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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.

The same idea applies to fault lines that slide rock layers apart from each other; a fault that cuts across a set of strata must have occurred after the formation of that set.

Let's say we find out, through numerical dating, that the rock layer shown above is 70 million years old.

Inclusions are always older than the sedimentary rock within which they are found.Other times, geologists discover patterns in rock layers that give them confusing information.Once we assume that all rock layers were originally horizontal, we can make another assumption: that the oldest rock layers are furthest toward the bottom, and the youngest rock layers are closest to the top. The forest layer is younger than the mud layer, right? When scientists look at sedimentary rock strata, they essentially see a timeline stretching backwards through history.The highest layers tell them what happened more recently, and the lowest layers tell them what happened longer ago.As a member, you'll also get unlimited access to over 70,000 lessons in math, English, science, history, and more.

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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.

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.

Relative dating cannot establish absolute age, but it can establish whether one rock is older or younger than another.