Bearded craters and dunes

A piece of Mars: This 600×450 m (1969×1476 ft) scene has a complex sedimentary history. How are bearded craters and dunes formed? They weren’t always bearded. At some point, a deposit of bright material accumulated on this surface, and was then eroded so that all that remains of it is what is protected by topography (anything that pokes up like dunes or crater rims). Can you find the boulder that has tumbled downslope (it too has a beard!). (HiRISE ESP_038826_1700, NASA/JPL/Univ. of Arizona)