Chapter II : Physical Geography
Part 2 : Geological Observations.


from : J. Roland and E. Duchesne, Cours Complet de Geographie : Le Congo Belge, Namur 1914, pp.13



Geological Observations.

The Congo basin is a massive of very old land formations (of archaic and primary lands). Folded by force, these land formations formed in chains which were reduced by erosion to the state of submerged, levelled and shaved mountain ranges. - In the present state of things, they they constitute the ground of the elevated region of the basin's periphery and of the plateaus around the Great Lakes. They are found in the form of granitic formations, of rocks of gneiss, of micaschists, phyllades and of quartzites, and contain a number of systems we can observe in Belgium (cambrian, silurian, devonian). Metalliferous ore deposits or considerable content have been found, or iron, of copper, of tin, of lead and of gold.
In those regions where they constitute the sub-underground, the archaic and primary formations are generally masked by a layer of rubble and of alluvial deposits. Very rarely they resurfaced in strengthened posterior formations, in red sandstone, in psammites, in clay slates (Mont de Cristal, Katanga) or in white sandstone (the soft sandstones of Upper-Congo) : they are the depots left behind by the interior sea which, during the secondary and tertiary periods, covered all the Congo as a consequence of the great movements of the ground and of formidable dislocations. At many places, brittle sandstone formed layers of several hundreds of meters of rubble.
These formations vanished under a mantle of mobile lands, when a vast cover of inland waters found it's way out toward the ocean, the entire Congo basin presents the impression of an immense undulated plain. The forces of wind- and river-erosion which followed the drainage, the chemical alteration and the physical erosion of rocks subjected to atmospheric agents, the force of intensive deposition added new sediments, or rubble or of alluvium, which appeared on the surface of the soil. There they appeared, as a consequence of the amount of limonite or iron they include, in a reddish-bricklike colour which impresses all travelers and which in the Congo is called laterite (after Latin later, brick).
A new depression concerning the central regions of the basin, accompanied by a rise and bulging of the border regions, determined the formation of the actual basin or interior depression. It was resulted by a torrential climate, with falls and rapids, along the upper courses of the river affluents; the river itself, barred from emptying into the ocean by the coastal barrier (the Crystal Mountains) which were created by a lifting, energetically attacked the mountaineous massive and created for itself a massive gorge and outlet.
On the other side, the dislocations which were produced in very remote epochs, formed the great crevice, called in German Graben and which contains the Lakes Albert, Albert-Edward, Kivu and Tanganyika. Along the axle of the Graben an eruptive elevation gave birth to the volcanic massive of Virunga, which separates the crevice in two parts; in the north, the waters flow towards the Nile, in the South they empty into the basin of the Congo, Lake Kivu empties, via the Ruzizi, into Lake Tanganyika, and the latter, which has lifted the level of it's waters since it's formation in a relatively recent epoch, through it's actual outflow, the Lukuga.
Other depressions formed in the terrasse of Katanga, those of the Luapula, where Lake Moero again occuppies a basin, of the Lufira and the Upper Lualaba; their waters, when they rise and fall, and cut outlets through the threshold of the Mitumba Mountains and flow out into the lower terrasse, through the upper arm of the Congo, the Lubudi-Lualaba.





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