Bandelier National Monument is a 33,677-acre United States National Monument near Los Alamos in Sandoval and Los Alamos counties, New Mexico. The monument preserves the homes and territory of the Ancestral Puebloans of a later era in the Southwest. Most of the pueblo structures date to two eras, dating between 1150 and 1600 AD. The monument is 50 square miles of the Pajarito Plateau, on the slopes of the Jemez volcanic field in the Jemez Mountains.