National monument, south-central Colorado, U.S. At the eastern edge of the San Luis Valley, it parallels for about 10 mi (16 km) the western base of the Sangre de Cristo Range .
Established in 1932, the 60-sq-mi (155-sq-km) region contains some of the highest inland sand dunes in the U.S., with changing crests that rise to 700 ft (215 m).