Gypsum rarely occurs as sand because it is
water-soluble. Rain usually dissolves gypsum and rivers then carry it to the sea. The Tularosa Basin has no outlet to the sea, so it traps rain that dissolves gypsum from the surrounding
San Andres and
Sacramento Mountains. The rainwater either sinks into the ground, or forms shallow pools that subsequently dry out and leave gypsum on the surface in a crystalline form called
selenite. Groundwater that flows out of the Tularosa Basin flows south into the Hueco Basin.
[25] During the last
ice age, a lake now called Lake Otero covered much of the basin. When it dried out, a large flat area of selenite crystals remained, which is named the Alkali Flat.
Lake Lucero, a dry lake bed which occasionally fills with water, is located in the southwest corner of the park, at one of the lowest points of the basin.
The ground in the Alkali Flat and along Lake Lucero's shore is covered with selenite crystals that measure up to three feet (1 m).
Weathering and
erosion eventually break the crystals into sand-size grains that are carried away by the prevailing winds from the southwest, forming the white dunes. The dunes constantly change shape and slowly move downwind. Since gypsum is water-soluble, the sand that composes the dunes may dissolve and
cement together after rain, forming a layer of sand that is more solid, which increases the wind resistance of the dunes.
[26] The increased resistance does not prevent dunes from quickly covering the plants in their path. Some species of plants, however, can grow fast enough to avoid being buried by the dunes.
