The Pliocene - Pleistocene boundary, marking the onset of the Quaternary Period, is based upon the first appearance of cold tolerant species of mollusks within stratigraphic sections at 1.8 million years ago. Debate and discussion regarding the timing of the Pliocene - Pleistocene transition and sub - divisions of the Quaternary continues since the inception of ice ages began as early as 2.6 million years ago. The Quaternary Period, also known as the Quaternary Ice Age since it is characterized by many glacial advances and retreats (glacial and interglacial periods), comprises the Pleistocene epoch and is followed by the Holocene epoch (the most recent 10,000 years). Ice covered up to 30% of the Planet's land surface during the height of glacial expansion in the late Pleistocene (Oxygen Isotope Stage 2), but many previous glacial periods preceding this were much larger in extent. The Quaternary Period is characterized by drastic shifts in global climate and is recent enough so that many different types of paleoclimate records are preserved: Loess, marine sediment, glacial ice, and terrestrial records such as raised marine terraces, and lake sediment and geomorphologic features.
