Geoscience Reference
In-Depth Information
tributaries are the Blue Nile and the Atbara. The Blue Nile joins the White Nile
at Khartoum to form the Main Nile, or Saharan Nile, which then flows north into the
eastern Sahara Desert to be joined by the Atbara 320 km downstream, after which the
Nile has a waterless journey of 2,689 km until it debouches into the Mediterranean
some 256 km north of Cairo. The Atbara also rises in the highlands of Ethiopia not far
from the source of the Blue Nile but pursues a more northerly course through more
arid terrain, losing its water in the desert during the dry winter months, it failed to
reach the main Nile until construction of the dam at Khashm el Girba.
The Gezira is a clay-mantled, low-angle alluvial fan roughly 230 km in radius and
40,000 km
2
in area. It is bounded to the east by the present Blue Nile, to the west by
the White Nile and to the south by the Managil Ridge, and is traversed by a series
of sandy paleochannels that originate between the towns of Sennar and Wad Medani
on the present-day Blue Nile (
Figure 10.10
). The Blue Nile fan consists of a veneer
of dark cracking clay one to five metres in thickness which mantles alluvial sands
and gravels with very large cross-beds indicative of very high-energy flow (Williams,
2012a
). Tothill (
1946
;
1948
) was the first to show that the Gezira clays were alluvial
and not formed by wind-blown dust, and considered that they had been laid down in
early Holocene times as a result of seasonal flooding from the Blue Nile. He based
his conclusions on the presence of aquatic gastropods (see
Chapter 16
) in the upper
two metres of Gezira clay and on the presence of pyroxene and other heavy minerals
indicative of a volcanic (i.e., Ethiopian) provenance for the alluvial clays. Williams
(
1966
) proposed that the Gezira clays had been deposited by seasonal floods from
distributary channels of the Blue Nile that radiated across the Gezira alluvial fan. He
also provided the first radiocarbon ages for two sites east of the White Nile, showing
that theseWhite Nile alluvial clays had been deposited during the terminal Pleistocene
and early Holocene.
Williams and Adamson (
1974
;
1980
;
1982
), Adamson et al. (
1980
;
1982
)and
Williams et al. (
1982
;
2000
) carried out a program of comprehensive radiocarbon
dating of gastropod and other shells fromBlue andWhite Nile alluvium. These authors
were able to show that there had been a series of Holocene phases of diminishing high
flood levels in both rivers, with the last moist phase at around 2 ka associated with
a unique form of pottery manufacture using the swamp-dwelling sponge
Eunapius
nitens
as temper (Adamson et al.,
1987a
). Later work by Talbot et al. (
2000
) and by
Williams et al. (
2006c
), based in part on the use of strontium analyses (see
Chapter 7
,
Figure 7.1), showed that the abrupt return of the summer monsoon in the Ugandan
headwaters of the White Nile around 14.5 ka was followed by widespread flooding
and clay deposition in the lower White Nile Valley. This major flood event caused
erosion of many of the sand dunes in the lower White Nile Valley (Williams,
2009b
)
and was followed by Blue and White Nile incision and progressively reduced flooding
until aridity set in around 4,500 years ago (Williams et al.,
2010b
).
