The Kalahari occupies an exceedingly flat tableland taking up much of the
central part of southern Africa. When the national gravity survey of Botswana
was mooted in 1970, absolute heights above seal level for this remote, dry and
thinly-populated area were unavailable and precision primary levelling was
still in progress. Earlier levelling work in pursuit of potential engineering
work on river courses (Du Toit, 1926; Brind, 1955) had proved inconclusive. A
by-product of the national gravity survey (Reeves & Hutchins, 1976) was the
first rough topographic contour map of Botswana derived from about 2500 heights
determined by barometric altimeters tied to the few available reliable datum
points. The lowest point in the inland drainage basin in the Makgadikgadi salt
flats was determined as 903 (+/- 5) metres. Almost all of northern Botswana lay
below 1000 m with the only escape for water from the annual flooding of the
Okavango delta in the NW of the country at about 930 m, located near the
administrative centre of Maun.
More recently, Shuttle imaging radar (SIR) has determined the topographic
surface with centimetre accuracy. Geomorphological studies, linked to SIR data,
have defined old shorelines at 973 m outlining a lake that occupied most of
northern Botswana - an area about half the size of France - in earlier pluvial
times. Archaeological studies of these ancient shorelines have revealed copious
stone tools, relics of a Stone Age civilisation living in a much more clement
climate than today's (Moore & Larkin, 2001). In a recent Nature publication
(Chan et al., 2019) it is claimed that studies of mitochondrial DNA lead back
to this earthly lakeshore paradise hosting the origin of Man. Migratory pulses
are interpreted to have left for east Africa 130 000 years ago, and for the
south 110 000 years ago.
The Mwembishi fault zone (see previous news item, below) separates the ancient
lake from the present-day watercourses of three large rivers, the Okavango, the
Cuando and the Zambezi, all drawing their water from the Angola highlands that,
even today, enjoy higher levels of rainfall than the Kalahari. It would seem
that movements on the Mwembishi shear zone in the Early Cretaceous set up the
topography of southern Africa, much of which persists to the present day.
Perhaps we should all be grateful for that!
In the figure, Karoo rocks are indicated in beige and Kalahari sand cover in
pale yellow. International boundaries are in red and the paleo-lake is shaded
2019 October 31
Brind, W.G., 1955. The Okavango delta: report on the 1951-3 field surveys. Chan, E.K.F., et al., 2019. Human origins in a southern Africa paleo-wetland and first migration. Nature. https://doi.org/10.1038/s41586-019-1714-1
Du Toit, A, 1926. Report of the Kalahari reconnaissance of 1925. Union of South Africa, Department of Irrigation.
Moore, A.E., and Larkin, P.A., 2001. Drainage evolution in south-central Africa since the break-up of Gondwana. South Africa journal of Geology, 104, 47-68.
Reeves, C.V. and Htuchins, D.G., 1976. The national gravity survey of Botswana, 1972- 73. Bulletin No. 3, Geological Survey Department, Botswana.