This 60-second animation depicts the formation of the dunes. Through the video, the corresponding dates scroll rapidly from 400,000 years ago to present day. There is no audio.

1) 400,000 years ago: The San Luis Valley is flanked by the San Juan Mountains on the left and Sangre de Cristo Mountains on the right. The valley is filled with sediments left behind by a large lake that once covered the valley. The text reads: "The subsiding rift valley created a depression that filled with sandy sediment."

2) 120,000 years ago: The mountains and valley are covered with glaciers and snow, which then melts away into intermittent lakes fed by streams. The text reads: "Over thousands of year, melting glaciers filled the valley with streams and lakes where sand was deposited."

3) 60,000 - 3,000 years ago: Repeated glaciation is followed by the intermittent appearance of lakes then sand deposits left behind as the lakes recede. Streaks of wind are depicted, blowing sand from the lakebeds into a pocket of the Sangre de Cristo Mountains. The text reads: "Prevailing winds pushed sand up against the base of the mountains. Migrating dunes moved easily in the open valley due to lack of vegetation."

4) 3,000 years ago to Present Day: The animation zooms into the forming dunefield, which continues to grow at the base of the mountains. Storm winds periodically reverse the dune ridges, causing them to oscillate with the opposing wind directions. The line of Medano Creek is on the southern edge of the dunefield. The text reads: "Multiple wind directions along the mountain front have caused the dunes to grow to their impressive height."

Animation: Argentine Productions https://argentineproductions.com/
for the National Park Service