Earlham College-Geology 211-Great Sand Dunes

Physical Geology 2002

by Ari Halberstadt
www.magiccookie.com/photography/photos/great-sand-dunes/199708-0934-lg.jpg

Related Links

www.usgs.gov United States Geological Survey. Good for all sorts of geology and earth info.

www.npca.org National Parks Conservation Association. Good stuff about keeping our wildlife wild and such. Find the perfect national park to visit.

www.americansouthwest.net/colorado/great_sand_dunes/
national_monument.html a travel-oriented site with some geologic information and pictures

www.sangres.com/sanddunes.htm "For your daily dose of the mountains" Again, more touristy than academic, but some info, a map, and pictures.

by CristoMedia
www.sangres.com/sanddunes.htm

Literature Cited

Andrews, S. "Geometry and Dynamics of Great Sand Dunes, San Luis Valley, Colorado." Abstracts with Programs. Geological Society of America (GSA), 1978

Burford, Arthur E. "Petrology of the Great Sand Dunes, Colorado." Proceedings of the West Virginia Academy of Science 1961. Morgantown: West Virginia University Bulletin, 1961

Johnson, Ross B. "The Great Sand Dunes of Southern Colorado." Mountain Geologist 1968: 23-29

Magee, A.W. & A. Valdez. "Seasonal Eolian Transport Directions and Volumes at Great Sand Dunes Nat'l Monument." Abstracts with Programs. GSA, 1992

Merk, George. "Dune Form and Structure at Great Sand Dunes Nat'l Monument." Abstracts with Programs. GSA, 1983

Tarbuck, Edward J. & Frederick K. Lutgens. Earth: An Introduction to Physical Geology. Upper Saddle River: Prentice Hall, 2002

Walker, A.S. Deserts: Geology and Resources. United States Geological Survey. 20 Mar. 2002 <http://pubs.usgs.gov/gip/deserts/contents/>

Walker, Alta S. Geomorphology From Space: A Global Overview of Regional Landforms. Ed. Nicholas M. Short, Sr. and Robert W. Blair, Jr. NASA: 1986. 20 Mar. 2002 <http://daac.gsfc.nasa.gov/DAAC_DOCS/
geomorphology/GEO_HOME_PAGE.html>

*Title Picture courtesy National Park Service brochure via Jeffrey Stehr www.meto.umd.edu/~stehr/outdoors.html

Great Sand Dunes National Monument

The Great Sand Dunes in Southern Colorado are the tallest sand dunes in North America, rising up to 750 feet, and cover approximately 40 square miles. They are located on the edge of the San Luis Valley, which lies between two mountain ranges: the Sangre de Cristos to the north and east, and the San Juans to the west. Medano Creek flows past the edge of the dunes from a canyon in the Sangre de Cristo mountains, and is a natural recycler of sand for the Dunes. The sand which created these dunes started accumulating only about 2 million years ago, making them relatively young. The dunes are constantly changing, due to wind, water, human activity and other elements, and migrating slowly, but so far they are not disappearing.

Geologic Processes

The sand which formed the dunes most likely came predominantly from the volcanic rocks of the San Juan mountains. Quartz is the most common mineral, but Plagioclase Feldspar, volcanic material, Hypersthene, and various Hornblende are very common as well. There are several explanations as to how material from the mountains was transported to it's current location on one side of the valley. One of the most widely accepted explanations (according to geologist Arthur E. Burford, "Petrology of the Great Sand Dunes, Colorado", in 1961) is that, after the formation of the valley and the surrounding mountains, a lava flow dammed the Rio Grande, creating a lake. Material from both mountain ranges, though mostly from the San Juans, collected in the lake. When the dam eventually broke, and the lake drained, the gathered material was left behind. The prevailing winds were then able to slowly carry the material Northeast across the valley, where it eventually accumulated at the foot of the Sangre de Cristos- because of the height of the mountains, most (though not all) of the sediment carried by the wind is dropped before the wind current goes up and over the mountains.

Products of the Geologic Process

There are three types of dunes at the monument: parabolic, longitudinal, and transverse. The transverse dunes are a long series of ridges and troughs that form at right angles to the wind. They are the tallest, and therefore tend to be the most stunning, of the three types. Because of the mountain barrier, the high transverse dunes form downwind, closest to the mountains. They are not nearly so migratory as most dunes, mostly due to changing wind directions. In the fall and winter, the winds start coming from the northeast, rather than the southwest, reversing the direction of the dune crest (the top). There is usually no vegetation in the area where these dunes accumulate; if any, it is found in the most moist places.
Longitudinal dunes form in places which have more vegetation, and are very long and low-laying, usually only barely above the valley floor. In this case, longitudinal dunes tend to form in the area around the path of the river which is southeast of the larger transverse and parabolic dunes.
Parabolic dunes are named for their shape: they are U shaped, with the bowl of the U in the direction the wind is moving. They tend to have some vegetation, and vary in height. One type, called parabolic dunes of deflation, are the most abundant. They are low, composed of loose sand, and are called dunes of deflation because the sand is being removed (mostly blown away). These dunes are on the southwest side, which means the sand is moving towards the higher dunes and being recycled. The other type is parabolic dunes of accumulation. These occur in between the dunes of deflation and the transverse dunes, and are as expected from the name, accumulating the sand blown over from the dunes of deflation. Parabolic dunes of accumulation can be anywhere from around the height of the longitudinal dunes to 100 to 200 feet high.

Much of the sand in the dunes is being recycled by natural processes. In between the high transverse dunes and the Sangre de Cristo mountains is Medano Creek. Medano Creek comes down from the mountains and is fed by melted snow or rain water, flows south and then west, and is only about 10 miles long. A large portion of the sand which falls down the dune faces is picked up by the water and carried back downstream, towards the area where the low dunes of deflation accumulate. It is re-deposited along the way, and will then be transported again by the wind to the northeast and into the dune field.

the little specks are people!

by Kenneth R. Neuhauser
www.fhsu.edu/geo/neuhauser/knimgex3.hmt

Author: Ariel Boone-Worthman
Creation/revision date: 25 February 2002/22 March 2002

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