Read this tip to make your life smarter, better, faster and wiser. LifeTips is the place to go when you need to know about Geologic Time and other Natural Science topics.
Here's another of those relative dating tips that seems obvious to us in our modern sophistication, but which was pretty clever when it was originally developed:
if you have layers of rock which are cut by something - a fault, say, or basalt dike - then the rock layers are obviously older than the fault or dike which cuts them. That's called the Principle of Cross-Cutting Relations.
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