Wind drops its load of sediment when its velocity falls. Wind commonly deposits sand in mounds or ridges called dunes. Windward slope is gently inclined and the steeper leeward slope is called the slip face. Velocity of wind, amount of material: types of sand dunes. Areas where there is a hard ground surface, a moderate supply of sand, and a constant wind direction. Form in areas where there is abundant supply of sand and a constant wind direction. Barchans merge into transverse dunes if supply of sand increases. Long straight dunes that form in areas with a limited sand supply and converging wind directions. Wind direction changing = converge into long line dunes. Also called blowout dunes are u shaped dunes with an open end facing upwind. Usually stabilized by vegetation, and occur where there is abundant vegetation, a constant wind direction, and abundant sand supply. Difference from barchans is the abundance of sand for parabolic: barchans = desert pavement.