Basic GEOLOGY
Story
Understanding how and why the Great Sand Dunes exist is an
ongoing puzzle to Earth scientists. Geologic research underway
today continues to shape our understanding of this remarkable
pile of sand!
How old are the dunes? While the exact age of the dunes is
unknown, geologists think they probably began to pile up perhaps
12,000 years ago or longer, as glaciers from the last Ice Age
began to melt. The meltwater and runoff from current rivers and
streams carried rocks, sand, and silt into the San Luis Valley.
The valley floor is very flat so the streamflow looses energy and
the sand and other materials that were being carried are
dreposited and over time have accumulated to great depths. When
the waters had either disappeared or changed course, the wind—
still a powerful factor in dunes formation--swept across the
valley from the southwest to the northeast, and carried the sand
toward a natural barrier, the Sangre de Cristo Mountains. These
mountains, and the three low passed found here, change the wind
patterns, and as a result of that, dune behavior. On the valley
floor the primary wind direction is from the southwest and dunes
that form there migrate steadily toward the Sangre de Cristo
Mountains. Near the mountain front, it’s common for winds to
blow in many directions as they approach and come from over this
barrier. The net result is that sand is now blown toward each
dune from every direction and the dune slows greatly and the dune
begins to grow vertically. These multiple winds are the primary
reason for the impressive height of Great Sand Dunes.
Where does the sand come from? Both the San Juan mountains and
the Sangre de Cristo mountains provide sand sources. Most of the
sand comes from the volcanic San Juans; this fine-grained sand is
composed of volcanic rock fragments and clear quartz. Sangre de
Cristo sand comprises many rock and mineral types - including
epidote, feldspar, milky quartz, and shale - and is more coarsegrained than San Juan sand, because it does not travel as far.
The exception is the minor amounts of magnitite sand, which is
very fined grained and sourced in the Sangres. Trace amounts of
gold is also present and also from the Sangres. Sand grains from
both sources range in size from 0.2 mm to 2 mm. Look at a
handful of sand when you visit: the larger, chunkier particles
are probably from the nearby Sangre de Cristo mountains, while
the smaller, smoother particles probably originated in the San
Juan mountains.
How does sand move?
suspension.
In three ways: saltation, surface creep, and
Saltation is a process where the sand grains bounce as they are
transported by wind. This leaping movement accounts for about
95% of sand grain movement. Sand grains usually are not carried
high into the atmosphere, even by the strongest winds. Rather,
they bounce along within a few feet or inches of the ground
surface.
A sand grain "creeps" when it collides with other grains causing
it to roll or make small jumps. Surface creep may account for
about 4% of sand movement.
Suspension involves movement of sand grains when sand is blown
high into the air. Suspension accounts for 1% or less of sand
grain movement.
How does the wind interact with the mountains? The wind here
blows prevalently from the southwest. As it blows against the
Sangre de Cristo mountains, they funnel through three low
mountain passes. The northernmost and highest pass is Music
Pass, blocked from view from the Visitor Center by Mount Herard.
Because of its height and location, Music Pass probably has the
least effect on the dunes. Medano Pass is the middle pass,
visible from the Visitor Center in front and to the right of Mt.
Herard. Of the three passes, it is the lowest, the widest, and
likely has the most effect on the Great Sand Dunes. Medano
(pronounced "MED-ah-no", since the original accent was on the
'e') is a Spanish word meaning "sandy or sand dune." Mosca Pass
is the southern pass. Narrower and farther from the dunes than
Medano Pass, the wind is less influenced by Mosca Pass, the
southernmost pass by the dunes. Mosca is a Spanish word meaning
“house fly”.
How much do the dunes change? Overall, not much. Dune shapes you
view today will probably be about what you would see if you
visited the dunes next year, 10 years from now, and even 100
years from now. Photographs taken about 1900 show the mass of
dunes shaped much as it is today. There is a great deal of
surface sand movement with each wind storm and the dune rigdes to
occillate back and forth, but since the winds come from many
directions, the net movement is minimal.
The wind does cause surface changes, however. Dunes have two
slopes or "faces." The shallow dipping side is the windward
face. Sand blown from the windward face is deposited on the
steeply dipping leeward side. Sand on the leeward side or
slipface is subject to constant change, avalanching when sand
reaches its "maximum angle of repose," at 34 degrees. No
slipface can exceed a 34-degree angle, though hikers may feel
they’re much steeper.
The northeasterly storms can also blow small ridges of sand onto
the crests of the dunes, creating what some call a ‘Chinese Wall’
effect, or mini-reversing dunes on top of larger dunes.
What are those patches of black we see on the dunes? Magnetite,
a iron oxide mineral, shows itself as black streaks in the sand.
As you can guess from the name, it is readily attracted to a
magnet. Because magnetite is heavier than most minerals, it
remains on the surface of the sand while the lighter materials
are carried away by the wind. The presence of magnetite
indicates the presence of gold. The sand contains minute pieces
of gold, called ‘flour gold.’ Prospectors in the early 1900s
tried to extract gold from the sand, but local citizens,
concerned about mining destruction of the dunes, lobbied Herbert
Hoover in 1932 to make the Great Sand Dunes a national monument.
Today protected as part of the national park system, the dunes
system is more valuable as a source of beauty, recreation, and
inspiration.
Why are the dunes so tall? The Great Sand Dunes are the tallest
dunes in North America, and among the tallest on Earth. (China
is home to the tallest dunes in the world.) The ‘recycling’
action of Medano Creek and Sand Creek is at least partly
responsible for the vertical height of the Great Sand Dunes.
Medano and Sand Creeks bound the dunefield on the east and west
sides, and wash grains of wind-blown sand downstream. As the
sand saltates from the dunefield upwind toward the mountains,
many grains land on wet sand or in one of the creeks, and are
washed back downstream. As the sand deposits in the lower parts
of both creeks dry out, they’re picked up by the wind and carried
back toward the mountains, piling higher and higher. Without
Medano and Sand Creeks, the dune field might cover more surface
area, but probably wouldn’t be as tall.
To better protect the entire geological system of the dunes especially the aquifer and related hydrological system from
commercial water export to Colorado Front Range cities - local
citizens and the Colorado congressional delegation worked
together and in November 2000 the "Great Sand Dunes National Park
and Preserve Act" was passed in Congress by an overwhelming
margin, and signed into law by the president. The act
immediately established Great Sand Dunes National Preserve,
containing the alpine watersheds of Sand Creek (Music Pass),
Medano Creek (Medano Pass), and Mosca Creek (Mosca Pass). It
also authorized Congress to purchase needed lands west of the
dunefield containing sand deposits and smaller dunes that have
not yet blown into the main dunefield, as well as many fragile
wetlands and part of Sand Creek. Once these lands west of the
dunes are purchased and added to the monument, it will be called
"Great Sand Dunes National Park and Preserve", and the whole
dunes system - sand deposits, sandy grasslands, streams,
wetlands, forests, alpine lakes and tundra - will be protected as
a geological and hydrological wonder.