Final answer:
Sand dunes are formed primarily by the accumulation of coarse sand grains carried by wind through the process of saltation.
Explanation:
Sand dunes are primarily composed of coarse grains of sand that are carried by wind. Wind transports small particles like silt and clay over long distances, leaving behind the heavier sand grains that accumulate to form dunes. The process of sand movement by wind, called saltation, accounts for a significant portion of sand transport over dunes.
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