Stratigraphic Interpretation in Archaeology

Core Idea

Stratigraphic layers in archaeological sites form a chronological sequence: lower layers are older, upper layers younger. Interpreting stratigraphy means distinguishing natural layers from human disturbance, understanding mixing and displacement, and recognizing post-depositional changes that can scramble the sequence.

Explainer

You've already worked with artifact examination techniques, which taught you to read individual objects for information about materials, manufacture, and use. Stratigraphy scales that skill up to the entire site: instead of reading one object, you read the sequence in which objects and materials accumulated in the earth, building a relative chronology directly from physical context.

The foundational principle is the law of superposition, borrowed from geology: in an undisturbed deposit, the material at the bottom was deposited first and is therefore older than material above it. Each distinct stratum — a layer distinguishable by color, texture, composition, or content — represents a period of deposition. A gray ash layer might mark a burning event; a yellow sandy layer might represent wind-blown accumulation during an abandonment phase; a dark organic layer might indicate a period of human habitation with organic waste. Reading a site profile (the exposed cross-section of an excavation) means reading this sequence from bottom to top as a timeline.

The complication — and the interpretive skill — comes from distinguishing the original depositional sequence from everything that has scrambled it since. Intrusive features like pits, post-holes, wells, and burials cut through earlier layers and deposit later material below the surface level of their period. A Roman pit dug into a Bronze Age deposit will insert Roman pottery into what otherwise appears to be Bronze Age stratigraphy: you must recognize the cut and associate the material with its feature rather than its layer. Bioturbation — disturbance by roots, worms, burrowing animals — can move small artifacts vertically across significant distances over centuries. Colluviation — downslope movement of material — can displace entire assemblages from their original position.

The concept of archaeological context captures what's at stake: an artifact's stratigraphic position, its spatial relationship to other artifacts, and its association with features together constitute its context, and context is what allows inference about date, function, and meaning. An arrowhead in a burial stratum means something different from the same arrowhead in a midden layer or a forge deposit. Context is irreplaceable: once a site is dug, the stratigraphic record is destroyed by the excavation itself. This is why archaeologists record meticulously — layer by layer, feature by feature, artifact by artifact — and why a poorly excavated site can never be re-excavated to recover the information that was lost. Stratigraphy is the reading of a document that is destroyed in the reading.