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 obliquity, or axial tilt, of the Earth. Ice coverage was relatively sparse during this period, confined to the land only on the continent of Gondwana.