Sands of Time:
A Natural History of Colorado’s Great Sand Dunes

Written By Quentin Septer

Note: This is a modified excerpt from my book, Where Land Becomes Sky: Life and Death Along the Colorado Trail, available now.

Late in the year of 1806, a 27-year-old United States Army Lieutenant by the name of Zebulon Pike led an expedition to Colorado’s Rocky Mountains. Commissioned to explore the region in search of the headwaters of the Arkansas River, Pike led his men and mules over the Sangre de Cristos and down the western slope of Mosca Pass in the dead of winter. Half starved and severely frostbitten after weeks of frigid travel, Lt. Pike penned the world’s earliest written words describing Colorado’s Great Sand Dunes in the pages of his journal, dated January 28, 1807:

“After marching some miles, we discovered…at the foot of the White Mountains [known today as the Sangre de Cristos] which we were then descending, sandy hills…When we encamped, I ascended one of the largest hills of sand, and with my glass could discover a large river…The sand-hills extended up and down the foot of the White Mountains about 15 miles, and appeared to be about 5 miles in width. Their appearance was exactly that of the sea in a storm, except as to color, not the least sign of vegetation existing thereon.”

Colorado’s Great Sand Dunes National Park and Preserve is an otherworldly landscape. On the western flank of the Sangre de Cristo Mountains, the continent’s tallest sand dunes rise some 755 feet above the floor of the San Luis Valley. The dunes span an area of about thirty square miles, at the site of a bygone lake. The high peaks of the Sangre de Cristos—the Blood of Christ Mountains—enclose the dunes in a half-circle. Mount Herard sails to the north. Mount Zwischen rises to the east, and to the south is North Zapata Ridge. To the west, the San Luis Valley stretches endlessly toward the San Juan Mountains. The peaks loom prominent and picturesque in the distance. 

This place serves as the continent’s hourglass; sands of geologic time stretching back through the epochs. Fine and silky smooth in appearance, the awe-inspiring expanse of the Great Sand Dunes and its five billion cubic centimeters of sand formed not by way of ground-breaking geologic events, but grain-by-grain, with the flow of time. How long the dunes have existed in their current form, no one can say with certainty. Where to begin the story of the Great Sand Dunes’ genesis is simply a matter of perspective.

Around 80 million years ago, a tectonic plate underlying the Pacific Ocean collided with the continental crust of North America. Like a near miss in a head on collision, one plate zigged, and one zagged. The Farallon Tectonic Oceanic Plate, as it’s known among geologists, slipped beneath the crust of the continent, grinding along the underside of California and the western United States as the plate moved eastward. The crust of the American West wrinkled and rose, forming volcanoes in modern-day California and giving shape to John Muir’s beloved Sierra Nevada. And the plates kept moving. Earth’s crust crumpled and ascended still higher, shaping a new mountain range in the middle of the continent. This range became known as the Rocky Mountains. This collision of tectonic plates and the subsequent uplift of the landscape became known as the Laramide Orogeny.

The Farallon plate penetrated nearly one thousand miles into the North American mainland before sinking toward Earth’s mantle. As the plate sank, magma swelled between these two layers in the Earth. Hydrothermal and volcanic forces ascended toward the planet’s crust, and the landscape rose like a buoy on the surface of the sea. Volcanoes began to erupt. About 35 million years ago, “a large body of molten rock moved into place deep beneath the area and thrust up the high peaks of the Rocky Mountains still farther,” geologists Bruce Bryant and Peter L. Martin wrote in a report for the United States Geological Survey. “In many places this magma broke through to the surface, and volcanoes covered the terrain with vast flows of lava and ash.” All told, nearly a third of Colorado was layered by ash flows. Today, that lava and ash has hardened to form the igneous rocks of the San Juan Mountains.

Meanwhile, the crust of North America continued to drift westward atop the Farallon plate—as it still does today, albeit only very, very slightly—and faults began to develop in the landscape. One such fault was the Rio Grande Rift. The Rio Grande Rift thrust the region skyward. The San Luis Valley grew wider. To the west of what would become the Great Sand Dunes, the San Juans rose to towering heights. To the east rose the Sangre de Cristos.

For tens of millions of years, the mountains were weathered and eroded. Southwesterly winds gusted across the San Juans, carrying sediments of igneous stone and quartz to the floor of the San Luis Valley. Visit Colorado’s Great Sand Dunes, and take a close look at a handful of sand, and you’ll see a mixture of fine, white quartz grains and dark, volcanic sediment blown in off the San Juans. Rivers and streams flowed with heightening power from the newly risen ranges, too. During the Pleistocene, some two million years ago, the Rio Grande surged with glacial runoff, carrying untold volumes of sediment down into the San Luis Valley. Sediment piled some three miles thick on the valley floor. Today, we see that sediment as nothing more than the region’s soil—an intrinsic, stagnant, stable part of the landscape. It’s anything but.

Winds continued to blow. Sand swept across the floor of the San Luis Valley, toward the Sangre de Cristo Mountains. Three passes in the Sangre de Cristos—Medano, Music, and Mosca Pass—form a funnel into which southwesterly winds and their cargo of sediments flow. Here, the southwesterlies are met with south-east blowing winds, and airborne sand swirls and settles on the valley floor. A body of water known as Lake Alamosa once flanked the Sangre de Cristos, at the site of today’s Great Sand Dunes National Park and Preserve. But as wind continued to funnel into the region, and sediment continued to accumulate on the floor of Lake Alamosa, sand dunes rose from the lake’s surface, like a lotus flower.

Lake Alamosa disappeared about 440,000 years ago. The lake was drowned in sand, so to speak. Shortly thereafter—in geologic time, that is—the climate began to change. “Dramatic natural climate change,” in the words of the National Park Service, led to the drying of still other lakes on the floor of the San Luis Valley. Sand left in these lakes’ wake, too, was blown toward the Sangre de Cristos. For hundreds of thousands of years, sand heaped upon the playa of Lake Alamosa, giving form to Colorado’s Great Sand Dunes.

The Utes called the Great Sand Dunes Saa waap maa nache, “sand that moves.” It’s an apt designation. The prevailing southwesterly winds of the region, met by southeasterly winds gusting off the Sangre de Cristos, has shaped, patterned and textured the region's dunes. Reverse dunes, as they’re known among geologists, form where wind frequently reverses direction. Sand is blown eastward, forming a dune. Then it's blown westward. Sand piles back upon itself, and the dune grows in height. Such winds sculpt smooth, symmetrical sand dunes. These “reverse dunes” are the most common kind of sand dune in the Great Sand Dunes National Park and Preserve. Transverse dunes can be found along the eastern flank of the main dunefield, too, where tons of sand is blown into a pocket of the park that’s shielded from easterly winds coming off the Sangre de Cristos. Barchan dunes are found along the rim of the main dunefield—the remnants of larger, bygone sand dunes. And star dunes can be seen at the center of the dunefield, where winds are omnidirectional and omnipresent.

Medano Creek flows along the southern edge of the Great Sand Dunes. This stream, along with the nearby Sand Creek, carries dune sand down to the floor of the San Luis Valley, where southwesterly winds blow the sand back toward the main dunefield of the Great Sand Dunes National Park and Preserve. In this manner, the sand is recycled, and the Great Sand Dunes have remained remarkably stable over time. The earliest photographs of the area, captured by William Henry Jackson of the United States Geological Survey in 1874, show that the dunes of the late nineteenth century look much the same as the dunes of today.

Hike to the west of Colorado’s Great Sand Dunes, just west of the main dunefield, and you’ll be met with southwesterly winds. You’ll see a fine mist of sand flow across the landscape, toward the Great Sand Dunes themselves. 

And you’ll see miniature transverse dunes, left in the wake of the gales.