PALEONTOLOGICAL DIVISIONS GEOLOGICAL DURATION IN CUMULATIVE DIVISIONS YEARS TOTALS (_Round numbers_)
CENOZOIC (“recent life”) _Quaternary_ Holocene 25,000 Age of Man (“wholly recent”) Pleistocene 1,000,000 1,000,000 (“most recent”) or Great Ice Age _Tertiary_ Pliocene 11,000,000 Age of Mammals (“more recent”) Miocene 16,000,000 (“less recent”) Oligocene 11,000,000 75,000,000 (“little recent”) Eocene 19,000,000 (“dawn of recent”) Paleocene 17,000,000 (“ancient recent”) MESOZOIC (“middle life”) Three periods 130,000,000 205,000,000 *Age of Reptiles PALEOZOIC (“ancient life”) Six or seven 300,000,000 505,000,000 periods beginning with the Cambrian *Age of Fishes, Amphibians, and Primitive Marine Invertebrates PROTEROZOIC (“earlier life”) Pre-Cambrian 1,250,000,000 1,750,000,000 Age, presumably, of soft-bodied animals ARCHAEOZOIC (“primordial life”) EOZOIC (“dawn of life”) Problematic signs of life, indicated by presence of carbon Unrecorded Interval Since the Origin of the Unknown 2,000,000,000 Earth to 10,000,000,000
In this book we are concerned with two divisions of the Quaternary which are also growing vaguer in outline, less precise in time. They are the Pleistocene, or Glacial Period, or Great Ice Age, and the Holocene, Recent, or Postglacial Period in which we now live. (If your Greek is rusty, you will be amused to discover that those scientific-sounding terms are merely translations of “wholly recent” and “most recent.”) Most geologists believe that these two areas of time covered about 1,000,000 years; but some give them half a million more, and a few limit them to the 600,000 years, or even 300,000 years, of the last four glaciations. Some start the Postglacial 25,000 years ago, when the ice began to shrink toward its present limits; some start it 9,000 years ago, when a relatively modern climate appeared. Some geologists say we are still in the Pleistocene, and merely enjoying a warm spell before another glaciation.
By definition—or lack of it—the Pleistocene is rather vaguely bounded, and quite as much at its beginning as at its end. To the paleontologist, the Pleistocene is the time of certain large and picturesque mammals that are now extinct. To the geologist, it is the time of the waxing and waning of the great glaciers. The beginnings and the ends of these two definitions of the Pleistocene do not correspond too closely. We shall use the term as little as possible, substituting the Great Ice Age.