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.
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.