They travel near the surface by bouncing or rolling along the surface by traction. Finer clay and silt-sized particles are winnowed away and may be carried high into the atmosphere. Larger gravel-sized particles are too heavy to be transported by normal wind systems. Thus, the sand in eolian fields does not show a large variation in grain size. This size-sorting is one way a geologist can recognize ancient eolian systems in buried strata.
Dunes are asymmetrical in cross section.
Wind carries sand up the gently sloping side of the dune. It then avalanches down the steep lee side of the dune. Thus, the main wind direction responsible for forming dunes and surface ripples is easily determined. Surface ripples formed by the most recent winds may be in a completely different orientation from larger dunes formed by prevailing winds. New sand is not arriving at Kelso Dunes today.