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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 consolidated <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 /> PJ9 9390206.00D 5 <br />