Geoscience Reference
In-Depth Information
other had not (Avni et al.,
2006
). The stone terraces constructed by the early Byzantine
farmers in the cultivated valley had helped trap several metres of colluvial-alluvial
loess and gravel, and where these structures had survived undamaged, those parts of
the valley were still cultivated by the local Bedouin.
The methodology used in these carefully devised studies is relatively straightfor-
ward and could profitably be applied more widely. The first step is to date the valley
fills using a combination of radiocarbon and luminescence dating methods allied with
detailed mapping and logging of the main landforms and stratigraphic units. This
allows the timing of the change from a regime dominated by loess and alluvial loess
deposition to one of erosion to be determined reasonably accurately and compared
with the archaeological evidence of human settlement in the region. A further by-
product of this approach is that it allows rates of sedimentation to be determined
for different intervals of time, together with quantitative estimates of rates of gully
headwall retreat.
The conclusion from each of the three studies was that land degradation caused by
gully erosion is indeed contributing to desertification but was not in any way brought
about by human actions. On the contrary, well-designed and properly maintained soil
and water conservation measures helped arrest the process in part of one valley for
more than a thousand years, indicating that human actions can be both beneficial and
enduring.
One unexpected cause of gully erosion that had nothing to do with either climate
or humans was an earthquake that led to the formation of a deep fissure in the welded
tuffs and overlying sediments on the floor of one of the calderas within the K'one
volcanic complex near the eastern margin of the Ethiopian Rift Valley where it begins
to merge into the southern Afar Rift (Williams,
2003
). The earthquake occurred some
5,000 years ago, and the gully erosion that followed created a veritable 'badland'
topography incised into horizontally stratified sediments that contain Middle Stone
Age obsidian artefacts in the lower units and Late Stone Age and Neolithic artefacts
and charcoal (dated by
14
C) in the uppermost units. Although the government of the
day was inclined to blame the local charcoal burners for the gully erosion, they were
unaware that the gullies were no longer active and had probably been stable for several
thousand years, as shown by a detailed plane table survey of the entire gully complex
(Williams,
2003
).
The vegetated dunes between Tahoua and Abalak, located about midway between
Niamey and Agades in central Niger provide another instance of gullies formed during
extreme events (Talbot and Williams,
1978
; Talbot and Williams,
1979
). During the
course of a camel survey of the Wadi Azouak Basin in late 1974, near the end of a long
interval of drought, they observed a series of sandy alluvial fans located on the lower
slopes of some fixed dunes near Janjari village. Older fan sediments were exposed
in the banks of incised channels upstream of the most recent fan, which had formed
during an intense rainfall event in the previous wet season earlier in 1974. Using the
