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II . GEOLOGIC AND HYDROGEOLOGIC SETTING A. GEOLOGY Located along the eastern edge of the San Joaquin Valley, the proposed landfill site is situated within an area of gently rolling topography (see Figure 2) . The geology of the area is characterized by alluvial sediments deposited by westward-flowing rivers and streams emanating from the Sierra Nevada Mountains. The youngest geologic units in the vicinity of the site are semicon- solidated alluvial deposits of the Pleistocene Turlock Lake and Riverbank Formations (Wagner et al . , 1981) . These formations consist of alluvial deposits of clays, silts, arkosic sands, and gravels. The deposits are structurally undeformed and lithologically gradational , both vertically and laterally. Underlying the Turlock Lake and Riverbank Formations are the Arroyo Seco Gravel (Pleistocene) , Laguna Formation (Pliocene) , and the Mehrten Formation (Miocene-Pliocene) . Differentiation of alluvial deposits in this region into formations is difficult, due to similar origins and common lithologies (Marchand and Allwardt, 1981) . DuringEMCON's assessment of this site for development opment into a Class III landfill , the geologic units were characterized by reviewing existing geologic information and drilling six borings ranging from 40-1/2 to 215 feet in depth. A seismic refraction survey was conduct- ed to evaluate the rippability of subsurface materials. In addition, an evaluation was completed to determine the Maximum Probable Earth- quake and associated peak ground acceleration. Drilling information indicates that site surficial soils consist of silts and clays to a depth of 2 to 3 feet. These soils are commonly 1 underlain by a thin layer of cemented soil called hardpan. The under- lying Turlock Lake and Riverbank Formations consist of heterogeneous PJ9 9390207.00D 2