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4-8.24.7.1 Regional Geology The landfill is located near the transition between the foothills of the Sierra Nevada province to the east and the flat Great Valley province to the west. In the area surrounding the landfill, continental sediments derived from the Sierra Nevada were deposited in the valley with several alluvial and debris flow units overlapping the older crystalline and metamorphic bedrock units of the Sierra Nevada. Quaternary alluvial deposits of the Modesto Formation (Qm) and the slightly older Riverbank Formation (Qr) are primarily located north and west of the site and are typically found adjacent to Mormon Slough (Appendix D). Two geologic formations are mapped at the landfill and adjacent land surrounding the landfill (Wagner, D.L., et. al, 1981). These formations consist of the Tertiary Laguna Formation (TI) and the Tertiary Mehrten Formation (Tm). The Mehrten is mapped throughout the landfill property, with the exception of the northeast corner mapped as Laguna (WateFigure X110). 48.2.4.7.1.1 Laguna Formation (TI) The Tertiary Laguna Formation consists of alluvial fan deposits which were derived from fluvial systems originating from the Sierra Nevada. The soils associated with the Laguna are typically weakly cemented such that permeability is generally low to moderate. Surface soils can contain expansive clay. The Laguna Formation soils generally consist of sand and gravel with minor silt. 4.8. 4.7.1.2 Mehrten Formation (Tm) The Tertiary Mehrten Formation contains moderately cemented non -marine agglomerate, conglomerate, tuffaceous sandstone, and siltstone. Permeability is generally low. The Mehrten Formation also contains some andesite mudflow breccia (lahar). The formation is consolidated to the point of a very dense, Foothill Sanitary Landfill Page 31 Public Works/Solid Waste Joint Technical Document County of San Joaquin — Revised 7/30/10