You will also notice that both mountain ranges have the same striped appearance. The stripes are really thick layers of sedimentary rock. The layers are easy to see because they are different colors. Each layer is made of a different material. The darker ones are layers of shale.
Lighter bands are limestone.
These are the ones that are important to our story because they contain gypsum. The sand at White Sands National Monument is almost pure gypsum. Gypsum is different from many other rocks because it is readily soluble. That means it will dissolve in water, just like sugar or salt. When rain falls on the mountains, the layers of gypsum start to dissolve, and the gypsum runs down the mountains as fast as the water can carry it.