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' Boyett Petroleum June 29, 2001 419 South Main Street,Manteca, California Page 7 Final Report of Findings for Lateral Extent of Groundwater Contamination the State allowable level. The laboratory analytical data and chain of custody can be found in Appendix C and a summary in Table 2 of this report. 6.0 Geology The subject property is located on the western side of California's Central Valley. The Central Valley is approximately 400 miles long and averages 50 miles wide encompassing ' approximately 20,000 square miles. The valley is a large asymmetric trough that is bounded by granitic, metamorphic and marine sedimentary rocks of pre-tertiary age. This trough has been filled with as much as 30,000 feet of sediment in the San Joaquin Valley portion to the south, and as much as 60,000 feet of sediment in the Sacramento Valley portion to the north. The age of the sediments range from Jurassic to Holocene and include both marine and continental rocks and sedimentary deposits(Olmstead and Davis, 1961). Manteca lies in the Central Valley Physiographic Province of California. The Valley is approximately 50 miles wide. It is bounded to the west by the foothills of the Coast lieRanges, and to the east by the foothills of the Sierra Nevada Range. The Manteca area is nearly devoid of structural geologic features. ' The Valley is floored by unconsolidated Quaternary sediments to depths of at least 400 or more feet in the Manteca area. All of these units can be considered soils in the engineering sense, because they are unconsolidated. Quaternary sediments in the Central ' Valley were deposited as a series of coalescing alluvial fans. The fans originated where valleys of the ' major streams which drained the Sierra Nevada Range emptied into the broad expanses of the valley. The coarser sediments which comprise the fans are mainly arkosic in composition and were derived from erosion associated with glacial stages in the ' mountains. The finer grained sediments are predominantly composed of rock flour washed out of the former extensive glaciers in the Sierras, (Arkley, 1964). ' The axis of the Central Valley Trough was a marshy, wet area throughout much of the Pleistocene Epoch. Lacustrine, flood plain and marshy depositional environments predominated along the axial portions of the trough, in the toe areas of the fans, throughout ' much of this time period. Coarser grained sediments of the upper portions of the fans occasionally prograded over the axial area of the trough in response to major climatic .• nmi.:_ t___ _ __ti_.] �...].......:........... .-.F r-- rr..n;—A nil+n n"A rInvrc ' changes in diu Siciias. 1i11S 11[lS leJUlLGIt lAi GL �71 G\tVllllSlcuaVV vi uxi�. bauui..0 vuw c.aau va.,.).. in the subsurface of the area. 1 1