Most dunes are aligned to almost parallel sand mounds with undulated dune ridges. They form when the prevailing wind direction is fairly constant. In contrast, variable wind directions create radially symmetric star dunes. Obstacles like sparse vegetation can enhance the development of dunes as the wind speed behind a bush is lowered and the sand can accumulate. Thereby crescent-shaped dunes can develop, called barchans.
Canon 20D, EF-S 10-22mm, f/22, 1/30sec, ISO 100, tripod