White Sands National Monument is well-known for its fields of white sand dunes composed of gypsum crystals. The White Sands National Monument is a U.S. National Monument located 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 form of sand because it is water soluble. Normally, 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 subsequently dry out and leave gypsum in a crystalline form called selenite on the surface. During the last ice age a lake called Lake Otero covered 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 occasionally 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 eventually breaks the crystals in to sand-size grains that are carried away by the prevailing winds from the south-west, forming white dunes.
The dunes constantly change shape and slowly move downwind, covering the plants in their path. Some species of plants, however, can grow rapidly enough to avoid being buried by the dunes.
From the visitor center at the entrance of the park, the Dunes Drive leads 8 miles (12 km) into the dunes. Four marked trails allow to explore the dunes by foot. During the summer, there are also Ranger-guided orientation and nature walks. In areas accessible by car kids frequently use the dunes for sledding downhill.
The park lies completely within the White Sands Missile Range, and both the park and the U.S. 70 between the park and Las Cruces, New Mexico may be closed for safety reasons while tests are conducted on the missile range. On average, this happens about twice a week, for a duration of one to two hours.
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