Geological and archaeological layer sequencing, unconformities, and relative dating
Steno's law of superposition (1669) states that in undisturbed sequences, older layers lie below younger ones. The principle of original horizontality states that sediments are deposited horizontally; tilted or folded strata indicate subsequent deformation. Unconformities — gaps in the stratigraphic record — represent periods of erosion or non-deposition and were recognized by James Hutton at Siccar Point (1788) as evidence for vast geological time ("no vestige of a beginning, no prospect of an end"). In archaeology, stratigraphy distinguishes primary contexts (undisturbed deposits) from secondary contexts (redeposited material), and Harris matrices encode the temporal relationships between units.