Types of Sand Dunes
In places where tons of sand accumulate, seas of sand bury the landscape. Wind shapes the sand into dunes that exist in a variety of shapes and sizes, depending on the character of the wind and the sand supply.
When the wind blows steadily in one direction, crescent-shape dunes called barchan dunes form, with each tip of the dune pointing in the direction of the wind. (Barchan dunes are also known as crescentic dunes.)
Constant shifting of wind in similar conditions will form a group of crescent-shaped dunes pointing in different directions. These dunes will overlap each other and create a constantly changing star dune.
When enough sand accumulates for the ground to be completely buried and moderate winds are blowing, simple, wave-like dunes called transverse dunes are formed. The crests of transverse dunes lie perpendicular to the wind direction.
Powerful winds may break through transverse dunes and change them into parabolic dunes. Usually, they are much larger than barchan dunes (because there is more sand), and their ends point against the direction of the wind.
If there is abundant sand and a strong, steady wind, the sand streams into longitudinal dunes (also called seif dunes, after the Arabic word for "sword"), whose axis lies parallel to the wind direction. The Rub al Khali, an erg in the Arabian Peninsula, contains seif dunes that stretch for nearly 200 km and reach height of over 300 km.
Reversing dunes refer to dunes that occur in places where winds periodically reverse direction. They can exist as varieties of any of the above types of dunes.
On a somewhat unrelated note,
Great Sand Dunes National Park of
Colorado was just recently (
September 13, 2004) made the 58th national park of the
United States, the first to be created under the
Bush Administration. There are still legal fights over the land involving rights to subsurface water bought by private companies.
Marshak, Steven. Earth: Portrait of a Planet
http://pubs.usgs.gov/gip/deserts/dunes/
Great Sand Dunes National Park:
http://www.denverpost.com/Stories/0,1413,36~154~2406748,00.html