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4.6.6 Wind Rose The mean annual wind speed is 8.3 mph in a Westerly direction. Monthly predominant wind direction varied between southeast in January, February, and December and west during the remaining months (Figure 10). The Western Regional Climate Center was contacted for historical information. The closest station to the landfill is the Stockton Airport. 4.7. Geology 4.7.1 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 (Figure 11). 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 Foothill Sanitary Landfill Page 10 of 57 Public Works/Solid Waste Joint Technical Document County of San Joaquin—Revised 7/30/10