How does the law of superposition determine the relative ages of sedimentary rock layers?

The law of superposition is a fundamental principle of stratigraphy used to determine the relative ages of sedimentary rock layers. Formulated by Nicolaus Steno, it states that in an undisturbed sequence of sedimentary strata, deposition occurs from the bottom to the top. Consequently, the oldest layer is found at the base, and each successive layer above it is progressively younger.12

This principle relies on the observation that sediments accumulate in ascending order over time. However, geologists must be cautious, as geological processes can complicate this record. For instance, crustal deformation may tilt or overturn rock layers, and thrust faults can sometimes displace older units on top of younger ones. Additionally, erosion may remove portions of the sequence, making it difficult to distinguish the original top from the bottom. To accurately apply the law, experts often look for depositional features—such as cross-beds, ripple marks, or fossil orientations—to confirm the original orientation of the strata.312