In this example above, we have rock layers A - E and Fault F showing.According to superposition, A is the oldest rock layer, while E is the youngest rock layer. According to cross-cutting relations, the fault is younger than those layers it cuts.Free 5-day trial Geologic time extends back 4.6 billion years.Because the earth is so old and rocks formed millions of years ago, geologists needed a way to date rocks and rock units called strata.As a member, you'll also get unlimited access to over 70,000 lessons in math, English, science, history, and more.Plus, get practice tests, quizzes, and personalized coaching to help you succeed.
From these principles, we can also learn about the geologic history of the rock or fossil.
In this case, the principle states that the item doing the cutting is younger than what is being cut.
If we return to our example of the layered cake, the cake has to be there first before you can cut with a knife.
To understand this better, consider a four-layer cake. It wouldn't be possible for the other layers to be added without the bottom layer first.
This same process applies with each layer to the cake added.
Today there are two common practices for dating rocks and strata. Geologists use what they see and some simple strategies to relative date the rock layers found in the Grand Canyon.