1 Of or relating to a glacier.

2 An extended length of time during which Earth’s glaciers expanded widely. The most recent glacial epoch, better known as the Pleistocene glacial epoch, previously the Quaternary, covered at least the last 3 000 000 years. A. Penck and E. Brückner (1909) established the classical sequence of four glacial periods in the Alps: the First Glaciation, at its peak some 550 000 years ago, the Second Glaciation, at its peak 450 000 years ago, the Third Glacial Period (Riss) with a climax 185 000 years ago, and the Fourth Glaciation (Wurm) at its height 72 000 years ago. Ice-sheets grew only during parts of these so-called ‘glacials’, each of which was followed by an interglacial period.