The plateau is composed largely of sandstone, and the natural erosion has resulted in hundreds of natural rock arches and other spectacular land formations - the ‘forests of stone’. Because of the altitude and the water-holding properties of the sandstone, the vegetation is somewhat richer than in the surrounding desert, and includes scattered woodland of the endangered endemic species of the Saharan cypress - one of the oldest trees in the world - and the Saharan myrtle. The literal English translation of Tassili n'Ajjer is 'plateau of rivers'. Relict populations of the West African crocodile persisted in the Tassili n'Ajjer until the twentieth century. Various other fauna still reside on the plateau, including Barbary sheep, the only surviving type of the larger mammals depicted in the rock art of the area.
Between 12,000 and 7,000 years ago the Sahara's climate was far wetter than it is today. Water flowing from the mountainous regions fed the savanna and woodland, which housed much wildlife. This in turn attracted hunter-gatherers, and pottery found in Niger’s nearby Aïr Mountains has been dated to 11,500 years old. Roughly 7,000 years ago, domesticated animals such as cattle, goats and sheep began to appear, so whilst hunting and gathering continued, some Saharans adopted a pastoral lifestyle. By 6,000 years ago, the climate began to change, becomming much drier; people and their livestock moved away. By 4,500 years ago the Sahara began to resemble the picture we see today.
The art consists of paintings and engravings on exposed rock faces. The art is representative but almost certainly not simply a reflection of daily life. Compositions depict large animals with diminutive human figures or humans with animal heads. The geometric symbols certainly held meaning for the artists and the people. Dating of the rock art has proved difficult; researchers have used style, content, degrees of fading, superimposition, associated archaeological dates and changing climate to construct a chronology (Coulson & Campbell, 2017).
David Coulson, of the Trust for African Rock Art, provides the current and generally accepted chronology, whilst accepting that this may well change in the future:
Without doubt, more sites are yet to be found at Tassili n'Ajjer, and one may well expect the rock art to be proved even older in some cases as direct dating techniques become more reliable.