Page 319 - Fundamentals of Geomorphology

302 PROCESS AND FORM

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 become established between the airflow and the evolving
the wind than in other environments. 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 Traditionally, geomorphologists studied dune form and
and sand seas) or loess or as characteristic dunes.It the texture of dune sediments. Since around 1980,
is a popular misconception that the world’s deserts are emphasis has shifted to investigations of sediment
vast seas of sand. Sandy desert (or erg) covers just 25 transport and deposition and of their connection to
per cent of the Sahara, and little more than a quarter dune inception, growth, and maintenance. Research has
of the world’s deserts. Smaller sand accumulations and involved field work and wind-tunnel experiments, as well
dune fields are found in almost all the world’s arid and as mathematical models that simulate dune formation
semi-arid regions. and development (see Nickling and McKenna Neuman
Sand accumulations, in sand seas and in smaller fea- 1999). Nonetheless, it is still not fully clear how wind,
tures, usually evolve bedforms. They are called bedforms blowing freely over a desert plain, fashions dunes out of
because they are produced on the ‘bed’ of the atmo- sand. The interactions between the plain and the flow
sphere as a result of fluid movement – airflow. They of sand in which regular turbulent patterns are set up
often develop regular and repeating patterns in response are probably the key. Plainly, it is essential that wind
to the shearing force of the wind interacting with the velocity is reduced to allow grains to fall out of the con-
sediment on the ground surface. The wind moulds veying wind. Airflow rates are much reduced in the lee of
the sediment into various landforms. In turn, the land- obstacles and in hollows. In addition, subtle influences
forms modify the airflow. A kind of equilibrium may of surface roughness, caused by grain size differences,