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43 GEOLOGY AND SOILS <br /> ENVIRONMENTAL SETTING <br /> Regional Geology <br /> The project site lies in the center of the Great Valley of California near the junction of the San Joaquin <br /> and Sacramento River systems. It is located in the southern portion of the Great Valley, known as the <br /> San Joaquin Valley. To the west are the "delta lands," an area near sea level where the San Joaquin and <br /> Sacramento Rivers join together. To the east are the slopes of the Sierra Nevada. <br /> The proposed project site is on a broad valley plain consisting of gently rolling terraces, the floodplain <br /> of the Mokelumne River and two dry lake beds known as the North and South Tracy Lakes. <br /> The San Joaquin Valley is on northwest-trending asymmetric structural trough filled with thick <br /> sedimentary deposits ranging in age from Eucene to Present. Quaternary alluvium from the riverbank <br /> formation covers the site with silt, sand, and minor gravel deposited chiefly by the Mokelumne River <br /> (Marchand, 1977). <br /> Soil Conditions <br /> Soil investigations have been conducted on the project site by the Soil Conservation Service(SCS),Lowry <br /> and Associates and Questa Engineering, Inc.,consultants for this EIR. In the San Joaquin County Soil <br /> Survey (1990), the SCS identified five soil types (Figure 4.3-1). About two-thirds of the area is on <br /> terraces dominated by soils that have an impermeable hardpan (San Joaquin and Kimball hardpan <br /> substratum). These soils are well drained and have a claypan on top of the hardpan. These hardpan soils <br /> are so impermeable that vernal pools form on them in depressions and minor drainageways. <br /> The remaining soils are very deep and loamy textured(Columbia,Bruella, and Xerofluvents). Drainage <br /> varies from somewhat excessively drained (Xerofluvents),well drained (Bruella) and somewhat poorly <br /> drained (Columbia). <br /> 4.3-1 <br />