Sand dunes are mounds of sand formed along the deserts or beaches. Two things are needed for their formation – a plentiful supply of sand and persistent wind. Depending on such things as the nature of the sand, the lay of the land and the strength of the wind, they take on a variety of shapes.
One common type called transverse dunes forms long ridges across the path of the prevailing wind, like waves at sea. The dunes along most seashore are transverse dunes. Seif dunes, in contrast, lie parallel to the prevailing wind direction. Sculpted by very powerful winds, they are often many miles long and up to 300 feet high. The bare, windswept troughs between rows of seif dunes have traditionally been used as highways by nomadic desert people.
Barchans are shaped like crescents, with their tips pointing downwind. Found where sand is relatively scarce, they travel across gravelly surfaces or even bare bedrock. Barchans are among the most active dunes, migrating up to 100 feet per year.
Some dunes are mountainous, star shaped formations up to 1000 feet high. Created by winds that blow alternately from various directions, these formations tend to remain where they were formed.