Why do pebbles rarely form true sand dunes?

Pebbles rarely form true sand dunes because they are too large and heavy to be transported and shaped by the wind in the same manner as sand grains. While sand grains—typically ranging from 0.02 to 0.04 cm in diameter—are easily moved and sorted by wind to create dunes, particles as coarse as pebbles generally require exceptionally strong and persistent winds to move at all.1

When such coarse-grained features do form under these intense conditions, they are classified as granule ripples rather than true dunes. True dunes are defined by the accumulation of windblown sand into specific mound or ridge shapes, a process that relies on the ability of the wind to effectively transport and deposit finer particles. Larger materials, such as small boulders, are even more restricted, as they can only be moved across specific surfaces like ice or wet saline mud and are incapable of forming dune structures.12