How Four Hundred and Forty Thousand Years of Sand Got Here

The sand dunes at Great Sand Dunes National Park stand up to seven hundred and fifty feet tall, taller than any other dunes in North America, and they are sitting in a place that seems geometrically wrong for dunes to exist. This is the San Luis Valley, a high-altitude flat basin at roughly seventy-four hundred feet elevation, ringed by mountain ranges, sitting in the middle of Colorado. Dunes belong at sea level, in coastal deserts, where the wind has long fetches and no mountains to stop it. These ones appear to have not gotten the message.

They formed because three systems converged in one place over a very long time. The Rio Grande rift, an ancient crack in the continental crust, pulled the valley floor down and left the mountain ranges on either side standing higher. The mountains shed glacial sediment into the valley for hundreds of thousands of years, building an enormous flat apron of sand and gravel across the basin floor. Prevailing southwesterly winds picked up the lightest particles and carried them toward the Sangre de Cristo range. The range stopped them. Where the sand piled up against the mountains and the wind could not carry it further, the dunes grew.

The creek system keeps them from advancing. Medano Creek and Sand Creek flow along the base of the dunes each spring, carrying sand back down the valley when it creeps too far upslope. The dunes are not static. They migrate, grow, flatten, and rebuild. Individual dunes have been observed to move hundreds of feet in a single season. The overall field stays roughly where it is because the system is balanced, a sand budget in approximate equilibrium for several hundred thousand years.

The park was designated in two thousand four, converted from a national monument. The dunes were forming here for four hundred and forty thousand years before either designation.