Gypsum deposits washed down from the San Andres mountain range are the main source of the dunes in White Sands National Monument. White gypsum dunes engulf 275 square miles of New Mexico's desert forming the largest gypsum dune field on earth. White Sands National Monument preserves a major portion of these unique dunes. The San Andres mountain range lies outside the national monument to the west. It extends about 75 miles (120 km) north to south, but is only about 12 miles (19 km) wide from east to west at the widest point.