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San Joaquin County Community Environmental Setting,Impacts,and Mitigation Measures Development Department Geology,Soils,and Seismicity Coast Range on the west. The valley is about 450 miles long and has an average width of about 50 miles. Elevations of the alluvial plain are generally just a few hundred feet above mean sea level(MSL),with extremes ranging from a few feet — below MSL to about 1,000 feet above MSL(Hackel 1966). Regional and Project Area Topography The main topographic features of the region consist of dissected uplands, low alluvial plains and fans,river floodplain and channels,and the Sacramento-San Joaquin Delta to the north. The topography of the project area is typical of an alluvial fan, sloping uniformly away from and formed by sediment from the nearby Coast Ranges to the west. Elevations in the project area presently range from approximately 185 feet on the western edge of the project area to approximately 160 feet on the eastern edge of the project area. — Regional and Project Area Geology Geologically,the Great Valley geomorphic province is a large, elongate, — northwest-trending asymmetric structural trough that has been filled with an extremely thick sequence of sediments ranging in age from Jurassic to Recent. This asymmetric geosyncline has a long, stable eastern shelf supported by the _ subsurface continuation of the granitic Sierran slope and a short western flank expressed by the upturned edges of the basin sediments (Hackel 1966). The project area consists of thick Holocene(early Quaternary)non-marine (continental)sedimentary alluvial fan deposits. These are sediments deposited from former streams emerging from highlands surrounding the Great Valley geomorphic province, specifically the Coast Range. This unit is composed of gravel, sand,silt, and, clay(Wagner et al. 1990). Kleinfelder(2003a) summarizes geologic condition within the project area. The subsoil parent material encountered in 10 boreholes on-site consists of sandy clay to clayey sand to depths ranging from about 2.5-29.5 feet below the existing ground surface,underlain by partially cemented,gravelly alluvium with — occasional cobbles and thin,partially cemented lenses of clay,silt,and sand to the maximum depths explored, 90-120 feet. The alluvium encountered generally has a greater content of clay and silt than alluvium encountered in previous investigations in the project area(i.e., at the nearby Teichert and DSS pits and along PG&E's Line 108). Otherwise,the soils — encountered during Kleinfelder's 2003 investigation are generally consistent with the soils encountered in previous investigations. Based on well sounding information,groundwater may be expected in J excavations deeper than between about 70-83 feet in the northern portion of the project area and deeper than about 95 feet in the southern portion of the project area(Kleinfelder 2003b). — DeSilva Gates Quarry Project — Draft Environmental Impact Report 3.F-2 As 05105 05