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' rolling hills toward the valley. ' The alluvial fans and plains unit comprises most of the valley. Virtually every drainage channel that enters the San Joaquin Valley from the Coast Range has produced or contributed to the piedmont slopes of coalescing alluvial fans. ' The Corral Hollow Sanitary Landfill lies within the low alluvial plains and fans unit. This unit is comprised of a series of coalescing alluvial fans that have built out from the ' Coast Range toward the valley.Terrace deposits that cap the hills to the northwest of the site represent the erosional remnant of broad terraces that existed along the Coast Range mountain front. The alluvial fans and terraces lap onto bedrock exposed in the Coast ' Range to the west. Active deposition of alluvial fans and terraces has ceased in the past several thousand years and the study area is currently undergoing a period of erosion ' and downcutting (Hotchkiss, 1971). ' Two general types of deposits, consolidated and unconsolidated, outcrop in the Coast Range and underlies the study area. The consolidated deposits are composed of pre- Tertiary and Tertiary sedimentary and crystalline rocks. The sedimentary deposits are mostly marine deposits while the basement complex consists of granitic rocks and ultramafic intrusive rocks. Unconsolidated deposits consist of the Tulare Formation of ' Tertiary and Quaternary age and terrace deposits, alluvium, and flood-basin deposits of Quaternary age. The Tulare Formation is composed of beds,lenses, and tongues of clay,silt, sand, and gravel. The sedimentary material is derived from the Franciscan Formation and Cretaceous and Tertiary sedimentary rocks of the adjacent Coast Range. One ubiquitous clay bed, the Corcoran Clay Member,underlies most of the study area. The Corcoran Clay is a lacustrine deposit which consists of sandy silts and clays. The ' unit pinches out against the foothills of the Coast Range and thickens rapidly eastward toward the center of the valley. The Corcoran Clay generally lies in the upper half of ' the Tulare Formation and acts as a confining bed(Hotchkiss, 1971). The coarser water- bearing deposits above and below the clay have been termed the upper section and the ' lower section (Hotchkiss, 1971). STL 9390314 6 Rev. 0 06/30/88 tEmcon Associates