Contra Costa County

Native Americans have lived in the San Francisco Bay area for over 10,000 years. Rather than a single nation, they lived in many independent tribes and spoke different languages. The various tribes are generally referred to as one group called the Ohlone or Costanoan. In 1769, Spain expanded their territory north of Mexico and established military garrisons (Presidios), Franciscan missions (Missions), and civil settlements (Pueblos). They reached the San Francisco Bay area in 1776. In 1821, Mexico was declared an independent state. In 1846, the United States took possession of Northern California during the Mexican-American war and established a military territory. California was admitted to the United States on September 9, 1850.

Contra Costa County was one of the original counties when California was admitted to the United States in 1850. A southern portion of the original county (all of the bayside area) was given up to form part of Alameda County in 1853. The county seat is Martinez.

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