A Study of the Great Sand Dunes National Monument
By Nipunika Bose
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| Figure 1. Great Sand Dunes National Monument [1] |
LOCATION
The
Great Sand Dunes National Park is located in the high desert of the San Luis
Valley at an elevation of 8175 ft. From the south, the park is located 90 miles
from the junction of US 160 and Colorado 150 [2]. The Great Sand Dunes is not
only known for its dramatic landform structure but also is a great topic of study for its unusual location across the San
Luis Valley. It has
been a wonderful site for geological study for scientists in the past and in the
present.
THE INITIAL FORMATION
The Hayden survey conducted in 1873
made the first attempts of studying the minerals in these sand dunes,
potentially to answer the biggest question - where did all the sand come
from? It was found by radiological analysts that the San Juan Mountains
were composed of volcanic rocks that dated between 18 to 35 million years in
time [4]. Some geologists believe that these bits of volcanic materials proved
that the San Juans were the original source of all the sand.
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| Figure 2: Location of Great Sand Dunes National Park [3] |
THE INITIAL FORMATION
As of November 25th, 2012, The
United States Geological Survey [5] stated in its website,
- "The dunes are a remnant of the Miocene-Pliocene Santa Fe Formation (the principal basin-fill sediment in the Rio Grande rift).
- The sand was derived from the late Pliocene-early Pleistocene Alamosa Formation, which is at or near the surface over much of the San Luis Valley.
- The sand was derived from late Pleistocene flood-plain deposits of the Rio Grande.
- The sand was derived
from dry lake-beds on the valley floor intermittently over a time span of
unknown duration, but which probably included much of the Pleistocene."
Various
theories were proposed and scientists debated on the origins of these dunes.
Some said it was a result of the flood plain deposits, however, some said it
was the result of the multiple forces of nature like the transportation of sand
controlled by the rising and declining water table over time. The answers to
all these theories lay in the sand itself. Scientists dated the mineral
crystals mostly to the Tertiary Age, during the prevalant volcanic activity in
the San Juans. However they also found traces of minerals that dated to as old
as the Pre-Cambrian age rocks from the Sangre de Cristos [4]. They consequently
established facts that the streams from the Pleistocene glaciers in the San
Juan Mountains carried deposits of sand in the valley. It was during this time
that scientists found compelling evidence that the Great Sand Dunes were formed
much earlier, around 130,000 years ago [4].
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| Figure 3: The winds that shape the dunes formation [6] |
It was consequently believed that the lava flows from the San Juan Mountains resulted in much of the material in this region. Over a period of time water erosion contributed in carrying these sediments down into the valley depositing the materials at the base. These sandy deposits were exposed to the winds and blown to the large dunes system. Extreme gust of winds further broke down the sand grains thereby creating a sifting effect [7]. The heavier grains settled down in the valley while the lighter ones were carried away. It is these complex wind patterns in these regions resulting from the three low passes namely Medano, Music, and Mosca that formed the Great Sand Dunes. The variations in the dune shapes also reflect these complex wind patterns in this region.
A DETAILED ANALYSIS OF THE GEOLOGY
The San Luis Valley is an enormous depression that is
mostly flat, and lies between the San Juan Mountains and the Sangre de Cristo
mountains. If we study closely the structure of the geology and the rock types
of this valley, we would come to know the actual formation process of the Great
sand dunes [16]. Millions of years ago, when
the earth’s crust began splitting apart into large surface plates in a process
called rifting, these mountains were created. This was followed by a high
volcanic activity causing an extreme eruption of lava. Geologists call this the
La Garita Caldera, the largest volcanic explosion on earth [17]. Due its impact the
San Juan Mountains were formed in the west. It is around this time that a long
deep rift appeared, known as the Rio Grande Rift due to the earth’s intense
tectonic movements. Consequently, around 80 million years ago, the Sangre de
Cristo Mountains were formed, caused by the rotation of a large tectonic plate [17].
This further widened the rift, thereby forming the current San Louis Valley.
In 2002 geologists found large chunks of lake bed
deposits in this valley. On further investigations they found that millions of
years ago there existed a huge water body in the area known as Lake Alamosa
today [18]. The Lake Alamosa later disappeared due to its water flowing into the
cutting edges of the volcanic deposits on the south. Scientists believe that
the Lake Alamosa eventually merged into the Rio Grande River through a deep
gorge around 440 thousand years ago.
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| Figure 4: Lake Alamosa in the San Luis Valley [19] |
THE FORMATION OF THE DUNES
Geologists consider that it is during the Pleistocene Age
when the actual dunes started forming. It is during this period that there was
a huge climate change that considerably consumed all of the water in this
region and left behind sandy deposits in form of sheets, which is known as
sabkha today. Scientist Ferdinand V. Hayden proposed a theory that the sand
deposits came from principle sediment in the Rio Grande Rift called the Santa
Fe formation [20]. After much research and investigation it was established
that the dunes system evolved from the mineral debris left behind by the
disappearing waters in the San Luis Valley. Other scientists believed that the
sand came from an ancient dry lake bed of the Closed Basin (a lake bed that had
no drainage downwards). Around 12,000 years ago towards the end of the
Pleistocene when the earth’s climate became warmer causing many glaciers to
melt, huge quantities of sand, silt were carried by the rivers and streams into
this valley [21]. It is this multitude of factors – the dry warm climate of the San
Luis Valley, the hydrological structure if the Closed Basin, the disappearing
lake beds in the region that brought all the sand in the valley.
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| Figure 5: Hayden's map of San Luis Valley, compiled from survey conducted in 1874 [22] |
PERSISTENCE OF THE DUNES
Over centuries the great Sand Dunes were stored intact in
the valley by the opposing wind directions within the mountains and the wind
funnels towards the mountain passes of Medano, Mosca, and Music [21]. This caused all
the sand to settle down together in a closed confined region. Moreover, as the
sand particles are bounced off the ground through the process of saltation the
sand carrying wind gradually loses its speed towards the passes [22]. This constant process results in deposits of
huge layers of sand at the base of the Sangre de Cristo Mountains. The Medano
and the Sand Creeks serve as the constant obstacles to the shifting dunes and
play a pivotal role in transporting and recycling the sand. The drifting sand
caused by the opposing winds result in formation of very tall dunes. It is
therefore both wind and water erosion that contributes to the persistent of the
Great Sand Dunes.
THE GEOGRAPHICAL ELEMENTS
As stated above the Great sand Dunes were formed largely
by wind erosion. The topography in such a dry land largely involves the different Aeolian processes. Aeolian processes are processes related to wind action. They are broadly classified into,
(1) Aeolian erosion - This phenomenon is again broadly classified into deflation (meaning blowout and rolling of finer particles along the ground) and abrasion (meaning the less effective erosion of airborne sand and dust particles).
(2) Aeolian transportation - Larger sediment particles either slide or roll along the ground following the process of traction while the smaller particles are transported by the wind by the process of bouncing and jumping on the ground known as saltation. The minuscule sediments are carried by the winds in suspension in forms of dust [23].
(3) Aeolian deposition - This phenomenon consists of the process of deposition of sand and dust particles forming the dune shaped structures. This is how the sand dunes gets its different shapes like parabolic, star shaped and barchan dunes as discussed below. The Sand Dunes in the San Luis Valley is a product of the amount of the sand in this area and the persistence of the local wind directions. This helps in forming dunes and cross beds that are formed by the shifting position of the down current side of the dune. The dune slopes are usually flatter towards the direction of the wind, in other words the angle of repose is much smaller towards the current. The sand grains gradually drop of the other side causing steep slopes, at times even forming sand avalanches.
THE DUNE STRUCTURE
The Medano creek and the sand creek are vastly responsible for the persistence of these dunes over time. The water from these creeks enters the Closed Basin and flows below these dunes emerging later as Big Spring and Little Spring Creeks. Surprisingly, geologists have found that each drop of water from the creek takes over 60 years to make its way to these springs. Scientists studying these complex water patterns in the Closed Basin believe that it is this moist sand that is nourished by the mobile groundwater from the unconfined water body that provides the cushion keeping the whole dune system in place [8]. The Sand Dunes National Park contains a huge variety of dunes that result from the complex wind patterns in the dune fields. When the strong winds reverse their usual directions, they form steep ridge tops called Chinese Walls. The Star Dunes are formed in places where the wind blows from all different directions. The Star Dunes are known to have multiple arms. When the winds erode vegetated sands, it forms parabolic dunes [13]. Wherever the wind blows in a single direction and there is very little vegetation, arc shaped Barchan Dunes are formed. Transverse Dunes are forward marching ridges that are formed when the Barchan dunes get aligned in a straight line along a plane perpendicular to the wind.
(1) Aeolian erosion - This phenomenon is again broadly classified into deflation (meaning blowout and rolling of finer particles along the ground) and abrasion (meaning the less effective erosion of airborne sand and dust particles).
(2) Aeolian transportation - Larger sediment particles either slide or roll along the ground following the process of traction while the smaller particles are transported by the wind by the process of bouncing and jumping on the ground known as saltation. The minuscule sediments are carried by the winds in suspension in forms of dust [23].
(3) Aeolian deposition - This phenomenon consists of the process of deposition of sand and dust particles forming the dune shaped structures. This is how the sand dunes gets its different shapes like parabolic, star shaped and barchan dunes as discussed below. The Sand Dunes in the San Luis Valley is a product of the amount of the sand in this area and the persistence of the local wind directions. This helps in forming dunes and cross beds that are formed by the shifting position of the down current side of the dune. The dune slopes are usually flatter towards the direction of the wind, in other words the angle of repose is much smaller towards the current. The sand grains gradually drop of the other side causing steep slopes, at times even forming sand avalanches.
THE DUNE STRUCTURE
The Medano creek and the sand creek are vastly responsible for the persistence of these dunes over time. The water from these creeks enters the Closed Basin and flows below these dunes emerging later as Big Spring and Little Spring Creeks. Surprisingly, geologists have found that each drop of water from the creek takes over 60 years to make its way to these springs. Scientists studying these complex water patterns in the Closed Basin believe that it is this moist sand that is nourished by the mobile groundwater from the unconfined water body that provides the cushion keeping the whole dune system in place [8]. The Sand Dunes National Park contains a huge variety of dunes that result from the complex wind patterns in the dune fields. When the strong winds reverse their usual directions, they form steep ridge tops called Chinese Walls. The Star Dunes are formed in places where the wind blows from all different directions. The Star Dunes are known to have multiple arms. When the winds erode vegetated sands, it forms parabolic dunes [13]. Wherever the wind blows in a single direction and there is very little vegetation, arc shaped Barchan Dunes are formed. Transverse Dunes are forward marching ridges that are formed when the Barchan dunes get aligned in a straight line along a plane perpendicular to the wind.
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| Figure 6: Chinese Walls in Great Sand Dunes National Park [9] |
The
climate in this region is highly complex because the temperature ranges from
warm to mild to cold on the scale in different regions of the valley. The
highest temperature recorded was 96° F in 1982 while the lowest was -25° F in 1963 [10]. The San
Luis valley is usually sunny during most of the year but receives a good
amount of rainfall during the months of July and August marked by thunderstorms
lightning and hail. The winters on the other hand are quite severe especially
in March, which sees the maximum snowfall. With the high blowing winds in these
severe wintery conditions the temperatures drop quite below zero [10]. This
region however has become dryer and more arid over time. The shift from the Pleistocene
to Modern climates, vegetation patterns and fauna was marked by a dramatic
shift in the lifestyle for the people living in these regions. In recent days
the precipitation measures on the dunes is only about 11 inches of rainfall
every year due to fast evaporation rates. In summer months the sand surface
temperature may increase to as high as 140° F in the afternoons while it drops into the 40s at
nights [10].
FLORA
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| Figure 7: Sunflowers in the sub alpine region [11] |
The
Great Sand Dunes area has a very diverse flora that ranges from grasslands to
dunes, to riparian, to sub- alpine and finally to alpine tundra. Blow-out
grass, Indian rice grass, and scurf pea grow on the dunes, while the prairie
sunflowers, the slender spider flowers grow in the adjoining grasslands during
summers. Also found in the dry areas are cacti like the prickly pear and yucca.
Along the stream courses of the Medano creek, the most common trees are of
cotton woods, wild roses and orchids. The wild strawberries grow in huge
numbers along various trails at higher elevations. As one walks towards the
mountains the ponderosa pines, douglas firs and aspens become common. These
areas are full of different varieties of pines. The lower elevation is
suitable for junipers and pinion pines. The alpine tundra region mostly
consists of grassy layers of alpine phlox. Other plants like the dwarf clover
are also seen on the sunnier side of the hills. The alpine forget-me-nots are
blue, dwarf plants that grow on small branches on the lower alpine region. The
other common tundra plants are the small yellow butter cups called Alpine Avens
and the magenta lavender daisy like flowers called the Fairy Primrose.
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| Figure 8: Alpine Dwarf Clover in alpine tundra [12] |
FAUNA
This
region is a witness to diverse animal life and a great treat to Ornithologists.
Birds like Wilson's phalaropes flock together on the breeding grounds on the
San Luis Valley every spring. The blue American avocets are found swishing
their up-curved bills on the water moving their black and white bodies. Other
bird species include the black necked stilts, the white faced ibis and flocks
of snowy clovers. The little clovers usually nest in loose colonies along the
Blanca wetlands at the far end of the playa. It is also quite common to see
ravens like falcons and golden eagles as well as owls in the grasslands.
As one walks towards the golden river of the Sand Creek, one may come across foot tracks of bisons. The sandy dunes and the adjoining grasslands provide nice habitat to various rodents, coyotes, bobcats and jackrabbits. The kangaroo rats and the rabbits attract the large variety of snakes in the area like the bull snakes and the rattlesnakes. At higher elevation in the San Juan and the Sangre de Cristo mountains animals like black bears, mountain lions, porcupines, and big horn sheep are commonly spotted. These animals at times might come down to the valley and even hover round the dunes. The sand dunes area is a home to the rare insect species, The Great Sand Dunes Tiger Beetle. These bugs are specific to this region only and are not found anywhere else. Other insects include the Wolf Spider, the Giant Sand Treader Camel Cricket, the Circus Beetle, the Flower Beetle and the fierce looking large fly called the Robber Fly.
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| Figure 9: Sand Hill Cranes in Great Sand Dunes National Park [15] |
As one walks towards the golden river of the Sand Creek, one may come across foot tracks of bisons. The sandy dunes and the adjoining grasslands provide nice habitat to various rodents, coyotes, bobcats and jackrabbits. The kangaroo rats and the rabbits attract the large variety of snakes in the area like the bull snakes and the rattlesnakes. At higher elevation in the San Juan and the Sangre de Cristo mountains animals like black bears, mountain lions, porcupines, and big horn sheep are commonly spotted. These animals at times might come down to the valley and even hover round the dunes. The sand dunes area is a home to the rare insect species, The Great Sand Dunes Tiger Beetle. These bugs are specific to this region only and are not found anywhere else. Other insects include the Wolf Spider, the Giant Sand Treader Camel Cricket, the Circus Beetle, the Flower Beetle and the fierce looking large fly called the Robber Fly.
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| Figure 10: Great Sand Dunes Tiger Beetle [14] |
BIBLIOGRAPHY
[1] National Parks Guides - http://www.nationalparkguides.com/images/great-sand-dunes/great-sand-dunes-l.jpg
[2] Charlie and Diane Winger, The Essential Guide to Great Sand Dunes National Park & Preserve (CMC Press), 35
[3] "United State Geological Survey, Sitemap", http://esp.cr.usgs.gov/info/dunes/images/sitemap.jpg
[4] Photography by Bob Rozinski and Wendy Shattil, Text by Audrey DeLella Benedict,
Valley of the Dunes (Revised Edition, 2010, Cloud Ridge Publishing), 38
[5] "United States Geological Survey, Surficial Geology and Geomorphology of the Great Sand Dunes Area, South-Central Colorado", http://esp.cr.usgs.gov/info/dunes/origin.html
[6] National Park Service, Great Sand Dunes, http://
m.localguides.com/site/home/apk/grsa/naturescience/images/valley_geology_dune_formati.jpg
[7] Charlie and Diane Winger, The Essential Guide to Great Sand Dunes National Park & Preserve (CMC Press), 40-41
[8] Photography by Bob Rozinski and Wendy Shattil, Text by Audrey DeLella Benedict,
Valley of the Dunes (Revised Edition, 2010, Cloud Ridge Publishing), 52
[9] National Park Service, Great Sand Dunes, http://www.nps.gov/grsa/photosmultimedia/images/grsa-two_people_chinese_wall-2011-jpg_1.jpg
[10] Charlie and Diane Winger, The Essential Guide to Great Sand Dunes National Park & Preserve (CMC Press), 36-37
[11] Great Sand Dunes Pictures, Tundra Plants, http://0.tqn.com/d/gocalifornia/1/0/5/8/dunes5-a.jpg
[12] National Park Service, Great Sand Dunes, Plants, http://www.nps.gov/grsa/naturescience/images/alpine_dwarf_clover.jpg
[13] National Park Service, Great Sand Dunes, Dune Types, http://www.nps.gov/grsa/naturescience/dune-types.htm
[14] Hands On The Lands, Resources, http://www.handsontheland.org/grsa/resources/curriculum/mid/insects/images/cith01.jpg
[15] National Park Service, Great Sand Dunes, http://www.nps.gov/grsa/planyourvisit/images/sandhill_cranes_dunes285.jpg
[16] Ross B. Johnson, The Great Sand Dunes of Southern Colorado, U.S Geological Survey, Denver, Colorado, http://nmgs.nmt.edu/publications/guidebooks/downloads/22/22_p0123_p0128.pdf, 1
[17] Michael Geary, Sea of Sand, A History of Sand Dunes National Park and Preserve, http://publiclands.colostate.edu/docs/seaofsand2.pdf, 9
[18] Great Sand Dunes National Preserve, Formation of Great Sand Dunes and San Luis Valley, http://greatsanddunes.areaparks.com/parkinfo.html?pid=14179
[19] Great Sand Dunes National Preserve, Formation of Great Sand Dunes and San Luis Valley, http://m.localguides.com/site/home/apk/grsa/naturescience/images/lake_alamosa285.jpg
[20] Michael Geary, Sea of Sand, A History of Sand Dunes National Park and Preserve, http://publiclands.colostate.edu/docs/seaofsand2.pdf, 10
[21] Great Sand Dunes National Park Information, http://www.great.sand.dunes.national-park.com/info.htm
[22] Ferdinand Hayden’s map of the San Luis Valley, compiled from survey1 undertaken in 1874-1875, Michael Geary, Sea of Sand, A History of Sand Dunes National Park and Preserve, http://publiclands.colostate.edu/docs/seaofsand2.pdf, 11
[23] McKnight’s Physical Geography: A Landscape Appreciation, Chapter 18, The Topography of Arid Lands, http://media.pearsoncmg.com/bc/bc_0media_geo/interactiveanimations/017_CrossBed_HS_GG_Stu.html?mpg10#/Dune-Formation-and-Cross-bedding
