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development of layers of unconsolidated sand, silty sands, silts and clays <br />throughout most of the Delta. <br />The geologic subsidence rate during the later part of Holocene has <br />been in equilibrium with the sedimentation rate causing prolonged periods <br />of marsh development. As a result, a thick layer of peat was developed in <br />most of the San Joaquin Delta. The thickness of peat varies from zero at <br />the outer rim of the Delta to as much as 60 feet near the center. <br />Construction of man-made levees or raising of natural levees at the turn of <br />the century to develop islands and tracts has modified the natural <br />sedimentary processes causing a net subsidence rate of 2 to 3 inches per <br />year as a result of oxidation of the peat and wind erosion and <br />consolidation of peat and other sediments. At present most of the area is <br />below mean sea level from the western parts of Stockton to Indian Slough <br />near the Bixler area, and as far south as 5 miles north of Tracy. <br />2.3 Local Soil Conditions <br />Previous investigations in the Lower San Joaquin Delta Regiona (1,2)* <br />have indicated a generally consistent vertical sequence of subsurface <br />soils. In general the subsurface soils are divided into five separate <br />"horizons" on the basis of similarities in composition and depositional <br />environment. A brief description of each horizon is presented below. The <br />occurrence and thicknesses of the first three soil horizons may be found in <br />Plate 2 of Appendix B as drawn by A.L. Franks, consultant for Arcady Oil <br />Company. <br />Horizon 1 comprises the levee fill. Composed primarily of dredged <br />material from the adjacent sloughs, this horizon consists of variably mixed <br />* The numbers in parentheses refer to the list of References presented at <br />the end of this report. <br />2-2