Smithsonian National Museum of Natural History Geologic Time The Story of a Changing Earth
Presented by the Department of Paleobiology.
The Pleistocene
Contents
Overview
Defining and Dating the Pleistocene Boundary
Pleistocene Glacial Events
Pleistocene Ecosystems and Extinctions
Human Evolution during the Pleistocene
Evidence
Skeleton of the woolly mammoth
learn more
sample
Walrus
Saber-toothed cat
Human ancestor
references and links
Foundational Concepts
Dating Methods
Earth Processes
Life Processes
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Defining and Dating the Pleistocene Boundary
Typically, geologists can identify the boundaries of different time periods by locating changing rock characteristics or layers where fossils may be seen for the first or last time. However, the Pliocene-Pleistocene boundary proved to be exceptionally difficult for geologists to pinpoint. The rocks containing the earliest Pleistocene fossils were mainly terrestrial sediments, whereas those in the latest Pliocene were mostly marine. The rocks at Crotone in Calabria, southern Italy, however, preserve distinct marine faunas from both time periods. In 1940 the International Geological Congress decided that the Pliocene-Pleistocene boundary would be based on the marine faunas located at this site (called the type section).

Once the boundary between the Pleistocene and Pliocene had been specified, scientists could calculate its age. This depended on their ability to date changes in ocean faunas and correlate them with the rocks in Calabria, Italy.

Dating the beginning of the Pleistocene is beyond the range of radiocarbon dating and also beyond the limits of other methods that use organic materials. An alternative method is magnetostratigraphy (or paleomagnetism). By systematically collecting and measuring the orientation of magnetic grains in volcanic rocks, ashes, and other rock types, a record of Earth's magnetic history can be constructed. New localities can be placed into the context of this history if they contain rocks with magnetic grains. For the Pliocene-Pleistocene boundary, biostratigraphic information could be correlated to a magnetostratigraphic event called the Olduvai Normal. This suggested that the lower Pleistocene boundary could be no older than 1.8 million years ago.


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Overview | Defining and Dating the Pleistocene Boundary | Pleistocene Glacial Events
Pleistocene Ecosystems and Extinctions | Human Evolution during the Pleistocene



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