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f Draft Environmental Impact Report Page IV.G-6 Forward Landfill Expansion Geologic Conditions Regional Geology This discussion of the regional geology of the proposed landfill is based on regional geologic studies compiled by the California Division of Mines and Geology (CDMG, 1982). The landfill is located near the geographical center of the Great Valley province of California. The Great Valley separates the Sierra Nevada Range on the east from the Coast Range on the west. It is structurally an asymmetrical trough filled with a thick sequence of flat-lying accumulated marine and continental sediments. The marine sediments of the Great Valley are Cretaceous to Early Tertiary (approximately 35 to 150 million years before present) deposits of marine clays, silts, and sands that have been consolidated into sandstones, siltstones, and shale. This sequence is about one mile thick beneath the site. The marine sediments were deposited during the period when the Sierra Nevada range was developing and the Pacific plate was being subducted to the west. Continental sediments in the Great Valley sequence were derived from rocks eroded from the Sierra Nevada to the east and the newly forming Coast Ranges to the west. At the end of the Eocene period, the Great Valley around Stockton was uplifted and eventually cut off from the Pacific Ocean by the accreting Coast Ranges,forming an inland sea area. Sediments from the eroding Sierra Nevada and Coast Ranges were deposited in the Great Valley by rivers and streams starting about 35 million years ago. Site Geology The geology of the site and site vicinity is based on regional studies augmented by the numerous exploratory bore and well installation programs completed at the site. The geology of the landfill has been described in several reports, including Kleinfelder & Associates (1991, 1997, 1999); CH2M Hill (1992);EMCON (1972); and CDM (1999). A summary of the geologic setting and associated data is presented in the Joint Technical Document UTD by Bryan Stirrat & Associates and Geologic Associates (2007) and Geo-Logic Associates (2008). These studies confirm that the geology as well as the hydrogeology of the landfill area are consistent with regional conditions. As determined in exploratory boring and well installation programs performed at the site, geologic materials beneath the site occur as lenticular units composed of clay, silt, and sand with minor amounts of gravel. Bedding is typically discontinuous over distance and ranges in thickness from a few inches to approximately 20 feet. This variability is consistent with deposition in a fluvial environment where the course of rivers or streams meanders and is subject to frequent flooding. The sediments observed in borings at the site appear to correspond with the Pleistocene Victor Formation and can be divided into three large groups t based on lithology and pervasive distinctions in color. • Group A.: Surface to approximately 28 to 40 feet below ground - Highly interbedded sandy silt, clayey silt, silt, and clay. A few layers of sand. Generally sands are fine grained. Most ' of the sediments have an oxidized appearance as an orange brown color. • Group B: Top: 28 to 40 feet deep, to bottom: approximately 55 to 75 feet deep (25 to 35 feet thick) - Interbedded sand, silty sand, silt, and a little clay. Sands are fine grained. Overall a T