Stratigraphy and the Harris Matrix: How Archaeologists Read Time in the Ground
Stratigraphy — the study of layers — is the foundational interpretive principle of both geology and archaeology. In archaeology, it provides the primary chronological framework for any excavation: the relative dating of contexts (individual units of deposit, construction, or cut) against each other based on their physical relationships. The principle was borrowed from geology and applied systematically to archaeological excavation by the British General Augustus Lane Fox Pitt Rivers in the 1880s; formalised into a recording system by the American W.C. Wren and subsequently developed into the Harris Matrix by Edward Harris in Winchester in the 1970s. The Harris Matrix is now the universal standard tool for recording and analysing complex urban stratigraphic sequences.
The Law of Superposition
The fundamental principle of stratigraphy is the law of superposition: in an undisturbed sequence of deposits, lower layers were deposited before upper ones. A layer sealed beneath another layer must be older than the layer that seals it. This is intuitive but has profound implications: it means that the physical relationships between layers encode chronological information that can be read systematically.
In practice, archaeological stratigraphy is rarely undisturbed. Pits are dug through earlier layers; walls are built cutting through existing surfaces; robber trenches are cut to remove earlier masonry; bioturbation (root action, burrowing animals, earthworm activity) moves material between layers. Understanding stratigraphy requires identifying not only depositional layers (layers of material that accumulated passively) but also cut features (pits, ditches, postholes, robber trenches) and their fills, and interfaces (surfaces that represent exposed ground at a particular time, such as floor surfaces and occupation horizons).
The Harris Matrix
The Harris Matrix is a diagram that represents the stratigraphic relationships between all recorded contexts at a site in the form of a flowchart. Each context (a layer, a fill, a cut, a floor surface, a wall) is represented by a rectangle containing the context number; the physical relationships between contexts are shown by connecting lines — with a horizontal line indicating contemporaneity and a vertical line indicating that one context is above (and therefore later than) another.
Edward Harris developed the matrix in response to the complexity of urban stratigraphy at Winchester in the 1970s, where hundreds of intercutting features and dozens of construction phases made a simple visual cross-section inadequate for representing the full network of chronological relationships. By constructing a Harris Matrix, an archaeologist can identify the full sequence of events at a site — which walls were built before which others, which pits were dug through which floors, which fills belong to which cuts — in a way that allows the sequence to be read and tested before post-excavation analysis begins.
Recording Systems
The recording of stratigraphy is done at the level of individual contexts: each context is recorded on a pro-forma context record sheet noting its type (layer, fill, cut, structure, surface), its description (colour, texture, inclusions), its physical relationships to adjoining contexts (what is it above, below, equal to, cut by), and its dimensions. The context sheet is the basic data unit of an excavation; all subsequent analysis and synthesis depends on the quality and completeness of context recording.
Photography and measured drawing supplement the written context record. Sections (vertical drawings of the faces of excavated trenches) show the physical relationships of layers as exposed in the trench wall; plan drawings (horizontal maps of the exposed surface at each stage of excavation) show the spatial layout of features at each level. Together with the context sheets and Harris Matrix, these documents constitute the site archive — the permanent record of the excavation from which analysis and publication are derived.
Urban Stratigraphy
The most complex stratigraphic sequences occur at urban sites with long occupation histories. At York (Roman Eboracum and Viking Jorvik), the excavations at Coppergate in the 1970s–1980s produced deposits up to 8 metres deep, representing continuous occupation from the Roman period to the present, with hundreds of individually recorded contexts. The Harris Matrix for this excavation extended to thousands of entries. The preservation of organic material in the waterlogged lower levels — leather shoes, wooden combs, textile fragments, food remains — made Coppergate one of the most informative Viking-period urban sequences in Europe.
London (Roman Londinium and medieval and post-medieval London) presents stratigraphy of similar complexity. The Museum of London Archaeology (MOLA) has developed GIS-integrated recording systems that allow stratigraphic data to be analysed spatially, temporally, and thematically across thousands of excavation interventions across the city, building up a composite understanding of the urban landscape that no single excavation could provide.
Absolute and Relative Dating
Stratigraphy provides relative dating — context A is earlier than context B — but not absolute dating (calendar dates). Absolute dates are assigned to stratigraphic contexts through the dating of associated artefacts (pottery, coins, glass, metalwork with known date ranges), radiocarbon dating of organic material in deposits, dendrochronology of structural timbers, and other scientific dating methods. The integration of absolute dates into the stratigraphic sequence allows individual contexts to be assigned calendar date ranges and the overall site sequence to be expressed in real time.
The relationship between stratigraphic sequence and absolute dating is not always straightforward: a coin in a layer provides a terminus post quem (earliest possible date) for the layer's deposition, since the coin must have been minted before it was deposited, but cannot by itself indicate how long after minting it was deposited. Pottery, with its changing styles and production periods, provides broader but sometimes more reliable date ranges when multiple vessels from the same type series are present.
Teaching Stratigraphy
Stratigraphy is taught to archaeology students through practical exercises — constructing Harris Matrices from prepared multi-layer examples, reading sections in the field, recording contexts on site — and is refined through practice. The best stratigraphers develop an intuitive understanding of how layers form and relate to each other that goes beyond the formal rules; they can read the physical character of deposits (their texture, compaction, and boundary characteristics) to identify formation processes that would otherwise be ambiguous.