How does the law of superposition help geologists determine the relative ages of sedimentary rocks?

The law of superposition is a fundamental principle of stratigraphy used to determine the relative ages of sedimentary rock layers. Formulated by the Danish geologist Nicolaus Steno, the principle states that in an undisturbed sequence of sedimentary strata, each layer is deposited on top of an older one. Consequently, the oldest layers are found at the base of the sequence, while the layers become progressively younger as one moves toward the top.1234

This method allows geologists to establish a chronological order of rock units through simple visual observation, providing a basis for understanding Earth's history without requiring complex analytical equipment. However, the principle assumes the sequence has not been significantly disturbed. In cases where geological forces—such as tectonic tilting, overturning, or thrust faults—have altered the original orientation of the strata, geologists must use additional criteria, such as analyzing asymmetric depositional features or fossil patterns, to correctly identify the original top and bottom of the layers.341