Relative age dating inclusions
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.Different species of ammonites lived at different times within the Mesozoic, so identifying a fossil species can help narrow down when a rock was formed.Correlation can involve matching an undated rock with a dated one at another location.The five principles that are used in relative age dating are: original horizontality, superposition, lateral continuity, crosscutting relationship and inclusions.
Many of these organisms have left their remains as fossils in sedimentary rocks.Geologists have studied the order in which fossils appeared and disappeared through time and rocks. Fossils can help to match rocks of the same age, even when you find those rocks a long way apart.This matching process is called correlation, which has been an important process in constructing geological timescales.Some of the most useful fossils for dating purposes are very small ones.For example, microscopic dinoflagellates have been studied and dated in great detail around the world.
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Suppose you find a fossil at one place that cannot be dated using absolute methods.