Contra Costa County’s physical geography is dominated by the bayside alluvial plain, the Oakland Hills–Berkeley Hills, several inland valleys, and Mount Diablo, an isolated 3,849-foot (1,173 m) upthrust peak at the north end of the Diablo Range of hills. The summit of Mount Diablo is the origin of the Mount Diablo Meridian and Base Line, on which the surveys of much of California and western Nevada are based.
The most notable natural landmark in the county is 3,849 feet (1,173 m) Mount Diablo, at the northerly end of the Diablo Range. Mount Diablo and its neighboring North Peak are the centerpiece of Mt. Diablo State Park (MDSP), created legislatively in 1921 and rededicated in 1931 after land acquisitions had been completed. At the time this comprised a very small portion of the mountain.
Contra Costa County Cities include: Alamo, Antioch, Blackhawk, Brentwood, Clayton, Concord, Danville, Diablo, Discovery Bay, El Cerrito, Hercules, Lafayette, Martinez, Moraga, Oakley, Orinda, Pinole, Pittsburgh, Pleasant Hill, Richmond, San Pablo, San Ramon and Walnut Creek.