Weathered sandstone column in Tassili n'Ajjer National Park, Algeria. Tassili is a sandstone plateau at the centre of the Sahara. Fluctuations between moist and arid periods in past climates has resulted in the formation of steep sided valleys and gorges across the plateau. The climate of this region at the end of the Upper Pleistocene was considerably wetter and the sandstone was incised by rivers flowing to great lakes. This arch is called Allagh Arch.