Geology Reference
In-Depth Information
Plate 12.4
Mega-ripples formed on a hard sebkha surface in the United Arab Emirates.
(
Photograph by Dave Thomas
)
Free dunes
of anchored dunes (Livingstone and Warren 1996, 75)
(Table 12.3).
Free dunes
may be classed according to orientation
(transverse) or form (linear, star, and sheet) (Figure 12.6).
All types of
transverse dune
cover about 40 per cent
of active and stabilized sand seas. The transverse vari-
ety (Table 12.3) is produced by unidirectional winds
and forms asymmetric ridges that look like a series of
barchan dunes whose horns are joined, with their slip
faces all facing roughly in the same direction.
Barchans
are isolated forms that are some 0.5-100 m high and
30-300 m wide (Plate 12.5). They rest on firm desert
surfaces, such as stone pavements, and move in the direc-
tion of the horns, sometimes as much as 40 m/yr. They
form under conditions of limited sand supply and unidi-
rectional winds. Other transverse dune types are domes
and reversing dunes.
Domes
lack slip faces but have an
orientation and pattern of sand transport allied to trans-
verse dunes.
Reversing dunes
, which have slip faces on
opposite sides of the crest that form in response to wind
coming from two opposing directions, are included in
the transverse class because net sand transport runs at
right-angles to the crest.
Dunes
are collections of loose sand built piecemeal by
the wind (Figure 12.5). They usually range from a few
metres across and a few centimetres high to 2 km across
and 400 m high. Typical sizes are 5-30 m high and spaced
at 50-500-m intervals. The largest dunes are called
draa
or
mega-dunes
and may stand 400 m high and sit more
than 500 m apart, with some displaying a spacing of up
to 4 km.
Dunes may occur singly or in dune fields. They may
be active or else fixed by vegetation. And they may be
free dunes or dunes anchored in the lee of an obstacle
(impeded dunes). The form of free dunes is deter-
mined largely by wind characteristics, while the form
of anchored dunes is strongly influenced by vegetation,
topography, or highly local sediment sources. Classi-
fications of dune forms are many and varied, with
local names often being used to describe the same
forms. A recent classification is based upon dune for-
mation and identifies two primary forms - free and
anchored - with secondary forms being established
according to morphology or orientation, in the case of
free dunes, and vegetation and topography, in the case
