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Artwork showing a map of the south polar regions of Earth during the late Ordovician Period (480 MYa). Scientists have identified two distinctive cyclic episodes of glaciation, each due to a different cause and having different effects on ice distribution. This illustration shows the extent of the ice during a time when the glaciation was dominated by changes in the eccentricity - or elongation - of the Earth's orbit around the Sun. Ice coverage was relatively extensive during this period, covering the entire south of the continent of Gondwana, and extending out to sea as far as 50 degrees south