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TABLE III-6 The rocks in the study area range in age from Precambrian to Holocene. The oldest rocks are the Precambrian gneisses, schists, and quartzites that form the core of the Front Range west of the study area. Beneath the study area, the Precambrian rocks are at a depth of about 12,000 feet. Stratigraphically above these rocks are sedimentary bedrock formations ranging in age from Pennsylvanian to Late Cretaceous, and surficial deposits that rest unconformable on the eroded surface of the folded and faulted bedrock formations. COMPARISON OF NEDERLAND AND VALMONT SERIES SOILS [After Moreland and Moreland, (SCS, 1975)] -cc . .. Property Soil Serles . . Nederland Vlllmont Parent material Old high terraces and alluvial fans. Old high terraces and benches in gravelly and cobb ley, loamy alluvium. Drainage Deep, well-drained soils. Deep, well-drained soils. Surface layer Four inches of brown, very cobbley, Four inches of grayish-brown, light sandy loam. clay loam containing varying amounts of cobbles and gravel. Subsoil Upper Brown and reddish-brown, heavy, Brown clay loam, 3 inches thick. coarse, sandy loam and very Middle cobbley, sandy, clay loam, 16 Brown light clay, 13 inches inches. Underlying these materials thick. Lower to a depth of 60 inches or more is Calcareous, light-brown, gravelly, reddish-brown, very cobbley, coarse sandy loam. clay loam, 4 inches thick. Underlying material Calcareous, pinkish-white and light- brown, very gravelly loam The oldest surficial deposit in the study area is the pre-Rocky Flats Alluvium, which caps some hills and terrace remnants along the western edge of the area. Next oldest is the Rocky Flats Alluvium, which dominates the hydrology of the entire study area. The Rocky Flats Alluvium is a broad, alluvial fan deposit that slopes gently east away from the mouth of Coal Creek Canyon. Contact springs commonly issue from the base of the Rocky Flats Alluvium. The next oldest formations are the Verdos Alluvium and Slocum Alluvium. GROUNDWATER The groundwater in the area of the Rocky Flats Plant originates primarily from three sources: infiltration of stream and irrigation ditch flow, infiltration of irrigation water, and infiltration from precipitation. Coal Creek enters the Rocky Flats Alluvium to the west of the Rocky Flats Plant site and flows northeast in contact with the Rocky Flats Alluvium for a distance of 3 to 4 miles. The elevation of Coal Creek west of the Plant is approximately 200 feet higher than the Rocky Flats Plant site. Several ditches and canals traverse the Rocky Flats Alluvium on the western side of the plant site and flow to the east toward the major site drainages. Seepage from such ditches contributes recharge to the Rocky Flats Alluvium. The surface irrigation and land application which take place on the plant site also contribute to the recharge. The amount of recharge is dependent on the intensity of the irrigation and land application. A third source of recharge is from precipitation. The recharge is directly related to storm intensity and duration, and occurs during and immediately following these events (USGS, 1976).