Geoscience Reference
In-Depth Information
climatically restricted areas characterised by a long dry season and a distinct wet
season, flanked upstream by often high upland catchments. The landform assemblage
of these fans is complex, and includes extensive level clay plains, several generations
of former stream channels, sand dunes, closed depressions and back swamps (
Chapter
10
,
Figures 10.3
-
10.5
). Although they display a high degree of geomorphic variability,
these regions possess many common attributes. They are areas of low, erratic rainfall
and high rates of evaporation. Inputs of cyclic salt have contributed to the naturally
high levels of soil and subsoil salinity. When combined with inadequate leaching
and drainage, they offer peculiar problems for agricultural development that are
exacerbated by the general lack of surface water and the presence of groundwater of
variable quality.
In addition, they have undergone a complex depositional history that exerts a
powerful control over the distribution of soils and their physical and chemical char-
acteristics. As a result of the variable depositional history of their parent materials,
the soils are equally variable in texture, salinity and permeability. Indeed, many of
the soils in semi-arid areas have physical and chemical characteristics that have little
to do with present-day climatic conditions in those regions (Williams,
1968a
; Wil-
liams,
1968b
). On the contrary, they owe much to their past history, especially the
depositional history of their parent materials.
The cracking clay soils of central Sudan illustrate this point very well (Buursink,
1971
; Williams et al.,
1982
; Blokhuis,
1993
). Attributed variously to deposition from
wind-blown dust or to in situ weathering of the underlying Proterozoic Basement
Complex rocks, it was not until the pioneering work of Tothill (
1946
;
1948
)thattheir
alluvial origin was recognised. Tothill based his conclusions on the heavy mineral
content of the clay soils, which denoted a volcanic source from the Ethiopian High-
lands via the Blue Nile, and on the presence of freshwater gastropod shells found down
to depths of 2 m, including the aptly named
Cleopatra bulimoides
. Williams (
1966
)
built on this work by providing the first radiocarbon ages of shells at 1.5-2.0 m depth
at two sites adjacent to the present White Nile. Considerable effort has been devoted
since then to developing a detailed late Pleistocene and Holocene flood history for
both the White and the Blue Nile rivers (Williams and Adamson,
1973
; Williams
and Adamson,
1974
; Williams and Adamson,
1980
; Williams et al.,
2006c
; Williams,
2009b
; Williams et al.,
2010b
). Indeed, some puzzling features of the soil landscapes
in the lower White Nile Valley are not explicable without a detailed understanding of
its geologically recent history (Williams et al.,
2000
). These features include a braided
channel pattern despite a remarkably low gradient, a flood gradient of 1 in 100,000,
the strongly localised presence of highly saline subsoils, the presence of subsurface
carbonates and evaporites, the presence of widespread cracking clays overlying quartz
sand dunes, the presence of lake and swamp fossil faunas in now arid areas west of the
river and the presence of buried shell-beds located up to 4 metres above present-day
