The first known inhabitants of the Walnut Creek area were Bolbones Indians. In March 1772, the first Spanish explorers arrived to the region. California became a possession of Mexico following the Mexican Revolution in 1821. To encourage settlement in its newly-acquired territory, Mexico made large land grants, four of which were in the Walnut Creek area. A grant of nearly 18,000 acres in what today is the Ygnacio Valley was made to Dona Juana Sanchez de Pacheco, whose husband, Miguel, had been a war hero for Mexico. She deeded her property to two grandsons, Ygnacio and Ysidro Sibrian. Ygnacio Sibrian, the namesake of the Ygnacio Valley, built the first roofed residence in the valley around 1850. Following the Mexican-American War, California became a United States territory and subsequent