Anchored Dunes

This chapter examines dunes that are 'anchored' to an obstacle, whose geometric characteristics then control the shape of the dunes. There are many kinds and sizes of obstacle: plants, organic debris of various kinds, pebbles, boulders, or isolated hills. Nebkhas are discrete dunes formed by the trapping of sand, silt, clay, and coarse particles up to the size of small gravel, and/or snow by a plant. Shadow dunes are pyramidal through triangular to tear-drop-shaped dunes. They are formed downwind, or in the shelter of nebkhas, plants, or other obstacles. Lunettes are typically crescentic or arcuate shaped dunes formed on pan, playa, and lake margins, usually mimicking the plan shape of the adjacent palaeo or present shoreline, with the horns pointing upwind. Blowouts are erosional hollows typically found on vegetated to semi-vegetated sediment deposits. Transgressive dunefields are commonly colonised and stabilised naturally by vegetation over time. Topography variably influences dune orientation and migration rate.