Geoscience Reference
In-Depth Information
north-west India, the Alashan Plateau of Inner Mongolia in northern China and the
Namib Desert.
Along the strategically important railway between Baotou and Hohot in Inner
Mongolia, dune encroachment onto the tracks is a perennial problem. To overcome
this problem, the Chinese have adopted an ingenious albeit labour-intensive method of
dune stabilisation. Rows of dried wheat stalks (or, in the northern Taklamakan Desert,
dried reed stems from the piedmont marshes south of the Tian Shan) are pushed well
into the sand to form small square hedges about a metre wide and up to 0.5 m high.
These 'chequer-boards' act as temporary windbreaks, allowing wind-blown dust to
accumulate within each square. The dust is washed down into the sand during the
sparse summer rains and, within a few decades, a soil enriched in silt and clay forms,
soil moisture becomes trapped and seeds of ephemeral and perennial desert shrubs
and grasses germinate and stabilise the previously mobile dunes.
Many of the dunes along the southern margins of the Sahara are now vegetated and
stable (Grove and Warren,
1968
; Talbot,
1980
). The dunes have weakly developed
A-horizons enriched in silt and clay from many centuries of dust storms. Such soils
are very vulnerable to gully erosion during intense rainstorms. One such extreme
event occurred during the wet season in mid-1974 near the village of Janjari in central
Niger, about midway between Niamey and Agades (Talbot andWilliams,
1978
; Talbot
and Williams,
1979
). The fixed dunes in a small area 10-15 km in radius between
Tahoua and Abalak were eroded during this storm, and a series of sandy alluvial fans
formed along their foot-slopes. Older fan sediments were visible in the banks of a
channel incised upstream of one of the most recent fans, and buried soils exposed in
the banks showed that previous fans had become vegetated and stable. The young-
est buried soil was stratigraphically earlier than charcoal from a fireplace dated to
335
60 yr BP (N-2129). This same soil was found to be widespread throughout
the region and may have formed during a slightly wetter interval evident elsewhere in
the Sahel and dated to about 150-350 years ago (Talbot and Williams,
1978
; Talbot
and Williams,
1979
). One conclusion to emerge from this work is that episodic dune
dissection, fan deposition, soil formation and fan stabilisation have been typical of
the last few thousand years, indicating that humans were not responsible for the dune
erosion and that desertification processes are reversible.
±
15.6 Attributes of soils and soil landscapes in semi-arid regions
Desert margins are often characterised by very gently sloping alluvial fans which can
occupy vast areas, such as the Gezira alluvial fan between the lower Blue and White
Nile rivers (40,000 km
2
) and the Riverine Plain in the lower Murray-Darling Basin
of south-east Australia (77,000 km
2
). These vast, low-angle, inland alluvial fans have
been termed 'megafans' by Leier et al. (
2005
), and they appear to be confined to
