Relative dating is used to arrange geological events, and the rocks they leave behind, in a sequence.
Throughout the history of life, different organisms have appeared, flourished and become extinct.
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 fossils, called index fossils, are particularly useful in correlating rocks.