| Summary: | A thesis submitted to the Faculty of Science, University of the Witwatersrand,
in fulfillment of the requirements for the degree of Doctor of Philosophy.
Johannesburg, 1998 === The appearance of the dry Molopo River with its generally wide and shallow valley cut into the
Kalahari Group sediments, but more especially its few impressive rock-cut gorges, has intrigued
many people over the ages, and led to many theories concerning its history. The rock-cut
gorges, in particular, have traditionally been attributed to ancient, previously much more
extended drainage lines, more or less in the same geographical position as the present Molopo
Valley. An analysis of the sediment body of the Kalahari Group, and both, alluvial gravels and
geomorphic features on the rim of the Cainozoic Kalahari Basin, have indicated that the Molopo
drainage has gradually shifted westward over time, in response to a tilting of the drainage area,
which lies across the south-eastern rim of the Kalahari Basin, The present geographical position
of the Molopo River is thus a relatively recent one in its evolution. The earliest traces of
drainage lines in the area predate the Permo-Carboniferous glaciation of Gondwana. A preglaciation
valley system with only some similarities to the present-one flowed in a northwesterly
direction. There is a long hiatus in the evidence from the end of the glaciation to the
beginning of the formation of the Calnozoic Kalahari Basin. The most significant feature of that
intermediate period, is a large meteorite impact which occurred near Morokweng at the J-K
boundary.
The Cainozoic evolution of the Molopo drainage has been strongly influenced by both tectonics
and climatic change. Tectonics, which include both the initial formation of the Kalahari Basin
and Neogene warpings of the intra-continental axes of uplift, caused the interruption of a pre-
Kalahari southward flowing drainage system, an extended upper Molopo, the existence of the
Molopo as an endoreic system for an extended period of time, and a westward shift of the entire
Molopo drainage system. Progressively more arid conditions interrupted by humid climatic
pulses of decreasing intensity have occurred since the Cretaceous. The aridifying conditions
caused the existence of playa-like conditions over long periods oftime in the back-tilted section
of the proto-Molopo. This was followed by a rapid sediment infilling of the sub-basin and a
major rejuvenation phase, which caused the incision of the Molopo River into the duricrusted
sediments of the Kalahari Group, and the re-establishment of the Molopo River as an exoreic
drainage system in its present position. Rock-cut terrace remnants in the upper Molopo give
evidence of much smaller climatic changes during the Quaternary. There has been no integrated
flow in living memory.
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