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How do scientists use the law of superposition to determine the relative ages of sedimentary layers?

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Quaternary paleogeography — Quaternary Distribution of landmasses, mountainous regions, shallow seas, and deep ocean basins during the Quaternary Period. Included in the paleogeographic reconstruction are the locations of the interval's subduction zones.

The law of superposition is a fundamental principle of stratigraphy used to determine the relative ages of sedimentary rock layers. It states that in an undisturbed sequence of sedimentary strata, the oldest layers are deposited at the base, while progressively younger layers are deposited in ascending order toward the top.123

Geologists apply this principle by observing the vertical sequence of rock units. Because deposition naturally proceeds from the bottom up, the physical position of a layer within a series serves as a direct indicator of its relative age.3

However, scientists must account for potential geological disturbances. If crustal deformation has caused layers to tilt or overturn, or if erosion has removed significant portions of the record, determining the original top and bottom of a layer becomes necessary. To resolve this, geologists look for evidence of depositional asymmetry—such as cross-beds, ripple marks, or specific fossil orientations—to confirm the original orientation of the strata before assigning relative ages.13