Page:Africa by Élisée Reclus, Volume 4.djvu/302

 Some traces still survive of the ancient inland sea which flooded this region of the continent between the uplands skirting the Limpopo and the Damara highlands, before the waters of this vast basin were drawn off through the gorges of the Zambese. Over the old lacustrine bed are still scattered numerous flooded depressions, which become displaced, enlarged, or reduced in size according to the abundance or scarcity of the rains and the deposit of alluvial matter. The long presence of water in a vast continental lake is clearly shown, not only by the almost perfectly level disposition of the land, but also by the formation of extensive lacustrine deposits. The whole plain is floored, as it were, by a kind of

tufa more or less soft according as it is exposed to the air or covered with organic débris. Wherever the soil is turned up freshwater shells are brought to the surface, analogous to those still found in the Zambese.

The bed of the Ku-Bango, as well as those of the streams flowing from the Damara uplands, and ramifying over the great plain, are flanked by depressions where the surplus waters are gathered in temporary lakes during the rainy season. Moreover, these rivers branch off into distinct channels, the so-called molollas of the natives and laagten of the Dutch Boers, which also receive much of the periodical overflow, but in which the current sets in the opposite direction and thus rejoins the main stream during the dry season. In this way is produced a sort of ebb and flow, regularly following the annual alternations of the climate.