The Work of Wind and Deserts - The Changing Earth: Exploring Geology and Evolution - page 386

and around an obstruction,
resulting in the deposition
of sand grains, which accu-
mulate and build up a de-
posit of sand. As they grow,
these sand deposits become
self-generating in that they
form ever-larger wind bar-
riers that further reduce the
wind's velocity, resulting in
more sand deposition and
growth of the dune.
Most dunes have an
asymmetrical profile, with a
gentle windward slope and a
steeper downwind or leeward
slope that is inclined in the
direction of the prevailing
wind (

Figure 15.8a). Sand
grains move up the gentle
windward slope by saltation
and accumulate on the lee-
ward side, forming an angle
of 30 to 34 degrees from the
horizontal, which is the angle
of repose of dry sand. When this angle is exceeded by accu-
mulating sand, the slope collapses, and the sand slides down
the leeward slope, coming to rest at its base. As sand moves
from a dune's windward side and periodically slides down its

◗

◗

Figure 15.5 Defl ation Hollow A defl ation hollow, the low area, between two sand dunes in Death
Valley, California. Defl ation hollows result when loose surface sediment is differentially removed by wind.

The Formation and Migration of Dunes
The most characteristic features in sand-covered regions
are dunes, which are mounds or ridges of wind-deposited
sand (

◗ Figure 15.7). Dunes form when wind flows over

Deflation

Wind

◗ Figure 15.6 Desert Pavement

a Fine-grained material is removed by wind,

Desert pavement
(deflation ends)

Wind

c Desert pavement in the Mojave Desert, California. Numerous
ventifacts are visible in this photo. Desert pavement prevents
further erosion and transport of a desert's surface materials by
forming a protective layer of close-fi tting, larger rocks.

b leaving a concentration of larger particles that form desert
pavement.