Grain by grain, the dune moves downwind. Accumulations of sediment blown by the wind into a mound or ridge, dunes have gentle upwind slopes on the wind-facing side. The downwind portion of the dune, the lee slope, is commonly a steep avalanche slope referred to as a slipface . Dunes may have more than one slipface. The minimum height of a slipface is about 30 centimeters.
Sand grains move up the dune's gentle upwind slope by saltation and creep.
When particles at the brink of the dune exceed the angle of repose, they spill over in a tiny landslide or avalanche that reforms the slipface. As the avalanching continues, the dune moves in the direction of the wind. Bagnold investigated the physics of particles moving through the atmosphere and deposited by wind.