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Foothill Sanitary Landfill Page 10 of 53 San Joaquin County DPW - Solid Waste Joint Technical Document Revised 6/09/2020 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 (Tl) and the Tertiary Mehrten Formation (Tm). The Mehrten is mapped throughout the landfill property, with the exception of the northeast corner mapped as Laguna (Figure 11). 4.7.1.1. Laguna Formation (Tl) 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. 4.7.1.2. 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, partially to fully cemented mudflow in many areas. The Mehrten Formation soils are believed to be reworked volcanic mudflow deposits such that they contain stream deposited silt, sand, and minor gravel. Surface soils can contain expansive clay soil. Beyond formational contacts, the geology mapped in the near vicinity and at the landfill does not indicate significant geologic structural features (i.e. folding, attitude of beds, faulting, etc). A review of the Wagner, 1981 geologic map and boring logs from previous site investigations (Kleinfelder, 1985-1990) suggested that the primary mechanisms for the deposition of the soil