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Mosca, Colorado USA
Geologic Timeline (animation):
 During
the Paleozoic Era, over 500 million years ago, shallow seas
and river systems deposited sedimentary materials throughout
the region. The Sangre de Cristo mountain range east of the
dunes is composed of Paleozoic rocks as well as metamorphic
rocks from the much older Precambrian period.
Eons later, pressures from within the Earth
pushed the landscape high above sea level, limiting sedimentary
deposition and leaving a gap in the region's Mesozoic record.
Around 40 million years ago, an arid landscape
dominated where rivers and intermittent lakes emerged and
subsided upon the land. Evidence of this period are found
in the silts, clays, sands, and gravels of the early-Tertiary
Blanco Basin Formation.
Recurring lava flows poured onto the landscape
about 34 million years ago from volcanic vents in the San
Juan mountains, creating a subsurface layer that geologists
call the Conejos Formation.
After the San Juan Mountain lava flows subsided,
volcanic activity continued and for four million years ash
flows blanketed the earlier lavas of the Conejos Formation.
These ash-flow tuffs are visible on the west side of the Sand
Luis Valley.
Rapid uplift of the Sangre de Cristo Mountains
began about 19 million years ago. Rifting associated with
the uplift widened the valley and caused its floor to drop
significantly. (This process continues today.) Water-born
and eolian sediments were deposited in the subsiding San Luis
Valley, creating the Santa Fe Formation.
Around five million years ago, a new series
of volcanic activity rocked the region, at times damming rivers
that flowed out of the southern half of the San Luis basin.
The Alamosa Formation formed during this time, made of lake,
river, marsh sediments and wind-deposited layers of clay,
sands and silts. Locally known as the "blue clay layer,"
the Alamosa Formation is a confining layer between aquifers
found under the valley floor.
The Pleistocene began 1.8 million years ago
as climate changed globally. Glaciers grew in mountain valleys,
some pouring ice and rock far into the San Luis Valley. Many
scientists consider the Pleistocene to be the period in which
dune formation began in the San Luis Valley.
Cooler temperatures also caused the Earth's
oceans to contract, creating, land bridges that permitted
a worldwide diaspora of animals between North America and
Asia.
Only about 12,000 years ago, a warming climate
melted many glaciers worldwide and signaled the end of the
Pleistocene. Large quantities of silt, gravel and sand were
carried by rivers and streams into the San Luis Valley. Although
the dunes existed long before the Pleistocene ended, glacial
outwash materials provided new sources of sand for winds to
rework into even greater sand dunes.
Today, the rivers and creeks continue to transport
sediment into playa lake systems which are sources of sediment
for dune-forming winds of the San Luis
Valley.
Geologic Cross Section (animation):
The Rio Grande Rift, one of the most
notable geologic forces in the region, extends northward from
Mexico into central Colorado. For almost 20 million years,
this gaping crack in the Earth's crust has widened and the
San Luis Valley floor has slowly dropped while "hinged"
on the west to the San Juan Mountains. The sinking San Luis
Valley has created a catch basin for river gravels, eolian
sands, volcanic ash-flow tuffs and lake deposits and clays.
About 15,000 feet of sedimentary fill is now nestled below
the Great Sand Dunes, all of which has been deposited within
the past 35 million years.
Wind Regime: Thirty-four miles away
by car in Alamosa, prevailing winds are from the southwest
throughout the year?as illustrated by arrowhead in the wind
rose diagram. This wind pattern continues to the southwest
corner of the dunefield.
Over the years, sand and small
dunes migrate in this southwestern,
unimodal wind pattern toward the main dunefield. Close to
the dunefield, however, wind patterns change. It is not uncommon
for winds to emerge from any of the three mountain passes?Music,
Medano and Mosca?any time of the year. The dunefield's bimodal
and complex wind regime contribute to two distinctive dune
types: reversing and star dunes.
If you scrutinize the relief map above, take
notice of the Sand Creek Star Dune complex in the northwest
corner of the dunefield. Star dunes are oriented in many directions,
which indicate the occurrence of a complex wind regime. And
notice that a bimodal wind regime is apparent in the southern
region of the dunefield by the existence of the north-south
trending Medano Creek dune ridges.
Dune Types: reversing (animation), star, parabolic, barchan,
transverse
Sand Recycling Grand Sand Recycling: The
main dune field grows to enormous heights not only because
of a complex wind regime, but also due to sand carried by
Medano and Sand Creeks. These streams seasonally carve out
and carry sand from the east and north side of the dunefield
and re-deposit it where wind can transport it back to the
dune field.
Saltation: Individual sand grains move
in three ways: saltation, creeping, and suspension. Sand grains
bounce or "saltate" slightly above the surface when
blown by the wind. With each bounce, the grains usually reach
heights of less than 5 cm above the surface, and move horizontally
5-10 cm. The average distance a grain bounces is the width
between the crests of two ripples in the sand. Saltation accounts
for about 95% of a sand grain's movement. A sand grain "creeps"
when it collides with other grains, causing it to roll or
make small jumps. Creep may account for about 4% of a grain's
movement. Finally, a sand grain moves by "suspension"
when it is blown high into the air. Suspension accounts for
1% or less of an individual sand grain's movement.
How Much Sand is There? More
Sand than Meets the Eye!
How much sand is in the sand dunes? The simple answer is "a
lot," approximately 4.8 billion cubic meters, but that
is really just the tip of the iceberg. The dunefield is impressive
with its tall dunes and scenic backdrop, but it is the smallest
of three sand deposits in the Great Sand Dunes system, measuring
just 30 mi2 (78 km2). In the valley surrounding the dunefield
is the sand sheet, covering 180 mi2 (466 km2). Close examination
shows how the sand sheet got its name; it is almost 100 percent
sand and, despite some small hills and depressions, is primarily
flat. Farther west in the valley, near the San Luis Lakes,
lies the third sand deposit, the sabkha. It extends over 120
mi2 (311 km2) and is characterized by sand grains cemented
together by minerals.
 Why
are dunes only found on 30 of 330 mi2 (855 km2) of sand deposits?
Dune formation requires several conditions. First, there must
be sand! Wind strong enough to move the sand (>13 mph or
21 kmph) must also be present. Both sand and wind are plentiful,
so the key factor is that sand must be loose and moveable.
The sand in the sabkha is hardened by minerals, so the wind
cannot shape it into dunes. On the sand sheet, vegetation
anchors sand, preventing dune formation. If you look across
the sand sheet, however, you can see tan-colored areas where
the vegetation has lost its foothold and dunes are forming.
Sand in the dunefield is completely exposed to the wind and,
as a result, dune formation is very active.
The sand sheet and sabkha are subtle features,
but you can discover them as areas undeveloped for human use.
Coming from the west on Lane Six, the farmlands end where
the sabkha starts. Travelers using Highway 150 will notice
a lack of houses west of the highway until they near Highway
160; that is the sand sheet. Since the land in both areas
is sand, it is not suitable for farming or home sites.
These varied sand deposits played an important
part in the legislation passed in late 2000. The condition
of the two larger deposits greatly influences whether new
sand comes into the dunefield; the expansion is intended to
help protect the natural processes of sand replacement occurring
in those areas. by Andrew Valdez
[This website is created to show the beauty
and wonder of a great american natural treasure. The contents
are courtesy of the National Park Service. It is created in
English and in Japanese. All the photo images shown here are
taken by Kiyomichi Koike. The copy rights are reserved.]
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