White Sand Monument is located at the northern end of the Chihuahuan Desert lies a mountain-ringed valley, the Tularosa Basin.
This sand is called gypsum, which dissolve like salt and sugar in water. In this case, when the rain falls on the mountain layers of the gypsum, it runs down the mountain and dissolve. On the other side of the mountain this does not happens because there is rivers that carries rainwater out into the Gulf of Mexico.
In the Tularosa Basin all the rainwater stays there and collects in the lowest spot on the basin floor. This is known as Lake Lucero, located at the southwest corner of the dune field. This is were the sand beings to form. Lake Lucero is a playa or seasonally lake, which means water is only there during raining seasons. When water is there it is only about 12 inches deep, therefore you can not go swimming or fishing. Once the lake evaporate, the gypsum turns into curst on the lakebed. Some of the gypsum turn into crystals called selenite.
Selenite is very soft, in fact you starch your finger nails across it. When the wind blows, the selenite is broken down into small pieces. These small pieces can be picked up in the wind and carried short distances. When the small pieces are between 1/400 to 1/12 of an inch, it is called sand.