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6.2: Principle of Horizontality

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    What is the pattern of the Grand Canyon rock layers?

    If you go to the Grand Canyon, you'll see layers of sedimentary rocks. These rocks are extremely well exposed for viewing. Some people call this "layer cake geology." It looks like a layer cake, but not as tasty. Just like a cake, the bottom layer is put down first. Subsequent layers are deposited next. Where the layers are not horizontal, there was deformation.

    Sedimentary Rock Rules

    Sedimentary rocks follow certain rules.

    1. Sedimentary rocks form with the oldest layers on the bottom and the youngest on top.
    2. Sediments are deposited horizontally so sedimentary rock layers are originally horizontal.
    3. Sedimentary rock layers that are not horizontal are deformed.

    Sedimentary rocks start out horizontal with the oldest on the bottom. Sedimentary rocks that are not horizontal must be deformed. This deformation produces geologic structures such as folds, joints, and faults that are caused by stresses. We can look at the deformation and structures out the deformation history of the rock.

    Geologic Structures

    Sedimentary rocks are formed in horizontal layers. This is magnificently displayed around the southwestern United States. The arid climate allows rock layers to be well exposed (Figure below). The lowest layers are the oldest, and the higher layers are younger. This concept is called superposition since it deals with the positions of rock layers.

    Layers of different types of rocks are exposed in this photo from Grand Staircase-Escalante National Monument

    Layers of different types of rocks are exposed in this photo from Grand Staircase-Escalante National Monument. White layers of limestone are hard and form cliffs. Red layers of shale are flakier and form slopes.

    Folds, joints, and faults are caused by stresses. If a sedimentary rock is tilted or folded, we know that stresses have changed the rock.


    • Sedimentary rocks are laid down horizontally with the oldest at the bottom.
    • Sedimentary rocks that are not horizontal have been deformed.
    • Sedimentary rocks are very useful for determining the deformation history of an area.


    1. Why are sediments laid down horizontally?
    2. Why are sediments laid down from oldest to youngest?
    3. Why are sedimentary rocks so good for studying the geology of a region?

    Explore More

    Use the resource below to answer the questions that follow.

    1. How is rock laid down?
    2. What is the law of superposition?
    3. Why is the law of superposition important?
    4. Where is the oldest rock found?
    5. Where is the youngest rock found?
    6. Why do we know that the fault is younger than the three rock layers?
    7. Is the intrusion the youngest rock in the section? How do you know?

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