Merced County was formed in 1855 from Mariposa County. The county derived its name from the Merced River of El Rio de Nuestra Senora de la Merced (River of Our Lady of Mercy); named in 1806 by an expedition, headed by Gabriel Moraga, which came upon it at the end of a hot dusty ride. Territory which at one time was in Merced County is now in Fresno, Madera, San Benito and Stanislaus counties.

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