Where does martian sand come from?

A piece of Mars: The source of dune sand on Mars is normally something of a mystery. But here on the interior wall of a small crater it appears that small gullies have eroded sediment from the wall and carried it down toward the center of the crater (toward the upper right). The bluish and tan regions are the material the gullies have transported, and the bluish sediments have formed into small aeolian ripples. The mystery is solved for this one small region of Mars: is it the same story elsewhere? (HiRISE ESP_030915_1290, NASA/JPL/University of Arizona)