The sand dunes began to form just under a half a million years ago, as sedimentary outwash from the floodplains of the Rio Grande and surrounding watersheds of the San Luis Valley were picked up by strong westerly winds. The winds would lose force after passing over the Sange de Christo mountains, before heading east across the plains, depositing the sand particles at the base of the range. These geomorphological processes are still at work, today, and the sand dunes still continue to grow.