Overview
“Although a complete excavation record should have been established as far as possible on-site, both time pressures and practical problems, plus matters such as cross referencing, mean that certain aspects will need to be finished after the event” (Roskams 2001, 241).
For most excavations the post-excavation process has to begin with a process of consolidating and validating the site archive records compiled during the on-site recording of the excavation. In a single context methodology this involves ‘checking the record’ by reviewing the main written, graphic, photographic and (more recently) digital components. Record checking would ordinarily involve the systematic review and validation of the primary data (context sheets, plans, databases, images, etc.) to make sure that there are no errors or omissions, and the cross-checking and cross-referencing of registers to ensure the integrity of the data-structure and the unique identifiers associated with the archive. In most stratigraphic recording systems (such as: single context recording) this will usually hinge upon the unique ‘context number’ associated with each stratigraphic unit, but may also include an array of finds numbers, sample numbers, photograph or drawing numbers, survey control records, and indeed spreadsheets or databases (e.g. see examples in Stratigraphic Documentation Outputs listings) .
This process of checking the record is about checking archival integrity for the subsequent analysis of the sequence and the finds it yields. Hammer (2002, Section 2.2) notes that “Although analysis may bring changes to the original (checked and amended) interpretation, the original state of a site record must remain unaltered or as a minimum, the original version must be readable and any changes must be initialled and dated. Reinterpretation during analysis will often occur and be discussed in the report. It has become common practice that all records from excavation (and post-excavation) have their own integrity and must be indexed and accessible for future work.”
As a minimum record, all stratigraphic relationships (above, below, equals) between identified stratigraphic units should be recorded in some format,
using stratigraphic principles (Harris 1989). This should clearly record the relevant relationships between every context that is to be part of the analysis.
A – No Stratigraphic Relationship
B – Stratigraphically Above (After) / Below (Before) (spatiotemporal sequence)
C – Equals (at some time stratigraphically whole)
The use of the Harris Matrix diagram is a highly effective method and organisational tool for checking of data retrieved in plan, against the evidence of any sections and the observations recorded on the context sheet by the original excavator, any of which can show disparities between separate elements of the primary record.
Once this is completed or achieved the matrix diagram also helps direct the gaze of the stratigraphic analyst towards any areas of contradiction. These are interesting and exciting challenges, rather than problems, and they invite the archaeologists to think more closely about their assumptions. Once these challenges have been addressed then the Harris Matrix provides a visual synthesis of the sequence of deposition of events of truncation or deposition from which one can start interpreting meaning.
The matrix provides a means of addressing assumptions and can provide focal points for critical discussion during analysis. The generation of a stratigraphic matrix is not simply about the end product or a diagram, but rather the compilation of a matrix is part of a process, as Harris himself says, “to understand stratigraphy as its excavation is happening”.
When deciding in analysis upon the need for a site-wide Harris matrix diagram, its relevance and usefulness must be considered based on the nature of the investigation and the complexity and scale of the archaeology encountered. For stratigraphic records with any depth or complexity a matrix diagram provides a great tool to organise and sense check the primary record.
The stratigraphic principles set out by Harris (Harris 1989) are fundamental and should be adhered to, but not all archaeological investigations may necessarily need a site wide and phased Harris matrix diagram to be drawn, if there is literally no stratigraphic record to analyse. Or a fully phased matrix diagram may only be most useful for specific areas of stratigraphic complexity within an otherwise very shallow, or uniform, site.
The purpose of a Harris matrix diagram is to provide an initial sense-check of primary recording. The end product, the matrix diagram, provides a visual representation of the stratigraphic sequence. It is central to understanding the chronological sequence of deposition and truncation and is a fundamental tool in understanding the development or changes on a site over time.
The matrix, even on what one may assume is a simple site, acts as a means of organising, but more importantly thinking about data, making sure data is consistent. In some cases, the stratigraphic relationships may be seen to be straightforward but one should still critically examine any assumptions. Without informed critical thinking about stratigraphy then easily understood generalist interpretations may become a norm and leave no latitude for understanding the extraordinary, or even the ordinary, in different contexts. The tendency for over simplification is to be guarded against as much as the tendency to make it more complicated than it is.
In deeper stratified sites, most often in urban situations or deeply stratified rural sites, that exhibit vertical complexity in the sequence, ‘running’ or draft Harris matrices should have been created on-site, to support the initial observations and interpretations of the excavator. These draft matrices serve as working documents and are continuously updated as excavation progresses. They allow archaeologists to document and analyse perceived stratigraphic relationships in real-time, and can inform decision-making on excavation strategies. This earliest period of post-excavation record checking then, is the point at which the Harris Matrix might be initially compiled from the primary record and on-site draft in something approaching its final form (so-called ‘running’ matrices are drawn out either by hand or digitally using specialised tools). Without careful checking the records may produce circular stratigraphic relationships or “H” relationships which, if not checked at the trowel’s edge, will cause greater problems later.
For shallower sites, with less ‘vertical’ depth of stratigraphy (and more spatial horizontality in the contexts recorded), often these types of sites present an opportunity to make good use of Geographic Information Systems (GIS) for managing the site’s spatial record, together with temporal and stratigraphic documentation. In this case the process of consolidating the archaeological archive and preparing for analysis is characterised by slightly different but intimately related steps. A key aspect of this process is the thorough examination and critical understanding of any digital survey or site plan, ensuring its accuracy and completeness. Survey control and surveyed data serves, in these instances, as the verification of the location of units of stratigraphy and the basis for “grouping” and then “phasing”, and the correctness of the digitised spatial record is crucial to any stratigraphic analysis. The use of GIS technology has become increasingly common in archaeology for this purpose, however, regardless of the GIS implementation, the verification of recorded relationships remains a laborious and difficult process in the digital realm. It entails meticulous scrutiny of the entire database to ensure its coherence and functionality and underscores the need to carefully document running matrices as the site is being excavated.
In analysis, the decision to construct a site-wide and phased Harris matrix should be based on an informed and reflexive assessment of the site’s stratigraphic complexity and the specific research objectives, including potential to associate stratigraphy on known excavations nearby. It is important to weigh the potential benefits of using a collated matrix diagram for site-wide analysis against the time and resources required to create and maintain it. Adaptability and flexibility in choosing the most suitable methods for stratigraphic analysis based on the archaeology encountered should ensure that the overall methodology employed aligns with the specific requirements of each archaeological site.
Methods
Generally when completing a site record, either with seemingly simple spatial stratigraphy on shallower “topsoil/natural sites” or more complex sites with deeper vertical stratigraphy, there are iterative stages that the supervisor should undertake. Supervisors or excavators should check context sheets and plans, sections, photography and sample for primary data consistency to ensure there are no internal contradictions. Draft or ‘running’ matrices should be critically checked so that a 4-dimensional conception of “what is going on” can be achieved (note: ideally, especially on simpler sites, much of this work will have been completed in the field so this may be little more than a straightforward data validation check).
It can be useful to consider the nature of identified events, as a working interpretation, but this should not be fixed and should always be open to critical self-analysis. If one is confident in the interpretations and they are justified then one can begin to create subgroups (and/or groups) based on interpreted events or activities identified in the stratigraphic record. The creation of subgroups, utilising this framework of thinking is really useful, but it is an interpretive move and must be underpinned by explicit and overtly stated sets of justifications. Ideally further analysis would continue after spot-dates are returned from specialists and subgroups and groups (as appropriate), can be finalised.
The construction of the stratigraphic matrix is always closely linked to the record checking process and can be broadly divided into a few key tasks depending, again, on the type of stratigraphic recording system adopted on site. For single context recording, the process generally follows the following pattern :
- Compiling a plan matrix (single context planning only)
- Checking and validating the stratigraphic relationships.
- Correlating stratigraphic contexts.
However, with the uptake of spatial technologies to handle the spatial data on excavations 5m by 5m plans might be digitised and merged in GIS before the excavator attends to the matrix; in which case the plan matrix is less critical (serving as little more than a failsafe, to check ambiguities). Furthermore on excavations where much of the recording is handled digitally, either through primary data acquisition (by survey or 3D recording), or digitised during or directly on leaving site then first steps would be to systematically consolidate and verify the written archive, by cross-referencing the digital survey or site plan with the physical artefacts and stratigraphic relationships collected during excavations noting any discrepancies or missing information. Use of GIS technology can significantly aid in visualising the spatial relationships and contextual information of the recorded features and context.
Compiling a plan matrix (optional analogue single context method only)
The process of checking and validating the basic stratigraphy will vary depending on the complexity of the site and recording system used. Pro-forma single context planning sheets generally have a space for documenting a plan matrix for the grid square being recorded. Therefore if a strict single context recording system has been deployed in the recording of an excavation, then a common first step in this process of checking the stratigraphic record and creating a site matrix, is to collate a separate plan matrix for each 5m grid square excavated. These can then be cross-referenced and combined to form a complete plan matrix for the whole site or area. This would form a basis for a fully phased matrix, which would be filled out with any context numbers which may not have been planned (e.g. contexts only recorded in section, unplanned fills of cut features), before being checked and validated (see below). But it should be noted that this approach can present challenges and create more work for the stratigrapher, and piecing together ‘blocks’ of matrix is harder in post-excavation. As noted above, plan matrices are generally rendered unnecessary by digitising the plans and using a GIS. As such it is worth reiterating again that it is easier and more logical to produce the complete stratigraphic matrix as you excavate (as a ‘running’ matrix) wherever possible.
Checking and validating the stratigraphic relationships
When a matrix is needed for stratigraphic analysis, whatever the mode of graphical or spatial recording and whether a plan matrix has been recorded or not, the excavation team should have kept a draft or ‘running’ matrix of excavated units on-site, compiled as stratigraphic contexts were defined, but these may be incomplete and/or area specific (i.e. focused upon more complex areas of excavated stratigraphy). if so, the next task for the stratigraphic analyst (often the field director, project officer or area supervisor) will involve transposing the stratigraphic relationships from the written (or digital) archive into a final draft matrix diagram and checking the validity of these relationships against the graphic or spatial record (whether these are pro-forma context sheets and draft matrices or digital stratigraphic relationships in a database and matrix software). Ambiguous stratigraphic relationships might be further checked against the photographic archive, site diaries and interpretative notes, 3D models, or other aspects of the archive. At the end of this process the aim is to produce a clear and final Harris matrix diagram for further analysis, the relationships which it articulates should be updated in the primary record. It should also be clear in records where the top and bottom extents of the matrix are and consistent conventions should be used to show if the bottom of the matrix represents simply a limit of the extent of excavation carried out, perhaps due to building limits (eg. NFE – No Further Excavation), or the actual lowest extents of Anthropocene activities (e.g. SoN – Surface of Natural).
The most commonly occurring protocol for denoting the top of the stratigraphic sequence is the use of the ‘+’ symbol to denote the uppermost context (e.g. in MOLA archives). In some archives, this ‘+’ corresponds to a ‘top of sequence’. In other examples (mostly from rural excavations), the term ‘Topsoil’ denotes the uppermost context in the stratigraphic sequence.
A common protocol for the bottom of the stratigraphic sequence is the term ‘NFE’, which is an abbreviation for ‘No Further Excavation’. Other terms encountered are ‘limit of excavation’, ‘excavation extent’ and variations on a similar theme.
It would be useful to adopt a uniform way of representing the bottom of an excavation in a Matrix diagram and the associated data records, once archaeologists have finished excavating. Similarly, various terms have been used for where the surface of underlying geological layers and non-anthropogenic strata were encountered. Most commonly ‘Natural’, or ‘Surface of Natural’ (SoN) is used, but it is not always clear if archaeologists are consistent in distinguishing between ‘NFE’ and ‘SoN’. On many sites, particularly in urban locations, archaeologists finish the excavation at NFE and it would be useful within development schemes to record whether SoN has also been encountered.
There are some considerations about when and how to construct the Harris Matrix from the top of the sequence down, or from the bottom up.
● Constructing from the top down:
This approach involves starting with the most recent or uppermost layers of the stratigraphic sequence and progressively working downwards. This is most often the way in which running, partial, or draft matrices are constructed as they represent the order of excavation on site. In this way a matrix is built in the sequence that the stratigraphic units are encountered in the ground. As such the matrix diagram is used for maintaining stratigraphic control of the excavation archival record.
● Constructing from the bottom up:
This approach involves beginning with the earliest or deepest layers of the stratigraphic sequence and moving upwards. It can therefore only be done efficiently once excavation has been completed of all the contexts to be included in the diagram and may only be effective on smaller sites, or areas. Working through and checking an already complete stratigraphic relationships from the bottom up can be advantageous when trying to make sense of more complex stratigraphy, as it will reflect the order of deposition of the stratigraphic sequence and essentially underpins the structure of interpretive Phases and the stratigraphic narrative (which should be written following the order of deposition). Phase numbering is usually applied starting from earliest levels moving upwards to the latest levels in the matrix.
Correlating stratigraphic contexts
Simultaneously the stratigrapher will be looking for correlations between contexts. That is, those units which might be aligned horizontally in the matrix on the basis of one of three (normally) forms of association:
- Shared space (in time) (i.e. a series of pits which were cut at the same time)
- Shared function (i.e. a cluster of post holes that formed a structure)
- Identicality or former wholeness (i.e. a single spatiotemporal deposit which was divided by a later truncation, or only excavated in separate segments) – see illustration
(for an extended discussion of correlation see: Roskams 2001, 246-253)
All correlations are a form of interpretation of the stratigraphic sequence and should be justifiable, many may (ideally) have been noted in the primary record as they were excavated, some may be assigned in post-excavation as the stratigrapher constructs the record and builds a holistic understanding of the sequence. It should be noted that Harris also explores the importance of correlations (Harris 1989, 105-107) and he explicitly recognises the correlation of “a once-whole deposit or feature interface” (association 3 above) as one of the three core stratigraphic relationships in the Harris Matrix, alongside having no direct relationship with any other stratigraphic unit, or being in superposition (ibid., 36). This would imply that the first two associations are more interpretative in their fundamental nature and may not be recognised by purists as a core stratigraphic relationship. These correlations are often represented as ‘=’ or double headed arrows in the matrix diagram.
In practice elements of this process may blur as practitioners often do this in conjunction with the grouping (and perhaps phasing), especially upon sites where units are distributed over a larger horizontal area. So, in fact, the process of validation and correlation is not necessarily sequential, but rather may be iterative. Practitioners may often take an area of the site, look at it, check it and understand any stratigraphic relationships, whilst simultaneously grouping and phasing that area, before moving on. The result of this process is a checked and validated stratigraphic sequence diagram where strings of contexts are ordered according to the interpretive correlations as defined by the stratigrapher.
Recommended Outputs
Site Excavation ‘Archive’
The comprehensive management and organisation of archaeological site archives involves a diverse range of analogue and digital components, each contributing to the documentation, analysis, and preservation of valuable archaeological data. Following is a list of key archaeological site archive elements, with indications of whether they might be expected to be analogue or digital outputs:
Site Documentation:
| Output Category | Description | Example Output Format |
| List of all trenches/areas and their addresses (Digital) | Comprehensive list detailing each trench/area and its corresponding address. | Digital list (e.g. .docx, .pdf, .csv or .xlsx) |
| Archive/HER reference numbers (Digital) | Reference numbers associated with archives or Historic Environment Records, specific to regions (like ECB numbers in Cambridgeshire). | Digital reference list (e.g. .txt, .pdf or .xlsx) |
| Watching brief, test pit, and evaluation trench records (Analogue/Digital) | Documentation and records associated with specific excavation methodologies and evaluations. | Analogue/Digital records (e.g. .pdf, .docx or .odt) |
Stratigraphic Documentation:
| Output Category | Description | Example Output Format |
| Context (plan), section, finds, and sample registers of all areas/sites (Analogue/Digital) | Registers detailing plans, sections, finds, and samples across sites or areas. | Analogue/Digital registers (e.g. .pdf, .docx or .xlsx) |
| Context sheets (notebooks) including primary stratigraphic relationships (Analogue/Digital) (Scanned if no alternative) | Database files or proforma sheets (or notebooks) used for recording context details. | Analogue/Digital/ (e.g. .csv, .xlsx ) Scanned sheets (.pdf if no alternative) |
| Plan sheets (Analogue/Digital/Scanned) | Sheets used for documenting plans. | Analogue/Digital/Scanned sheets (e.g. .pdf, .jpg or .png; or as CAD files: .dxf or .dwg) |
| Sections/elevations (Analogue/Scanned) | Visual representations of sections or elevations of the excavation area. | Analogue/Scanned representations (e.g. .pdf .jpg or .png; or as CAD files: .dxf or .dwg) |
Matrices and Registers:
| Output Category | Description | Example Output Format |
| Draft and final site wide or area matrices (Analogue/Digital) | Preliminary and final matrices for an entire site or specific areas. | Analogue/Digital matrices (e.g. .pdf, .docx, .xlsx, .dwg or .json) |
| Plan matrices (Analogue/Digital) | Atomised plan matrices for individual grid squares (single context recording only). | Analogue/Digital matrices (e.g. .pdf, .docx, .xlsx, .dwg or .json) |
| Notebooks (eg. survey day books) (Analogue/Digital) | Notebooks, such as ones used during surveys. | Analogue/Digital notebooks (e.g. .pdf, .docx or .odt) |
Visual Documentation:
| Output Category | Description | Example Output Format |
| Photos (Digital, possibly Analogue, scanned where necessary) and Metadata | Photographs of the site, features, or artefacts. Metadata provides additional information or context for each photo. | Digital/Analogue photos with metadata (e.g. .jpg or .dng or .tiff) |
| Scanned context sheets with appropriate metadata | Digitised context sheets, paired with corresponding metadata. | Scanned sheets with metadata (e.g. .pdf, .jpg or .png) |
| Drawing scans with appropriate metadata | Digitised versions of drawings with associated metadata. | Scanned drawings with metadata (e.g. .pdf, .jpg, .png, .svg, .dwg or .dxf) |
Digital Data:
| Output Category | Description | Example Output Format |
| Spreadsheets (Digital) | Various ‘flat file’ digital data. | Flat file tables (e.g. .csv, .xlsx, or .ods) |
| Database downloads/clones (Digital) with metadata | Digital databases, accompanied by their metadata. | Database files with metadata (e.g. .csv or .db) |
GIS Data:
| Output Category | Description | Example Output Format |
| GIS and Survey Data (Digital) | Digital data related to GIS and surveys, including attributes for grouping, phasing, and periods. Should be accessible and non-proprietary. | Data tables, shapefiles, and spatial raster data Metadata (e.g. .csv, .shp, .kml, .gdb, .gml, .geojson, .geotiff) |
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).
Header Image Credits: left to right – CC BY 4.0 Colchester Archaeological Trust (2024) Site Data from an Archaeological Evaluation and Excavation at Bulse Grange, Rookery Lane, Wendens Ambo, Essex, November 2022 https://doi.org/10.5284/1117248. CC BY 4.0 Colchester Archaeological Trust (2024) Images from an Archaeological Evaluation and Excavation at Cross Cottages, Cooks Hill, Boxted Cross, Essex, December 2021 https://doi.org/10.5284/1117246. CC BY 4.0 South West Archaeology Ltd. in Morris, B. W. (2019). Copplestone Farm, West Manley, Tiverton, Mid Devon Results of an Archaeological Evaluation and Heritage Impact Assessment. South West Archaeology Ltd. https://doi.org/10.5284/1075353.