The county was formed on March 25, 1853, from a large portion of Contra Costa County and a smaller portion of Santa Clara County. Much of what is now considered an intensively urban region, with major cities, was developed as a trolley car suburb of San Francisco in the late 19th and early 20th centuries. The historical progression from Native American tribal lands to Spanish, then Mexican ranches, then to farms, ranches, and orchards, then multiple city centers and suburbs, is shared with the adjacent and closely associated Contra Costa County. This detailed narrative gives an in-depth view of the county's history.