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Great Sand Dunes National Park & Preserve
Frequently Asked Questions About Sand Dunes Geology

Why are the dunes in this particular spot? Since dunefields
develop downwind of their sand supply, we need to know what is supplying the sand
into this dune system to answer this question. In the past, the sand supplier was
thought to be the Rio Grande, but if that were the case we would expect to find
dunes along much of its course, not just in one spot. A better explanation is that
the sand supply comes from a lake that occasionally fills the northern part of the
San Luis Valley. This lake varies in size depending on climatic conditions and can
range from being totally dry to being about 16 miles long when completely full.
Currently, it is about 1 mile long and is known as San Luis Lake. Streams flow into
this lake bringing water and sand. As the lake dries, the sand is exposed to the
wind and gets blown toward the northeast. This lake is directly downwind from the
dunes and the wind-blown sand starts in the area covered by the lake. The entire
valley floor, from the dunefield to the lake area, is covered by sand. Since the
lake covers a limited area, the sand blown off it is only found upwind of it. That
is why there are sand dunes only in this section of the mountain front.

Why are the dunes so tall? Sand dune shape and behavior are
the result of the wind patterns that create the dune. In areas where the wind blows
mainly from one direction, dunes tend to migrate and are small. In areas like this
where the wind blows abundantly in two or more directions, dunes tend to oscillate
and get taller over time.

Why do the dunes stop at the base of the mountains? Many
people think that the mountains are the barriers that stop the dunes, but the real
key is the northeasterly winds and, to a lesser extent, Medano Creek. The northeast
winds counter the southwest wind’s ability to push the dunes toward the mountains.
As we learned in the previous question, dunes in areas with multiple wind directions
tend to stop migrating and grow vertically. The position of the large dunes in the
dunefield is relatively stable. Along the eastern edges of the dunefield, water
flowing down Medano Creek truncates the dunefield and keeps it separated from the
mountain front.

Are these dunes unique? Yes, in the sense that every dunefield
is unique. However, there are similar dunefields. Look closely at the dunes and
notice that there are big sand ridges with smaller sand ridges developed on them.
Every dune is connected to the dune next to it, there is no flat space between the
dunes.

The Great Sand Dunes are actually one giant dune with lots of smaller dunes superimposed
on it. This is a feature called a draa (giant sand dune).
There are also draas in California (Imperial Dunes, Eureka
Dunes, Kelso Dunes, and Dumont Dunes) and Nevada (Sand Mountain). They have big
dunes with similar dune configurations as Great Sand Dunes. They do look a bit like
the dunes here. The difference is that they are in very dry areas and the mountains
surrounding them are barren. At Great Sand Dunes, the mountains are lush with vegetation
and that creates a sense of contrasting environments, as if the dunes shouldn’t
be here. But we can also find other dunes surrounded by abundant vegetation in Oregon
(Oregon Dunes), Idaho (St. Anthony’s Dunes), Michigan (Sleeping Bear Dunes) and
Maine (the Desert of Maine). There are even dunes on Mars! There is a crater with
dunes along the edge that, from space, looks like Great Sand Dunes.

Where are the biggest sand dunes in the world? I did a search
for other large dunes and this is what I found. In China’s Badain Jaran Desert the
dunes are reported to reach 1640 feet. The tallest dunes in the Sahara Desert are
around 1500 feet, and in the Namibian Desert they grow to around 1200 feet.

These answers reflect our current scientific thinking about some of the questions
asked about this place. When you stop in to visit, ask at the Visitor Center for
our free information sheet on geology for answers to other common questions.

Courtesy Andrew Valdez, National Park Service geologist at Great Sand
Dunes.

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