Summary Analysis Paper

1. Russell 1
Thomas Russell
GEO 111-500
26 July 2012
Geomorphology: The Great Sand Dunes National Park
Declared a national monument by the National Parks and Preserve Act of 2000, the Great
Sand Dunes are the largest dunes in North America, rising a staggering 750 feet; the dunes are
located at the base of an alcove in the Sangre de Cristo Mountain Range in Colorado. For
geographical clarity, it is important to note that both the San Juan and Sangre de Cristo Mountain
Ranges are part of the larger Rocky Mountains and are separated by the San Luis Valley.
Building the Great Sand Dunes
While it is commonly accepted that the Great Sand Dunes are a young landform, a
precise age has not been determined. “U.S. Geological Survey geologists estimate that the dunes
are between a couple of thousand years and 12,000 years old….” 1 What is lucidly understood,
however, is the process and structure by which the sand dunes are created and maintained.
Building the Great Sand Dunes National Park requires four predominant factors—
plentiful loose sediment deposits, strong winds, a permanent sand trap structure, and a method by
which to control the expansion or migration of the dunes. Loose sediment in the form of sand,
silt, and gravel exists in abundance across the floor of the San Luis Valley; “[t]he sand is blown
by prevailing southwesterly wind from alluvial sediments of the San Luis Valley. Sand
mineralogy indicates that much of the sediment was derived from volcanic rocks of the San Juan
Mountains, transported by the Rio Grande, and deposited in the river's huge alluvial fan on the
western side of the valley.”2 Sand carried by the wind is directed by the Sangre de Cristo
Mountains north and funneled into a naturally occurring sand trap—an alcove in the mountain
1
National Park Service. “Geology Fieldnotes: Great Sand Dunes National Park and Reserve.”
http://nature.nps.gov/geology/parks/grsa/. (Accessed July 18, 2012).
2
James Arber. “San Luis Valley, Colorado.” Emporia State University.
http://academic.emporia.edu/aberjame/field/rocky_mt/zapata.htm#dune. (Accessed July 18, 2012).

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range whose concave side opens westward. Anchored to the mountains, the deposited
sedimentary material is bounded on all other sides by geologic forces; more specifically,
prevailing winds blow from the west and the south while Sand Creek boarders the north and
Medano Creek boarders the south. Collectively, these provide the methods by which expansion
of the dunes can be moderated; evidently, it would prove impossible for the dunes to expand east
or west, and the creeks to the north and south progressively erode the dunes as they attempt to
migrate across. Crucial to the region‟s process of sedimentrecycling, the Sand and Medano
creeks return sediment to the plains of the San Luis Valley, where the process is free to
repeat.The aggregate result is a relatively stagnant dunefield composed of fine-grained sediments
and whose individual dunes change shape daily with the winds.
Geologic Structure of the Great Sand Dunes National Park
While it may appear counter-intuitive at face value, the sand dunes have a structure more
complex than simply “piles of sand.” Beginning with the watershed to the east—Sangre de
Cristo mountains—this is the primary source of water for the sand dunes; water originates as
rainfall and snowmelt which then flows down the west side of the mountains towards the sand
dunes. Infiltration, “[t]he movement of surface water into porous soil,”3occurs where water from
the watershed meets the dunefield and sinks underground. The dunefield pertains to an area
covering about forty square miles and containing the largest dunes. “For a place formed of sand,
the Great Sand Dunes National Park is surprisingly wet. You can actually reach wet sand just by
digging a few inches in the dunes, even on their tops.”4 Water underneath the dunefield
continues flowing westward, gradually approaching the surface.
3
“Geological Terms Beginning with „I‟.” http://geology.com/dictionary/glossary-i.shtml. (Accessed July 18,
2012).
4
“The Great Sand Dunes National Park.” http://denvercolorado.org/history-places-of-interest/the-great-sand-dunes-
national-park/. (Accessed July 18, 2012).

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Outside the dunefield is a wide perimeter of sandy grasslands which actually contains the
majority of the sand attributed to the Great Sand Dunes National Park, though it contains
relatively few and miniscule dunes. Known as the sand sheet, it contains the majority of
sediment because of its vegetation; “[w]ith just 10 percent plant cover, movement across the
sand sheet drop by 50 percent. With 50 percent coverage by vegetation, movement ceases
altogether.”5 Continuing outward from the dunefield and passed the sand sheet, the final
structural component of the park is the Sabkha. “When the water evaporates away in late
summer, minerals similar to baking soda cement sand grains together into a hard, white crust.
Areas of sabkha can be found throughout western portions of the sand sheet, wherever the water
table meets the surface.”6
Weathering and Erosion of the Great Sand Dunes
Having established a firm background regarding the structure and processes underlying
the sand dunes, consider a closer analysis of weathering and erosion related to the sand dunes.
Beginning with mechanical weathering processes in the San Juan Mountains to the west, large
rocks can be cut down by frost wedging; this process occurs when liquid water enters small holes
and cracks in the rock and is later frozen by cold temperatures. Since water expands in volume
nearly ten percent when frozen, it widens the openings in the rock and will eventually cause a
fracture. Additionally, biological weathering in the form of lichens may weaken and break rocks
as the organisms extract minerals.
Fluvial forces such as rainfall or snowmelt will often displace smaller rock fragments into
a stream or river. This same result can also be accomplished through a mass wasting process
5
Stephen Trimble. Great Sand Dunes National Monument: The Shape of the Wind. (Tucson: Western National
Parks Association, 2000), 6.
6
National Park Service. “Great Sand Dunes System.” http://www.nps.gov/grsa/naturescience/sand_system.htm.
(Accessed July 18, 2012).

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such as a mudslide, landslide, or rockfall. Once part of a stream, it is likely that the sediment
will be flushed into the Rio Grande River, which bisects the San Luis Valley and contains a
broad shoreline upon which fine sediments can be deposited and retrieved by the wind.
Destroying the Great Sand Dunes
Despite the awesome beauty and seeming immobility of some billions of tons of sand, the
Great Sand Dunes are ultimately doomed. Responsible for their fate millions of years in the
future is the Rio Grande Rift. “Over the last 25 million years, a gigantic rift has been opening up
at the southern end of the rocky mountains. It stretches over 160,000 square miles and is known
as the Rio Grande Valley.”7 The rift continues spreading north while widening from the south as
the surrounding mountains erode into the valley. Rifting is caused by a sinking and thinning of
the earth‟s crust by “…lava from a source deep in the mantle [that] periodically spreads across
the surface. In the near geologic future, several million years or so, a youthful ocean basin may
occupy this area.”8
As the Rocky Mountains—and its Sangre de Cristo Mountains—erode and collapse into
the rift, shifting wind patterns will displace and ultimately destroy the Great Sand Dunes. In the
distant geologic future, the same sediments that compose the Great Sand Dunes today may be
relocated to the bottom of the ocean only to be later subducted into the mantle.
Produced by a rare conglomeration of geologic features, the San Luis Valley in Colorado
boasts what is, without doubt, one of the most striking landforms on the continent—The Great
Sand Dunes—and will continue to so until the whole of the Rocky Mountains erode into the Rio
Grande Rift.
7
“How the Earth was Made: The Rocky Mountains.” http://www.history.com/shows/how-the-earth-was-
made/episodes#slide-9. (Accessed July 18, 2012).
8
“A Tapestry of Time and Terrain: The Rio Grande Rift.” http://tapestry.usgs.gov/features/28riogrande.html.
(Accessed July 18, 2012).

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Bibliography
Arber, James. “San Luis Valley, Colorado.” Emporia State University.
http://academic.emporia.edu/aberjame/field/rocky_mt/zapata.htm#dune.
“Geological Terms Beginning with „I‟.” http://geology.com/dictionary/glossary-i.shtml.
“The Great Sand Dunes National Park.” http://denvercolorado.org/history-places-of-interest/the-
great-sand-dunes-national-park/.
“How the Earth was Made: The Rocky Mountains.” http://www.history.com/shows/how-the-
earth-was-made/episodes#slide-9.
National Park Service. “Geology Fieldnotes: Great Sand Dunes National Park and Reserve”
http://nature.nps.gov/geology/parks/grsa/.
National Park Service. “Great Sand Dunes System.”
http://www.nps.gov/grsa/naturescience/sand_system.htm.
“A Tapestry of Time and Terrain: The Rio Grande Rift.”
http://tapestry.usgs.gov/features/28riogrande.html.
Trimble, Stephen. Great Sand Dunes National Monument: The Shape of the Wind. Tucson:
Western National Parks Association, 2000.