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.

Inclusions are always older than the sedimentary rock within which they are found.

The highest layers tell them what happened more recently, and the lowest layers tell them what happened longer ago.

How do we use the Law of Superposition to establish relative dates?

We could assume that this igneous intrusion must have happened after the formation of the strata.

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.

Learn how inclusions and unconformities can tell us stories about the geologic past.

We'll even visit the Grand Canyon to solve the mystery of the Great Unconformity!

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.

Geologists use this type of method all the time to establish relative ages of rocks.

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.