Sand dunes and Sangre de Cristo Mountains, Great Sand Dunes National Park and Preserve, Colorado, USA

Sands of time. The towering sand dunes of Colorado

These ancient sand dunes are pictured in front of the Sangre de Cristo Mountains, a subrange of the Rocky Mountains here in southern Colorado, in the western USA. Humans have lived around the edge of the fertile San Luis Valley just east of the source of the Rio Grande river, for at least 11,000 years. But that's just a few grains in the hourglass for this sand field which formed when huge glacial lakes dried up, leaving sediments that were blown against the mountains by the wind. Slowly, they formed the tallest sand dunes in North America – up to 750ft (229m) high.

Now known as Great Sand Dunes National Park and Preserve, it was made a US national park 16 years ago, although it has been a national monument (a protected area) since 1932.