A process-based sedimentological analysis of Upper Ordovician glacial-marine rocks in Africa suggests that full glaciation of the continental shelf started in the late extraordinarius Zone of the Hirnantian. Two cycles of ice-sheet growth are represented during full glaciation. Initial terrestrial ice-sheet growth in the early extraordinarius Zone influenced the first event of the Late Ordovician mass extinction. Retreat of the ice sheet from the shelf ended by the persculptus Zone, when the second event of the Late Ordovician mass extinction began. Eccentricity controlled ice-sheet growth is assumed (periodicity 0.1 m.y.). Hence, two cycles of full glaciation lasted 0.2 m.y. The duration of the extraordinarius Zone is estimated as 0.5 m.y. Therefore, the minimum duration of the first extinction event was 0.3 m.y.