Our Part of the Geologic Time Scale
The dead hand of another system of classification lies across a still larger area than the Stone Age itself or the Age of Man. This area is the entire life of our earth since it took sufficient shape to support cellular life. As it is so large an area and much of it is so remote in time, changes in the definition of most of its various divisions do not much affect the present discussion.
Once upon a time there were four great divisions, neatly numbered in Latin as the Primary, the Secondary, the Tertiary, and the Quaternary. The first two went by the board when newer scientists found older ages and stretched the life of the earth a couple of billion years. The Tertiary is still a respected appellation, but the good name of the Quaternary—the area of time with which this book is mainly concerned—is seriously questioned. Defined as the Age of Man, it was supposed to harbor all evidence of his existence; but hints of his presence in the Tertiary have rather sullied the scientific standing of the later period.
THE LIFE STORY OF THE EARTH
This summary of the story of the earth is a combination of charts in Arthur Holmes’ Principles of Physical Geology, Earnest A. Hooton’s Up from the Ape, and George Gaylord Simpson’s The Meaning of Evolution, with modifications by William C. Putnam and James Gilluly. *The divisions marked with an asterisk used to be called, respectively, Secondary and Primary.
| PALEONTOLOGICAL DIVISIONS | GEOLOGICAL DIVISIONS | DURATION IN YEARS | CUMULATIVE TOTALS (Round numbers) | |
|---|---|---|---|---|
| CENOZOIC (“recent life”) | ||||
| Quaternary Age of Man | Holocene (“wholly recent”) | 25,000 | ||
| Pleistocene (“most recent”) or Great Ice Age | 1,000,000 | 1,000,000 | ||
| Tertiary Age of Mammals | Pliocene (“more recent”) | 11,000,000 | ||
| Miocene (“less recent”) | 16,000,000 | |||
| Oligocene (“little recent”) | 11,000,000 | 75,000,000 | ||
| Eocene (“dawn of recent”) | 19,000,000 | |||
| Paleocene (“ancient recent”) | 17,000,000 | |||
| MESOZOIC (“middle life”) | Three periods | 130,000,000 | 205,000,000 | |
| *Age of Reptiles | ||||
| PALEOZOIC (“ancient life”) | Six or seven periods beginning with the Cambrian | 300,000,000 | 505,000,000 | |
| *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 Earth | Unknown | 2,000,000,000 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.