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Let's say, in this set of rock strata, that we found a single intrusion of igneous rock punching through the sedimentary layers.We could assume that this igneous intrusion must have happened after the formation of the strata.There may be a layer missing in the strata, or a set of sedimentary rock on top of metamorphic rock.These interfaces between discontinuous layers of rock are called unconformities.
We follow this same idea, with a few variations, when we talk about cross-cutting relationships in rock.Let's say we find out, through numerical dating, that the rock layer shown above is 70 million years old.We're not so sure about the next layer down, but the one below it is 100 million years old. Not exactly, but we do know that it's somewhere between 70 and 100 million years old.If it had happened before the layers had formed, then we wouldn't see it punching through all the layers; we would only see it going through the layers that had existed at the time that it happened. The Principle of Cross-Cutting Relationships states that rock formations that cut across other rocks must be younger than the rocks that they cut across.The same idea applies to fault lines that slide rock layers apart from each other; a fault that cuts across a set of strata must have occurred after the formation of that set.