White Sands

The White Sands National Monument is a U.S. National Monument situated about 25km (15 miles) southwest of Alamogordo in the state of New Mexico. The area is in the mountain-ringed Tularosa Basin valley area and comprises the southern part of a 710km² (275-square mile) field of white sand dunes composed of gypsum crystals.
Gypsum is rarely found in the shape of sand because it is water soluble. generally, rain would dissolve the gypsum and carry it to the sea. Since the Tularosa Basin has no outlet to the sea, rain that dissolves gypsum from the surrounding San Andres and Sacramento Mountains is trapped within the basin and either sinks into the ground or forms shallow pools that next dry out and leave gypsum in a crystalline form called selenite on the surface. During the last ice age a lake called Lake Otero enclosed much of the basin. When it dried out it left a large flat area of selenite crystals which is now the Alkali Flat. Lake Lucero, at the south-west corner of the park, is a dry lake bed at one of the lowest points of the basin that infrequently fills with water.
The ground in the Alkali Flat and along Lake Lucero's shore is covered with selenite crystals which reach lengths of up to three feet. Weather erosion ultimately breaks the crystals into sand-size grains that are carried away by the prevailing winds from the south-west, forming white dunes.