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(Arkell,
1949
). Incision by the Blue Nile amounts to at least 10 m since 15 ka, with
associated incision by the White Nile of around 4 m since that time (Arkell,
1949
;
Arkell,
1953
; Williams and Adamson,
1980
; Williams et al.,
2000
).
The very gentle flood gradient of the White Nile (1: 100 000) has meant that the
post-LGMflood deposits in the lower White Nile Valley are unusually well-preserved,
in contrast to those of the Blue Nile. High White Nile flood levels have calibrated
radiocarbon ages of around 14.7-13.1, 9.7-9.0, 7.9-7.6, 6.3 and 3.2-2.8 ka obtained
on freshwater gastropod and amphibious
Pila
shells and fish bones (Williams,
2009b
).
The more fragmentary Blue Nile record shows very high flood levels at around 13.9-
13.2, 8.6, 7.7 and 6.3 ka (Williams,
2009b
).
Mayewski et al. (
2004
) examined fifty globally distributed paleoclimate records
spanning the interval from 11.5 ka to present and identified six significant periods of
rapid climate change at 9-8, 6-5, 4.2-3.8, 3.5-2.5, 1.2-1.0 and 0.6-0.15 ka. They
observed that the first five of these episodes coincided with polar cooling and tropical
aridity. The intervals in between were wetter in the tropics and are broadly similar to
the intervals of high Blue and White Nile floods identified here. At the site of Erkowit
in the Red Sea Hills (Mawson and Williams,
1984
), there is evidence of permanent
stream flow around 1.8-1.6 ka, coinciding with high White Nile flows but not as yet
evident in the much more incomplete Blue Nile sedimentary record.
Verschuren et al. (
2009
) identified four episodes of low Holocene lake levels at
Lake Challa on the slopes of Kilimanjaro with ages of 8.0-6.7, 5.9-4.7, 3.6-3.0 and
0.7-0.6 ka. These ages also roughly coincide with times of low flow in the White Nile
(Williams,
2009b
). The dry interval starting at 3.6 ka at Lake Challa may be coeval
with the sharp decrease in rainfall along the southern Dead Sea at around 3.9 ka
(Frumkin,
2009
), although this may simply be a coincidence.
Unlike the sinuous suspension load channel characteristic of the early Holocene
Blue Nile, the LGM Blue Nile and main Nile were highly seasonal rivers which
carried a substantial bed load of sand and gravel, much of which they deposited in
northern Sudan and southern Egypt during the long dry season, when they lost the
competence to transport coarse sediment (Adamson et al.,
1980
; Williams,
2012a
). A
prime reason for this was the LGM desiccation of Lake Victoria and the curtailment
of flow in the White Nile to a trickle. The White Nile today maintains perennial flow
in the main Nile during years of drought in the Ethiopian headwaters of the Blue Nile
and Atbara rivers.
18.9 Cenozoic evolution of the Namib and Kalahari deserts
The geomorphic evolution of southern Africa appears to have been far less complex
than that of the Sahara, Afar and Negev deserts described earlier in this chapter.
From west to east, the major elements of the landscape are the Namib plains, the
Namaqua highlands, the Kalahari Basin, the Drakensberg escarpment and the Natal
