When the Sahara was green (I)The Sahara has not been always the most impressive desert in the world. Instead of this kingdom of rocks and sand, desolated and arid, existed in remote times a verdant and populated region. This is the testimony of the civilizations that flourished on the Central Sahara, civilizations that were extinguished when the wind of the desert covered the greenery with a shroud of sand.
Already in the most ancient written documents, the Sahara is described as a strange, mysterious and inhuman region. Herodotus, the first author that mentioned the Sahara, around 430 BC, attributed extravagant habits to its inhabitants, describing it as an already desertized region, composed of dunes, waterless vast spaces, oases and mounds of salt. Confused by the presence of salt mounds, mollusc shells and immense extensions of sand, the roman generals that entered the Sahara believed to be witnessing the bed of an ancient, extinct sea.
The modern exploration of the Sahara begun during the 19th century. French explorer René Caillié, who crossed the Sahara from Tombouctou to Morocco in 1828, had to face a torrid heat, endure sand storms, exhausting marchs and a devouring thirst that could not ease, lacking enough reserves of water. He experienced that famous mirage that induces to believe in lakes surrounded by trees, that vanish as the traveler approach. During six years the German explorer Heinrich Barth traveled the Central Sahara, departing from Tripoli in 1849, crossing the Fezzan, Tassili-n-Ajjer, the Air mountains and the Kaouar cliffs, arriving to Chad on the east and Tombouctou to the west. He created a map of the Sahara that offered the first faithful vision of the relief, including the location of high mountains, such as the Ahaggar, Tassili-n-Ajjer or the Air, and the dried basins that depart from them. Besides, he did mention engraved bovids in the rocks of the Fezzan and the Air. This made him to think in a saharan past with a climate much more favorable for life. He supposed the existence of a people of cattle herders and noticed that did not existed even a single camel represented in the group of engravings, from which he deduced that this animal appeared much later in the Sahara. With this, he had posed the bases of the saharan archaeology, articulated around a Sahara previous to the camel and a Sahara with camels.
Morphology of the SaharaThe French occupation of the Sahara brought the apparition of the definitive maps of the Sahara, revealing that the sandy areas (ergs) covered only a minor part of the topography, being the remaining ones formed by mountains, rocky plateaus, stony fields (regs) and a certain number of oases. The mountainous massifs such as the Ahaggar (up to 3000 meters), the Tibesti (up to 3400 meters), the Tassili-n-Ajjer (up to 2000 meters) or the Air (up to 2000 meters), contain numerous volcanoes and vast effusions of lava have created immense volcanic fields, being the eruptive systems relatively young and some of them potentially active. The natural hydrographic network is very complex and the morphology of the mountainous massifs is due mainly to the effect of running waters, being the erosive wind a secondary factor. And precisely is the water who explains the mystery of the origin of all the sand that partially covers the ancient valleys.
The study of the saharan relief shows that a majority of the watercourses, instead of leading into the sea, lead into closed basins; so, along the time, the elements drawn by the waters, instead of being drawn to the sea, are deposited in the bottom of those closed basins, raising the level of the base and decreasing the slope. The stream loses strenght, it cannot evacuate normally the alluvium and these are abandoned in the sections of lesser inclination, eventually causing the obstruction of the riverbed. The waters border the obstacles seeking for an exit, leaving the course, and expand around forming marshes, that are easily evaporated. This way died the rivers and creeks of the Inner Sahara. The abundant salt deposits of the Sahara, such as the ones in Amadrar, Teghaza or Taoudenni, are vestiges of these ancient closed basins on which the waters evaporated and deposited the molecules of sodium chloride created from the dissolution of the rocks. Some of these riverbeds, such as the Tilemsi in the Adrar des Iforas, disembogue into the Niger river and suffer less this effect. Of the ancient saharan lakes, only the Lake Chad remains, reduced to a surface that is at the present around thirteen times lesser than the ancient one (up to 330000 km2). The death of the watercourses left uncovered the alluvium on the riverbeds, being exposed to the action of wind; the heavier particles fell to the lower zones, where the grains of quartz eventually accumulated in great quantity, being slowly formed the sand dunes and the ergs. Regarding the stony fields, these are sometimes product of the dissemination of the alluvium during the season of the overflows (in the desert the rainfall is scarce, but torrential and very erosive), and sometimes caused by the disintegration of ancient sandstone massifs by effect of the waters.
A valley from the Central Sahara, that meanders at the foot of the mountains along the regs, immense stony extensions where alluvium are accumulated. In the far distance are visible the peaks of the southern Teffedest, a massif belonging to the Ahaggar.
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