Answer :
Final answer:
Dune sands have a uniform grain size because the wind selects for sand-sized particles and finer particles like clay are blown farther away. Rounded sand grains facilitate transportation, leading to similar sizes within dune sands. Dunes lack clay because strong winds move these smaller particles great distances.
Explanation:
Dune sands tend to vary little in grain size and are generally free of clay because the physical processes of wind transportation select for certain particle sizes. Sand-sized particles, which are between 63 µm and 2 mm, are most readily transported by wind. Larger particles like gravel are too heavy for suspension, while finer particles such as silt and clay are carried further away. The rounded shape of these sand grains also facilitates their transportation as rounded grains roll easier than angular grains. This leads to a uniform size distribution within dune sands.
Dunes also lack clay because they are formed in environments with strong winds that tend to move smaller particles great distances. In terms of depositional forms, such as star dunes and barchan dunes, they have specific shapes related to the wind patterns and sand availability. With migration of sand dunes, the wind's consistent directionality facilitates the dune's gradual movement across the landscape through a process called saltation where sand grains bounce along the surface and slowly shift the dune's position.
