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5.3.1. Geologic Setting <br /> 5.3.1.1. Regional Geology <br /> The landfill is located near the transition between the foothills of the Sierra Nevada province to <br /> the east and the flat Great Valley province to the west. In the area surrounding the landfill, <br /> continental sediments derived from the Sierra Nevada were deposited in the valley with several <br /> alluvial and debris flow units overlapping the older crystalline and metamorphic bedrock units of <br /> the Sierra Nevada. Quaternary alluvial deposits of the Modesto Formation (Qm) and the slightly <br /> older Riverbank Formation (Qr) are primarily located north and west of the site and are typically <br /> found adjacent to Mormon Slough (Appendix D). <br /> Two geologic formations are mapped at the landfill and adjacent land surrounding the landfill <br /> (Wagner, D.L., et. al, 1981). These formations consist of the Tertiary Laguna Formation (Tl) <br /> and the Tertiary Mehrten Formation (Tin). The Mehrten is mapped throughout the landfill <br /> property, with the exception of the northeast corner mapped as Laguna (Plate 10). <br /> ® Laguna Formation (Tl) <br /> The Tertiary Laguna Formation consists of alluvial fan deposits which were derived from fluvial <br /> systems originating from the Sierra Nevada. The soils associated with the Laguna are typically <br /> weakly cemented such that permeability is generally low to moderate. Surface soils can contain <br /> expansive clay. The Laguna Formation soils generally consist of sand and gravel with minor silt. <br /> Mehrten Formation (Tm) <br /> The Tertiary Mehrten Formation contains moderately cemented non-marine agglomerate, <br /> conglomerate, tuffaceous sandstone, and siltstone. Permeability is generally low. The Mehrten <br /> Formation also contains some andesite mudflow breccia (lahar). The formation is consolidated to <br /> the point of a very dense, partially to fully cemented mudflow in many areas. The Mehrten <br /> Formation soils are believed to be reworked volcanic mudflow deposits such that they contain <br /> stream deposited silt, sand, and minor gravel. Surface soils can contain expansive clay soil. <br /> Foothill Sanitary Landfill Page 22 of 62 Public Works/Solid Waste <br /> JTD County of San Joaquin—Revised 10/6/05 <br />