Great Sand Dunes National Monument Geographic Formation

By: Michelle Suarez

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Internal Earth Processes• The Great Sand Dunes lie in the San

Luis Valley Basin.• The Valley lies between the Sangre De

Cristo Mountains and the San Juan Mountains.

• The Valley Basin is part of the Rio Grande Rift.

• The San Juan Mountains were created by volcanic activity. This same volcanic activity was part of the continental spreading that created the rift. This area is a divergent boundary.

• The Sangre de Cristo Mountains formed from the rotation of a continental plate as a result of the rifting.

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Glacial and Fluvial Processes• Pleistocene glaciation of the Sangre de

Cristo’s and the San Juan’s left moraines that closed of the south end of the valley.

• Glacial meltwater and drainage from the Rio Grande formed mountain streams.

• Runoff from mountain streams flowed into the basin and formed a lake. This lake is known as Lake Alamosa.

• Sediments from the stream runoff filled the bottom of this lake.

• Lake Alamosa dried up as the climate of the area changed from wetland to highland desert, leaving behind a great sand sheet from the sediments that accumulated at the bottom of the lake.

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Aeolian Processes• The sand left behind from the dried up lake

was blown into a piedmont of the Sangre de Cristo Mountains by deflation.

• The winds blow predominantly from the southwest, building the dunes up at the piedmont zone.

• Wind in the valley change direction often and have built the dunes up to their current heights.

• The creeks around the dunes flow through the dunes seasonally. Even though the area of the dunes is considered highland desert, there is plenty of water in the surrounding environment.

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Star dunes can be found at the northeast corner of the dune field. Star dunes are pyramid shaped dunes that occur with frequently changing wind directions. These dunes occur at the Great Sand Dunes where the wind enters through the Sangre De Cristo Range in opposition to the strong southwesterly winds. The largest dune measures 750 feet high and is called The Star.

The majority of the tall dunes on the dune field are transverse dunes. The constant changing of wind direction results in many different dune formations on the dune field. Reversing dunes are also prominent here. The dunes continuously change in formation and size because of the active wind.

There are parabolic dunes southwest of the dune field in the sand sheet. Here, close to the edge of the Medano Creek, there is more vegetation which helps to hold the arms of the dune as the wind blows into the dunes. Much of the sand here is transported by deflation to feed the main dune field and the parabolic dunes are fed by sand from the creek.

BibliographyMadole, Richard F., Joe H. Romig, John N. Aleinikoff, D. Paco Vansistine, Ezra Y. Yacob. “On

the Origin and Age of the Great Sand Dunes, Colorado,” Science Direct Geomorphology

99(2008): 115-117

James P. McCalpin. “General Geology of the Northern San Luis Valley, Colorado.” Geo-Haz.

Consulting. n.d. accessed March 3, 2013.

“Great Sand Dunes.” National Park Service. accessed March 3,

2013. http://www.nps.gov/grsa/naturescience/sanddunes.htm

“Great Sand Dunes National Park and Preserve.” Wikipedia. last modified March 21,

2013. http://en.wikipedia.org/wiki/Great_Sand_Dunes_National_Park_and_Preserve

Johnson, Ross B. “Great Sand Dunes of Southern Colorado.” U.S. Geologial Survey Denver,

Colorado. n.d. accessed March 3, 2013.

“Geology Fieldnotes: Great Sand Dunes National Park and Reserve.” National Park Services.

n.d. http://www.nature.nps.gov/geology/parks/grsa/

Hess, Darrel. Mcknight’s Physical Geography. Upper Saddle River: Pearson, 2011