If you look around you as you stand on top of a big
dune at White Sands National Monument, you will notice that you are surrounded
by mountains. The San Andres Mountains rise above the basin to the west,
and the Sacramento Mountains border it to the east. 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. You may wonder why there aren't gypsum sand dunes on the other sides of the mountains. The rain certainly washes gypsum down there too. Why is it only in the Tularosa Basin that the gypsum forms sand dunes?
The Tularosa basin has no rivers running out of it. There is no way for water entering it to get out. There are large rivers to the east and west of the basin that carry rainwater to the Gulf of Mexico, but all the rainwater that falls inside the basin stays here. It collects at the lowest spot on the basin floor. This spot is Lake Lucero, and it's located at the southwest corner of the dune field.
It is here at Lake Lucero that the sand dunes begin to form. Lake Lucero is a playa, or seasonal lake. Water only collects there during the rainy season, and the rest of the year is it dry. Even when the lake has water, it is only about 12 inches deep, so it is not a place you could go fishing, or swimming. As the water begins to evaporate, the gypsum that is in the water begins to form a crust on the lake bed. Some of the gypsum forms beautiful crystals called selenite.
Selenite crystals are very soft. You can scratch them
with your fingernail. They are so soft that it doesn't take very long before
the wind and rain begin to break them apart into smaller and smaller and
smaller pieces. Soon they become so small that the wind can pick them up
and carry them for short distances. When the pieces get to be this size,
1/16 to 1 millimeter in diameter, they are called sand.
We can see that there are many things that help to build the gypsum dunes at White Sands National Monuments. The mountains supply the gypsum. The rain washes the gypsum into the basin. The shape of the basin keeps the gypsum from running into the sea, and the wind breaks up the gypsum crystals into sand grains.