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(a)
NE
SW
TIBESTI
MASSIF
Erg
Calanscio
Erg
Rebiana
Erg
Bilma
Erg
Ténéré
Erg
Azouak
TAMGAUT
MOUNTAINS
Benghazi,
Libya
0
500 km
(b)
Reduction of total
sandflow energy
Reduction of resultant
sandflow energy
1
4
2
5
3
Figure 17.8
(a) The relationships of ergs in part of North Africa to topography (after Wilson, 1973). (b) Models of topographic
influences on sand sea development (after Fryberger and Ahlbrandt, 1979). (1) In the shadow of a topographic barrier, (2) in
shallow desert depressions and (3) by the direct blocking of wind, all leading to a total reduction in sand transporting energy.
Resultant energy may be reduced (4) when surface winds are deflected, leading to sites of favourable accumulation, and (5) when
katabatic winds off a highland oppose a dominant wind. Sand seas can also develop where water bodies interrupt regional sand
flow patterns.
aeolian sand supply, or playa, sebkha or coastal sand sea
situations (Fryberger, Schenk and Krystinik, 1988), where
periodic
or
seasonal
flooding can also prevent dune devel-
opment. Langford (1989) identified a number of ways in
which interactions between aeolian and fluvial processes
could lead to different sedimentary deposits in areas with
fluctuating groundwater tables. The formation of
surface
crusts
, e.g. salcretes (see Chapter 6), or the growth of al-
gal mats (Fryberger, Schenk and Krystinik,1988) can also
inhibit deflation and contribute to the formation of sand
sheets, as in the White Sands, New Mexico.
With the exception of the presence of coarse sand, the
probably inapplicable to the development of the hyper-
arid Selima Sand Sheet, because their effectiveness re-
quires either the presence of some moisture or proximity
to a dunefield. Rather, Breed, McCauley and Davis (1987)
ascribe its development to the blanketing of an ancient flu-
vial landscape by aeolian redistribution of the sand frac-
tion contained within widespread alluvium deposits. The
absence of major topographic barriers able to inhibit long-
distance wind transport accounts for the great size of this
sand sheet, while the coarseness of the sand has prob-
ably caused the general lack of dune development and
the formation of gently undulating subhorizontal tabular
