Clayton is a city in Contra Costa County, California, United States. The population was 10,897 as of the 2010 census.GeographyAccording to the United States Census Bureau, the city has a total area of 3.8sqmi, all land.Clayton is located at the foot of Mt. Diablo State Park.ClimateThis region experiences hot and dry summers. According to the Köppen Climate Classification system, Clayton has a warm-summer Mediterranean climate, abbreviated "Csb" on climate maps.HistoryIn 1857, the town of Clayton was laid out and founded by Joel Henry Clayton (1812–1872) and his two younger brothers. Clayton was born in Bugsworth, now Buxworth, in the United Kingdom, and emigrated to the United States in 1837. After years in other states he settled down with his wife Margaret (1820–1908) at his town at the foot of Mount Diablo, where he and his family prospered. Clayton was named after Joel Henry Clayton, although only by the flip of a coin. Joel Clayton and Charles Rhine cofounded the town, and each wanted to name it after himself. If Charles had won it would have become Rhinesville, but Joel Clayton won. Joel and his wife Margaret both died in Clayton, and were buried in Live Oak Cemetery in what is now Concord, CA.Clayton prospered during the coal mining boom in eastern Contra Costa County. The post office opened in 1861. Following a previous incorporation attempt in 1960, Clayton incorporated in 1964 in order to stave off an annexation attempt in 1963 of the Cardinet Glen neighborhood by nearby Concord. After steady expansion during the 1970s to the east and west from its original boundaries, Clayton's land area more than doubled in 1987 to near its present-day boundaries with the annexations of the Dana Hills/Dana Ridge and Clayton Wood subdivisions, as well as the former Keller Ranch property that was developed during the 1990s with the Oakhurst Country Club.