Earth Science

Geological Activity from Plate Tectonics

Principle of Horizontality

Study Tip
You can think of the layers of a rock as layers in a cake. Layers of rock are naturally placed flat on top of each other.

Sedimentary Rock Rules

The Principle of Original Horizontality has parallels to the formation of sedimentary rocks; it is one of the four major principles of geology.

  • Sedimentary rocks are formed with the oldest layers on the bottom and the youngest on top.
  • Sediments are deposited horizontally.
  • Sedimentary rock layers that are not horizontal are deformed.

Geologic History

Sedimentary rock layers are useful in deciphering geological history. If any layers within a rock have been tilted, it means that this deformation was a result of geologic activity that occurred after deposition.

Study Tip
You can think of the layers of a rock as layers in a cake. Layers of rock are naturally placed flat on top of each other.
Study Tip
The same way a person may cast a shadow over another person when they stand under the sun, planets or celestial bodies that have aligned themselves cast shadows over one another as well.
Sedimentary rock layers
Concept Check
  • Discuss the Principle of Original Horizontality.
  • What are sedimentary rock layers useful for?