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Try it risk-free Discover how geologists study the layers in sedimentary rock to establish relative age.

Learn how inclusions and unconformities can tell us stories about the geologic past.

These items are called inclusions - foreign bodies of rock or mineral enclosed within another rock.

It sounds like common sense to you and me, but geologists have to define the Principle of Original Horizontality in order to make assumptions about the relative ages of sedimentary rocks. Say you have a layer of mud accumulating at the bottom of a lake. More sediment accumulates from the leaf litter and waste of the forest, until you have a second layer.

Since we assume all the layers were originally horizontal, then anything that made them not horizontal had to have happened after the fact.

We follow this same idea, with a few variations, when we talk about cross-cutting relationships in rock.

Your goal is to study the smooth, parallel layers of rock to learn how the land built up over geologic time.

Now imagine that you come upon a formation like this: What do you think of it? How can you make any conclusions about rock layers that make such a crazy arrangement?

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