Contra Costa County is a county in the state of California in the United States. As of the 2010 census, the population was 1,049,025, the name is Spanish for opposite coast, referring to its position on the other side of the bay from San Francisco. Contra Costa County is included in the San Francisco–Oakland–Hayward, CA Metropolitan Statistical Area and it occupies the northern portion of the East Bay region and is primarily suburban. In the northern part of the county, significant coal and sand deposits were formed in even earlier geologic eras, other areas of the county have ridges exposing ancient but intact 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. This county is an agglomeration of several distinct geologic terranes, as is most of the greater San Francisco Bay Area, younger deposits at middle altitudes include pillow lavas, the product of undersea volcanic eruptions. There is an extensive but little recorded human history pre-European settlement in this area, the earliest definitively established occupation by modern man appears to have occurred six to ten thousand years ago. However, there may have been human presence far earlier, at least as far as non–settling populations are concerned, extensive trading from tribe to tribe transferred exotic materials such as obsidian throughout the region from far distant Californian tribes. Unlike the nomadic Native American of the Great Plains it appears that these tribes did not incorporate warfare into their culture but were generally cooperative. Within these cultures the concept of individual or collective land ownership was nonexistent, early European settlers in the region, however, did not record much about the culture of the natives. Most of what is known comes from preserved contemporaneous and excavated artifacts. Although there were no missions established within this county, Spanish influence here was direct and extensive, in 1821 Mexico gained independence from Spain. Mission lands extended throughout the Bay Area, including portions of Contra Costa County, between 1836 and 1846, during the era when California was a province of independent Mexico, the following 15 land grants were made in Contra Costa County. Rough surveying was based on a map, or diseño, measured by streams, shorelines, and/or horseman who marked it with rope, lands outside rancho grants were designated el sobrante, as in surplus or excess, and considered common lands. The law required the construction of a house within a year, fences were not required and were forbidden where they might interfere with roads or trails. Locally a large family required roughly 2000 head of cattle and two leagues of land to live comfortably. Foreign entrepreneurs came to the area to provide goods that Mexico couldn’t, Rancho Canada de los Vaqueros was granted to Francisco Alviso, Antonio Higuera, and Manuel Miranda. Two ranchos, both called Rancho San Ramon, were granted by the Mexican government in the San Ramon Valley, in 1833, Bartolome Pacheco and Mariano Castro shared the two square league Rancho San Ramon
The west face of Mount Diablo, the most notable natural landmark in Contra Costa County
Postcard showing the Contra Costa County Courthouse in 1906.