Sand+Dunes

 = Sand Dunes =

Have you ever seen the hills in the desert that look like an ocean of sand? These are called sand dunes. They are formed in many different sizes and different shapes. A sand dune needs three different things to form:

When all of these things merge, a sand dune is formed. When the wind picks up the sand, the sand travels, but only about an inch or two above the ground. There are three ways in which wind moves sand:
 * A large amount of loose sand in an area with little vegetation -- usually on coast or in a dried-up river, lake or sea bed.
 * A wind or breeze to move the grains of sand.
 * An obstacle that causes the sand to lose momentum and settle. This obstacle could be as small as a rock or as big as a tree.
 * 1) Saltation-when the sand grains bounce along in the wind, this is the most common way in which the sand travels.
 * 2) Creep-when sand grains collide with other grains, such as clay or gravel, causing them to move.
 * 3) Suspension- when sand grains blow high in the air and then settle, this is the least common way in which the sand travels.

__How Do They Form?__ Sand dunes (sandhills) are formed from windblown sand blown off exposed sandy beaches. The sand accumulates into ridges which originally lie parallel to the direction of the prevailing winds. Over time these ridges may break up and a more irregular arrangement of hills (dunes) and hollows (dune-slacks) forms. The formation of sand dunes is not continuous - those around the N. Ireland coast were formed thousands of years ago and little or no new dune formation now takes place. In addition, erosion by the sea can “bite off” portions of dune systems closest to the shore, a process likely to accelerate with the projected sea-level rise associated with rising global temperatures.

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 __Sources:__ http://geography.howstuffworks.com/terms-and-associations/sand-dune1.htm http://www.habitas.org.uk/flora/habitats/sanddunes.htm http://www.youtube.com/watch?v=9p6VtkVKrKU&feature=related