Describe relative absolute dating techniques
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.
Following the Principle of Original Horizontality, he could say that whatever forces caused the deformation, like an earthquake, must have occurred after the formation of all the rock 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.
Since we assume all the layers were originally horizontal, then anything that made them not horizontal had to have happened after the fact.
We follow this same idea, with a few variations, when we talk about cross-cutting relationships in rock.
Geologists find the cross-cutting principle especially useful for establishing the relative ages of faults and igneous intrusions in sedimentary rocks.