How does the law of superposition help date sedimentary rocks?

The law of superposition is a fundamental principle of stratigraphy used to determine the relative ages of sedimentary rocks. It states that in an undisturbed sequence of rock layers, each layer is deposited on top of the one preceding it; therefore, the oldest strata are located at the base, and the layers become progressively younger toward the top.12

This principle allows geologists to establish a relative chronology of events without needing complex analytical equipment. However, the law assumes the sequence has not been disturbed by geological processes. In some cases, crustal deformation can tilt or overturn rock layers, or thrust faults may displace older units on top of younger ones. When such disturbances occur, geologists must use additional visual criteria—such as cross-beds, ripple marks, or fossil patterns—to identify the original top and bottom of the layers to accurately apply the law.321