Definition
Stratigraphy is the branch of geology concerned with rock layers (strata) — their composition, sequence, lateral extent, and the timing of their deposition. Its founding principle, the law of superposition, states that in an undisturbed sequence, younger layers lie above older ones. From this single insight a precise framework of relative dating follows.
In paleoanthropology, stratigraphy provides the spatial and temporal context for every fossil. A hand bone is just a bone until its stratigraphic position — which layer, at what depth, bracketed by which datable units — places it in a chronological and environmental setting.
Why it matters
How it works
Field stratigraphy begins with measuring sections — describing each layer's thickness, composition, colour, sedimentary structures, and contacts with adjacent layers. Fossils are recorded by their exact position within the section. Datable units (volcanic ashes, magnetic reversals, biostratigraphic markers) provide age control. Lateral correlation across outcrops or boreholes extends the framework over a region.
When a new fossil is found, its stratigraphic position is plotted onto the measured section, and its age is interpolated from the bracketing dated units. If the fossil sits between a 3.0 mya tuff above and a 3.4 mya tuff below, its age is somewhere in that range, with proportional placement narrowing it further.