The Great Sand Dunes lie in the San Luis Valley Basin between the Sangre De Cristo Mountains and the San Juan Mountains. Glacial meltwater formed Lake Alamosa, and sediments from mountain streams filled the lake bottom. As the climate changed, Lake Alamosa dried up, leaving behind a great sand sheet. Winds from the southwest then blew the sand into large dunes piled against the Sangre De Cristo Mountains. The dunes continue to change shape as the prevailing winds vary in direction.

1. Great Sand Dunes National Monument
Geographic Formation
By: Michelle Suarez
http://files.myopera.com/cofoppyplop/albums/371184/Great%20Sand
%20Dunes%20National%20Monument.jpg

2. http://3.bp.blogspot.com/-
IOBq8OevqDg/UTPP8vxkKsI/AAAAAAAAC7c/FT6hYpREKjc/s1600/dunes+col+plat+3.j
pg
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.

3. http://www.nps.gov/grsa/naturescience/images/lake_alamosa285.j
pg
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.

4. http://3.bp.blogspot.com/-
x6vf2CPk5ks/UU9mXriIZuI/AAAAAAAADCs/hVAwauiTGnA/s1600/dunes
+map+with+camp+ellipse+and+arrows.jpg
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.

5. http://www.environmentalatlas.ae/images/geographi
cInheritance/static/3.4.3.jpg
http://earthsci.org/education/teacher/basicgeol/windes/tr
ansverse.gif
http://www.geo.msu.edu/geogmich/images/parabolic.jpeg
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.

6. Bibliography
Madole, 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