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C <br />3.0 GEOLOGY AND HYDROGEOLOGY <br />3.1 GEOLOGY <br />The North County Sanitary Landfill is located near the northeastern <br />edge of the San Joaquin Valley. The valley is a deep, asymmetric <br />structural trough filled with sediments derived from adjacent mountain <br />ranges. The geology of the eastern portion of the San Joaquin Valley <br />is characterized by alluvial sediments deposited by rivers and streams <br />emanating from source terrains in the Sierra Nevada Mountains to the <br />east. <br />The youngest geologic units underlying the site are semi consolidated <br />alluvial deposits of the Turlock Lake and Riverbank Formations. These <br />two formations consist of heterogeneous deposits of gravels, sands, <br />silts, and clays. Because of similar origins and common lithologies, <br />differentiation of alluvial deposits into formations is difficult. <br />Regionally, lateral and vertical lithologic variations within a <br />formation can be pronounced. <br />The geologic units beneath the site were characterized by reviewing <br />existing local data and drilling six borings ranging from 40.5 to <br />215 feet in depth (see Appendix A, Geologic and Hydrogeologic Report). <br />In addition to the borings drilled by EMCON, previous drilling was <br />conducted by another consultant on the same property and on the half - <br />section parcel immediately to the west. Data from these earlier <br />borings were also analyzed in studying the subsurface geology of the <br />site. <br />The exploratory drilling performed during this and previous investiga- <br />tions confirmed that subsurface conditions are consistent with the <br />regional information described above. Borings penetrated interlayered <br />silts, clay, and fine sand to their entire depths. Finer -grained <br />materials (silt, clay, and silty sand) predominate. Boring logs are <br />contained in Appendix A. <br />