Blowing in the Wind: Moving Sediments without Water - Geology - page 211

in the rocks formed by these sediments and are snapshots of dunes that existed when
the sediments were first deposited.
Figure 14-4:
Creation of bed-
ding on the dune
slip face and the
appearance of
cross-beds.
The ongoing process of wind moving sediments up the windward face of a dune by salta-
tion, and down the leeward face of a dune by slipping, causes the dune to move across
the surface in the direction the wind is blowing. This movement is called dune migration.
Shaping sand
You may think the formation of dunes by wind creates a never-ending array of different
shapes. However, only a few major shapes or types of dunes are consistently created by
wind. I describe them here, and they are illustrated in Figure 14-5.
Barchan dunes
Barchan dunes are created when the sand supply is limited and the land surface is flat.
They are crescent-shaped, like a half moon, with two points or horns pointed downwind,
or in the direction the wind is blowing. Most commonly the horns are the same length,
but occasionally one is longer than the other.
Parabolic dunes
Parabolic dunes appear similar to barchan dunes in shape, except the horns of their
crescent shape point upwind, in the direction the wind flows from, instead of downwind.
Parabolic dunes are most common along coasts, where the sand is partially anchored by
vegetation. The result is that sand is moved forward in areas without vegetation (the
curve of the crescent) and remains in place where vegetation is present (at the horns).