Figure 1 shows layers of rock that have been offset by a fault, a fracture in rock along which displacement occurs. It is clear that the strata must be older than the fault that broke them. The principle of cross-cutting relationships states that geologic features that cut across rocks must form after the rocks they cut through. Igneous intrusions provide another example.
The dikes shown in Figure 2 are tabular masses of igneous rock that cut through the surrounding rock. The magmatic heat from igneous intrusions often creates a narrow “baked” zone of contact metamorphism on the adjacent rock, also indicating that the intrusion occurred after the surrounding rocks were in place.