Overview
Stratigraphic phasing in archaeology refers to the process of interpreting and determining the sequential order of archaeological remains. This concept is grounded in the principle of superposition, which states that in undisturbed stratigraphic sequences, the oldest layers are at the bottom, while the newest are at the top.
Each phase denotes a distinct timespan, most often with explicit or implied start and end dates, and will contain specific chronologically related contexts that fall within the identified timespan. As such the phase forms a key stage in the interpretation and meta-grouping of the stratigraphic sequence. This analytical technique facilitates and enables the analytical analysis of material culture across a site and generally underpins the chronological narrative of human habitation and cultural change at a particular site over time.
Phasing then, is the broad grouping of archaeological stratigraphy by relative chronology and in its simplest form generally involves seeking out related features, which are situated at the same stratigraphic level and ordering them chronologically. As such it is a higher level interpretative process and would ordinarily require a complete and checked stratigraphic sequence and matrix diagram. Spatial location and function can often play a part in deciding whether features belong to one phase or another, but the most crucial factor is the relationship between the chronology of the site/area and the stratigraphy. Hammer (2002, Section 3.6) suggests that ‘it may be necessary to clarify the stages of construction or deposition by dividing phases into subphases as necessary’.
An archaeological subphase might be seen as a more refined segment of time within a main archaeological phase. Just as a phase represents a distinct period of human activity across a specific site, a subphase further breaks down this time period to allow for a more detailed understanding of the sequence of events or changes. Subphases might denote differences in types of activity, changes in the style of artefacts or architecture, or subtle shifts in the nature of occupation within the broader time span of the main phase. This more granular stratigraphic chronological grouping can be used to highlight crucial nuances in the evolution of a site’s cultural, technological, or architectural development.
It should be noted that on large sites, or sites where the stratigraphic phasing is spatially fragmented, it may be necessary to offer localised sub-phasing (of specific areas or trenches). Localised phase analysis focuses on the unique stratigraphic sequences and contexts within these areas to create a bespoke timeline, which might be tied into a broader site-wide phasing or just assigned to even broader regional periods. By definition a period does not necessarily form a discrete phase on a site, however all phases can probably be assigned to specific regional periods.
Note for purposes of this Handbook:
Phases here are defined as being distinct from Periods. An archaeological phase refers to a specific time frame at a particular archaeological site or part of a site. It is an analytical unit defined by archaeologists to interpret the stratigraphic and artefact sequences on a local scale.
An archaeological period, on the other hand, is a broader concept. It refers to a span of time defined across a large region, often encompassing multiple sites, and characterised by distinctive technology, culture, or type of social organisation (i.e. the Three Age System, and their regional subdivisions).
Methods
At its core the physical process of phasing is very simple, once it has been decided which key (‘nodal’) features mark the interface between each phase, you simply move the blocks of stratigraphy that lie above or below those features downwards or upwards in your matrix respectively. Pearson and Williams (1993, 95) describe this process in terms of “context-series”; that is: discrete strings of linked contexts or stratigraphic units identified within the Harris Matrix sequence diagram. Roskams (2001, 257-261) discusses the merits of this approach in some detail, as it relates to grouping stratigraphy. He also suggests that the result of ordering the matrix by phase can be visualised in the matrix diagram itself (ibid.I, 265).
In essence once the matrix is ordered and the ‘context-series’ are pulled ‘into phase’, it should be possible to draw a horizontal line across the stratigraphic matrix, either above or below the defining features (dependent upon whether that feature marks the end of a phase or the beginning of another).
Identifying the defining stratigraphic units of a phase is the interpretative element of the process and it is largely up to the original excavator, or individual responsible for analysing the stratigraphy, to define this. Examples of phase indicators (which may form ‘nodal’ points in the matrix are floors throughout a structure which mark the beginning of an occupation sequence (‘Occupation Phase’), architecture which was founded at the same stratigraphic level (both within one structure or across many) (‘Construction Phase’), robbing pit clusters or tumble sequences (‘Post-Abandonment’ or ‘Demolition Phase’), etc.
Phasing often also falls into broad interpretative categories such as ‘Natural’, ‘Occupation’, ‘Construction’, ‘Demolition’, ‘Preparation’, ‘Abandonment’ or ‘Modern’, and may even respect broader periodisation on less complex sites or parts of the sequence (i.e. ‘Medieval Pitting’; all of which should be clearly defined in the stratigraphic narrative). The exact terminology can vary between sites and projects because identified phases are often site or area specific, since they are implicitly tied to the local stratigraphy. On larger sites one might make a distinction between ‘Site-Wide Phasing’ (where the stratigraphy is essentially proven between various archaeological interventions or structures across the site) and ‘Local Phasing’ (where these relationships aren’t fully understood across the site or are missing in places). In theory phasing can always change, subject to further work (this is especially true of local phasing), where projects run from season to season, or are otherwise revisited in subsequent archaeological interventions.
It should be noted that grouping can technically be done at any time after the matrix is established, either before or after the phasing is finalised. However, it is generally easier to phase with a simpler matrix, particularly in/on larger areas and sites, in which case it might be helpful to group your stratigraphy before phasing and then phase the group matrix. As such it is generally better, if you are using groups, to complete the grouping first. That said, phasing can help you to finalise groups so the process of grouping and phasing is closely related and may be iterative. It is important to remember however, that phasing has to be done during the post-excavation analysis, whereas stratigraphic grouping can be optional. Groups (and particularly subgroups if applied) can later be used to help structure the site’s stratigraphic narrative (see discussion in the section on subgrouping and producing a narrative).
Phase numbering is often assigned upon a site by site basis, but may be subject to project or organisational conventions which should be decided upon at the outset of any stratigraphic analysis. Generally it is good practice to number phases in the order of sequential depositions (ie. from the bottom of the sequence up) as this will generally align with the stratigraphic narrative.
Current Good Practice
Some general rules for Phases
- Phases are a special form of a spatial (plans) & temporal (dating evidence) grouping of a spatiotemporal data set (the sequence), usually operating at an intra-site scale;
- as such Phases may be defined by a date range predefined by site/organisation.
- Phases must contain at least one Unit/Context.
- all Unit/Contexts, Group/Features and Sub-Groups should (as a final check) be assigned to a Phase. This would be a final check when all phasing is considered complete.
- All Phases should be allocated a timespan with start date and end date.
- Phases should be allocated to a Period/Level (if Periods are used).
- Phases require some clearly defined Title or Code, in order to rationalise their existence (these may be predefined by site/organisation).
- Phases should be across all strands of the matrix (i.e. Site-wide) unless using sub-phases.
Recommended Outputs
Please note that the formats mentioned are examples, and the actual output formats can vary depending on project requirements and preferences.
| Output Category | Description | Example Output Format |
| Phased Plans (Analogue/Digital) | Maps illustrating the key contexts or groupings present in each phase. | .jpg images, or ideally GIS compatible format including data tables, shapefiles, and spatial raster data Metadata (e.g. .csv, .shp, .kml, .gdb, .gml, .geojson, .geotiff), or vector graphics format (e.g. .dxf, .dwg or svg) |
| Phased, Revised and Updated Harris Matrix (Analogue/Digital) | Modifications made to the Harris matrix to reflect the results of the phasing analysis. | Analogue/Digital matrices (e.g. .pdf, .docx, .xslx, .dwg or .json) |
| Phasing Index or Index of Archaeological Association (Digital) | A complete table that documents the key stratigraphic, grouping and phasing relationships between contexts upon an archaeological site. | Flat file tables (e.g. .csv, .xlsx, or .ods) |
| Phasing Report (Digital) | A comprehensive document outlining the results and interpretations of the phasing analysis, including the sequence of archaeological deposits and features (may form part of the narrative output). | Document (e.g. .docx or .ods) |
| Phasing Interpretations/Conclusions (Digital) | Interpretive conclusions drawn from the phasing analysis, providing insights into the development and occupation history of the site (may form part of the narrative output). | Document (e.g. .docx or .ods) |
| Stratigraphic Sequencing Descriptions (Digital) | Detailed descriptions of the stratigraphic sequencing and relationships between different layers and deposits (may form part of the narrative output). | Text Document (e.g. .docx, or .ods) or spreadsheet (e.g. .csv, .xlsx, or .ods) |
| Chronological Framework (Digital) | A framework establishing the chronological context and sequence of the archaeological site (a definitive list of periods). | Text Document (e.g. .rtf, .docx, or .ods), or graphical representation, or timeline visualisation in digital format (e.g. .jpg, .png or interactive web-based format) |
It is important to note that some elements can exist in both analogue and digital formats, depending on the specific project requirements (example file types above are for digital formats only).
Resources
Index of Archaeological Association
Example of an Index of Archaeological Association deposited on Archaeology Data Service (ADS) website from Liverpool Street Worksite, Liverpool Street (Crossrail XSM10) Museum of London Archaeology, 2019. https://doi.org/10.5284/1055107
References
Hammer, F. 2002 Post-Ex. Manual [Online], Interpreting Stratigraphy Group. http://www.york.ac.uk/archaeology/strat/ [Last Accessed: 1 March 2023. Website currently not supported].
Pearson, N. and Williams, T. (1993) ‘Single-context planning: its role in on-site recording procedures and in post-excavation analysis at York’, in E.C. Harris, M.R. Brown and G.J Brown (eds) Practices of Archaeological Stratigraphy, London: Academic Press. 89-103. https://doi.org/10.1016/B978-0-12-326445-9.50013
Roskams, S. 2001 Excavation, Cambridge: Cambridge University Press.
Header Image Credits: left to right – CC BY 4.0 Cotswold Archaeology (2023) Data from Archaeological Works at St. Sidwell’s Point Leisure Centre and Exeter Bus Station, Exeter https://doi.org/10.5284/1116182. CC BY 4.0 South West Archaeology Ltd (2024) Images and Site Records from an Archaeological Evaluation Trenching at South Molton Tannery, East Street, South Molton, Devon, September 2019 https://doi.org/10.5284/1117189. boris, Public domain, via Wikimedia Commons https://commons.wikimedia.org/wiki/File:Phase.001.png.
