Cliff-top dunes, also known as perched dunes, are dunes that occur on the tops of cliffs. They are uncommon in most parts of the world, because they only develop under unusual geomorphological conditions. Processes by which they may be formed include: * a dune advances up a pre-existing slope, which is then eroded to form a cliff; * a dune forms during a period of high sea level, then sea level drops, exposing a cliff face.