Gypsum sand is being made all the time. The dunes continue to form and move under the influence of water and wind. Water continues to wash down from the mountains carrying dissolved gypsum into the Tularosa Basin. Wind continues to blow across the Basin carrying the gypsum.
The gypsum sand grains crash into each other. The crash creates tiny lines or scratches on the surface of the sand. These scratches change the way light shines off the surface. This makes the sand appear white. The sand dunes look like great masses of bright white snow. But they are not cold and wet. It only rains about eighteen centimeters each year.
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VOICE TWO:
There are four kinds of sand dunes at White Sands National Monument. Some of the dunes are small and fast-moving. They are called dome dunes because they are shaped like a half-circle. Few if any plants grow on them. These dunes move the fastest, up to twelve meters a year.
Other dunes are called transverse dunes. They form in long lines across the dune field. They can grow to be one-hundred-twenty meters thick and eighteen meters high.
Another kind of dunes are barchan dunes. They form in areas with strong winds but a limited supply of sand. These dunes have sand in three parts, like a body in the center and two arms on the sides. The sand in the two arms moves faster than the sand in the center.
Parabolic dunes are the opposite of barchan dunes. They form when plants hold sand in the outer parts of the dune but the center of the dune continues to move.
最新
2013-11-25
2013-11-25
2013-11-25
2013-11-25
2013-11-25
2013-11-25