Rock is often disturbed as a result of tectonic processes. A break in the rock, known as a fault, often results in the broken rock layers sliding past one another. This fault must be younger than the displaced rock that results, thus indicating its relative age.

Igneous rock may also intrude into existing rock. Again, the age of the intrusion must be younger than that of the surrounding rock.

The concept that a fault or an intrusion is always younger than the rock layers that it cuts through is known as the principle of cross-cutting relationships. Cross-cutting relationships are just one way of determining relative age.