Geology Reference
In-Depth Information
particles lifted into its flow, significant
sorting
of sediments by size occurs as wind trans-
ports particles. Sorting means that sediments of similar size are grouped together when
they are deposited.
Sand particles are the largest sediments moved by wind. As soon as the wind flow slows
down, the sand particles fall to the surface, while the smaller particles of clay and silt re-
main carried as suspended load in the wind current. The result is that some features of
wind deposition, the
dunes,
are created almost entirely of sand-sized particles. The smal-
ler particles of silt are deposited when the wind stops moving completely, forming lay-
ers of
loess.
In this section I describe these distinct features and explain how they are created by
wind deposition.
Migrating piles of sand: Dunes
Sand dunes form in areas where the wind blows almost constantly and there is a large
supply of sand that is not held in place by vegetation or moisture (such as deserts or
sandy coastlines). When the wind flow slows, the sand in its bed load is deposited in
piles, creating dunes.
Sand dunes are usually asymmetric in appearance, with one side having a steeper slope
than the other. Figure 14-3 illustrates the different parts of a typical sand dune.
Figure 14-3:
A typ-
ical sand dune.
As wind moves sand up the
windward
side of a dune, most of the sand is deposited at
the
crest
or top of the dune. When this pile of sand is too heavy to be supported by the
steeper angle on the
leeward
side of the dune, the sediments slip downward in response
to the pull of gravity, creating a
slip face.
The sediments rest along the slip face at the
angle of repose
(which I describe in detail in Chapter 12).
The sliding of sediments down the slip face of a dune creates layers, as illustrated on the
left in Figure 14-4. Over time, multiple dunes are deposited above, creating a series of
cross-beds
such as illustrated on the right in Figure 14-4. These cross-beds are preserved
