* Wind Sorting: Wind is the primary agent that shapes sand dunes. Wind carries and deposits sand grains, but it's better at carrying finer grains. Stronger winds carry larger grains, but even then, finer sand is often more abundant in dunes.
* Saltation: Sand grains are transported by a process called saltation, where they bounce along the surface. Finer grains are easier to lift and transport by wind, leading to their dominance in dune formations.
* Dune Mobility: Dunes are constantly shifting, with sand being eroded from one side and deposited on the other. This constant movement further selects for finer grains, as they are more readily transported.
Exceptions:
* Location: Sand dunes in areas with abundant coarse sediment (like riverbeds or beaches with large rocks) might have a higher proportion of coarser grains.
* Dune Type: Some dune types, like barchan dunes (crescent-shaped), are known for having finer sand, while parabolic dunes (U-shaped) may have more varied grain sizes.
Overall: While there can be exceptions, sand dunes tend to have finer sediments due to wind sorting and the ease with which finer grains are transported.
