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1j SECTION 2.0 GEOLOGY AND HYDROGEOLOGY 21 REGIONAL AND LOCAL GEOLOGY The site is located near the center of the Great Valley of California, a geomorphic province, that is a nearly flat alluvial plain bounded on the east by the Sierra Nevada and on the west by the Coast Ranges The Great Valley is comprised of a thick sequence of sedimentary rocks of Jurassic to Recent age The Great Valley is an elongated, northwest trending, asymmetric trough with a long stable eastern shelf and a short western flank (Bailey, 1996) The Stockton Arch, an area of uplift, extends from the Sierran slope across to the western flank of the trough The uplift is thought to have occurred during Eocene time and may have continued into the Miocene as a result of upward movement on the south side of the Stockton fault The United States Department of Agriculture (USDA) classified soil in the vicinity of the site as fine, montmorillonitic, thermic typic pelloxerert The soils are vertisols, which indicates they are clayey soils that have cracks and have high bulk densities The soil is formed from transported material of mixed origin and occupies nearly level or very gently sloping plains Erosion is not a problem, but because of the basin position, some areas are subject to flooding Internal drainage is poor as a result of the heavy-textured soil which becomes plastic and swells when wet, and movement of moisture is further retarded because the substratum is comparatively impervious The soil forms cracks when it is dry When the soil becomes wet, the cracks close and the soil is then slowly pervious to moisture (Woodward-Clyde, 1987) Based on boring Iogs from previous investigations, the stratigraphy consists primarily of alluvial deposits of clays, silts, and sands Figure 2-1 shows a plan view of the location of geological cross-sections at the site Figures 2-2 through 2-4 show geological cross-sections of the site and the neighboring Caltrans Stockton Yard site across South B Street to the east Fill material is found near the surface and is underlain by a silty and sandy clay layer, which has been encountered to a depth of approximately 10 to 15 feet below ground surface (bgs) Below the clay layer are silts, silty sands, and clayey sands with zones of poorly graded sand and silty clay to a depth of approximately 55 feet bgs A zone of silty sands and poorly graded sands is found to the total depth of borings advanced at Shop 10 (approximately 95 feet bgs), as shown on the geologic cross-sections 2.2 REGIONAL AND LOCAL HYDROGEOLOGY The regional hydrogeology of the Central Valley is dominated by a large basin-fill aquifer system comprised of a structural trough that is about 400 miles long and from 20 to 70 miles wide and extends over more than 20,000 square miles The trough is filled to great depths by marine and continental sediments, which are the result of millions of years of inundation by the ocean and . erosion of the rocks that form the surrounding mountains Sand and gravel beds in this basin form the aquifer system From north to south, the aquifer system is divided into the Sacramento s 4PR014iouicoo4lstiopio 5HOPiosllDoc 2-1 URS Greiner Woodward Clyde