Linking Rocks and Time

mountain ridge and valley
The Middle Permian Capitan Reef system is one of the world’s largest and best preserved fossil reefs. Guadalupe Mountains National Park contains the Global Stratotype Section—the reference standard—for the Middle Permian, now known as the Guadalupian Epoch. Guadalupe Mountains National Park, Texas.


It would be convenient if somewhere in the world a single rock outcrop recorded all geologic history without interruption. Unfortunately this is not the case, so in order to reconstruct Earth’s history geologists have had to piece together rocks of different ages from all over the world. The representative outcrops that have been selected are called Type Sections or Stratotypes, and the areas in which the type sections occur are called Type Areas or Type Localities. For example, the representative outcrop for Middle Permian time occurs in Guadalupe Mountain National Park in Texas, pictured above.

In order for a rock outcrop to be recognized as a type section, it must be accessible and available for present and future scientific study. The scientific community has recognized many type sections for hundreds of years. However, formally recognized type sections for some portions of geologic time are still in the process of being selected. In these cases, an international committee of scientists, namely the International Union of Geological Sciences’ International Committee on Stratigraphy, must agree upon the best representative example of an outcrop in the world for a particular time period. In this way, type sections are the definitive examples of rocks that represent a particular segment of geologic time. Moreover, people from all over the world can go to these outcrops to find out what the “standard” for a particular time period looks like.

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Last updated: October 29, 2019


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