The Soft Sunset 2 of White Sands National Monument, New Mexico
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The white sands are not ordinary sand, but are made of almost pure gypsum. According to Wikipedia, 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. White Sands National Monument is enclosed, meaning that it has no outlet to the sea and that the 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. Wind erosion breaks any gypsum crust into sand particles. Unlike dunes made of quartz-based sand crystals, the gypsum does not readily convert the sun's energy into heat and thus can be walked upon safely with bare feet, even in the hottest summer months.