Perched dunes exist
on a surface that is above the level of Lake Michigan. Most perched
dunes form when wind travels up a coastal bluff or cliff and removes
fine material from the slope along the way. Sand-sized materials
are deposited on top of the bluff to form a dune. Finer material
(clay and silt) can be removed from the area entirely by the wind.
Sleeping Bear Dunes National
Lakeshore has some spectacular examples of perched dunes. The dunes
sit on top of glacial moraines which rise 30 to 120 meters (90 to
360 feet) above Lake Michigan.
Wind travels up the bluff (from right to left
in photo), picking up sand along the way and depositing it on
top of the bluff. The grass-covered mounds of sand are dunes;
the rest of the slope from the dunes down is a glacial deposit.(Sleeping
Bear Dunes National Lakeshore. Photo from R. Stearley, 1997).