3 . 0 ENVIRONMENTAL SETTING 3. 1 GEOLOGY The site lies in the center of the Great Central Valley geomorphic province. The valley has been down-warped and filled with material eroded from the surrounding mountains since pre-Cretaceous times. As a result, the central part of the valley is filled with several hundred feet of consolidated and unconsolidated sediment's (Calif. DWR, 1978; Olmsted et al. , 1961 ; USGS, 1981 , 1984 ) . Important water--bearing units in the vicinity of Rainbo Bakery include the Mehxten, Laguna, Tulare, and Victor Formations. The top of the Mehrten ranges from 820 to 1 ,000 ft deep in the central part of the valley and is about 590 ft thick. The nonvolcanic Laguna Formation overlies the andesitic Mehrten on the eastern side of San Joaquin County, and varies from 430 to 1 , 000 ft in thickness. The Laguna unit is a stream-laid unit of discontinuous sand and silt lenses with lesser amounts of clay and gravel . The Laguna sediments become finer grained toward the center of the basin where the formation merges with the equivalent west-side formation, the Tulare. The contact is near the San Joaquin River. A thick confining layer, the Corcoran clay (usually logged as "blue clay" by drillers) , is found near the top of the Tulare Formation. The thickness of the clay varies from approximately 0 ft to 160 ft ( Calif. DWR, 1978; Olmsted et al. , 1961 ; USGS, 1981 , 1984 ) . The Victor Formation lies over the Laguna on the eastern side of the county. It varies from 0 to 50 ft thick. The sediments are alluvial fan and interfan deposits, which are generally coarser than the Laguna Formation. However, the interfan areas contain large amounts of fine sand, silt, and clay, making the two formations practically indistinguishable. Tho aquifer is considered semiconfined because of 3-1