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i <br /> 3.2 Regional Geology and Soils <br /> The site is located in a relatively flat area of the southern portion of the Sacramento.Valley Valley <br /> (Figures 1 and 2). The site is located in the east-central portion of the Great Valley geomorphic <br /> province, a north-south-trending valley approximately 400 miles long by 50 miles wide. The <br /> surface of the southern Sacramento Valley is composed primarily of unconsolidated, Pleistocene <br /> (1.6 million to 11,000 years ago) and.Recent(11,000 years ago to the present)alluvial sediments <br /> (Wagner and others,.1991;Wagner and others,-1981; Jennings, 1977). <br /> The ST&E site is located on an old Calaveras River and tributary flood plain that extends <br /> westward from the Sierran foothills into central.San Joaquin County. This flood plain has been <br />` exposed and leveled by progressive down-cutting of the ancestral Calaveras River and its <br /> tributaries and by wind erosion (Wagner and others, 1991 and 1981). <br /> Quaternary-age sediments extend from the ground surface to about 400 feet bgs at the site. The <br /> upper 60 feet of the site is comprised of eolian and flood plain sediments of the Modesto <br /> Formation (Wagner_, 1981). The Modesto and underlying Riverbank Formations consist of silts, <br /> generally become coarser with depth. <br /> sands, and gravels that ge y p <br /> i <br /> f Immediately below the Modesto and Riverbank Formations is the Pleistocene-age Turlock Lake <br /> Formation, consisting of low-permeability, fine-grained siltstone and claystone with interbedded <br /> E and intermittent coarse-grained sand zones present as channel sands and gravels that have <br /> extremely high permeabilities. This unit is derived almost principally from the Sierran granitic <br /> rock and formed under a flood-plain environment. <br /> Environmental\STE1PARSCR0910200S.wpd 13 <br />