Relative dating

Relative dating places rocks, fossils, or geological events in chronological order without assigning them numerical ages.

Law of superposition: In an undisturbed sequence of layered rocks, lower layers are older than layers above them. It supplies the basic age-order rule for stacked sedimentary strata.
Stratigraphy: The study of rock layers and their relationships in space and time. Relative dating is built largely on interpreting the order of strata.
Geological time scale: A system dividing Earth's history into units defined by stratigraphy and geological events. Relative dating underpins the ordering of the scale's major divisions.
Absolute dating: Methods that assign numerical ages or age ranges to materials and events. It provides numerical estimates, unlike the sequence alone established here.
Principle of original horizontality: Sedimentary layers are generally deposited in approximately horizontal sheets. Tilted layers can be recognized as having moved after their deposition.
Sedimentary rock: Rock formed from deposited sediment or from minerals precipitated from water or air. Its layered deposits commonly preserve an interpretable sequence.
Stratigraphic correlation: The matching of rock layers or units across different locations. It extends local age sequences into regional and global comparisons.
Radiometric dating: A method of numerical dating based on predictable radioactive decay in minerals or other materials. It can attach numerical ages to parts of a relative sequence.
Principle of cross-cutting relationships: A geological feature that cuts another feature formed after the feature it cuts. Faults and intrusions can be ordered against the rocks they disrupt.
Fossil: Preserved remains, traces, or other evidence of past life. Fossils provide biological markers for ordering and correlating rock layers.

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