7.2 Principle of Horizontality

Why does the Grand Canyon resemble these cakes?

If you go to the Grand Canyon, you'll see a layer cake of geological formations. Some people call this "layer cake geology." Just like the cake, the bottom layer is put down first and then subsequent layers moving upward. If a layer is not horizontal it must have been deformed. We'll learn about deformation in the next several concepts.

Sedimentary Rock Rules

Sedimentary rocks follow certain rules.

1. Sedimentary rocks are formed with the oldest layers on the bottom and the youngest on top.
2. Sediments are deposited horizontally, so sedimentary rock layers are originally horizontal, as are some volcanic rocks, such as ash falls.
3. Sedimentary rock layers that are not horizontal are deformed.

Since sedimentary rocks follow these rules, they are useful for seeing the effects of stress on rocks. Sedimentary rocks that are not horizontal must have been deformed.

You can trace the deformation a rock has experienced by seeing how it differs from its original horizontal, oldest-on-bottom position. This deformation produces geologic structures such as folds, joints, and faults that are caused by stresses.

Geologic History

You're standing in the Grand Canyon and you see rocks like those in the Figure below. Using the rules listed above, try to figure out the geologic history of the geologic column. The Grand Canyon is full mostly of sedimentary rocks, which are important for deciphering the geologic history of a region.

In the Grand Canyon, the rock layers are exposed like a layer cake. Each layer is made of sediments that were deposited in a particular environment - perhaps a lake bed, shallow offshore region, or a sand dune.

In this geologic column of the Grand Canyon, the sedimentary rocks of groups 3 through 6 are still horizontal. Group 2 rocks have been tilted. Group 1 rocks are not sedimentary. The oldest layers are on the bottom and youngest are on the top.

The ways geologists figure out the geological history of an area will be explored more in the chapter Earth History.

Summary

• Sedimentary rocks are laid down horizontally.
• Rocks are laid down from oldest to youngest.
• Sedimentary rocks that are not horizontal have been deformed.
• Sedimentary rocks are very useful for deciphering the geological history of an area.

Review

1. In the Grand Canyon section, what do you think happened to the rocks between layers 12 and 11?
2. For what reason are the rocks 16, 17 and 18 not layered?
3. What are the oldest rocks in the Grand Canyon and what type are they? What are the youngest rocks and what type are they?

Explore More

Use this resource to answer the questions that follow.

1. What is the law of superposition?
2. How are the sediments laid down?
3. How can you determine the oldest rock?
4. What can happen to disturb the layers?
5. What does erosion do?
6. What is an intrusion?