"The whole landscape was on the move."
Ralph Alger Bagnold
Author of The Physics of Blown Sand and Desert Dunes, 1941
Having left the lofty San Juan Mountains in July, my colleague Wayne Ranney and I headed due east across the alternately arid and irrigated San Luis Valley. Our destination was the Great Sand Dunes of south-central Colorado. The dunefield, located at the extreme east side of the valley, struck me as being somewhat out of context with its surroundings, in a state traditionally characterized by its alpine nature. Yet, it’s the heart of an exquisitely balanced geologic, geographic and climatic system that includes the watershed and windshed of the Sangre de Cristo Mountains, the valley’s sand sheet, and its shallow playa lakes or sabkha.
This panoramic photo was taken from the south side of the dunefield looking north. The sand sheet is in the foreground and the Sangre de Cristo Range is in the background. The sabkha (not shown) is off to the west. The earth's curvature is an artifact of Photoshop post-processing. Click on the panorama for a larger view.
The system originated when Lake Alamosa, in the valley to the west of the dunefield, drained to the south about 440,000 thousand years ago. Sand from its paleo-lakebed was blown to the east by prevailing southwest winds off the San Juan Mountains. An alcove within the Sangre de Cristo’s accommodated the developing dunefield, assisted by northeast seasonal storm winds and watershed streams that re-cycled the sand back to the dunes. The cycle is actually quite simple, but predicting changes within has been far more complex. For more of the juicy geological details, please visit my previous post Part I entitled the “The Great Sand Dunes – Its Geological Evolution here.
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| San Luis Basin flanked by the San Juan Mountains on the west and the Sangre de Cristo Range on the east. The Great Sand Dunes is tucked into a mountain-alcove on the east side of the valley. Inset map shows relationship to the four basins within the Rio Grande Rift. Modified from USGS |
In 1932 President Herbert Hoover officially created the Great Sand Dunes National Monument, and in 2004 Congress established the 233 square mile-region as a National Park and Preserve. Its 84,670 acres contain more than just sand, and includes high mountain peaks, tundra and lakes, pine and spruce forests, stands of aspen, grassland and wetlands. Recently, Mark Udall, who chairs the U.S. Senate National Parks Committee, proposed the establishment of the Sangre de Cristo National Historic Park to protect many historically and culturally significant sites within the Sangre de Cristo Mountains and the neighboring San Luis Valley region.
The dunefield’s geological assembly is tied to the operation of the Rio Grande rift that became active some 26 million years ago. Hinging on the west along the San Juan Mountains, it gradually dropped almost four miles on the east. The downdropping of the half-graben forced the horst-block of the Sangre de Cristo’s skyward. Rifting created the accommodation space for the dunefield’s eolian sands that originated in the San Juan’s far to the west and secondarily blown in from the valley’s sand sheet by the prevailing southwesterlies. In conjunction with seasonal, storm-related northeasterlies from the Sangre de Cristo’s, a bimodal wind regime (red arrows) now confines the dunefield to a relatively fixed footprint.
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| The Great Sand Dunes are nestled within an embayment of the Sangre de Cristo Mountains and surrounded by the seasonal streams of Sand and Medano Creeks. The prevailing southwesterlies and seasonal northeasterlies confine the dunefield. Our camp was located at the red ellipse. Modified from USGS Map |
With the Sangre de Cristo Mountains at our backs, we’re looking southwest from camp past the sand sheet, here vegetated with patches of grass and shrubs, at the dunefield’s southern edge. A fiery Colorado sunset showcases the San Juan Mountains on the horizon to the west, the original source of most of the sand to the dunefield. The remainder, about 10%, comes from the Sangre de Cristo’s, at our backs to the east. Surprisingly, only about 10% of the system’s sand is actually contained within the dunefield. The rest is housed within the sand sheet that surrounds the dunefield on three sides.
Looking north at sunrise, the tall peak to the right of center in the Sangre de Cristo’s is Mount Herard at 13,340 feet, whose watershed supplies Medano Creek. Piedmont streams such as Medano, Sand and Spring Creeks are essential to the replenishment of Great Sand Dunes by returning sand to the sand sheet beyond the dunes so that the southwesterlies can return it to the dunefield. They also recharge the valley’s aquifer and sustain the extensive wetlands that border the dunes further to the west and south.
Here’s the same view only a few minutes later showing welcomed blue skies. The colors and contrasts of the landforms were fantastic! Notice the low-relief extension of the dunefield toward the mountain front. Wind and water work in concert to replenish the dunefield and keep it confined. When the water table is low in the valley, sand is made available for transport to the dunes from the sand sheet via the prevailing southwesterlies. Thus, the sandscape is replenished and may even migrate outside its normal footprint.
The recycling action of wind and water also contributes to the astounding height of the dunefield and serves to stabilize it with a 7% moisture content below the surface. In addition, the opposing wind regime creates the dunefield’s varying architecture such as reversing, transverse, star and barchan dunes.
We made a pre-breakfast ascent onto the dunefield from camp, crossing Medano Creek and trudging our way up High Dune, which is actually ranked as a Colorado Peak because of its elevation at 8,691 feet. To the far right in the photo, Star Dune rises 100 feet higher off the valley floor making it the tallest dune in the park. It's all because of the hinging-tilt the valley has experienced due to the Rio Grande rift, although it’s visually imperceptible having been filled with 15,000 feet of alluvium from the mountains.
The complex system of winds that converge at Great Sand Dunes has conspired to create a variety of dune types within the dunefield. Star Dune is characterized by three or more ridges that radiate from its center, the product of wind convergence. This causes the dunes to grow upward rather than migrate laterally. Star dunes are located on the north and southeast edges of the dunefield. Winds that reverse direction produce transverse or barchanoid dunes with foresets facing in opposite directions. Reverse dunes mantle underlying dunes when the whim of the wind changes its trend. Notice the distant San Juan Mountains sixty five miles across the San Luis Valley making their own weather and desiccating the winds that reach down to the valley.
The composition of all sand betrays its source. The darker appearance of the dunefield’s eolian sand is due to sediments dominated by volcaniclastic rock fragments (51.7%) from the San Juan's. It's dark sand is a good absorber of the sun’s heat. When the air temperature is 80 degrees, the surface can reach a scalding 140 degrees! Notice the depressions or swales within the dunefield. They are sheltered from the wind, and some are close to the watertable. They serve as refugia for plant and wildlife.
Sand is moved about by the wind via three mechanisms: by bodily moving the sand in suspension (providing the wind speed is at least 15 mph); by saltation (with grains leapfrogging, bouncing and hopping along the surface that are too large to be moved by the wind alone); and by surface creep (nudging sand grains along by lightly lifting them briefly off the surface).
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| From Wikipedia |
The result of these eolian processes (named after the Greek god Aeolus, keeper of the winds) is sand dunes that migrate across the landscape. As the wind assembles the sand, a dune forms. Sand climbs a long, gently-sloping, windward slope and cascades over the crest onto the shorter downwind side of the dune called the slip-face in the lee of the wind. Sand is deposited there, as the wind’s speed diminishes and loses its capacity to carry sand. Thus, the slip-face is steep and forms an angle of repose that doesn’t exceed 34º. As each new layer of sand falls down the slip-face, cross beds are gradually formed, one layer after another. Over a period of time, the sand dunes advance down wind.
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| From Wikipedia |
Notice the "active" dunes that have migrated beyond the dunefield’s perimeter onto the piedmont slope that drapes from the mountain front. As they migrate, they bury vegetated areas on the slope and form “ghost forests” of dead, tree stumps (right of center). The low-lying, grassy vegetation acts as a baffle to nullify the movement of the wind at ground level. Saltation ceases when sand grains enter its “dead air” space, which then stabilizes the dunes horizontally.
Eventually, stray sand will be returned to the dunefield by the combined efforts of the Sangre de Cristo’s wind and water regime, the re-cycling process that has confined the dunefield to its seemingly stable footprint.
We're standing atop High Dune on the eastern front of the dunefield. That’s the braided-channel of Medano Creek running from left to right (here north to south). It has already begun to retreat back toward the mountains, typical of July, and will be completely gone by August or September. In drier years, streams are lost to infiltration within a few kilometers of the mountain front. Again, notice the sand that has invaded the vegetated region beyond the creek onto the shallow alluvial apron.
April is one of the snowiest months at Great Sand Dunes. This is when the seasonal stream of Medano Creek begins to trickle down as the snowpack begins to melt, recycling sand back to the valley floor. By mid to late May, the creek reaches its annual peak. Because the Medano’s sandy creekbed is so wide, the water depth is very shallow. Consequently, small rises in the bed are enough to block the flow. Once the pent-up water rises high enough, it breaks over the dam and creates a “surge flow” with pulses of waves with some reaching 16 inches in height. It’s a popular summer locale to experience waves “breaking” far from the ocean. The creek flows past the dunefield for an additional 8 km and then sinks into the valley floor.
Facing southeast, the Sangre de Cristo's reach to the south into New Mexico. A multitude of alluvial fans meet the sand sheet. Once again, notice the stray, parabolic dunes migrating past the dunefield’s perimeter. The sand sheet’s grassy vegetation holds the “arms” of the dunes in place as the leeward “nose” of the dune migrates forward. The dunes on which I’m standing are reversing dunes, the most common dune on the dunefield, formed during the summer as the wind changes direction. This creates a “Chinese Wall” at the crest of the dunes and also contributes to their great height.
The bands of black sand on many of the dunes are deposits of the heavy mineral magnetite, a crystalline oxide of iron. Brought by the wind from the distant San Juan Mountains, the iron-rich, volcanic minerals become sorted and concentrated by virtue of their greater density. The specific gravity of quartz is 2.7; whereas, magnetite is 5.2. Sorting by density is called placering, with wind being as effective a sorting agent as water (with a specific gravity of 1). Placering is also very apparent on coastal beaches after a storm, and it’s what miners used to pan for gold (SPG 19.3).
Thomas Edison once made the discovery of magnetite bands on a coastal beach in Long Island, something all beachcombers are familiar with. Recognizing its potential commercial value, his enthusiasm preceded his business sense when he purchased the beach and the separation machinery to extract the ore. On his return, he discovered that a storm had reworked the beach and removed the ore for him. Of course, he did come up with another bright idea. Indeed, sand is on the move everywhere by its very nature. You can also read about magnetite on Wayne Ranney’s Great Sand Dunes post here.
Both moving water and wind have the capacity to transport sand long distances before it’s deposited. Fine-grained particles of sediment become airborne in suspension. Along the ground, there is surprisingly little motion due to the wind. We've all seen a car travelling on an unpaved road pulling along a cloud of dust while leaving the loose road surface relatively unscathed. As previously mentioned, sand is moved on the ground by saltation (Latin “to leap”). As a “heavy” grain of sand gets knocked into the air, it falls back down and bumps along another grain. Thus, sand moves along the dune floor and creates secondary wind ripples, seen here. The ripples and the entire dune are an indication of the prevailing wind direction. Again, notice the magnetite-banding.
At 7,800 feet, seasonal conditions include snow and sub-zero temperatures on the dunefield during winter. By definition, a desert is a dry, often sandy region of little rainfall, extreme temperatures and sparse vegetation. Deserts characteristically receive less than 10 inches of rainfall annually. Park rangers refer to the arid and depauperate Great Sand Dunes as “desert-like” with an annual rainfall of about 11 inches. What a contrast of landforms are juxtaposed in this photo!
When we think of deserts, we generally envision a dry, barren lifeless place. In truth, deserts support an amazing variety of life and are places of stunning beauty and lots of activity. Its seemingly inhospitable environment actually plays host to insects that are well-suited to the harsh conditions. Severe temperatures, high winds, water scarcity and shifting sands all challenge the plants and animals that live on the dunes. This adult Conchuela stink bug with its distinctive red border and red spot feeds on the plants that grow on the dunes but also loves mesquite and alfalfa, the latter grown in the irrigated-valley to the west. Notice the faint trackway left by this dune traveler.
Tiny trackways can be found everywhere in the early morning. After sunset, surface temperatures drop and humidity increases. At night, cooler air from the mountains causes the surface temperatures to fall. Burrowing insects that spent the day undercover to escape the dune’s inhospitable surface conditions emerge to feed and mate in the cool night air. If you walk the dune at night with a flashlight and follow a trackway, you'll find a burrow in one direction and possibly an insect out for a stroll in the other. Notice the human tracks from the previous day and the insect trackway from last night! Reptilian trackways often show a tell-tale tail-drag (pun intended).
A busy nocturnal creature, likely an arthropod, hesitated on a dune and then crossed over its knife-edge crest. On the leeward side, it crossed a narrow band of volcanic magnetite.
Photographed only four inches from the surface, the wind has sculpted the crest of a dune into a sharp, razor-edge, held intact by the sand’s elevated moisture content. The natural world possesses incredible beauty at every scale of magnification.
Sand brings out the playfulness in us all, aptly demonstrated by geologist Wayne Ranney.
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| Photoshop post-processing by John Parmley of http://www.photographybyparmley.com |
INFORMATIVE RESOURCES
Ancient Landscapes of the Colorado Plateau by Ron Blakey and Wayne Ranney, 2008.
On the Origin and Age of the Great Sand Dunes, Colorado by Richard F. Madole et al, 2008.
Plateau – The Land and People of the Colorado Plateau by Wayne Ranney, Museum of Northern Arizona, 2009.
Sand – The Never Ending Story by Michael Welland, 2009.
The Geologic History of Colorado's Sangre de Cristo Range by David A. Lindsey, USGS 1349.
The Physics of Blown Sand and Desert Dunes by Ralph A. Bagnold, 1941.
SPECIAL THANKS
I want to personally thank and highly recommend John Parmley of "Photography by Parmley" for his outstanding Photoshop expertise in achieving the multiple image composition pictured above. His website can be found here.
