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USGS CMG InfoBank: Relative Age Dating of Rocks

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Comment: 11:23 - 12:24 (01:01)
Source: Annenberg/CPB Resources - Earth Revealed - 10. Geologic Time
Keywords: "relative age dating", "Principles of Relative Age Dating", "James Sadd", correlation, "Kiabab Limestone", "Grand Canyon", "rock formation", Arizona, Minnesota, "Mount Saint Helens", "sedimentary rock"

Our transcription: The principles of relative age dating are simple and yet so powerful that they were used by early geologists in the Eighteenth and Nineteenth Centuries to construct a complete historic sequence of all of the rocks in Europe.
And because rock formations in a sequence like this often extend over a large area, they can be used to correlate rocks from one sequence with historically equivalent strata in other regions.
This is easy to see in a place like the Grand Canyon.
This formation, the Kiabab Limestone can clearly be recognized in the sequence of sedimentary rocks on the opposite side of the Canyon, and because it's the same formation, it marks the same period of geologic time in both places.
Some rock layers persist laterally for thousands of kilometers and can be recognized in different places by their thickness, mineral composition, and other characteristics.
For example, volcanic ash from the 1980 eruption of Mount Saint Helens has been identified in sedimentary sequences as far away as Minnesota and Arizona.

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