Sand dunes, blown by relentless desert winds, long have been considered the chief means by which deserts move, sometimes encroaching on good agricultural land. Typically shaped like a croissant 50 feet high and one-third of a mile long, individual dunes have been watched for decades, moving steadily over the desert floor.

New research, inspired by photographs of the Sahara by the Landsat satellite, has revealed a previously unknown sand-moving mechanism that transports more than five times as much sand as a dune.

Instead of a high dune, the new mechanism involves broad but shallow layers of sand. Such formations -- some three miles wide and a half-mile long but only four inches thick -- may contain a half-million cubic yards of sand and travel 500 yards a year. A dune, by contrast, has much less sand and moves only about 10 yards a year.

The fast-moving formation, which looks like rows of zig-zags in computer-enhanced images from space, has been named a chevron by its discoverers, Ted Maxwell, head of the Air and Space Museum's Center for Earth and Planetary Studies, and C. Vance Haynes of the University of Arizona. The rate of movement was determined by comparing images of the same chevron formations taken years apart.

Maxwell said that, although winds are responsible for dunes and chevrons, the sand grains move in different ways. In a dune, sand grains blow up one slope and fall down the leeward side, then remain immobile until the dune passes over them and they are on the windward side again. In a chevron, the grains of sand appear to move simultaneously over a broad area.