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(Fig. 5.16b). Eventually this pattern will give rise
to a dune network of nearly linked ridges trend-
ing at 10 -20° to the prevailing wind direction
(see Warren (1979) for more details on airflow
over dunes).
In contrast, under similar unidirectional wind
conditions to those that form transverse and
barchanoid ridges, but where sand supply is less
abundant, barchan dunes will form (Figs 5.15
& 5.16a). The lower outer ridges will move
more quickly than the higher, central portions,
leading to significant arms that extend down-
wind. The width and spacing of the dunes will
reflect the effects of airflow patterns, which may
be related to upwind dune forms.
In areas of high wind variability, but relatively
limited sediment supply, linear dunes are created
(Fig. 5.15). Linear dunes (also called longitudinal
dunes, and including sinuous linear dunes or seif
dunes) are by far the most common dunes on
Earth. They are highly elongated in form and con-
tain two opposing slip faces either side of a crest
line. Linear dunes can be kilometres in length
and coalesce downwind into Y-shaped junctions.
They are typically associated with obliquely con-
verging wind directions, although the details of
their formation are still debated (see discussions
in Thomas 1997b).
Star dunes can be 300 - 400 m in height and
contain the largest sand volume (Figs 5.15 &
5.16c). This dune form requires abundant
sediment supply and multiple dominant wind
directions (Fig. 5.15). The latter wind directions
are typically associated with seasonal variations,
which cause other dunes to merge and modify.
Thus star dunes are common in the centre of
sand seas, although they are also associated
with topographic barriers that influence regional
windflow (Lancaster 1994). Once sufficiently
large, star dunes can modify their own windflow
patterns.
The most common type of these dunes is the
parabolic dune. Parabolic dunes tend to develop
where blowouts occur. These are formed by
the deflation of sand (often where vegetation
has been disturbed), which leads to areas of
higher wind speed over the associated lower
roughness surfaces. The increased wind speed
further dries the sand making it more prone
to mobilization. With increased deflation the
blowout will increase in size and the leeward
rim will migrate downwind, leaving trailing
limbs which are 'anchored' by vegetation. These
tend to limit the main sand movement in the
higher, central part.
5.3.4.3 Internal structures of dunes
Dunes form through the accretion of tractional
deposits or a drop in wind velocities (and thus
reduction in shear stress) leading to grain-fall
deposition. Once the initial sand has become
trapped, sediment will continue up the stoss
(windward) slope to the crest of the dune. Here
material will avalanche down a slip face. Thus
most dune types are associated with particular
sedimentological characteristics (Table 5.2). The
internal structure of sand dunes is dealt with in
more detail in McKee (1979). This and other
research is summarized in Nickling (1994).
Free dunes (which dominate in arid to hyper-
arid climates) are typically associated with well
developed cross-strata which dip downwind at
30 -34°. Dune deposits are also associated with
often massive, tabular to planar cross-strata that
thin from the base upward. Another common
feature is the development of bounding surfaces
which occur between sets of cross-strata, which
result from the migration of dunes over inter-
dune areas.
Vegetated dunes (which are typically associated
with semi-arid climates) tend to show a bimodal
distribution of steeper angle cross-strata and
minor truncation surfaces. Most dips are low
(about 12°). Parabolic dunes are dominated by
steeper cross-strata (similar to transverse dunes),
which often have concave slip-faces. They may
also contain some concave-upward sets that have
been deposited in hollows. Vegetated dunes are
5.3.4.2 Impeded dunes
Impeded (or anchored) dunes are dominated
by topographic barriers that inhibit sand move-
ment and lead to sand build-up. The variety of
forms is illustrated in Table 5.3 and Fig. 5.14.
