The Great Sand Dunes of Colorado: Part II – Climbing the Geology of the Dunes

"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.

Taken from the same perspective, this is the east edge of the Great Sand Dunes.

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.



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.  

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

It’s no wonder why the Spanish explorer Antonio Valverde y Cosio in 1719 christened the Sangre de Cristo’s “Blood of Christ” with its granitic feldspars ablaze in a reddish glow at sunset. The range forms the eastern backdrop beyond the Great Sand Dunes.

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).



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. 

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.

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.

Lithified Sand Dunes of the Ancient Bahamian Landscape

On a recent vacation to the Bahamas, Paradise Island in particular, while the rest of my crew was swimming, reading and kicking back, I did some exploring down beach and out onto a narrow "rocky" spit of land. I was surprised to find that the spit was a platform composed of sand dunes. Not only were they lithified but cross-bedded, reminiscent of the eolian Coconino and Wingate Sandstones on the Colorado Plateau, but on a vastly smaller scale.

A LITTLE BACKGROUND ON THE BAHAMIAN ISLANDS
Positioned a mere 50 miles off the coast of Florida at its nearest point, the Bahamian Islands, of which there are 700, form a northwest-southeast trending archipelago. The climate of the region is sub-tropical with hot summers, warm temperate winters and an average yearly rainfall of about 30 inches. The islands of the Bahamas rest on a shallow carbonate platform, which during the Pleistocene, had been intermittently exposed and submerged in conjunction with glacially-induced high and low sea level-stands. Glacial maxima favored lower sea levels that exposed bank sediment. In turn, this favored eolianite deposition which possessed the capacity for lithification under the right circumstances.


This is a Google Earth image taken from about a 15,000 foot-altitude showing the location of the lithified dunes on Paradise Island, and showing the relationship to much larger New Providence Island and its populated capital city of Nassau. The total length of the platform measured about 1/5 of a mile and the greatest width was 150 feet.
It's highest elevation above sea level is perhaps 20-25 feet. 

Location of the lithified dunes on Paradise Island. The spit is connected to the main body
of Paradise Island by a narrow neck of a sandy beach.



This extreme close-up is taken from a distance of about one foot. It provides a good view of the lithified dune's macroscopic structure. Although the surface of the dune is severely weathered, you can clearly make out its bedding planes and its oolitic composition.

INTRIGUING QUESTIONS
Interestingly and totally unanticipated (as an avocational geologist), the dune’s composition wasn’t the typical silica-sand variety (in the form of quartz) but instead a carbonate (a limestone). Upon close inspection, the sand grains had an oolitic (egg-shaped), spherical shape, like fish roe. Indeed, silica sand-dunes are typical of inland continental and non-tropical coastal settings, while tropical coastal settings possess sands of eroded limestone. How did the dunes lithify, while above ground (subaerially) or did they? And, how did the sand acquire its oolitic shape? Here’s the intriguing answer.

GENESIS OF THE LITHIFIED DUNES
The Bahamas are not of volcanic origin, typical of many of the Caribbean islands. There are no igneous and metamorphic rocks to be found. Shallow-water carbonates are ubiquitous, having formed near the surface for 200 million years. The Bahamas are a vast “carbonate factory,” producing sediment at a fairly rapid rate on a slowly subsiding crustal platform (keeping the water deep enough for the process to continue). Oolitic limestone is precipitated directly from sea water, although containing carbonate forms from other sources such as skeletal remains.

The sand dunes formed on land when global sea level fell during the Pleistocene Ice Age. As sea level rose and fell during each of four glacial-interglacial periods, new sediments washed up onto new beaches forming a new line of dunes with classic bedding planes and erosive bounding surfaces. Cementation of the dunes with calcium carbonate occurred both during interglacial-period, marine submergence and glacial-period, rainwater exposure by both crystallization and recrystallization. The process of converting the sediments to the rocks is called diagenesis.

Looking down the coast, it appears that several “fossil platforms” are on higher ground. During the Ice Ages, continental glaciers tied-up great quantities of water making global sea levels lower. This exposed more shoreline to undercutting-erosion. During interglacials, the melted glaciers freed-up water making global sea levels rise. This created wave-cut platforms above the normal level of the sea. Since the region exhibits no folding, tilting or faulting, we can safely assume that glaciation-induced subsidence rather than geological uplift is the only causative explanation for the “elevated” platforms.



 A fossilized tree and root structure preserved within the lithified dune
adds testimony to its origin as a terrestrial sand dune.

PHYSICAL AND CHEMICAL WEATHERING
Weathering is the breaking down of the Earth's rocks, soils and minerals through direct contact with the atmosphere. Weathering occurs in situ without "movement" and is not to be confused with erosion, which involves the movement of rocks and minerals by agents such as water, ice, wind and gravity. Physical weathering involves "breakdown through direct contact with atmospheric conditions such as heat, water, ice and pressure," whereas, chemical weathering involves the direct effect of atmospheric chemicals or biologically produced chemicals (Wikipedia).

The spit is essentially a narrow, rocky carbonate platform forming a small portion of the coast. It is evident here, in contrast to the neighboring beach itself, that morphologic change is a slow and gradual process dominated by physical,  biologic and chemical weathering processes. Tide, current and wave processes all yield change but not on temporal scales of hours and days compared to the beach. Both types of weathering can be found on the coastal carbonate platform but in varying degrees and at differing locations. The mechanisms yielding the various morphologies appear to be controlled by factors such as the position relative to sea level, the interface-distance between water and land, and the porosity and degree of cementation of the rock (which is undoubtedly directly proportional to its age) .  

PHYSICAL WEATHERING
On the oceanic margin of the spit, it has been eroded into cliffs which have been undercut everywhere by wave action forming wave-cut platforms that extend outward toward the sea. The most highly-dissected terrain was to be found in a zone that developed closest to the sea. In fact both physical and chemical weathering decreased as a function of distance from the edge of the platform.

An additional type of physical weathering includes haloclasty or salt crystallization which causes the disintegration of rocks when saline solutions seep into cracks and joints in the limestone. When the water evaporates, it leaves a residue of salt crystals behind. The salt crystals can expand up to 3 times their volume when they become heated, exerting pressure on the confining rock. It's reminiscent of the 9% expansion of water when it freezes. Salt crystallization can also occur when solutions decompose rocks, which likewise leaves a salt residue that can expand. This phenomenon is common in arid climates and along coasts.

It can readily be seen that physical and chemical weathering go hand-in-hand. On the platform, the delicately etched textures of the rocks were seen to develop within reach of frequent salt spray and are absent amongst identical rocks further away from the influence of the sea.  



Undercutting of the platform by wave action. The surface exhibits solution weathering.

 Further evidence of physical weathering and chemical dissolution contributes to the dramatic beauty.
The Pleistocene and Holocene-age limestones of the supratidal, coastal platform
are undergoing surficial meteoric diagenesis from weathering yielding "eogenetic" karst.



Watch the waves relentlessly breaking and eroding the coastal platform on the video below.

CHEMICAL WEATHERING

Rainfall is inherently acidic because of atmospheric carbon dioxide (although other atmospheric gases can be absorbed which may increase the acidity additionally). This produces a weak carbonic acid which leads to solution weathering on highly-susceptible rocks such as limestone. In addition, coastal platforms such as these are in the spray-zone. Over considerable time, the limestone undergoes chemical dissolution to the extent that its appearance becomes sharply-jagged with numerous voids, small excavations and holes, and razor-sharp edges. The holes tend to link up and gradually enlarge which gives the surface a pitted, honey-combed and drilled-out appearance. Also, kamenitza or solution pans tend to form which are shallow, rounded relatively flat-bottomed basins on exposed surfaces that develop via dissolution of limestone by standing water. These surface phenomena are generically known as karst. Subterranean karstic landforms (not the subject of this post) exist in the Bahamian tropics but  differ somewhat from traditional karstic landscapes formed in temperate climates.

Digressing briefly, classical karst terrains have distinctive landforms and drainage arising from greater rock solubility in natural water that is derived elsewhere. They are characterized by numerous caves, subterranean caverns, sinkholes, solution valleys, fissures and underground rivers and streams. Karst topography usually forms in regions of plentiful rainfall (cold and humid mid-latitude, temperate climates) where the bedrock consists of carbonate-rich rock such as limestone (CaCo3)  and dolomite (MgCaCO3), which is easily dissolved. Examples of classical karst terrains are the Dinaric Kras region (the type locality) of the Adriatic between Slovenia and Italy, and the Appalachian mountainous regions of the Mid-Atlantic States).

Most karstic features are created by carbonic acid (carbonation) which forms from the absorption of carbon dioxide (CO2) by rain (meteoric) water. Biological activity (such as plants, algae and lichen) can secrete acids that dissolve soluble bedrock. In addition, blue-green algae can produce a plant-generated surface karst (called phytokarst)  characterized by pitting and a sharp-edged, spongy lattice of ridges and pinnacles.    

The following is the main mechanism of calcium carbonate dissolution in limestone: Rain passes through the atmosphere picking up CO2 which dissolves in water. Once on the ground, the water containing the weak carbonic acid in solution passes through the bedrock and dissolves calcium carbonate.

Further attacks on the landscape occur as a result of fossilized plant-roots called rhizomorphs that once grew in the dunes long ago. Their roots may harden the soil around them via their secretions. Upon weathering, the resistant limestone can form thin, jagged edges. Biological weathering (biokarst or bioerosion) can further add to the jagged, etched and honey-combed effect from boring blue-green bacteria and invertebrate grazers (mainly molluscs such as gastropods), especially along the regions that are regularly wetted by waves and sea spray. Such plants produce acids and their filaments penetrate the rock promoting its disintegration.

It appears that distinct geomorphic zones exist on the platform that are discernible by their color, degree of  weathering and proximity to the land-marine interface.

 An extreme close-up of the razor-sharp, jagged and honey-combed surface on the platform.
This surface was virtually impossible to traverse safely in bare feet.

LET THERE BE LIFE
Not surprisingly, many creatures make their home in the intertidal zone of the platform which was teeming with life. Here are just a few inhabitants that I stumbled upon.

On the platform, a female polyplacophora (chiton) with 8-articulating shelly plates and her associated eggs are attached to the bottom of a shallow pool. Interestingly, the male chiton releases his sperm into the sea which finds (hopefully) a receptive egg-release. Various colorful gastropods (marine snails) were everywhere. Both chitons and snails are members of the Mollusc Phylum along with clams, mussels, oysters, squid and octopus.

A prickly-looking sea urchin, a member of the Echinoderm phylum (eg. starfish, sand dollars and brittle-stars),
bides its time.

A crab, a crustacean and member of the Arthropod Phylum (along with insects, spiders and extinct trilobites)

In summary, the Bahamas are largely a depositional landscape, unlike the more common, eroded landscapes of the continents, with their own unique carbonate signature. Both physical and chemical weathering can be observed on the platform that appear unique to marine coastal environments.

My casual stroll down the beach at Paradise Island turned out to be an unanticipated lesson for me in Bahamian dune composition, formation, lithification and weathering.  

P.S. Bahamian Landscapes by Neil Sealey is a great introduction to the geology and geography of the Bahamas with tremendous photos and illustrations!