  Spatiotemporal patterns in nature, such as ripples or dunes, formed by a
fluid streaming over a sandy surface show complex behavior despite their simple
forms. Below the surface, the granular structure of the sand particles is
subject to self-organization processes, exhibiting such phenomena as reverse
grading when larger particles are found on top of smaller ones. Here we report
results of an experimental investigation with downscaled model dunes revealing
that, if the particles differ not in size but in density, the heavier
particles, surprisingly, accumulate in the central core close to the top of the
dune. This finding contributes to the understanding of sedimentary structures
found in nature and might be helpful to improve existing dating methods for
desert dunes.
