Tassili n'Ajjer (Berber: Tassili n Ajjer, Arabic: طاسيلي ناجر, romanized: ṭāssīlī naʾjir; "Plateau of rivers") is a mountain range in the Sahara desert, located in south-eastern Algeria. It holds one of the most important groupings of prehistoric cave art in the world and covers an area of more than 72,000 km2 (28,000 sq mi). The site has been designated a national park and a Biosphere Reserve, and was inducted into the UNESCO World Heritage Site list in 1982. The literal English translation of Tassili n'Ajjer is 'plateau of rivers'.
Geography
Tassili n'Ajjer is a plateau in south-eastern Algeria at the borders of Libya and Niger, covering an area of 72,000 km2. It ranges from 26°20′N 5°00′E east-south-east to 24°00′N 10°00′E. Its highest point is the Adrar Afao that peaks at 2,158 m (7,080 ft), located at 25°10′N 8°11′E. The nearest towns are Djanet on the southeastern edge and Illizi on the northern edge. The plateau is of great geological and aesthetic interest. Its panorama of geological formations consisting of rock forests, composed of eroded sandstone, resembles a lunar landscape and hosts a range of rock art styles.
Geology The range is composed largely of sandstone. The sandstone is stained by desert varnish, a thin outer layer of deposited metallic oxides that colour the rock formations variously from near-black to dull red. Erosion in the area has resulted in nearly 300 natural rock arches being formed in the south east, along with deep gorges and permanent water pools in the north.
The Tuareg Shield, an accumulation of terranes that formed during the Neoproterozoic era between 750 and 550 million years ago, encompassed the area of today's Ahaggar Mountains in Algeria, the Aïr Mountains in Niger, and the Adrar des Ifoghas in Mali. To the west lies the West African Craton, to the east the Saharan Metacraton. It expanded through accretion in the north and south, so that it also encompassed the area of today's Tassili n'Ajjer. During this epoch, the supercontinent Gondwana formed, which also included all the cratons and shields of what would later become Africa. During the subsequent Cambrian period (541 to 485 million years ago), the first sediment deposits still present today were formed in the Tassili n'Ajjer region. During this epoch, Gondwana shifted from the equatorial zone to the southern hemisphere, so that the Sahara region was located in the South Pole region during the Upper Ordovician (458 to 443 million years ago). At the end of this period, the 'Hirnantian’ ice age prevailed for a million years. An ice sheet formed, covering much of the northern half of Africa. In the early Silurian period (starting 443 million years ago), the global climate changed, causing the ice of the Tuareg Shield to melt. This left behind meltwater channels that had formed under the ice. This completed the formation of the geological stratigraphy of the Inner Tassili Group. It consists mainly of sandstone and some layers of clay.
During the Silurian and Devonian periods, transgression (the advance of the sea) from the north and northwest covered large areas of the Sahara with further layers of sediment. At that time, the Tassili n'Ajjer was the seabed near the coast, later becoming a fluvial floodplain. During the Silurian period, the stratigraphic sequence of the Intra Tassili Group was formed, which contains a lot of bioturbated clay and some sandstone. During the Devonian period, the formations of the External Tassili Group were formed, which contain a lot of sandstone alongside clays with macrofauna and some limestone. It is primarily from these two stratigraphic sequences of the Silurian and Devonian periods that petroleum and natural gas have been and continue to be extracted in the Illizi Basin, north of Illizi. During the following 200 million years, this region remained geologically a rather quiet, fairly flat mainland area without major tectonic changes or sedimentation. - Probably as early as the Cretaceous period, but especially since the Miocene (23 to 5,3 million years ago), the Tuareg Shield began to arch as a result of magmatic uplift. The uplift in the Ahaggar region ultimately reached 4,000 to 5,000 m. The Tassili n'Ajjer region was also affected, and the uplift caused its layers to tilt toward the north. Parallel to the uplift, several basins formed north and south of the Tuareg Shield, which now served as deposition areas for the eroded sediment, for example the Illizi Basin. Especially in the Miocene, the sedimentary layers of the External Tassili (the uppermost layer sequence) were completely eroded in the southern and eastern parts of the Tassili n'Ajjer. In the northern part, on the plateau of this layer group, the river systems dug up to 400 m deep into the sandstone and clay layers due to the now greater gradient. The last major geological change occurred during the Pliocene (5,3 to 2,58 million years ago). Volcanism covered the Adrar region in southwestern Tassili n'Ajjer with craters, volcanic cones, and lava flows. This development continued into the early Quaternary period.
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