page_title: Great Sand Dunes National Park and Preserve
section_title: Geology
hierarchical_section_title: Great Sand Dunes National Park and Preserve / Geology
caption_attribution_description: English: Summit of Star Dune, the tallest dune in Great Sand Dunes National Park, Colorado, United States.
context_page_description: Great Sand Dunes National Park and Preserve is an American national park that conserves an area of large sand dunes up to 750 feet tall on the eastern edge of the San Luis Valley, and an adjacent national preserve in the Sangre de Cristo Range, in south-central Colorado, United States. The park was originally designated Great Sand Dunes National Monument on March 17, 1932, by President Herbert Hoover. The original boundaries protected an area of 35,528 acres. A boundary change and redesignation as a national park and preserve was authorized on November 22, 2000, and then established by an act of Congress on September 24, 2004. The park encompasses 107,342 acres while the preserve protects an additional 41,686 acres for a total of 149,028 acres. The recreational visitor total was 527,546 in 2019.
  The park contains the tallest sand dunes in North America. The dunes cover an area of about 30 sq mi and are estimated to contain over 1.2 cubic miles of sand. Sediments from the surrounding mountains filled the valley over geologic time periods.
context_section_description: The park contains the tallest sand dunes in North America, rising to a maximum height of 750 feet (229 m) from the floor of the San Luis Valley on the western base of the Sangre de Cristo Range. The dunes cover an area of about 30 sq mi (78 km²) and are estimated to contain over 1.2 cubic miles (5 billion cubic metres) of sand.
  Creation of the San Luis Valley began when the Sangre de Cristo Range was uplifted in the rotation of a large tectonic plate. The San Juan Mountains to the west of the valley were created through extended and dramatic volcanic activity. The San Luis Valley encompasses the area between the two mountain ranges and is roughly the size of the state of Connecticut. Sediments from both mountain ranges filled the deep chasm of the valley, along with huge amounts of water from melting glaciers and rain. The presence of larger rocks along Medano Creek at the base of the dunes, elsewhere on the valley floor, and in buried deposits indicates that some of the sediment has been washed down in torrential flash floods.
  In 2002, geologists discovered lakebed deposits on hills in the southern part of the valley, confirming theories of a huge lake that once covered much of the San Luis Valley floor. The body of water was named Lake Alamosa after the largest town in the valley. Lake Alamosa suddenly receded after its extreme water pressure broke through volcanic deposits in the southern end of the valley. The water then drained through the Rio Grande, likely forming the steep Rio Grande Gorge near Taos, New Mexico. Smaller lakes still covered the valley floor, including two broad lakes in the northeastern side of the valley. Large amounts of sediment from the volcanic San Juan Mountains continued to wash down into these lakes, along with some sand from the Sangre de Cristo Range. Dramatic natural climate change later significantly reduced these lakes, leaving behind the sand sheet. Remnants of these lakes still exist in the form of sabkha wetlands.
  Sand that was left behind after the lakes receded blew with the predominant southwest winds toward a low curve in the Sangre de Cristo Range. The wind funnels toward three mountain passes—Mosca, Medano, and Music Passes—and the sand accumulates in this natural pocket. The winds blow from the valley floor toward the mountains, but during storms the winds blow back toward the valley. These opposing wind directions cause the dunes to grow vertically. Two mountain streams—Medano and Sand Creeks—also capture sand from the mountain side of the dunefield and carry it around the dunes and back to the valley floor. The creeks then disappear into the sand sheet, and the sand blows back into the dunefield. Barchan and transverse dunes form near these creeks. The combination of opposing winds, a huge supply of sand from the valley floor, and the sand recycling action of the creeks, are all part of the reason that these are the tallest dunes in North America.
  Sufficient vegetation has grown on the valley floor that there is little sand blowing into the main dunefield from the valley; however, small parabolic dunes continue to originate in the sand sheet and migrate across grasslands, joining the main dunefield. Some of these migrating dunes become covered by grasses and shrubs and stop migrating. The dunes system is fairly stable as the opposing wind directions balance each other out over time. Also, the main dunefield is moist beneath a thin layer of dry surface sand. While the top few inches of sand are blown around during windstorms, the moist sand remains largely in place.
  Scientists estimate that Lake Alamosa disappeared about 440,000 years ago, but the dunes themselves apparently originate from sand deposits from later, smaller lakes. A relatively new dating process, optically stimulated luminescence (OSL), is still in development. This method takes core samples of sand from deep within a dune, and attempts to measure how long quartz grains have been buried in the dark. If the deepest sand deposits can be accurately dated, the age of the dunes could be determined. Samples of san
caption: Summit of Star Dune, the tallest dune in the park, and the Sangre de Cristo Range
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