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Reading the Rocks: Law of Crosscutting

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The second rule we will use in reading the rocks is the Law of Crosscutting. This one is so simple that you may think it is silly to even mention it. As we go along, we will see that it is a very important tool in reading rocks.

The Law of Crosscutting tells us that when one geologic structure cuts through another, the structure that was cut is older than the one that cut through it. Again, we can do a very simple activity to understand this. You will need:

  • a sheet of paper

Pick up the piece of paper and tear it in half. Now lets apply the Law of Crosscutting. Which came first, the paper or the rip? The paper was there first, of course. You could not have the rip without something to tear. I told you it would be simple.

Now, lets think about that in terms of the rocks. The photograph on the right shows layers of rock and a fault where the rocks have broken and moved. Did the fault occur before or after the layers of rock were deposited? It is the same as the paper. The rocks had to be there first. If the earthquake that caused the fault had happened first, the layers of rock would not have been broken by it.