The first step in this process is to assess the type of radioactive decay that is occurring.
Although some isotopes are stable, others are unstable because the forces binding protons and neutrons together are not so strong.
We can even go further by correlating rock types using their fossil contents.
Geologists study the natural history of an area by investigating outcrops, which are rock formations exposed aboveground.
Some fossils, called index fossils, are particularly useful in correlating rocks.