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KLEINFELDER 1 REGIONAL CHARACTERISTICS 1.1 TOPOGRAPHY Foothill Sanitary Landfill is in the eastern part of San Joaquin County on Waverly Road (Plate 1-1). It is located on the edge of the Sierra Nevada Foothills, at an elevation of 250 to 300 feet, mean sea level (MSL). Regional topography is shown on Plate 1-2. 1.2 GEOLOGY Foothill Landfill is located on the eastern edge of the Great Central Valley geologic province. The Great Valley is an asymmetrical structural trough filled with a thick sequence (approximately 10,000 feet) of flat-lying marine and continental sediments. A brief summary of the type and origin of sediments underling the landfill, from oldest to youngest, follows. A geologic time scale is presented on Plate 1-3 for convenience. Marine Sequence The marine sediments of the Great Valley were deposited during the Cretaceous and Early Tertiary Periods (approximately 35 to 150 million years before present). These marine sediments were probably deposited in a miogeocline bound on the east by the developing Sierra Nevada Mountain range and on the west by the subducting Pacific Plate., This sequence of marine clays, silts, and sands is approximately 1,000-2,000 feet thick under the site. Continental Sediment Sequence The Great Valley was uplifted at the end of the Eocene and was eventually cut off from the Pacific Ocean on the west by subduction and/or accretion events which uplifted the present day Coast Ranges.1,2 Sediments derived from the eroding Sierra Nevadas in the east and the Coast Ranges in the west were deposited in the Great Valley by rivers and streams from about 35 million years ago to the present. In the vicinity of the Foothill facility, the marine sediments are generally overlain by unconsolidated Tertiary and Quaternary sediments of the Valley Springs, Mehtren, and Laguna Formations. Sediments of these FOOTHILL 3