Other areas of the county have ridges exposing ancient but intact (not fossilized) seashells, embedded in sandstone layers alternating with limestone.

Layers of volcanic ash ejected from geologically recent but now extinct volcanoes, compacted and now tilted by compressive forces, may be seen at the site of some road excavations.

The Bay Miwok, Yokut and Ohlone people lived in the area now known as Contra Costa County before the arrival of Spanish colonists.

[10] The Volvon triblet of the Miwok lived on Tuyshtak (Mount Diablo near present day Walnut Creek) until they were forcibly moved from their land into the Spanish missions in the early 1800s.

John Marsh, owner of Rancho Los Meganos in Contra Costa County, sent letters to influential people in the eastern United States extolling the climate, soil, and potential for agriculture in California, with the purpose of encouraging Americans to immigrate to California and lead to its becoming part of the United States.

By 1850, California's population of over 100,000 was rapidly growing due to the gold rush and the large amount of gold being exported east, which gave California enough clout to choose its own boundaries, write its own constitution, and be admitted to the Union as a free state in 1850 without going through territorial status as required for most other states.

[17] During World War II, Richmond hosted one of the two Bay Area sites of Kaiser Shipyards and wartime pilots were trained at what is now Concord/Buchanan Field Airport.

Port Chicago was bought out and demolished by the Federal Government to form a safety zone near the Naval Weapons Station loading docks.

Some of the expansion of these suburban areas was clearly attributable to white flight from decaying areas of Alameda County and the consolidated city-county of San Francisco, but much was due to the postwar baby boom of the era creating demand for three- and four-bedroom houses with large yards that were unaffordable or unavailable in the established bayside cities.

That year, concerned residents formed the non-profit organization Save Mount Diablo to raise funds and awareness to protect more open space.

In addition to encouraging acquisition by the state and local authorities, SMD started fundraising and acquiring properties to transfer to the park.

It temporarily leaned toward the Republican Party in the 1970s and 1980s, with successive presidential wins by Richard Nixon in 1972, Gerald Ford in 1976, and Ronald Reagan in 1980 and 1984.

While no longer used for extensive irrigation, it is still possible for adjoining landowners (now large suburban lot owners) to obtain pumping permits.

Owing to its extensive waterfront on San Francisco, San Pablo, and Suisun bays the northwestern and northern segments have long been sites for heavy industry, including a number of still active oil refineries (particularly Chevron in Richmond, Phillips 66 in Rodeo, Shell Oil (now PBF) and Tesoro (now Marathon)- in Martinez), chemical plants (Dow Chemical) and a once substantial integrated steel plant, United States Steel, now reduced to secondary production of strip sheet and wire.

The major central county cities along Interstate 680 are Martinez, Concord, Pleasant Hill, Walnut Creek, Danville, San Ramon, and unincorporated Alamo.

Owing to the high quality of its public schools (due largely to both demographics and added support from prosperous parents), this area has become a magnet for well–off families with children.

Lower cost modern tract developments continue along Suisun Bay in the "East County" towns of Pittsburg, Antioch, and Oakley - new "bedroom" communities" to serve the now "edge cities".

The median income of a family in the two relatively affluent East County towns of Brentwood and Discovery Bay is approaching $100k/yr.

California Distinguished Schools, golf courses, vineyards, and upscale homes are found in Brentwood and Discovery Bay.

Discovery Bay is based on a waterfront community of 3,500+ homes with private docks with access to the Sacramento-San Joaquin River Delta.

Additionally, a county road from Brentwood to Tracy has received Federal Funds to be upgraded to a highway link between the areas.

[citation needed] By the early 1990s, 22 million square feet of office space had been built[55] along the 680 corridor, that segment of Interstate Highway 680 that extends from Concord in the north to San Ramon in the south, continuing into inland Alameda County from Dublin to Pleasanton.

[citation needed] The inland portions of the Concord Naval Weapons Station have been declared surplus by the Federal government and this area is expected to provide what is likely the last opportunity to plan and build city-sized development within the central county.

Much of the land to be developed is relatively flat grassland and the most prominent structures are ammunition bunkers that will be removed, so future uses of the property are largely unconstrained by previous uses.

In December 2019, there was a flurry of reports from reliable sources including the Associated Press and the San Francisco Chronicle that the 161-year-old Martinez News-Gazette, one of the longest-running newspapers in California, may have to cease publication.

The newspaper did cease publication of a print edition effective April 2, 2020, but this was characterized as a temporary measure arising from a lack of advertising revenue.

Since March 2020, this in turn arose as many local businesses were forced to suspended operations or even ceased to exist, when the area was under shelter-in-place regulations arising from COVID-19 pandemic.

In 1937, the two-bore Caldecott Tunnel for road vehicles was completed, making interior Contra Costa County much more accessible.

The line leaves Richmond through industrial and residential parts of West County before striking due east through Franklin Canyon and Martinez on its way to Stockton, Bakersfield, and Barstow.

Ohmer was a rail station located on the Oakland, Antioch and Eastern Railroad 6 miles (9.7 km) east of Martinez, which still appeared on maps as of 1947.