White Sands National Monument is composed not of silica sand but of grains of gypsum. Deposits of this mineral formed at the bottom of a shallow sea 250 million years ago. It eventually turned to stone and now covers a playa area (dry lake bed) which sometimes fills with water. Crystals of gypsum form, break down and eventually blow northeast to this area. I was there during a stormy week and there was standing water between the dunes.