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Rocks are the foundation of everything — your backyard, rivers, and streets. Where does that rock come from, and what can you learn about it? Stratigraphy is the study is of rock layers, which we call strata. It also is the study of the process that leads to the creation of these layers (stratification). It usually focuses on volcanic rocks and sedimentary rocks, or rocks formed by different minerals. Let’s rock on and learn more about what stratigraphy can tell us about the history behind rocks!
You can see the strata in this rock.

Strata make the beautiful lines on the rock

How Strata are Formed 

Strata can be long horizontal sections or thin layers. They range from only a few millimeters to many meters thick. Stratification occurs because the texture or form of the minerals, called sediment, change as they settle down into rock. Pressure, heat, and chemical reactions change the sediment into rocks. This process is crucial to the rock cycle.
Strata can be different colors because of it, too! Strata contain many types and patterns. Clay switches off with sandstone layers and carbonate switches off with shales. These layers create the combinations in the rocks you see below. Watch the video to see how these rock layers are created.

Stratigraphy and Relative Dating 

Stratigraphers divide the layers of rock into sections that can be connected to a specific time in history. The oldest layers are at located at the bottom. The layers on top of the stack are younger. This is because the new material usually settles on top of the old material to create new rock. This rule is called the law of superposition.
Although superposition does not tell you the exact age, you can learn the age of the rock compared to the other rocks around it. We call this process of finding out the age of strata based on the rocks around it, relative dating. For example, we know that chalk was formed in the Upper Cretaceous period. So if we find chalk in the strata, we can guess when it was laid down.
Diagram showing different strata.

Strata exposed on both sides of a valley. Each layer of strata matches a time period. The oldest rock is at the bottom.

How Archaeologists Use Strata and Fossils Together

We can find the history of Earth’s movements preserved in rock. Strata help stratigraphers understand different historical events when rock shifts, like earthquakes. Also, scientists use fossils and strata to find out the age of both the objects and the rock layers. 
Archaeologists try to find out how old an artifact is by looking at how deep in the rock they found the object. For example, fossil species appear in the same order in the rock because we know certain species went extinct and cannot show up in the younger rock.
Rocks can tell us an approximate age for fossils.

A fishy fossil surrounded by rock.

Other Great Resources:

Longer Video on Strata and Relative dating:

Another Great Video on Relative Dating: 

More About Fossils and Geologic Periods by Rock Layer: http://www.prehistoricplanet.com/features/index.php?id=48
Written By: Hayley Krolik.