Geology Reference
In-Depth Information
Plate 12.3
Zeugen, Farafra, Western Desert, Egypt. The limestone pillars are undercut by sand-blasting.
(
Photograph by Tony Waltham Geophotos
)
to the higher wind speeds, larger particles are carried by
the wind than in other environments.
become established between the airflow and the evolving
landforms, but it is readily disrupted by changes in sand
supply, wind direction, wind speed, and, where present,
vegetation.
AEOLIAN DEPOSITIONAL FORMS
Dune formation
Sand accumulations come in a range of sizes and forms.
Deposition may occur as
sheets of sand
(dune fields
and sand seas) or
loess
or as characteristic
dunes
.It
is a popular misconception that the world's deserts are
vast seas of sand. Sandy desert (or
erg
) covers just 25
per cent of the Sahara, and little more than a quarter
of the world's deserts. Smaller sand accumulations and
dune fields are found in almost all the world's arid and
semi-arid regions.
Sand accumulations, in sand seas and in smaller fea-
tures, usually evolve bedforms. They are called bedforms
because they are produced on the 'bed' of the atmo-
sphere as a result of fluid movement - airflow. They
often develop regular and repeating patterns in response
to the shearing force of the wind interacting with the
sediment on the ground surface. The wind moulds
the sediment into various landforms. In turn, the land-
forms modify the airflow. A kind of equilibrium may
Traditionally, geomorphologists studied dune form and
the texture of dune sediments. Since around 1980,
emphasis has shifted to investigations of sediment
transport and deposition and of their connection to
dune inception, growth, and maintenance. Research has
involved field work and wind-tunnel experiments, as well
as mathematical models that simulate dune formation
and development (see Nickling and McKenna Neuman
1999). Nonetheless, it is still not fully clear how wind,
blowing freely over a desert plain, fashions dunes out of
sand. The interactions between the plain and the flow
of sand in which regular turbulent patterns are set up
are probably the key. Plainly, it is essential that wind
velocity is reduced to allow grains to fall out of the con-
veying wind. Airflow rates are much reduced in the lee of
obstacles and in hollows. In addition, subtle influences
of surface roughness, caused by grain size differences,
