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III. GEOLOGY <br />A. REGIONAL GEOLOGY <br />The Landfill Development Site is located near the northeastern edge of <br />the San Joaquin Valley. The valley is a deep, asymmetric structural <br />trough filled with sediments derived from adjacent mountain ranges. <br />The geology of the eastern portion of the San Joaquin Valley is char- <br />acterized by alluvial sediments deposited by rivers and streams eman- <br />ating from source terrains in the Sierra Nevada Mountains to the east. <br />The Landfill Development Site is situated at the western edge of what <br />Piper et al. (1939) designated as the Arroyo Seco Pediment. This geo- <br />morphic feature was produced by the Mokolumne River and other streams <br />originating in the Sierra Nevada Mountains. The sediments were des- <br />cribed as fluviatile (stream or river) deposits consisting of sand, <br />gravel, silt, and clay. <br />Subsequent workers, most recently Marchand and Allwardt (1981), have <br />subdivided the sediments into thinner, less continuous units. The <br />youngest geologic units underlying the site are semi consol i dated <br />alluvial deposits of the Turlock Lake and Riverbank Formations. The <br />Turlock Lake and Riverbank Formations consist of arkosic alluvial <br />deposits of gravels, sands, silts, and clays. These deposits show <br />coarsening upward patterns, as is seen in most of the prograding <br />alluvial systems flanking the western Sierra Nevada. Due to similar <br />origins and common lithologies, differentiation of alluvial deposits <br />into formations is difficult. Lateral and vertical lithologic varia- <br />tions within a single formation are sometimes more pronounced than <br />differences between individual units (Marchand and Allwardt, 1981). <br />1-1 <br />