Quaternary geology of Contra Costa County, and surrounding parts of Alameda, Marin, Sonoma, Solano, Sacramento, and San Joaquin Counties, California: A digital database

Contra Costa County is located at the northern end of the Diablo
Range of Central California. It is bounded on the north by
Carquinez Strait, through which flows 27 percent of California's
surface water runoff. San Francisco Bay forms the western
boundary, the San Joaquin Valley borders it on the east and the
Livermore Valley forms the southern boundary. Contra Costa is one
of the nine Bay Area counties with streams that are tributaries to
San Francisco Bay. Most of the county is mountainous with steep
rugged topography. Mount Diablo, in the center of the county, is
one of the highest peaks in the Bay Area, reaching an elevation of
1173 meters (3,849 ft). Contra Costa County is covered by
twenty-five 7.5' topographic Quadrangles shown on the index map
(ccq_quad or Sheet 2). However, two of the quadrangles (Hayward
and Petaluma Point) contain no Quaternary deposits in Contra Costa
County, and so are not discussed herein.
The Quaternary deposits in Contra Costa County comprise two distinct
depositional environments. One, forming a transgressive sequence
of alluvial fan and fan-delta deposits, is mapped in the western
four-fifths of the county. The second, forming a combination of
eolian dune and river delta deposits, is mapped in the San Joaquin
Valley in the eastern part of the county.