GREAT SAND DUNES NATIONAL PARK & PRESERVE
FIELD TRIP STOP – GREAT SAND DUNES NATIONAL PARK (LARGEST SAND DUNES IN NORTH AMERICA)
LOCATION: San Luis Valley in Southern Colorado. Between the western base of the Sangre de Cristo Range and the San Juan Mountain Range to the west.
From Alamosa, CO take State Hwy 17 north to Mosca. Go east on County Lane 6 N and then State Hwy 150 to the park. Alternately, go east on US Hwy 160 and then North on State Hwy 150 to the park. Distance from Alamosa is 37 miles.
GEOLOGIC FEATURES: Sand Dunes. Dune migration.
DESCRIPTION: Dunes are the tallest in North America, reaching over 750 ft. and covering an area of about 30 square miles. Dunes are thought to have originated sometime within the past 400,000 years. The exact timing is speculative.
Sands were originally eroded from surrounding mountain ranges to form layers of sandy lake sediments within the San Luis Valley. After the lakes dried, sand was blown by southwest winds toward a bend in the Sangre de Cristo Mountains to the east, forming dunes. Creeks flowing from the east carry sand from the mountain side of the dune field back to the valley floor where it is blown back into the dune structures. Seasonal opposing storm winds from the northeast continue to squeeze the dunes and cause them to grow taller and taller while maintaining their same geographic position rather than migrating laterally over time. These opposing winds are also responsible for some asymmetry reversals and other changes of dune shape throughout the year.
STUDENT QUESTIONS:
(1) Describe the normal method of dune migration including the movement of sand along the windward and leeward dune faces.
(2) How are sand dunes recognized in the fossil record?
(3) Why have the dunes of Great Sand Dunes National Park remained essentially in place over the years despite the normal tendency of dunes to migrate?
(4) How might a geologist determine the source of sediments found within the dunes?
SELECTED REFERENCES:
-Great Sand Dunes National Park. Accessed on Dec. 22, 2022: https://greatsanddunesnationalpark.us
-National Park Service: Great Sand Dunes National Park and Preserve – Geology. Accessed on Dec. 22, 2022: https://www.nps.gov/grsa/learn/nature/sanddunes.htm
-Harris, D.V. and E.P. Kiver. 1985 (4th ed.). The Geologic Story of the National Parks and Monuments, John Wiley and Sons, New York, 464 pp.
PHOTOS:
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Figure 1 - Map of Great Sand Dunes National Park showing directions of Prevailing Winds (Large White Arrows), Storm Winds (Small White Arrows), and Sediment-earing Streams (Small Blue Arrows).
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Figure 2 - Two photos taken 147 years apart showing three dunes that have remained nearly stationary due to the action of opposing winds that cancel the common lateral movement of dunes.
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Figure 3 -- Dunes rise about 750 feet above the valley floor. The dark area seen in the foreground close to the base of the dunes is Medano Creek. This Creek helps to move sediment to the west where it will be “recycled” by prevailing westerly winds.
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Figure 4 - The dunes meet the Sangre de Cristo Mountains to the east of the dunefield. Medano Creek is in the foreground.
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Figure 5 - Close-up of the waters of Medano Creek moving sediment to the west (top of photo)
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Figure 6 - Tourists can slide down the dune faces using “surfboards.”
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Figure 7 - Photo taken inside the dune field.