Contra Costa County is one of California's original 27 counties, established in 1850 when the state was founded. It is an amalgamation of various geological terrains, making it one of the most geologically complex regions in the world. In prehistoric times, the area was populated by a wide range of now extinct mammals, as evidenced by fossil remains found in the southern part of the county. In the northern part, coal and sand deposits were formed in earlier geological eras.
Other areas have ridges that expose old but intact seashells embedded in layers of sandstone and limestone. At some road excavations, layers of volcanic ash can be seen ejected from geologically recent but now extinct volcanoes, compacted and tilted by compressive forces. The great local mountain, Monte Diablo, was formed and continues to rise due to plate tectonics. At its peak, it has old rocks from the seabed scraped from distant places of ocean sedimentation and accumulated and raised by these great forces.
Younger deposits at medium altitudes include pillow-type lavas, the product of underwater volcanic eruptions. The first occupation by modern man (Homo sapiens) seems to have occurred between six and ten thousand years ago. However, there is evidence that human presence may have existed much earlier. The well-known settled populations were hunter-gatherer societies that did not know metals and produced high quality, aesthetically attractive utilitarian handicrafts for daily use. Extensive trade from tribe to tribe transferred exotic materials such as obsidian throughout the region from distant California tribes. Most of what is known culturally comes from preserved contemporary and excavated artifacts and from intergenerational knowledge transmitted through peripheral tribes of the north.
Land titles in Contra Costa County can be attributed to multiple subdivisions of a few original land grants. The dealer's last names persist in some city and town names, such as Martínez, Pacheco, and Moraga, as well as in street names, residential subdivisions, and business parks. With the post-war baby boom and the desire to live in the suburbs, large housing developers bought large farms in the center of the county and developed them with roads, utilities, and housing. This area was once mostly walnut orchards and rural cattle ranches but was initially developed as low-cost large-lot suburbs with a typical low-cost home located on a quarter-acre (1,000 m) lot with 10,000 square feet (930 square meters).The establishment of a large population encouraged the development of large shopping centers and created the demand for extensive support infrastructure that included roads, schools, libraries, police, firefighting, water, sewerage, and flood control. Several large companies followed their employees to the suburbs filling large business parks. The county's most renowned natural monument is Mount Diablo at 3,849 feet (1,173 m) at the north end of the Diablo Range.
Diablo State Park (MDSP) was legislatively created in 1921 and rededicated in 1931 after land acquisitions were completed. In 1971 concerned residents formed the nonprofit organization Save Mount Diablo to raise funds and raise awareness to protect this area. The county has a total area of 804 square miles (2,080 km), of which 716 square miles (1,850 km) is land and 88 square miles (230 km) (11%) is water. The Hayward fault zone runs through the western part of the county from Kensington to Richmond while the Calaveras Fault extends in the south-central part from El Alamo to San Ramón. The Concord fault runs through part of Concord and Pacheco while Clayton-Marsh Creek-Greenville fault extends from Clayton at its north end to near Livermore.
These landslide faults and Diablo thrust fault near Danville can cause significantly destructive earthquakes. Contra Costa County is an incredible region with a rich history that dates back centuries. Its geological complexity makes it one of the most fascinating places on Earth with its diverse array of fossils, volcanic ash deposits, ancient seashells embedded in sandstone and limestone ridges, pillow-type lavas from underwater eruptions, and more. It has been home to hunter-gatherer societies for thousands of years who traded exotic materials such as obsidian throughout California tribes. After World War II saw an influx of people moving into suburbs which led to extensive development including roads, schools, libraries, police stations, firefighting services, water systems, sewerage systems and flood control measures. The county's most iconic landmark is Mount Diablo which stands at 3 849 feet tall at its peak with old rocks from seabeds scraped from distant places of ocean sedimentation accumulated by plate tectonics.
Diablo State Park was created in 1921 to protect this area which has been threatened by earthquakes caused by Hayward Fault Zone running through western parts of Contra Costa County as well as Calaveras Fault extending south-central parts from El Alamo to San Ramón.