There are several geographic physical features that are present on our earth. Sand dune is basically an accumulation of sand grains that are shaped into a mound or ridge by the winds and are under the influence of gravity.
There are several physical features that astound us with their beauty and the reasons of their occurrence. Sand dunes are comparable with other forms which appear when a fluid moves over a loose bed, like subaqueous “dunes” that are on the beds of rivers and tidal estuaries and sand waves that are on the continental shelves beneath shallow seas.
Dunes are present wherever loose sand is windblown i.e. in deserts, on beaches, and even on some of the eroded and also abandoned farm fields in semiarid regions, like northwest India and also parts of the southwestern United States.
Images of Mars that returned by the U.S. Mariner 9 and Viking space crafts have recently shown that dunes are widely spread on that planet both in the craters and in a sand sea that surroundes the north polar ice cap.
During the past two million years or even more there have been the conditions of extremely low rainfall under which the true dunes that are form expanded beyond the margins of the Sahara and other present-day arid regions that are into areas which are now more humid.
One of the best evidence for these changes is in the presence of sand seas that are immobilized by the vegetation. Dunes have formed under similar climatic challenges that were in the geologic past and also at certain times occupied deserts as extensive as modern ones. Rocks have been formed by the solidification of ancient sand seas occur. For example, in the walls of the Grand Canyon that are located in the southwestern United States, in the West Midlands of England, and further in southern Brazil.
By: Prerana Sharma