Geoscience Reference
In-Depth Information
11
Desert lakes
'What did he say?'
'He said there was a lake
Somewhere in Ireland on a mountain top.'
'But a lake's different. What about the spring?'
'He never got up high enough to see.'
Robert Frost (1874-1963)
North of Boston: 'The Mountain'
(1914)
11.1 Introduction
In the geographical heart of the Sahara within the Tenere Desert of Niger, there is
an isolated mountainous ring complex called Adrar Bous (see
Chapter 18
,
Figure
18.3
). If we were to draw an imaginary circle of radius 1,500 km centred on Adrar
Bous, the edge of the circle would only just meet the Mediterranean coast to the north
and the coast of West Africa to the south. Immediately south of the central granite
core of the mountain are the remains of two former lakes, one about 9,000 years old
and the other about 7,500-5,500 years old (Williams,
2008
). On the floors of both
former lakes, there are wind-eroded beds of diatomite and silt. Within the silts, there
are shells of freshwater snails indicating that the lakes were permanent bodies of
freshwater. Groups of Mesolithic hunter-gatherers lived near the edge of the older of
the two lakes until it finally dried out. It then refilled to a lower level, providing water
for subsequent bands of Neolithic pastoralists and their herds of short-horned cattle
(Williams et al.,
1987
; Williams,
2008
). The occupation sites and fireplaces left by
these prehistoric people contain bones of Nile perch, hippo and turtle. At one spot on
the edge of the former lake was the partly exposed skeleton of a hippo with a barbed
bone harpoon point embedded in its ribcage. Scattered across the Sahara at intervals
during the early to mid-Holocene were hundreds of small lakes similar to those at
Adrar Bous, offering eloquent witness to a time when the climate was considerably
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