The study of fluvial terraces has revealed coarse sand and gravel alternating with terraces of silt, clay and fine sand. This alternation highlights repeated changes in the dominant fluvial or wind patterns in the valleys of Tibesti during the Quaternary Period. The phases of erosion and sendimentation are indicative of the climate alternating between dry and wet conditions, the latter of which fostered dense vegetation in the Tibesti that was likely far more diverse than that which exists today (with the exception of grasses). Furthermore, the discovery of calcified charophyta (particularly of the family Characeae) and gastropod fossils in Trou au Natron indicates the presence of a lake at least deep during the Late Pleistocene. These phenomena are associated with various changes in climate, most notably during the last glacial maximum, which increased precipitation and reduced evaporation due to lower temperatures. In fact, the Tibesti supplied a considerable amount of water to the Paleolake Chad until the 5th millennium BC.