Sand dunes are formed as a result of the Earth's erosional and depositional forces. Rock is eroded by physical and chemical processes like wind and water. Sand dunes are formed as a resultof the wind blowing these particles along the Earth's surface. Depending on the size of the particle, the sand particles may stay in the wind for quite some time. At some point, they find their way to the Earth;s surface, largely due to the force of gravity. Dune formations may vary, according to the strength of the wind and the size of the sand particles. Particles may range in size from very fine silt, that remains in the air for long periods of time, to larger grain-sized particles that settle out relatively quickly.