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3 . 0 ENVI ONMENTAf, SETTING 3 . 1 GEOLOGY The site lies in 4he center of the Great Central valley geomorphic province. The valley has been dcwn-warped and filled with naterial eroded frcm the surrounding mountains sincr- pre-Cr.etacecus times. As a result, the central part of the valley is filled with several hundred feet of consolidated and unconsolidated sediments (Calif . DWR, 1978 ; Olmsted et „k al . , 1961 ; USGS, 1981 , 1984 ) . .aportant }rater-bearing units in tha vicinity of Rainbo Banery include the MQhrter, Laguna, Tulate, and Victor Formations. The tcp of the Mehrten ranges from 820 to 1 , 000 ft creep in the central part of the valley and is about 570 ft thick. The � = nonvcicanic Laguna 'ormation overlies the andesitic Mehrten on _ = the eastern side of San Joaquin County, and varies fron 4.30 to t3 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 sedi„ients 1,eccme firer grained toward the center of the basin where the formation merges with the equivalent gest-side formation, the Tulare. The contact is near the San Joaquin River. A thick confining layer, the c 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 , 1')84 ) . *� 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 depcaits, which are a 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. The aquifer is considered semiconfine_d because of 3- 1