Geoscience Reference
In-Depth Information
to several metres a year (Haynes,
1989
), under the influence of strong unidirectional
winds and are common in areas of limited sand supply. Should they migrate into areas
with a bidirectional wind regime, they may develop into linear dunes, depending on
sand supply (Bagnold,
1941
).
Luminescence dating of the linear dunes of the Simpson and Strzelecki deserts in
central Australia shows that the crests have been repeatedly active to depths of many
metres during the late Quaternary but appear virtually inactive today, a paradox noted
by Hesse and Simpson (
2006
). These authors investigated the relationship between
changes in vegetation cover and episodic sand movement on the dunes at three sites
located along a climatic gradient in which the ratio of evaporation to precipitation
increased from south to north. They surveyed the dune crests both during drought and
after renewed rain. They concluded that 'vegetation cover is very much a controlling
factor in sand movement on Australian dunes and that most Australian dunes are
inactive for most of the time because of the abundance of vegetation' (Hesse and
Simpson,
2006
, p. 287).
Ash and Wasson (
1983
) also found an inverse relationship between plant cover
and mobile sand and suggested that dunes become stable once the plant cover attains
a threshold value of about 30 per cent. In contrast to Hesse and Simpson (
2006
),
they concluded that sand movement is controlled by the frequency of strong winds. If
strong winds are more frequent during times of drought, as is often the case, then both
factors (reduced plant cover and high velocity winds) probably operate interactively,
so it will not be easy to distinguish the relative importance of wind speed and plant
cover as separate factors controlling sand movement. In addition, because the cover
of annual grasses and forbs will be reduced during dry years, plant cover acts as an
indirect index of precipitation and hence of aridity.
8.5 Dune orientation in relation to wind speed and direction
Linear dunes reflect the influence of the dominant sand-moving winds. In a pioneering
study of the Holocene and older climates of the Sudan, Warren (
1970
) investigated the
dominant sand-moving wind directions and the alignments of the vegetated dunes of
Kordofan Province west of theWhite Nile. From the alignments of several generations
of these now fixed linear dunes, he postulated a sequence of late Quaternary shifts in
the wind and rainfall zones. In the absence of any dates, Warren tentatively considered
the first and most arid phase to be late Pleistocene or older. He inferred a 450 km shift
of both wind and rainfall belts to the south of their present position during that time.
He regarded the wet phase that followed as equivalent to the terminal Pleistocene-
early Holocene (15-8 ka) wet phase dated elsewhere along the southern margins of
the Sahara. Warren estimated that during this time, there was a shift of the climatic
and vegetation belts to roughly 250 km north of their present position, a conclusion
accepted by Wickens (
1975a
; 1982) in his reconstruction of the hypothetical early
