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Some fossils, called index fossils, are particularly useful in correlating rocks.For a fossil to be a good index fossil, it needs to have lived during one specific time period, be easy to identify and have been abundant and found in many places. If you find ammonites in a rock in the South Island and also in a rock in the North Island, you can say that both rocks are Mesozoic.Using relative dating the fossil is compared to something for which an age is already known.

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The method of reading the order is called stratigraphy (layers of rock are called strata).Absolute dating is used to determine a precise age of a fossil by using radiometric dating to measure the decay of isotopes, either within the fossil or more often the rocks associated with it.The majority of the time fossils are dated using relative dating techniques.Sequencing the rock layers will show students how paleontologists use fossils to give relative dates to rock strata.Once students begin to grasp "relative" dating, they can extend their knowledge of geologic time by exploring radiometric dating and developing a timeline of Earth's history.

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