White Sands National Monument - Sand Dunes

The gypsum that forms the white sands was deposited at the bottom of a shallow sea that covered this area 250 million years ago. Eventually turned into stone, these gypsum-bearing marine deposits were uplifted into a giant dome 70 million years ago when the Rocky Mountains were formed. Beginning 10 million years ago, the center of this dome began to collapse and create the Tularosa Basin. The remaining sides of the original dome formation now form the San Andres and Sacramento mountain ranges that ring the basin.

A Rare Form of Sand

The common mineral gypsum, a hydrous form of calcium sulfate, is rarely found in the form of sand because it is soluble in water. Rain and snow that fall in the surrounding mountains dissolve gypsum from the rocks and carry it into the Tularosa Basin. Normally, dissolved gypsum would be carried by rivers to the sea. But no river drains the Tularosa Basin. The water, along with the gypsum and other sediments it contains, is trapped within the basin. A thick layer of recrystallized gypsum underlies much of the White Sands dunefield. The thickness of this layer at the playa lake at Lake Lucero ranges between 3 and 6.6 meters and up to 48.7 meters at a test well located in the alkali flats north of Lake Lucero.

Lake Lucero

With no outlet to the sea, water flowing into the Tularosa Basin either sinks into the ground or pools up in low spots. One of the lowest points in the basin is a large playa called Lake Lucero. Occasionally, this dry lake bed fills with water. As the water evaporates, the dissolved gypsum is deposited on the surface. Even more gypsum deposition occurred during the last Ice Age when a larger lake, Lake Otero, covered much of the basin. The Alkali Flat area is the exposed bed of this Ice Age lake. In wet periods, water evaporating slowly on the playa floor causes gypsum to be deposited in a crystalline form called selenite. Along Lake Lucero"s shore and in the Alkali Flat, beds of selenite crystals - some three feet long - cover the ground. The forces of nature - freezing and thawing, wetting and drying - eventually break down the crystals into sand-size particles light enough to be moved by the wind. Strong winds blowing across the playa pick up gypsum particles and carry them downwind. As the sand grains accumulate into a dune, they bounce up its gentle windward slope, rippling its surface. At the dune"s steep leading edge, sand builds up until gravity pulls it down the slip face, which moves the dune forward.

Four Types of Dunes at White Sands

Dome dunes - The first dunes to form downwind of Lake Lucero are low mounds of sand that move up to 30 feet per year.

Barchan dunes - Crescent-shaped dunes form in areas with strong winds but a limited supply of sand.

Transverse dunes - In areas with ample sand, barchan dunes join together into long ridges of sand.

Parabolic dunes - On the dune field edges, plants anchor the arms of barchans and invert their shape.