Landsat images, ASTER digital elevation models, aerial photographs, and field investigations in the western Murzuq Basin (Libya) and the adjacent Tassili N’Ajjers (Algeria) provide paleogeomorphological evidence for the existence of a Late Ordovician ice stream at least 200 km long and 80 km wide. This includes mega-scale glacial lineations, an associated subglacial meltwater drainage system, and ice-front features. This first comprehensive description of a pre-Cenozoic ice stream may help to identify other examples in the Proterozoic to Paleozoic glacial record. Reconstruction of the extent and behavior of former ice sheets and reservoir prospect analysis in glacially related successions have to take into account the potential occurrence of ice streams.