Principle of cross-cutting relations: Any geologic feature is younger than anything else that it cuts across.
Some elements have forms (called isotopes) with unstable atomic nuclei that have a tendency to change, or decay.
Where the amounts of parent and daughter isotopes can be accurately measured, the ratio can be used to determine how old the rock is, as shown in the following activities.
Part 2a Activity At any moment there is a small chance that each of the nuclei of U-235 will suddenly decay.
It wasn't until well into the 20th century that enough information had accumulated about the rate of radioactive decay that the age of rocks and fossils in number of years could be determined through radiometric age dating.