In front of a painted shelter in the Ennedi, the first instinct is to look for a scene. That is a mistake: what you are looking at is almost never a single composition, but a palimpsest — generations painting over their predecessors, sometimes thousands of years apart.
Look at the animals first. They are the most reliable marker. Elephant, giraffe, rhinoceros and hippopotamus belong to the Bubaline period, the oldest: a humid Sahara peopled by hunters. Great herds of piebald cattle signal the Bovidian period, the pastoral golden age. The horse harnessed to a chariot announces the Caballine period, when the desert sets in and trans-Saharan routes open. The dromedary, finally, marks the Cameline period — the Sahara as we know it.
Then the technique. Deep, polished engravings are generally the oldest. Polychrome paintings — red ochre, white, yellow — belong to the height of the Bovidian period. Cameline figures are often pale, quick strokes, sometimes simple incisions.
Finally, the superpositions. When a dromedary silhouette crosses a herd of cattle, the order is unambiguous. It is by accumulating such overlaps that prehistorians built the chronology of the central Sahara.
One absolute rule. Never wet a wall to bring out the colours. The practice is still widespread and it is destructive: it dissolves the binders, washes out the pigments and destroys in seconds what seven thousand years had left intact. No flash on painted panels either. Photograph early, in raking light — the colours come out far better than in full sun.