Sand seas and dune fields are comprised of dunes of different morphological types and sizes, areas of sand sheets, interdune deposits (including non-aeolian sediments), as well as extra-dune fluvial, lacustrine, and marine sediments. They represent the primary depositional sink of aeolian sand-transport systems. The greatest concentrations of desert sand seas and dune fields occur in mid-latitude arid regions, and the subtropical deserts and contain considerable volumes of aeolian sand. Sand seas and dune fields are generally located in topographic lows and in areas where wind energy and sand-transport rates are lower than those in adjacent sand source areas.