- Principle of Superposition: In a sequence of sedimentary rock layers, each layer must be younger than the one below.
- Principle of Original Horizontality: The surfaces on which sediments accumulate are nearly horizontal, therefore layers of sediment are also nearly horizontal when originally deposited
- Principle of Original Continuity: Sediments generally accumulate in continuous sheets that are continuous laterally.
- Principle of Cross-cutting Relations: If one geologic feature (dike, fault, erosional surface) cuts across another, the feature that has been cut must be older.
- Principle of Inclusions: A rock unit that is included within another rock is always older. If a layer of sediment overlying an igneous rock includes pieces of the igneous rock, the sedimentary layer must be younger. If an igneous intrusion contains fragments of another rock (xenoliths), the igneous intrusion must be younger.
- Principle of Faunal Succession: Fossil fauna (animals) and flora (plants) follow one another in a definite, recognizable order.