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How can you pick out older or younger rock layers?

Undisturbed Layers

If a sequence of rock layers (strata) does not seem to have been disturbed (that is, the units are still horizontal and haven't been tipped or faulted) you can assume that the oldest layers are on the bottom, and the youngest layers are on the top.

Yeah, it sounds obvious to us now, but in the early days of geology, that was a pretty astute observation known as the Principle of Superposition.

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