Located in the mountain-ringed Tularosa Basin, at about 25 km southwest of Alamogordo, at an elevation of 1,291m, White Sands National Monument comprises the southern part of a 710-km² field of white sand dunes composed of gypsum crystals (the largest gypsum dune field in the world). Gypsum is rarely found in the form of sand because it is water-soluble, and normally rain would dissolve the gypsum and carry it to the sea. The Tularosa Basin is enclosed, so that rain that dissolves gypsum from the surrounding San Andres and Sacramento Mountains is trapped within the basin. Thus water either sinks into the ground or forms shallow pools which subsequently dry out and leave gypsum in a crystalline form, called selenite, on the surface.
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