Two types of absolute dating

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.

In this lesson, we'll learn a few basic principles of stratigraphic succession and see whether we can find relative dates for those strange strata we found in the Grand Canyon.

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.

How do we use the Law of Superposition to establish relative dates?

Let's look at these rock strata here: We have five layers total.

Geologists find the cross-cutting principle especially useful for establishing the relative ages of faults and igneous intrusions in sedimentary rocks.

Sometimes, geologists find strange things inside the strata, like chunks of metamorphic or igneous rock.

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