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Determining Relative Ages

Using certain principles it is possible to determine the relative ages of rock units and reveal the geologic history of the region.

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John Day Fossil Beds

John Day Fossil Beds

Credit: Miguel Vieira
Source: http://www.flickr.com/photos/miguelvieira/6028394428/
License: CC BY-NC 3.0

John Day Fossil Beds National Monument in eastern Oregon has many more fossils and spans a longer period of time than almost any other fossil locale on Earth.

Why It Matters

  • The fossil beds contain fossils from 45 million to 5 million years ago – a tremendously long period of time.
  • The rock units tell of the types of habitats that existed when the rocks were laid down and processes that created them.
  • There are several rock units that bear fossils.
  • Credit: National Park Service; converted from PDF to PNG format by User:Finetooth using GNU Image Manipulation Program (GIMP) software
    Source: http://en.wikipedia.org/wiki/File:Joda_geologic_timeline.png
    License: CC BY-NC 3.0

    Rock strata and major fossil groups in the order they were deposited (bottom to top - oldest to youngest. MA - million years ago) - John Day Fossil Beds geologic timeline [Figure2]

     

  • Picture Gorge, a subdivision of the Sheep Rock Unit, ignimbrite has been radiometrically dated at 28.7 million years.

Explore More

With the links below, learn more about John Day Fossil Beds National Monument. Then answer the following questions.

  1. Which rock unit contains the Picture Gorge ignimbrite? Why do you think so?
  2. What are the relative ages of the Picture Gorge ignimbrite, Philotrox, and Enhydrocyon? What is your best estimate of the absolute ages of the three?
  3. What is the geologic history of the rock inclusions and the volcanic ash in the Clarno Formation?
  4. Why do the scientists think that the rocks exposed west of John Day Fossil Beds are the same units as those in the national monument?
  5. How would scientists test to see if the rock unit west of John Day is the same as the Picture Gorge ignimbrite within the monument?

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Image Attributions

  1. [1]^ Credit: Miguel Vieira; Source: http://www.flickr.com/photos/miguelvieira/6028394428/; License: CC BY-NC 3.0
  2. [2]^ Credit: National Park Service; converted from PDF to PNG format by User:Finetooth using GNU Image Manipulation Program (GIMP) software; Source: http://en.wikipedia.org/wiki/File:Joda_geologic_timeline.png; License: CC BY-NC 3.0

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