Geoscience Reference
In-Depth Information
desert exploration was well underway before then, often as a precursor to invasion or
military occupation. The story of the
prehistoric
settlement of our deserts is best left
to a later chapter in this volume (
Chapter 17
), but we touch here on aspects of the
prehistoric legacy that are pertinent to present-day concerns over desertification.
Herodotus (ca. 485-425 BC) may justifiably be considered the father of desert
studies. In the course of his travels in Egypt, he noted the presence of marine shells
on low hills near the delta, commenting that the sea must once have been there
before the delta had advanced out to sea. He observed that the pyramids were being
attacked by salt weathering and also commented on the customs and possible origins
of the desert tribes living in the coastal fringes of northern Libya. However, what
intrigued him most was why the Nile floods coincided with the hottest and driest
three months in Egypt. He inferred, correctly, that the black clay soils along the Nile
Valley had come from the Ethiopian Highlands. Following a boat trip north of the
delta during which he examined Nile mud carried out to sea, he speculated with
remarkable acumen that the abandoned distributaries in the Nile Delta could have
become choked with sediment within 10,000 to 20,000 years. He was intrigued by
the fact that in the reign of Moeris less than 900 years before his visit, the whole area
below Memphis used to become flooded when the Nile rose by only four metres, in
contrast to the rise of eight metres needed for flooding to occur when he was visiting
Egypt. He concluded that if the flood-plain continued to build upwards at this rate,
there would be a progressive reduction in the area flooded. He obtained his information
about the progressive decline in the extent of land flooded by the Nile from records
kept by the Egyptian priests, some of whose accounts he accepted while discarding
others.
Nearly five centuries after the death of Herodotus, another itinerant historian,
Diodorus Siculus ('the Sicilian') provided us with a vivid account of cattle rustlers
living in the Red SeaHills of eastern Sudanmore than 2,000 years agowho periodically
descended onto the plains, rounded up any stray cattle they could steal and disappeared
with their booty into the swampy fastnesses of the Red Sea Hills. There are no swamps
in these hills today, but there were then, as shown by the presence of permanent
freshwater mollusc and ostracod fossils in alluvial sediments dating to that time
(Mawson and Williams,
1984
).
At about this time, the Roman emperor Nero dispatched two centurions and a
cohort of soldiers with instructions to find the sources of the Nile. They failed in their
quest but seem to have reached as far south as two low granite hills (apparently Jebel
Ahmed Agha, just east of the White Nile, in latitude 11
N), where they reported their
way blocked by impenetrable swamps. The nearest swamps today are some 350 km
further south, consistent with the former presence of abundant freshwater sponge
spicules in sediments and pots of that age (
Figures 5.1
to
5.3
) at a site 10 km east of
the present-day village of Esh Shawal, situated 350 km south of Khartoum (Adamson
et al.,
1987a
).
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