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III. Environmental Setting, Impacts, and Mitigations J. GEOLOGY SOILS AND SEISMICITY SETTING Regional Context The Austin Road Landfill is located southeast of the City of Stockton,in the Great Valley Geomorphic province of California. The valley is a structural trough between the Coast Ranges and the Sierra Nevada that has been filled with thousands of feet of sediment. In the vicinity of the landfill site,the valley fill consists of a fairly complete stratigraphic sequence of Cretaceous, Tertiary and Quaternary units(ranging in age from 136 million years to less than 1.6 million years old)that includes marine and terrestrial sediments. Topography Prior to development as a landfill,the site was relatively flat and the lowest natural r P y al ground . elevation at the site is approximately 30 feet above mean sea level (MSL). Modifications to the topography have included excavations for borrow pits. The existing depressions include one on the southwest portion of the property with a bottom elevation approximately 14 feet MSL, and another along the western site boundary,with a bottom elevation approximately 15 feet MSL. The deposition of refuse has not been even over the ground surface of the existing landfill. In some locations the fill has reached the permitted fill height of 90 feet MSL(60 feet above the local ground level). At other locations there has been no refuse material placement and the elevation is approximately 35 feet MSL. Historical agricultural practices on the proposed landfill expansion areas may have included minor alterations to the surface, such as leveling and placement of irrigation ditches. Geology The uppermost geologic unit in the project vicinity is the Victor formation, comprised of sands, silts and clays that were derived from weathered rock formations in the Sierra Nevada. These materials were deposited primarily by flood waters of the Calaveras and Stanislaus River systems during the Quaternary(in the last 1.6 million years). The sediments observed in the site borings accumulated as floodplain and channel deposits. Such deposits may have lenticular shapes,and high laterial variability. Therefore,it is not III.J.1