CHAP. 34] THE PLEISTOCENE RECORD 891 



In conformity with ancient usage, post-Pliocene strata are grouped into a 

 Quaternary System. By many authorities that System is subdivided into a 

 Pleistocene Series and a Recent, Holocene, or Postglacial Series. Others view 

 the distinction between Pleistocene and Recent as arbitrary, and prefer to 

 consider the Pleistocene strata as representing the entire sequence from the 

 base of the Quaternary System to the sediments that are accumulating today. 

 The later classification is adopted, without prejudice, for the present contri- 

 bution because it is well adapted to the discussion of deep-sea sediments. 



The base of the Pleistocene Series (and also of the Quaternary System) has 

 been defined in various ways. A recent and closely reasoned definition sets the 

 base at the appearance in late Cenozoic sediments occurring in various parts of 

 Italy of a cold-water marine fauna (the Calabrian fauna) differing from the 

 underlying PHocene fauna. The newer fauna is characterized by the disappear- 

 ance of certain Pliocene species and by the apjDearance of a dozen species of 

 North Atlantic mollusks. In northern Italy the Calabrian marine strata grade 

 upward into continental sediments containing a distinctive mammal fauna 

 variously labeled Villafranchian and Upper Villafranchian. In some other areas 

 the mammal-bearing sediments contain a cool-climate continental flora, con- 

 trasting with the warmer. Pliocene flora in the underlying rocks. Thus, through 

 fossils of various kinds the base of the Pleistocene is extended by correlation 

 into widely separated regions. 



Subdivision of the continental Pleistocene is still based less on fossils than on 

 sediments implying alternation of glacial and interglacial conditions. In both 

 Europe and North America the glacial layers seem to fall naturally into major 

 subdivisions, some of which record a single glaciation, whereas others imply 

 repeated glaciation. Between the glacial units is found evidence, such as zones 

 of deep thorough weathering of glacial drift, or fossils denoting higher non- 

 glacial temperatures, from which long periods of warmer interglacial climate are 

 inferred. In classical North American terminology the major glacial and inter- 

 glacial subdivisions are stages, based only in part on fossils and otherwise on 

 evidence of glacial and interglacial climates. The stages imply at least four, and 

 probably more, major glacial events that seem, through similarity of sequence 

 and wide continuity, to be fairly well established in both Europe and North 

 America. As a result transatlantic correlations have been made with varying 

 degrees of confidence. 



Because few sheets of glacial drift possess distinctive characteristics by which 

 they can be identified throughout wide regions, the validity of the sequence of 

 stages is coming to depend more and more on distinctive characteristics within 

 the interglacial sediments that lie between them. The useful characteristics 

 consist chiefly of (1) distinctive fossil plants (commonly represented by pollen) 

 and animals, and (2) distinctive patterns of climatic fluctuations recorded by 

 changes in fossil pollen upward through the sequence. 



The areal extent of application of the terms glacial and interglacial is not 

 sharply defined. The terms are applied in the glaciated regions, where the former 

 presence of glacier ice is quite evident. They are applied also in areas of major 



