Dating metamorphic rocks
They complicate the task of relative dating, because they don't give an accurate picture of what happened in geologic history.
For example, say we have a layer missing from the rock strata.
How do we use the Law of Superposition to establish relative dates?
We follow this same idea, with a few variations, when we talk about cross-cutting relationships in rock.
These items are called inclusions - foreign bodies of rock or mineral enclosed within another rock.
Because the sedimentary rock had to have formed around the object for it to be encased within the layers, geologists can establish relative dates between the inclusions and the surrounding rock.
Relative dating cannot establish absolute age, but it can establish whether one rock is older or younger than another.
Relative dating requires an extensive knowledge of stratigraphic succession, a fancy term for the way rock strata are built up and changed by geologic processes.