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RIfE KLEINFELDER formations were deposited by the activity of the major rivers in this area (Calaveras, Stanislaus, Mokelumne and San Joaquin Rivers)which drained from the Sierra Nevada. The most recent formation in the area of Foothill Landfill is the Mehrten Formation of Miocene to Late Pliocene age. A regional geologic map is shown on Plate 1-4. The Mehrten Formation consists of sandstone and siltstone with claystone and conglomerates from andesitic source material. This sediment was deposited by southwest trending streams carrying andesitic detritus from the central and northern Sierra Nevada. The Mehrten Formation is dominated by sandy andesitic sediments of dark grey to black color. These beds commonly exhibit cross-bedding, fine laminations and foreset bedding. Within the sands, lenticular beds of stream gravel may occur. Layers of brown to blue silt and clay also occur interbedded with the sands. Structure The Mehrten Formation on the eastern side of the Great Central Valley generally dips on the order of one or two degrees toward the southwestll. It also thickens westward from the Sierra Nevadas. Faults And Seismicity Numerous active faults have been mapped in both the Coast Ranges (30 to 50 miles west) and the Sierra Foothills (10 to 20 miles east)4. The major faults which have historically produced earthquakes of greatest magnitude in Central California are the Calaveras, Hayward and San Andreas faults in the Coast Ranges; the Midland and Green Valley faults east of the Sierra crest. Local faults include numerous small shear zones and two major fault zones, the Bear Mountains and Melones, which make up the Foothills fault system. In the past, the Foothills fault system has been considered inactive, with no known creep or displacement of geologic features since Late Tertiary time. On August 1, 1975, an earthquake of magnitude 5.7 occurred approximately 6 miles southeast of Oroville. As a result of this event and subsequent aftershocks, a revaluation of the potential activity of the Foothills fault system took place. It has been proposed that a 6.5 magnitude earthquake may occur almost anywhere in the Foothills fault system4. FOOTHILL 4