There may be a layer missing in the strata, or a set of sedimentary rock on top of metamorphic rock.
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Geologists find the cross-cutting principle especially useful for establishing the relative ages of faults and igneous intrusions in sedimentary rocks.
Sometimes, geologists find strange things inside the strata, like chunks of metamorphic or igneous 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.
How do we use the Law of Superposition to establish relative dates?
Let's look at these rock strata here: We have five layers total.
We'll even visit the Grand Canyon to solve the mystery of the Great Unconformity!