KEY CONCEPT The geologic time scale divides Earths history based on major past events'

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KEY CONCEPT The geologic time scale divides
Earths history based on major past events.
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Index fossils are another tool to determine the
age of rock layers.

• Index fossils can provide the relative age of a
  rock layer.
• existed only during specific spans of time
• occurred in large geographic areas
• Index fossils include fusulinids and trilobites.

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The geologic time scale organizes Earths
history.

• The history of Earth is represented in the
  geologic time scale.

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• Eras last tens to hundreds of millions of years.

• consist of two or more periods
• three eras Cenozoic, Mesozoic, Paleozoic

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• Periods last tens of millions of years.

• most commonly used units of time on time scale
• associated with rock systems.

• Epochs last several million years.