THE GEOMORPHOLOGY OF AFRICA
During the Primary period of the Earth (some for billion years ago) the primitive crust, that would later become Africa, was essentially made up of eruptive or magmatic rocks (granite, syenite, gabbro...). They were formed deep in the earth's crust (later emerging as a result of erosion), and effusive rocks brought to the surface by volcanic activity. There is emergence of an immense mountain chain called Saharides. A billion years of erosion scraped away this first crust, leaving some platforms that eventually form the Nile cataracts. In the course of the Secondary period (some 200 million years ago) the vast continent called Gondwana begins to dislocate, and the different parts, Africa, Antarctica, Australia, America, Madagascar and the Indian subcontinent move away from each other, becoming individual units.
This phenomenon is accompanied by several events:
- The inversion of the magnetic poles, which will recur numerous times during the course of the geological eras.
- The presence of a new ice cap. The remains of ice caps are easily recognisable given the specific sediments that they engender. The continental masses (tectonic plates) having moved, glacial sediments can be found today on continents that no longer have a polar location.
- Volcanic eruptions and repeated sea invasions.
Africa fissures, floats and reaches humid equatorial latitudes.
During the course of the following millions of years, the marine transgressions on the edges of the African shield leave sediments of clay, sand or shell. They form the soil of the Sahara and of the Nile Valley.
During the Primary period, a levelling action leads to subsidence of the north-west of the future African continent. It is followed by the partial invasion of the continent by water. Thick layers of sandstone are then deposited by huge, unstable channels. Around 420 million years ago (middle of the Primary period), the sea covered the north of Africa and there accumulated schist with graptolites, which later was to become the future reservoirs of Saharan oil.
At the end of the Secondary period, Africa migrated towards the north, reaching equatorial latitudes and was then covered by conifers. The latter are the vestiges of the fossilized wood that is today found in the desert. Great rivers transported sand and clay, depositing them in lakes and swamps. These materials, once solidified, form the Nubian sandstone, the reservoir-rock of the current ground water.
The Mediterranean, the domain of the sea-deity Thetis, only opened up around 95 million years ago, and its depth increased during the Palaeogene, some 65 million years ago.
During the Tertiary (from 65 million years ago), an immense complex of lakes and rivers shaped the landscape. On the edges of the great plateaux, water run-off formed large valleys, creating large tabular mountains called gours.
The Sahara and the Nile Valley are both rare witnesses of the first formation of the Earth.