"On the day that followed they crossed a lake of gypsum so fine the ponies left no track upon it."
Gypsum sand dunes at White Sands National Monument, New Mexico
Creative Commons AttributionGypsum sand dunes at White Sands National Monument, New Mexico - Credit: Raul Diaz

Gypsum is a very soft mineral composed of calcium sulfate dihydrate.

It is one of the most common mineral compounds found on Earth but is rarely seen on the surface, as it dissolves in water. One exception is the Tularosa Basin in New Mexico (today known as the White Sands National Monument) where the unique conditions have created a 710 km2 (270 sq mi) expanse of white gypsum sand. The origin of the desert dates back to around 100 million years ago, during which it was covered by a shallow sea. As its waters gradually receded, saltwater lakes were left behind, which eventually evaporated in the sun. In addition to the salt, gypsum was also laid down in thick deposits on the old seabed.

The area was declared a government-protected U.S. National Monument in 1933.