Chapter 103: Pleistocene

Definition of the base of the Pleistocene has had a long and controversial history. Because the epoch is best recognized for glaciation and climatic change, many have suggested that its lower boundary should be based on climatic criteria—for example, the oldest glacial deposits or the first occurrence of a fossil of a cold-climate life-form in the sediment record. 

Other criteria that have been used to define the Pliocene–Pleistocene include the appearance of humans, the appearance of certain vertebrate fossils in Europe, and the appearance or extinction of certain microfossils in deep-sea sediments. These criteria continue to be considered locally, and some workers advocate a climatic boundary at about 2.4 million years.

Pre-Pleistocene intervals of time are defined on the basis of chronostratigraphic and geochronologic principles related to a marine sequence of strata.