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The Geology of Great Sand Dunes National Park and Preserve
Families visiting the Great Sand Dunes National Park and Preserve will find one of the most complex geological systems in the world.

Kids can learn all about it by earning their Junior Ranger badge or going on a ranger-led hike.




Photo: Kids play in Colorado's biggest sandbox at the Great Sand Dunes National Park.




Did You Know?

The tallest sand dunes in North America are in Colorado, at Great Sand Dunes
National Park.
The dune field  covers more than 30 square miles. The dunes themselves stretch over 700 feet high in places.


Photo: The Great Sand Dunes in Colorado lie in North America's rift valley, the Rio Grande Rift. This fissure in the earth stretches from Mexico as far north into Colorado as Leadville.

A Good Rift Valley

Colossal forces causing the earth’s crust to heave and buckle also make it stretch in some places. In one of those spots, all that crunching and stretching created a crack in the earth’s crust that has slowly widened into a rift. On either side of the rift, deep faults split the ground right down to the mantle, and the bit of land that happens to be between the faults dropped, unevenly, as much as 9,000 feet. And it hasn’t stopped. Even today, it is sliding downward along the faults. It so happens that the Great Sand Dunes National Park sits right on that bit of ground, known here as the San Luis Valley.

From the edge of the Sangre de Cristo Mountains, right behind the Sand Dunes, to the foothills of the San Juan Range across the valley, the flat surface of the San Luis Valley sprawls about 45 miles. Its elevation maintains a fairly consistent 7,500 feet above sea level and is deceiving. Below the surface lie layers and layers of lava, volcanic ash, clay, sand, and gravel, sometimes over two miles deep. These sedimentary layers have filled in the uneven spots in the rift, and in places the bedrock lies over 5,000 feet below sea level, and 13,000 feet below the current valley floor.

Geologists guess the deep faults lying on the western edge of the valley provided a way for molten rocks from the earth’s interior to push their way up. The volcanic San Juan Mountains lie right over the faults. The peaks are made in part from lava rocks containing lots of magnesium and iron, just like the earth’s mantle. When the ice age came, glaciers and their robust rivers ground away at the old San Juan volcanoes, washing their material out into the rift of the San Luis Valley. 

In this part of the world, the wind almost always blows from the southwest. And the San Luis Valley, lying in the rain shadow of the San Juans, is incredibly dry – a true desert. This allows the prevailing westerlies to pick up the volcanic material, now ground into sand, and blow it across the valley. Since the surface is nearly flat, there isn’t much to slow down the wind until it reaches the Sangre de Cristo Mountains. There, the air is pushed heavenward by the steep mountain flanks, and it is swirled around by a bend in the line of peaks. As the wind rises, it slows and drops its load of sand at the foot of the range.

Frequently in the late winter and spring, the wind switches direction and blows from the north through Music, Medano, and Mosca Passes, which pushes the sand back onto itself in big piles. Snowmelt streams pick up sand in the Sangre de Cristos and carry it to the dunes as well.

Only about one-tenth of the sand at the foot of the mountains is visible in the dune field, itself. Most of it lies beneath scrubby bushes and grasses on the surrounding sand sheet, and cemented by minerals brought to the surface in the springs of the sabkha. Together these two less obvious sandy regions cover nearly 300 square miles of ground.







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