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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 and rock types found at the site were fluvial (stream deposits) and fluvial deposits that were reworked by volcanic mudflows. Evidence of volcanic deposits included the presence of devitrified glass, tuff, and fine black inclusions (andesite detrital grains) characteristic of the Mehrten Formation was found in siltstone at the site. The attitude of bedding planes were not observed; however, the thickness of the bedding ranged from massive (>10 feet thick) to laminated (< 3/8 inch thick). Contacts between the various soil types ranged from gradational to sharp. Many of the soil types were weakly to moderately cemented. At depths greater than 150 feet bgs in MW-3, chert and quartz filled veins were encountered in siltstones and sandstones (Emcon, 2002). Foothill Sanitary Landfill Page 11 of 57 Public Works/Solid Waste Joint Technical Document County of San Joaquin—Revised 7/30/10 i