It is sometimes referred to as the “age of mammals”. Calling this span from roughly 66 Myr to 1.8 Myr the Tertiary Period is fairly common in geologic literature. In the time scale of Lutgens & Tarbuck, the Neogene Period and the Paleogene Period below are combined and called the Tertiary Period. In the time scale of Lutgens & Tarbuck, the Quaternary Period is further divided into the Pleistocene Epoch from 1.8 to 0.01 Myr and the most recent Holocene Epoch from 0.01 Myr to the present.īy the beginning of the Quaternary Period, most of the major plate tectonic movements which formed the North American continent had taken place, and the main modifications past that were those produced by glacial action and erosion processess. The division of the geologic column into different periods is largely based upon the varieties of fossils found, taken as indicators of a time period in Earth’s history. Research publications would give error bars for such division dates – it is not implied here that these boundaries are known to 3 or 4 significant digits. Note that the dates in millions of years are representative values. The brief outline below draws from that material and elsewhere to provide a brief sketch of Earth history. Lutgens & Tarbuck take on the task of surveying Earth history in one chapter, Chapter 19 of Essentials of Geology. Some descriptive information about the different divisions of geologic time is given below. Geologic Time and the Geologic Column from The times are in millions of years.įor examples that cover most of these time periods, see the outline of the Grand Canyon and Grand Staircase. The time data from radiometric dating is taken from that source. to organize the different periods of life since the beginning of the Cambrian period. This approach to the sweep of geologic time follows that in “The Grand Canyon”, C.Hill, et al., eds.
As a result, the history contained within these rocks cannot be as clearly interpreted. This is because older rocks have been buried deeply, intensely deformed and severely modified by long-term earth processes. Subdivision of periods into epochs can be done only for the most recent portion of the geologic time scale.
Epochsįiner subdivisions of time are possible, and the periods of the Cenozoic are frequently subdivided into epochs. In the time scale above you can see that the Paleozoic is subdivided into the Permian, Pennsylvanian, Mississippian, Devonian, Silurian, Ordovician and Cambrian periods. The events that bound the periods are widespread in their extent but are not as significant as those which bound the eras. Very significant events in Earth’s history are used to determine the boundaries of the eras. In the time scale above you can see that the Phanerozoic is divided into three eras: Cenozoic, Mesozoic and Paleozoic. ErasĮons are divided into smaller time intervals known as eras. Detailed geologic time scale: The United States Geological Survey has published “Divisions of Geologic Time: Major Chronostratigraphic and Geochronologic Units.” It is a much more detailed time scale than the simplified scale shown above.
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