In
southwestern Colorado sits a broad valley between the Sangre de Cristo and San
Juan Mountain Ranges. One hundred
million years ago, Colorado was located closer to the equator in the super
continent of Pangaea. The area that is
now the San Luis Valley lay at the edge of an inland sea that divided North
America into two sections. The Great
Basin is part of this depression. Sea creatures left their shells and bodies to
become limestone and deposits of sand and mud became part of the sea bottom.
These deposits were
disturbed during the Pennsylvanian period by what was to become known as the Ancestral
Rocky Mountains. There were two large uplifts that emerged from this inland
sea. The uplift created two large mountainous
islands, known to geologists as Frontrangia and Uncompahgria. The first, Frontrangia became the general
area of the Colorado Front Range and the second at the San Juan Mountains. They eroded and left behind their sands in the
basin of the former inland sea.
After a period of calm another mountain building period began known as the
Laramide Orogeny. This was the period that built the present day Rocky
Mountains.
During this period the San Luis Valley was a highland.
Over time the highlands eroded their
materials into the basin left by the, now drained inland sea.
Rifts began to form in the area and the
highlands dropped leaving the other side of the fault to form the Sangre de
Cristo Mountains.
The process began about
18 million years ago and continues in the area today.
There is a 16,000 ft displacement between the mountains and the valley. This
creates some spectacular views of both the mountains from the valley and the
other way around.
The main rift in the valley is the filled
with the Rio Grande River.
This rift is
still pulling apart.
Because of this
movement of tectonic plates the San Juan Mountains, the youngest in Colorado,
were formed from the folding on the outside edge of the rift. As pressure from
the folding occurred molten lava was spewed adding to the San Juan Range.
The unique structure of the valley with the
tall Sangre de Christos on the east and the San Juans to the west and the basin
of sediment and sand create the unique environment that produced the Great Sand
Dunes.
The Great Sand Dunes are the tallest sand dunes in
North America rising 750 feet from the valley floor. The dunes began forming
less than 440,000 years ago. The prevailing westerly winds racing down the San
Juans picked up the sediments from the basin of the San Luis Valley and blew
them into the barrier of the Sangre de Cristo Mountains. The winds slowed and the sand deposited began
to stack up against the base of the mountains.
The park is an important part of the San Luis Valley ecosystem which
helps maintain the underground aquifer that feeds water to the valley
agricultural interests. In this area of the valley there is no water above
ground do hold the sand down. The water
which is important to the area is all underground here in an aquifer made of
the sand and limestone that keeps it in the underground lake. The constant wind from the west changes the
sand dunes on a daily basis.
Streams that flow around the perimeter of the dunes go underground in this
area. The sand from these streams is also blown into the dunes. Digging a
couple inches into the dunes even at their peaks reveals wet sand. Part of the
motivation of turning the Monument into a National Park was the extra
protection of the water, which Colorado's cities and agriculture covet.
A very interesting interactive visiual on how these dunes were formed can be
found at
http://interactive-earth.com/resources/science-visualizations/10-geology-of-the-san-luis-valley.html
Walking the sand dunes is a very interesting experience.
Some people ski down them.
It is best to do so after a snow or
rain.
Also you find that when walking
the dunes by sing.
It is an eerie sound,
a kind of moan as you walk along.
Though
I could not find an complete explanation why the sands moan or sing as you walk
through them, it is thought that the grains are roughly spherical and as you disturb
them they rub together creating a sound.
This usually happens only with quartz sands and it doesn’t happen when
the sand is wet because water causes the particles stick together.
Some scientist learned that the reason that different dunes make different
sounds and volumes as they sing is due to the size of the sand that is moved.
The San Luis Valley and the Sand Dunes National Park are one of the most
mystical places I have visited.
The view
of the valley from the town of Crestone at the base of 14,000 ft Crestone Peak
in the Sangre de Christos is awe inspiring.
The view Blanca Peak peering over the top of the red/yellow sand dunes
is magic. It is well worth the trip.
Just be warned, if you drive through the town of Blanca please watch
your speed coming into the town from the west, you can’t see the sign until you
are already on it and there is a “friendly” officer there to make sure you slow
down!
