Glacial deposits in the McMurdo region record two series of major events. The first involves invasion of Taylor and Wright Valleys by tongues of the inland ice sheet dammed west of the Transantarctic Mountains. Advances prior to 4 million years ago carved the valleys; only four subsequent advances, all relatively minor, have occurred within the last 3.6 million years. The second series of events involves the repeated presence, within the last 1.2 million years, of grounded ice sheets in McMurdo Sound and in the eastern Ross Sea and Ross Ice Shelf areas.