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■40 INC <br /> 4 0 REGIONAL GEOLOGY AND HYDROGEOLOGY <br /> The investigation area is located in the Great Valley of California within the southern <br /> portion of the Sacramento Valley The subsurface geology generally consists of non- <br /> marine deposits, which are highly stratified, interbedded, coarse grained sediments and <br /> fine-grained cohesive to lithified sediments that are present in the subsurface to depths of <br /> approximately 2,000 feet bgs (California Department of Water Resources, 1974) Because <br /> of the lenticular nature of the subsurface stratigraphic units, it is difficult to determine the <br /> horizontal and vertical extent of formal and informal stratigraphic units within the <br /> subsurface <br /> Geologic formations and informal stratigraphic units underlying the site include, with <br /> respect to increasing depth the younger alluvium of Holocene age, the older alluvium of <br /> Pleistocene to Holocene age, the Turlock Lake Formation of Pleistocene age, the Laguna <br /> . Formation of Pliocene age, the Mehrten Formation of Pliocene age and the Valley Springs <br /> Formation of Miocene age (Page, 1986) These formations are present to the Lodi area <br /> and are presented as a geologic column in Table 2 and discussed in more detail below <br /> The younger alluvium, Turlock Lake and Laguna Formations are largely dominated by <br /> granitic source material These sediments tend to be brownish in color and are primarily <br /> composed of quartz, feldspar and mica from the igneous and metamorphic source rocks of <br /> the Sierra Nevada Mountain Range The Turlock Lake and Laguna Formations are also <br /> composed of poorly consolidated sands, gravels, silts and clays (California Department of <br /> Water Resources, 1974) The Turlock Lake Formation is primarily identified from <br /> topographic position and soil profile characteristics (Page, 1986) <br /> The late Pliocene Mehrten Formation is a volcanic (andesitic source) formation, which is <br /> characterized by abundant black volcanic sandstones and conglomerates with interbedded <br /> dark-colored mudstones and volcanic mud flow deposits (Page, 1986) The Miocene Valley <br /> Springs Formation, which overlies the deeper continental and marine deposits, is <br /> 19150011005 13 <br />