Dunes in the Winter
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A Landscape in Motion
Introduction to Great Sand Dunes Geology
To the historic Utes, these dunes were sowapophe-uvehe: "the
land that moves back and forth". For the Apaches, the word is sei-nanyedi:
"it goes up and down." To explorer Zebulon Pike in 1830, gazing
down at the dunes from the mountains, they appeared as a turbulent "...sea
in a storm, except as to color..."
At Great Sand Dunes, geology comes alive. Every day, sand grains are
moving—bouncing, avalanching, flying through the air, or flowing
in a stream. Every day, a little more sand erodes from the surrounding
mountains and is carried in water or blown by the wind toward the dunes.
And every day, scientists move closer to unraveling the many geologic
mysteries of the dunes—how they have been formed, how old they are,
and how they change over time.
A Unique Setting
The San Luis Valley is a vast, high-altitude desert plain surrounded
by two alpine mountain ranges. Mostly volcanic in origin, the San Juan
Mountains cover much of southwest Colorado. Bordering the valley on the
east is the narrow, jagged spine of the Sangre de Cristo Mountains, formed
by a sharp uplift stretching from central Colorado to northern New Mexico.
Both ranges contain extensive forest and tundra, including peaks that
rise over 14,000' in elevation.
The center of the valley is a true desert, receiving only 7" of
precipitation annually, while the surrounding mountains receive 40"-70"
in an average year, depending on elevation and location. In the Sangre
de Cristos, you can hike from desert to alpine tundra in a few steep miles.
Mountain runoff transforms parts of the valley floor into verdant wetlands,
especially in the northern half of the valley where surface waters flow
into a closed basin, never naturally reaching either ocean. This large
land basin, fed with water and sediments from the surrounding mountains,
may be the primary source of sand in the dunes.
Why are there Dunes Here?
The first time one sees these giant desert dunes piled up against the
snowcapped Sangre de Cristo mountains—especially when Medano Creek
is flowing in rhythmic waves around the dunes—the experience can
be inspiring and a bit unsettling. The combination doesn't seem to make
sense. Yet it is that very combination of high mountains, desert, and
water that makes the creation of the tallest dunes in North America possible.
The Great Sand Dunes have been formed over thousands of years, as sand
deposited by mountain streams and playa lakes on the San Luis Valley floor
is carried in the form of small dunes by predominant southwest winds toward
a low curve in the Sangre de Cristo Mountains. As the sand reaches the
mountains, it also encounters reverse storm winds from the northeast that
have a canceling effect on any gradual migration of the main dunes into
the mountains. Further barriers to migration of the main dune mass come
from Medano and Sand Creeks as they flow out of the mountains. In spring
and summer, these streams shave sand from the eastern and northern perimeters
of the dunefield, carrying it back toward the valley floor. The streams
then disappear into the sand on the western edge of the high dunes, allowing
the sand to be blown back and recycled into the dunefield.
Scientists originally thought that most of the sand in the dunes was
carried into the valley by the Rio Grande, flowing out of the San Juan
Mountains. Today new research suggests that a more likely source is the
closed basin, a stream and playa lake system north and west of the dunes.
Sand and sediments from the northern San Juans and Sangre de Cristos have
accumulated in the basin over time. As the lakes have dried, the sand
has become subject to the wind.
The Sand System
The Great Sand Dunes geological system consists of four basic parts:
Sabhka - This is a fragile plain where sand is cemented together
by minerals left behind from seasonal wetlands.
Sand Sheet - More than 90% of the sand here is found in this vast
desert of sandy grasslands and low dunes. Sand continues
to blow into the main dune mass from this area in the form of small migrating
dunes.
Main Dune Mass - This is the big sandpile: over 30 square miles
of dunes, reaching up to 750' in height, are cradled against the mountains.
They are pure sand from top to bottom.
Watershed - Mountain streams that begin here on alpine peaks are
crucial in recycling migrating sand back into the dunes, contributing
to the their impressive height.
How Old?
A new and complex process is beginning to provide dates for dune deposits,
based on how long sand grains have been in complete darkness deep under
the surface. Known as the optically stimulated luminescence process or
OSL, this process has recently provided estimated ages for some parts
of the dunefield. Samples were taken from a number of locations, including
200 feet deep into the tallest dune known as the Star Dune, and 200 feet
below the surface of Medano Creek. The Star Dune sample dated at approximately
750 years old—seemingly young, but tree ring samples tell us there
was a severe and prolonged drought in the Southwest at that time, which
could account for massive amounts of sand blowing into the dunefield.
The second sample from deep below the creekbed dated at roughly 18,000
years old. This was toward the end of the last Ice Age, when large amounts
of sand and sediment were washing into the valley, and began blowing in
the wind as the waters retreated. This may have been an early period of
major dune formation at Great Sand Dunes.
Sand Types
As you observe the main dune mass, you will notice three distinct types
of sand. Darker, fine-grained sand makes up the vast majority of the dunes;
lighter, coarse-grained sand is seen in lower pockets; and, depending
on wind speed and direction, black patches of sand are usually seen near
ridges.
These three basic sand types reflect the origins of Great Sand Dunes
sand. The fine-grained sand is primarily volcanic, traced to the San Juan
Mountains across the valley floor. It has traveled over 50 miles to reach
the dunes, first on water, then on wind. During their journey, these sand
grains have been bounced and tumbled until finely polished. In contrast,
the coarse grains in the dunes were washed down from the surrounding Sangre
de Cristo Range. Lighter colored metamorphic and sedimentary rocks make
up the majority of the Sangre de Cristos, although there are also some
volcanic intrusions and deposits.
Looking closely at a handful of San Juan or Sangre de Cristo sand, you
will also observe a variety of colors—a summary of the many minerals
found in each mountain range.
Black magnetite, a heavy mineral with its own magnetic charge, is often
left in large patches of black grains on the dunes after winds blow lighter
sand away. A compass will spin in confusion when placed next to a large
patch of magnetite, and a magnet will pick up fuzzy clumps of this strange
mineral.
How Much Do the Dunes Change?
During strong windstorms, sand streams off the dunes in large plumes.
Hikers brave enough to be in the dunes at those times are literally getting
sandblasted. Many visitors expect that each time they visit the park,
the landscape of the dunes will be totally different.
However, when a photograph of the dunes from 1874 is compared to one
taken at the same location from 1999, the close similarity is surprising.
While the smaller dune forms have changed, the massive high dunes are
in virtually the same places. The primary reason for this stability is
that winds are from opposing directions over the majority of the main
dune mass, canceling any significant movement. A giant dune may move as
much as 30 feet toward the mountains in one year, but the next year more
prevalent winds off the mountains may drive it back to the original location.
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