Contra Costa County is located at the northern end of the DiabloRange of Central California. It is bounded on the north byCarquinez Strait, through which flows 27 percent of California'ssurface water runoff. San Francisco Bay forms the westernboundary, the San Joaquin Valley borders it on the east and theLivermore Valley forms the southern boundary. Contra Costa is oneof the nine Bay Area counties with streams that are tributaries toSan Francisco Bay. Most of the county is mountainous with steeprugged topography. Mount Diablo, in the center of the county, isone of the highest peaks in the Bay Area, reaching an elevation of1173 meters (3,849 ft). Contra Costa County is covered bytwenty-five 7.5' topographic Quadrangles shown on the index map(ccq_quad or Sheet 2). However, two of the quadrangles (Haywardand Petaluma Point) contain no Quaternary deposits in Contra CostaCounty, and so are not discussed herein.The Quaternary deposits in Contra Costa County comprise two distinctdepositional environments. One, forming a transgressive sequenceof alluvial fan and fan-delta deposits, is mapped in the westernfour-fifths of the county. The second, forming a combination ofeolian dune and river delta deposits, is mapped in the San JoaquinValley in the eastern part of the county.