The monument is situated at an elevation of 4,235 feet (1,291 m) in the mountain-ringed Tularosa Basin and comprises the southern part of a 275 sq mi (710 km2) field of white sand dunes composed of gypsum crystals. The gypsum dune field is the largest of its kind on Earth. Gypsum rarely occurs as sand because it is water-soluble. Normally, rain would dissolve the gypsum and 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 Sacremento. Thus water 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.
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