Tassili n'Ajjer is a sandstone plateau in the Sahara of southeastern Algeria, and its rock shelters carry some fifteen thousand prehistoric paintings and engravings: giraffes, elephants, herds of cattle, and people swimming. They were painted here because this was green when the work began. Tuareg guides lead treks up onto the plateau from the oasis town of Djanet, and everything goes on the camels: water, food, bedding and fuel, because nothing is resupplied once you are up there. Camp is made where a rock wall breaks the wind, and your guides cook on a fire. The shelters are scattered rather than clustered, so most of each day goes on the ground between them, among eroded towers with close to three hundred natural arches worked through the rock. The route winds through narrow gaps, then opens into bare basins.
At Sefar, the richest of the sites, nothing is framed, labelled or lit. You stand at the shelter wall and pull the images apart by eye. The oldest layer is Round Head work, huge figures with smooth, blank faces, and later herders painted their cattle straight across them, so one panel can carry two or three ages at once. Telling which line belongs to which is what you are actually doing up here. The largest Round Head stands six metres tall. A French copyist in the 1950s nicknamed it the Great Martian God for its helmet-like head; archaeologists read these figures as masked dancers or spirits. The engravings work the other way round. The glossy black sheen on the rock is desert varnish, a wafer-thin coat of clay and iron oxides laid down over thousands of years, and the engravers chipped through it so the pale sandstone underneath became their line. A few hundred Saharan cypresses still hold on up here, some more than two thousand years old and hundreds of kilometres from the nearest other tree. They are the green Sahara too, and the desert is now so dry that almost none of their seeds come to anything.