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 Monument. 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.
Sand can only be moved by strong, steady winds. The air must be moving at least 15 miles per hour to be able to pick up sand grains.
In the Tularosa basin, it is primarily between February and April that the winds are strong enough. These winds are called unidirectional winds because they always move in the same direction, from the southwest to the northeast. As the wind blows, it pushes the sand ahead of it, so individual dunes are slowly moving to the northeast.
Sand is not as easy to move as you might think. Even very strong winds can't lift the sand any higher than three feet above the ground. As the wind blows, it lifts small sand grains a few feet off the ground, then drops them. When they hit the ground, they bump into other sand grains and cause them to jump up and be caught by the wind. It's almost as if the sand is playing leap-frog, jumping and bumping along. This kind of jumping movement is called saltation. You can see this kind of movement on the windward side of the dune, or the side facing the oncoming wind. But what about the leeward side of the dune, protected from blowing wind? What causes the sand to move on that side?
As the tiny sand grains slowly work their way up the windward dune face, they finally reach the crest, or top of the dune. They fall over the crest and start to pile up because they are protected from the wind. Now gravity steps in to move the dune. As more and more sand grains pile up, the angle of the leeward face becomes steeper and steeper. A pile of loose material, like sand, can only hold a slope of about 34 degrees. When the slope gets greater than 34 degrees, gravity pulls the loose sand down. Small avalanches occur. The sand might run down the leeward face like a waterfall, or the whole side might slip at once. When the entire face of the dune slips, it's called slumping.
How fast a sand dune moves depends on a number of things. Of course, the speed of the wind is a big factor. Wind that is blowing 45 miles per hour will move more sand than at 15 miles per hour. The size of the dune is also important. Smaller dunes with less sand move much more quickly than large ones. The vegetation also plays an important role. The dunes get caught on the plants that grow in the basin, and that slows them down.
There are 4 types of dunes at White Sands National Monument. The first ones to form, the "baby" dunes, are called embryonic dunes. They are usually not more than 20 feet. high, and speed along the basin floor as much as 40 feet a year. Transverse dunes form long ridges of sand, and can be very tall. They move much more slowly - usually between 8 to 12 feet a year. Barchan dunes are crescent shaped. This dune looks like a new moon. The arms or horns of the barchan dune always point in the direction the wind is blowing. Barchan dunes are also very large, and move between 8 and 12 feet per year. The last type is called a parabolic dune, and looks like an inside out barchan dune. The movement of this dune has been slowed down by vegetation, and rarely moves more than 5 feet per year.
The desert is not an easy place for plants to live. There is very little water in the Tularosa basin. Precipitation, or the amount of rain and snow that falls, averages only about 8 inches each year. Most of the rain falls in July, August and September. The rest of the year is very dry. Temperatures during the summer can soar to 110 degrees during the day, then drop to 50 degrees at night.
A lack of water is not the only thing that makes living in the dunes difficult. Windstorms come up from the southwest in February and March. They can last for several days, and wind speed can reach 45 miles per hour. During these windstorms, the plants are blasted by the blowing sand. Desert soils also have very few nutrients, like nitrogen, potassium, and phosphorous, that plants need to stay healthy.
You might think that desert plants are always struggling, always fighting to survive in these harsh conditions. But they do quite well here.
Desert plants have adapted to their surroundings. They have changed in ways that help them to conserve water, and grow in the very poor desert soils. The fourwing saltbush grows very well in the salty soil, and sends its roots deep into the ground to get to water. This is an easy plant to spot because its seeds have four, papery wings. The Russian thistle, also called tumbleweed, also likes salty soil. This plant uses the wind to help scatter its seeds as it rolls along the desert floor. In fact, if these plants were given more water and better soils, they would probably die.
Plants lose a great deal of water through their from leaves by evaporation. What do you notice about the leaves of desert plants? They are usually very small. Big leaves would allow too much water to escape. But small leaves helps the plant conserve water. You may also notice that the color of the leaves are different. They are rarely bright green. Instead, most are a dull gray. If you look closely, you will see why. The leaves of some plants are covered with tiny hairs. Leaf hairs help to shade the plant from the sun, and protect it from being dried out by the wind. Other plants have a thick, waxy covering on their leaves. This is called a cuticle, and it also helps keep the water in the leaves from evaporating.
A few plants look like they don't have any leaves. The Mormon Tea plant looks as if all its leaves have fallen off, and all that's left are bare twigs. The leaves of the Mormon Tea have become so small that you can hardly see them. They don't even work like the leaves on other plants. Most plants make food in their leaves, but the Mormon Tea makes food in its green stems.
Some plants, like those in the cactus family, don't have any leaves at all. Their leaves have changed into sharp spines that protect them from being eaten by animals, and also help to shade the plant. The members of the cactus family come in many different shapes and sizes. The cholla is very common at White Sands. It is very tall, with long branching arms that break off easily and usually fall all around the base of the plant. The prickly pear has wide, flat pads that look like paddles. Some native people like to eat the fruit of the prickly pear, called a tuna. The claret cup hedgehog has short, fat stems that look like little barrels.
There are many plants that have adapted to living in the desert. In the whole Tularosa basin there are 240 different kinds of vascular (stemmed) plants. As you get closer to the dune field, it becomes more and more difficult to survive, so fewer plants are found. Only 60 different kinds can live within the dune field. Most of these live in the interdunal flats between the dunes.
The big problem plants have in the dune field is being buried by moving sand dunes. Plants can't walk away from a dune that is starting to bury them. Most of the time the plants simply die because they can no longer get the oxygen they need once the sand has covered them. Living on top of the dunes requires very special adaptations. Only eight kinds of plants have managed to adapt in ways that will allow them to survive the moving sand.
One way a plant can survive is to grow faster than the dune is moving. The soaptree yucca uses this strategy. When it begins to get buried, the stem of the yucca starts to grow very rapidly and pushes the green leafy part above the surface of the sand. The stem can grow as much as 12" a year, and may get to be 30' tall. When you see a yucca plant on top of a sand dune, you can be sure that beneath it is a long stem that stretches all the way down through the sand.
Another way plants can survive in the moving sand is to try to hold on to a part of the dune. Plants like the Skunkbush Sumac send their roots out and down, deep into the sand. The roots hold on to the gypsum, and form a hard mound for the plant to grow on after the sand dune has moved away. We call these mounds pedestals. The Rio Grande Cottonwood, the only tree that survives in the dune field, also builds large pedestals by sending out roots from its buried trunk. The Rosemary Mint holds on to the sand with tiny, fast growing roots. If you look at the sides of these plant pedestals, you can see the exposed roots.
By growing faster than the dune can move, and holding on to the sand by building hard pedestals, a few plants are able to live very well on the sand dunes. But most plants growing in the dune field are found in the interdunal flats and around the edges of the dune field where the dunes are moving very slowly. In the heart of the dune field, it is a different story. Here the sand dunes are so big, and move so fast, that nothing can grow fast enough to escape.
Desert animals, like desert plants, have adapted in very special ways to live in this hot, dry environment. When finding water is a problem, many animals develop ways of living to help them use less water.
One way to save water is to stay as cool as possible. When people get too hot their bodies start to sweat. This water, coming from small pores in the skin, helps to cool the skin and keeps the body temperature from getting too high. Although most desert animals don't sweat, the same type of cooling happens when an animal pants. The body can lose a lot of water under the desert sun.
People are lucky because they can carry water with them in canteens and waterbottles. Animals must rely on the water that they can find. Most of the time little water is available, so animals have to be masters at keeping cool and saving water.
Almost all desert animals are smart enough to stay out of the sun during the hottest part of the day. They stay deep underground in burrows. There the sand is much cooler, and burrowing animals, like the kangaroo rat, the badger, the gopher, the coyote and the kit fox, sleep while you are playing on the dunes. At night, after the sun goes down and the sand cools off, the animals come out to hunt for food. When an animal is active at night and rests during the day, it is called nocturnal.
There are a few animals that can be seen during the day. Darkling beetles are crawling about, red-tailed hawks are flying overhead, and you might see a whiptail lizard darting past your feet. These animals, active during the day and inactive at night, are called diurnal. Diurnal animals protect themselves from the heat by spending most of the day in whatever shade they can find.
Because there are no permanent bodies of water in the dune field, most animals get their water from the food they eat. Green leaves contain lots of water. The animals that eat only plants, like jackrabbits and gophers, are called herbivores. The body of an animal also contains liquid in its blood and tissues. Animals that eat only other animals are called carnivores. The coyote will eat just about anything, plant or animal. That's what makes him a master at desert survival. Animals that eat both plants and other animals are called omnivores.
The best desert survivor is the kangroo rat. This little rodent eats only dry seeds, and is never seen drinking water. He is able to make water inside his body while digesting his food. This metabolic water is all he needs to survive. He is also a master at saving water. When in his burrow during the day, he seals all the entrances so none of the cool, moist air is lost.
Suppose you are a little brown mouse and you live in the dune field. You don't have long sharp claws, or chemical sprays, or even great speed to save you from being eaten by predators. Even when you wait until dark to go out to hunt for food, your color still makes you stand out against the white sand. Any owl sitting in the treetops can easily see you and swoop down to make you his dinner. You might think an animal like this has no chance to survive, but nature always comes up with a solution.
When a litter of mice is born, they're not all the same color. Some are a little darker, some are a little lighter. When the mice grow up and go into the dunes to hunt for food, which ones do you suppose get gobbled up first? Which ones blend with the color of the sand and are harder for predators to see and catch? If you guessed the dark ones wind up being the meal and the light ones escape, you're right. More of the light ones will always survive and have babies with light colored fur.
Because they are better adapted to their environment, because they blend into their surroundings and are less likely to be killed by predators, these lighter-colored animals stand a better chance of survival. This is what is meant by survival of the fittest. You don't have to be the biggest or strongest animal. You just have to be well adapted to your surroundings.
There are two animals living in the dunes that are nearly as white as the sand. They are the Apache pocket mouse and the bleached earless lizard. Their color has changed over many generations to help them survive. This change is called adaptive coloration.