Sculpted into graceful contours by countless centuries of wind and water, the Great Sand Dunes sprawl along the eastern fringes of the vast San Luis Valley of south-central Colorado. Covering an area of nearly thirty square miles, they are the tallest aeolian, or wind-produced, dunes in North America, towering 750 feet above the valley floor. With the addition of the enormous Baca Ranch and other adjacent lands, the dunes—originally designated as a National Monument in 1932—attained official National Park status in 2004. In Sea of Sand, Michael M. Geary guides readers on a historical journey through this unique ecosystem, which includes an array of natural and cultural wonders, from the main dunefield and verdant wetlands to the summits of the Sangre de Cristo Mountains. Described by explorer Zebulon Pike as “a sea in a storm” and by frontier photographer William Henry Jackson as “a curious and very singular phase of nature’s freak,” the Great Sand Dunes are a nexus of more than 10,000 years of human history, from Paleolithic big-game hunters to nomadic Native Americans, from Spanish conquistadores and transcontinental explorers to hard-rock miners and modern-day tourists in motor homes. Like these successive waves of visitors, Sea of Sand follows the water, analyzing its critical role in the settlement and development of the region. Geary also describes the profound impact that waves of human use and settlement have had on the land—which ultimately inspired the early grassroots efforts by San Luis Valley citizens to protect the dunes from further exploitation. He examines as well the more recent legislative effort led by an unprecedented coalition of local, state, and federal agencies and organizations, including The Nature Conservancy and the National Park Service, to secure the Great Sand Dunes’ national park designation. Amply illustrated, Sea of Sand is the definitive history of the natural, cultural, and political forces that helped shape this incomparable landscape.
About the Author
Michael M. Geary, author of A Quick History of Grand Lake, is currently a writer, researcher, and historian. He lives in Fort Collins, Colorado.
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Sea of Sand
A History of Great Sand Dunes National Park and Preserve
By Michael M. Geary
UNIVERSITY OF OKLAHOMA PRESSCopyright © 2016 University of Oklahoma Press
All rights reserved.
It is clear that the single most important factor governing the human habitation of the San Luis Valley is the availability of water.
Dennis Stanford, "History of Archaeological Research"
SCULPTED INTO THEIR PRESENT graceful contours by countless centuries of wind and water, the Great Sand Dunes sprawl along the eastern fringes of the vast San Luis Valley in south-central Colorado, covering an area of nearly thirty square miles. Hard against the rugged slopes of the Sangre de Cristo Mountains, they are the tallest aeolian (wind-produced, also spelled Aeolian or eolian) dunes in North America, heaping mounds of sand that tower up to 750 feet above the flat immensity of the valley floor. Despite their impressive height, the dunes are dwarfed beneath the lofty summits and thickly forested foothills of the Sangres. Strangely incongruous and otherworldly, seemingly isolated and remote at the base of the majestic mountains, the dunes are nonetheless intimately connected with the landscape that surrounds them, a massive intermontane basin that is often described as the largest alpine valley in the world.
Among the most enduring questions asked by generations of visitors to the massive dunefield is some variation of "How did all of this sand accumulate in the middle of the Rocky Mountains?" The answer to that query is constantly evolving, but most modern geologists agree that the origins of the Great Sand Dunes lie in the deepest recesses of geologic time, when large surface plates in the earth's crust beneath what is now the San Luis Valley began splitting apart in a process known as rifting. Crustal uplift accompanied this rifting, along with extreme volcanic activity that eventually unleashed the cataclysmic eruption of the La Garita Caldera about 27 million years ago, which many geologists believe was among the largest volcanic explosions in earth's history. In the aftermath, the San Juan Mountains began forming on the western fringes of the valley, born from the hot ashes and lava flows caused by the violent fracturing of the planetary shell. The Rio Grande Rift, a long, deep chasm that stretches from central Colorado all the way to northern Mexico, also appeared around this time, another product of the continual tectonic rending of the earth's crust. Around 18 to 19 million years ago, rotation of a large crustal plate forced the relatively rapid (geologically speaking) uplift of the Sangre de Cristo Mountains on the eastern edge of the valley, which further widened the rift and caused it to subside, deepening the chasm. The Rio Grande Rift has been collecting eroded sand, silt, gravel, and assorted detritus known as rift fill from the surrounding mountains ever since. In some places, this rift fill measures up to 17,000 feet thick, and it may go even deeper, with the very top layer visible as the floor of the modern San Luis Valley.
Geologic evidence further indicates that along with this rift fill, water has been accumulating in the valley for millions of years, creating ephemeral lakes and wetlands that appeared and disappeared as the climate changed over countless millennia. Most notable was an enormous body of water known to modern science as Lake Alamosa, which covered a considerable portion of the San Luis Valley beginning about 3.5 million years ago, expanding and contracting and depositing sediment in response to the fluctuating glacial and interglacial climates of the Pleistocene Epoch. In 2002, geologists confirmed the existence of Lake Alamosa with the discovery of lakebed deposits on low hills in the southern portion of the valley. At its fullest expanse, the ancient lake extended over sixty miles from north to south and nearly thirty miles east to west, making it one of the largest high-altitude lakes in North America. Lake Alamosa existed for roughly 3 million years, until around 440,000 years ago, when its waters over-topped a low point in the San Luis Hills, cut a deep gorge, and began draining southward, eventually joining the ancestral Rio Grande River.
Through untold eons of rifting and uplift, volcanic eruptions, sedimentary deposition, erosion, and the appearance and disappearance of transitory lakes both large and small, the material necessary for the creation of the Great Sand Dunes slowly accumulated on the floor of the San Luis Valley. Exactly how old this material is, as well as exactly where it originated, has long been debated by geologists, beginning in 1869 with Ferdinand V. Hayden's official United States Geological Survey report on the geology of Colorado and New Mexico. Hayden theorized that the sand came from "loose materials" of the Santa Fe Formation, the principal sediment in the Rio Grande Rift. These materials began accumulating sometime during the Miocene Epoch, which lasted from roughly 23 million to 5 million years ago. Curiously, Hayden, a renowned geologist who had traveled extensively throughout the American West, evidently found the prospect of sand dunes piled up to 750 feet high at the base of a range of mountains that included several 14,000-foot peaks to be so unremarkable that he devoted but a single sentence to them in his otherwise lengthy report on the region. Nonetheless, his map of the San Luis Valley, which first appeared in his Geological and Geographical Atlas of Colorado (originally published in 1877) formally labeled the vast dunefield the "Great Sand Dunes," which may be among the earliest uses in print of this now widely recognized term.
Geologist F. M. Endlich reached a different conclusion than Hayden, writing in 1877 that the "dunes seem to be of comparatively recent date, geologically speaking, and belong to the Post-Glacial age," which indicates that he thought the dunes began forming roughly 12,000 years ago, near the end of the last Ice Age. In addition, Endlich advanced the theory that the sand in the dunes came from eroded volcanic rocks in the San Juan Mountains. He declared, "The sand came across the valley from the mountains on the southwest, being driven to its present locality by the prevailing southwest winds ... especially collected in the reentrant angle in the mountain front near Mosca Pass as an effect of eddying currents in the winds, caused by the low gaps in the mountains near this point." Recent studies have confirmed Endlich's theory, indicating that roughly 70 percent of the sand in the Great Sand Dunes originated in the San Juans, and the remainder came from the Sangres. In 1910 geologist Claude Siebenthal produced a comprehensive report on the San Luis Valley and concluded, like Hayden, that the sand dunes were a remnant of the Miocene Santa Fe Formation that had been "broken down by the winds." He also theorized about the existence of an ancient lake in the middle of the valley, where in "the depression caused by the down warping in the center of the valley there was formed an extensive fresh-water lake, in which was deposited the series of sands and clays that make up the Alamosa formation." Siebenthal emphasized that he was not the first to propose the existence of such a lake; instead he conferred that honor upon an itinerant fur trapper named Jacob Fowler, who first visited the San Luis Valley in 1822 as he made his way up the Rio Grande. Siebenthal wrote of how "this observant trapper grasped the character and later geologic history of the valley" and quoted Fowler's observation that "I Have no doubt but the river from the Head of those Rocks up for about one Hundred miles Has once been a lake of about from forty to fifty miles wide and about two Hundred feet deep — and that the running and dashing of the Watter Has Woren a Way the Rocks So as to form the present Chanel [sic]." Nearly two centuries later, geologists confirmed the existence of the lake described by Fowler and Siebenthal and christened it Lake Alamosa. No doubt the fur trapper and the geologist would have been pleasantly surprised at how prominently their lake figured in future geologic studies and written descriptions of the San Luis Valley. Indeed, the idea of a large lake in the middle of the valley later became a recurring theme in the writings and advertisements of the so-called boosters who were trying to attract ranchers, farmers, and settlers to the valley during the latter half of the nineteenth century.
Between 1910 and 1962, several short papers were published in academic journals that essentially echoed or combined the ideas expressed by Hayden, Endlich, and Siebenthal regarding the Great Sand Dunes, and at least one author concluded that the age of the dunes was still open to question, given the conflicting opinions of previous geologists. Then, in 1967, geologist Ross Johnson proposed an entirely new interpretation for the origin and age of the sand dunes that became widely quoted and accepted for nearly thirty years. His theory, similar to Endlich's, suggested that the dune-building process began near the end of the last Ice Age, roughly 12,000 to 15,000 years ago, in an age known as the Late Pleistocene. Johnson surmised that during this period, as the last of the great continental ice sheets were retreating northward, the waters of what is now the Rio Grande River exited the San Juan Mountains near Del Norte and flowed directly eastward toward the vicinity of the present-day San Luis Lakes, where the river abruptly turned south. Over thousands of years, as melting glaciers poured their waters into the valley, the course of the Rio Grande gradually shifted to the southwest. In the process, the river deposited enormous quantities of eroded rock and sand from the San Juans in great alluvial fans across the center of the valley floor. According to Johnson's theory, this mineral debris provided the raw material for the eventual formation of the Great Sand Dunes.
As the understanding of aeolian geology and dune systems evolved throughout the late twentieth and early twenty-first centuries, a more rigorous scientific analysis of the geologic processes at work in the San Luis Valley presented alternatives to Johnson's hypothesis. For instance, a 2007 study that received considerable support in the scientific community, authored by U.S. Geological Survey (USGS) geologist Richard Madole and his colleagues, proposed that the sand dunes might actually be much older than the Late Pleistocene. Moreover, the geologists asserted that the sand in the dunes may not have originated in the alluvial-fan deposits of an ancestral Rio Grande River, but instead came from ancient dry lakebeds on the floor of the Closed Basin, a portion of the San Luis Valley known locally as the "sump" that has no natural drainage aside from downward seepage. The geologic study concluded that the dunes were the product of multiple episodes of sand deposition and transport that were influenced by climatically driven fluctuations of the underlying water table over tens, or even hundreds, of thousands of years. In other words, as the climate of the San Luis Valley continually changed over time, the peculiar hydrology of the Closed Basin caused a series of shallow lakes to repeatedly form and then disappear over a time span of unknown duration, eventually creating the extensive sand sheet that became the primary source for the sand in the Great Sand Dunes. West of the sand sheet, the mineralized hardpan of the sabkha (an Arabic word roughly translated as "salt flat") bears further witness to this process, as fluctuations in the water table repeatedly brought alkaline minerals to the surface and created a hardened crust of sand cemented together by the evaporative concentration of salts.
Since the dunes could only have begun forming when sufficient sand was available on the valley floor, and since the presence of an enormous prehistoric Lake Alamosa in the middle of the San Luis Valley would have necessarily precluded the movement of sand across the valley, Madole and his colleagues concluded that the unique geologic and climatic conditions most conducive to the initial formation of the sand sheet and the Great Sand Dunes probably began coalescing sometime after Lake Alamosa began draining, perhaps around 440,000 years ago. Thus the dunes could in fact be considerably older than the previously accepted estimate of approximately 12,000 years old. Using a process called optically stimulated luminescence (OSL), which measures how long sand grains have been in complete darkness deep under the surface, recent analysis of sand cores drilled at various locations in and around the main dunefield seems to confirm this conclusion, indicating sand ages between 18,000 and 67,000 years old. Testing of drill cores taken from gravel deposits elsewhere in the valley revealed sand up to 130,000 years old, placing the age of the dunes well into the Middle Pleistocene.
Despite over a century of shifting theories regarding the age and origin of the Great Sand Dunes, geologists unanimously agree that the valley's strong prevailing southwesterly winds bear primary responsibility for the initial formation and continual replenishment of the main dunefield. Loose sand on the floor of the San Luis Valley is continually swept up by the prevailing winds and transported eastward, either as airborne particles or bounced along the ground in a process known as saltation, toward a series of three low passes clustered together in a distinct pocket or indentation (known in geological terms as a reentrant) in the Sangre de Cristo Mountains: Music Pass to the north, Medano Pass in the middle, and Mosca Pass to the south. Funneled into these low passes by the broad flanks of the Sierra Blanca massif (also known as Blanca Peak and Mount Blanca) and by the formidable barrier of the high Sangres, the sand-laden wind loses its momentum to friction and turbulence as it rises. No longer capable of carrying its load, the wind continually deposits sand at the foot of the Sangres, resulting in the towering accumulation that modern visitors recognize as the Great Sand Dunes.
Far from static, the dunes are in a constant state of flux, their contours forever changing as the pervasive forces of erosion and deposition perpetually shape and reshape the shifting sands. Yet the dunes also exhibit a remarkable degree of long-term stability. Geological and photographic studies indicate that the main dune mass itself has moved very little over the last century, and quite possibly has remained that way for considerably longer. A number of factors account for this stability, including the fierce northeasterly "reversing winds" that occasionally gust through the low passes in the Sangres whenever low-pressure weather systems are located east of the San Luis Valley. These winds blow in the opposite direction of the valley's prevailing southwesterly winds, which temporarily halts the relentless eastward migration of sand and contributes to the dunes' tremendous height.
Intermittent clumps of vegetation (blowout grasses, scurfpea, Indian ricegrass) further anchor the sand in the active dunefield, although the role played by plant life in stabilizing the dunes is relatively minor. Far more crucial are the thin ribbons of water that course along the edges of the Great Sand Dunes, where centuries of natural geologic processes have produced a rather exceptional sand "recycling" system that partially relies on the waters of Medano and Sand Creeks. These waterways, so incongruous in the very midst of the heaping mountains of sand, serve both as barriers to the shifting dunes and as conduits for transporting sand. In a process that has continued ever since the dunes began forming, drifting sand constantly threatens to bury Medano and Sand Creeks, which respond by carving away the eastern and northern perimeters of the massive dunefield. The creeks then wash this eroded material out to the sand sheet to the west, where the valley's pervasive winds continually lift the sand and deposit it back on the dunes. Coupled with the reversing winds that blow opposite the prevailing southwesterlies and the vegetative life that struggles to survive in the depressions between the dunes, Medano and Sand Creeks are critical elements in the dynamic and elegant process of sand erosion, transport, and deposition that ensures the long-term development of large dune forms at the Great Sand Dunes. However, the presence of the "escape dunes" on the far eastern fringes of the main dunefield confirms that even this formidable combination of factors cannot hold back the entire, inexorable tide of shifting sand. The escape dunes have crossed Medano Creek and climbed the foothills of the Sangres, where they have buried, killed, then reexposed entire groves of ponderosa pine trees, creating the eerie Ghost Forest, which stands as stark testimony to the continual transformation of this remarkable landscape.
Excerpted from Sea of Sand by Michael M. Geary. Copyright © 2016 University of Oklahoma Press. Excerpted by permission of UNIVERSITY OF OKLAHOMA PRESS.
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Table of Contents
List of Illustrations ix
Reflections in the Sand 3
1 Prehistoric Dunes 10
2 Outskirts of Empire 32
3 Dunes in the Great Desert 50
4 Arid Eden 79
5 Monumental Dunes 102
6 Sand and Water 131
7 Racing for a National Park 157
8 Unimpaired for Generations 176
Lessons in the Sand 206