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Sand dunes form some of the most spectacular and dynamic landforms on the planet. Individual sand granules (grains of sand) accumulate through both water and wind (eolian) transportation, a process known as saltation. Individual saltating granules form transversely (perpendicular) to the wind's direction forming small ripples. As more granules collect, dunes form. Sand dunes can form in any landscape on Earth, not just deserts.
Formation of Sand Dunes
Sand itself is a type of soil particle. Its large size makes for swift transportation and high erodability. When granules accumulate, they form dunes under the following conditions:
1. Granules accumulate in an area devoid of vegetation.
2. There must be enough wind to transport the granules.
3. Granules will eventually settle into drifts and in larger quantities dunes when they accumulate against a stable barrier to the wind, such as vegetation or rocks.
Parts of a Sand Dune
Every sand dune has a windward (stoss) slope, crest, slipface, and leeward slope. The stoss side of the dune is transverse to the predominant wind direction. Saltating sand granules travel up the leeward slope, slowing as they accumulate other granules. The slipface forms right underneath the crest (the peak of the sand dune), where granules reach their maximum height and begin to slope steeply down the leeward side.
Types of Sand Dunes
Crescent sand dunes, also called barchan or transverse, are the most common sand dune shapes in the world. They form along the same direction as the predominant winds and have a single slipface. Since they are wider than they are long they can travel very quickly.
Linear dunes are straight and are often in the form of parallel ridges. Reversing dunes result from sand dunes that are impacted by wind that reverses direction. Star dunes are pyramid-shaped and have three or more sides. Dunes can also be comprised of smaller dunes of different types, called complex dunes.
Sand Dunes Around the World
Algeria's Grand Erg Oriental is one of the largest sea of dunes in the world. This portion of the vast Sahara Desert covers over 140,00 square kilometers in area. These predominantly linear dunes run north-south, with some complex dunes in the area as well.
The famous sand dunes at Great Sand Dune National Park in southern Colorado formed in a valley from ancient lake bed. Large quantities of sand remained in the area after the lake breached. Predominant winds blew the sands towards the nearby Sangre de Cristo mountains. Storm winds blew over the other side of the mountains towards the valley, causing the dunes to grow vertically. This resulted in the tallest sand dunes in North America at over 750 feet.
Several hundred miles north and east lie the Nebraska sandhills. Much of west and central Nebraska is covered by these ancient mostly transverse dunes, left over from when the Rocky Mountains formed. Agriculture can be difficult so ranching is the predominant land use in the area. Livestock graze these heavily vegetated hills. The sandhills are significant as they helped form the Ogallala Aquifer, which provides water for much of the Great Plains and central North America. Highly porous sandy soils collected centuries of rain and glacial meltwater, which helped form the massive unconfined aquifer. Today organizations such as the Sandhills Task Force strive to save water resources in this area.
Visitors and residents of one of the Midwest's largest cities can visit the Indiana Dunes National Lakeshore, along part of Lake Michigan's southern shore, about an hour southeast of Chicago. The dunes at this popular attraction resulted when the Wisconsin glacier formed Lake Michigan over 11,000 years ago. Sediments left behind formed the present dunes as the massive glacier melted during the Wisconsin Ice Age. Mount Baldy, the tallest dune in the park actually retreats south at a rate of about four feet per year as it is too tall to for vegetation to hold it in place. This kind of dune is known as a freedune.
Sand dunes are found around the world, in varying types of climates. Overall, every sand dune is created by the interaction of the wind with soil in the form of grains of sand.