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<br /> <br />Foothill Sanitary Landfill Page 10 of 52 San Joaquin County DPW - Solid Waste <br />Joint Technical Document Revised 12/04/2025 <br /> <br />4.7. Geology <br /> <br />4.7.1. Regional Geology <br /> <br />The landfill is located near the transition between the foothills of the Sierra Nevada <br />province to the east and the flat Great Valley province to the west. In the area surrounding the <br />landfill, continental sediments derived from the Sierra Nevada were deposited in the valley with <br />several alluvial and debris flow units overlapping the older crystalline and metamorphic bedrock <br />units of the Sierra Nevada. Quaternary alluvial deposits of the Modesto Formation (Qm) and <br />the slightly older Riverbank Formation (Qr) are primarily located north and west of the site and <br />are typically found adjacent to Mormon Slough (Appendix D). <br /> <br />Two geologic formations are mapped at the landfill and adjacent land surrounding the <br />landfill (Wagner, D.L., et. al, 1981). These formations consist of the Tertiary Laguna Formation <br />(Tl) and the Tertiary Mehrten Formation (Tm). The Mehrten is mapped throughout the landfill <br />property, with the exception of the northeast corner mapped as Laguna (Figure 11). <br /> <br />4.7.1.1. Laguna Formation (Tl) <br />The Tertiary Laguna Formation consists of alluvial fan deposits which were derived from <br />fluvial systems originating from the Sierra Nevada. The soils associated with the Laguna are <br />typically weakly cemented such that permeability is generally low to moderate. Surface soils can <br />contain expansive clay. The Laguna Formation soils generally consist of sand and gravel with <br />minor silt. <br /> <br />4.7.1.2. Mehrten Formation (Tm) <br />The Tertiary Mehrten Formation contains moderately cemented non-marine <br />agglomerate, conglomerate, tuffaceous sandstone, and siltstone. Permeability is generally low. <br />The Mehrten Formation also contains some andesite mudflow breccia (lahar). The formation is <br />consolidated to the point of a very dense, partially to fully cemented mudflow in many areas. <br />The Mehrten Formation soils are believed to be reworked volcanic mudflow deposits such that <br />they contain stream deposited silt, sand, and minor gravel. Surface soils can contain expansive <br />clay soil.