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.
Fossils can't form in the igneous rock that usually does contain the isotopes.
Since the moon and the Earth probably formed at the same time, this supports the current idea of the Earth's age.
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Correlation with them has helped geologists date many New Zealand rocks, including those containing dinosaurs.
The most widely known form of radiometric dating is carbon-14 dating.
So to determine the age of sedimentary rock layers, researchers first have to find neighboring layers of Earth that include igneous rock, such as volcanic ash.