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.
Since we assume all the layers were originally horizontal, then anything that made them not horizontal had to have happened after the fact.
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.
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.
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!
Imagine that you're a geologist, studying the amazing rock formations of the Grand Canyon.
Geologists use this type of method all the time to establish relative ages of rocks.
Now, what if instead of being horizontal, this rock layer was found in a tilted position?
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.