Relative dating cannot establish absolute age, but it can establish whether one rock is older or younger than another.Relative dating requires an extensive knowledge of stratigraphic succession, a fancy term for the way rock strata are built up and changed by geologic processes.

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Numerical dating determines the actual ages of rocks through the study of radioactive decay.

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.

In order to establish relative dates, geologists must make an initial assumption about the way rock strata are formed. sediments, which are deposited and compacted in one place over time.

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.

It's called the Principle of Original Horizontality, and it just means what it sounds like: that all rock layers were originally horizontal. As you can imagine, regular sediments, like sand, silt, and clay, tend to accumulate over a wide area with a generally consistent thickness.

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.

We'll even visit the Grand Canyon to solve the mystery of the Great Unconformity!

Imagine that you're a geologist, studying the amazing rock formations of the Grand Canyon.