Principle of Cross-Cutting Relationships
When a fault cuts through other rocks, or when magma intrudes and crystallizes, we can assume that the fault or intrusion is younger than the rocks affected. For example, in Figure 11.5, the faults and dikes clearly must have occurred after the sedimentary layers were deposited.
This is the principle of cross-cutting relationships. By applying the cross-cutting principle, you can see that fault A occurred after the sandstone layer was deposited because it broke" the layer. However, fault A occurred before the conglomerate was laid down because that layer is unbroken. We can also state that dike B and its associated sill are older than dike A because dike A cuts the sill. In the same manner, we know that the batholith was emplaced after movement occurred along fault B, but before dike B was formed. This is true because the batholith cuts across fault B, and dike B cuts across the batholith.