Three images in a row. From left to right - an archaeologists excavating a feature. A section photo of three sections of soil. A pile of ancient pottery sherds.

Dating & Periodisation

Overview

While the Harris Matrix itself does not provide absolute dates, it can be used in conjunction with other dating methods to establish a relative chronology for the site. Typically this would involve reviewing specialist data relating to the dating of material culture and ‘spot dates’ would be plugged into the matrix to establish date ranges for the stratigraphic phases, and ultimately periodise the site. It may be that on less complex sites the underlying stratigraphic phases align with, or respect the overall chronological periods present on the sites, but it should be noted that phases do not have to align specifically with historical (or prehistoric) periods, and this is a key distinction.

Spot dating in archaeology refers to the practice of assigning specific dates or narrow date ranges to archaeological contexts or artefacts. Derived from the concept of identifying a particular point or moment in time, spot dating involves the use of datable elements or objects associated with known historical periods or cultural contexts. By identifying diagnostic artefacts such as coins, pottery types, or architectural styles, archaeologists can estimate the approximate age or chronological placement of a specific context. While spot dating provides a preliminary dating framework, more advanced techniques like radiocarbon dating or Bayesian analysis may be employed for more precise chronological assessments.

If one is not planning to conduct detailed (Bayesian) chronological analysis (see below) then this process of applying the dating evidence to the stratigraphy would ordinarily be used to do three things:

  1. Adjusting the Harris Matrix: Spot dates can be used to refine the sequence represented by the Harris Matrix by highlighting specific correlations and potentially emphasising certain relationships.
  2. Validating and Modifying Stratigraphic Phases: The dating evidence can help validate and potentially modify established stratigraphic phases or groupings as part of the iterative process of phasing the site.
  3. Assigning Time Periods to Groups and Phases: Spot dates assist in assigning chronological groups and phases to specific time periods, aiding in the temporal organisation of the site.

Depending upon the complexity of the site it may be that dating needs to happen at a group level (rather than at the individual context level).

However, it is important to note the tension that arises between using spot dates in this manner and employing more detailed Bayesian chronological analysis. Bayesian chronologists would consider it bad practice to use spot dates, particularly any derived from scientific dating methods, to underpin the phasing and organisation of the matrix and associated chronological modelling. This is due to the tautological problem it presents, where the date being refined is used as part of the a priori logic that underpins the final output. In simpler terms, using a preliminary or initial date to inform the Bayesian process can bias the final, more refined date, by creating a circular logic.

Methods

Ultimately all dates should be assigned to a context number in the stratigraphic sequence, being sure to check for possible issues of residuality and contamination of material culture (e.g. the latest spot date in a cesspit which was backfilled with domestic rubbish many years after it was built, would be misleading). At this point the matrix diagram can be annotated or colour coded for ease of reference. This will establish relative ages of the corresponding contexts in the Harris Matrix.

To accomplish this level of analysis, a comprehensive list of dates from specialists is required. These dates encompass spot dates on specific objects such as coins or brooches, or date ranges of assemblages of material culture such as ceramics, associated with specific contexts. Additionally, scientific dates obtained from specific samples should be collected and organised. Throughout this process, it is essential to recognize and differentiate between relative and absolute dates. Relative dates provide information on the age of an object in relation to other items, while absolute dates offer more precise numerical estimations for chronological placement. 

Typical examples of relative archaeological dating involve establishing the sequence of stratigraphic layers and determining the relative chronology of different artefacts within a site. In contrast, absolute archaeological dating methods, such as radiocarbon dating or dendrochronology, provide numerical assessments of the age of archaeological materials or sites. It is also important to acknowledge that scientific dates often come with probability ranges. Thus, it is crucial to address these caveats and provide guidance on the appropriate utilisation of such dates in archaeological dating.

Once dating is established, at this point in the analysis it is often beneficial to cross-date contexts by seeking overlaps or associations between different contexts or stratigraphic units. These could share not only the same specific type of artefact but also artefacts of a similar character, thus allowing more flexibility and a broader understanding of the contexts in question. This process considers the issues of residuality and intrusion, wherein older artefacts may be found to have been redeposited in younger contexts (residual finds) and later objects are recorded in earlier deposits (intrusive finds) due to various natural or anthropogenic activities. By acknowledging these possible discrepancies in the dates of the finds, we can provide a more accurate analysis of the most likely dates for the deposition of the associated contexts.

If the stratigraphic relationship between two contexts is ambiguous, cross-dating with similar artefacts can offer critical clues about their relative order. This not only helps refine the relative chronology of the sequence but also aids in establishing temporal relationships between different parts of the Harris Matrix. By doing so, we can better understand the sequence and distribution of human activities in the area under study, considering all possibilities of artefact deposition and disturbance.

Throughout this process special attention should be paid to the problem of residuality. Each date should be considered in relation to its context within the sequence, with particular attention being paid to the security of its provenance, and the stratigrapher should ask ‘could this date be residual?’ If the answer is yes, then the date should probably be disregarded in terms of ordering the matrix or informing the phasing.

In a similar way it should be possible at this stage to assign absolute dating (if any has been acquired). This should suffer less from issues of residuality, because most absolute dates (with the possible exception of numismatic dates) will be derived from scientific methods (such as radiocarbon dating, dendrochronology, thermoluminescence), sampled from secure contexts. By dating specific contexts or associated materials, it is possible to establish fixed points within the relative chronology and refine the dating of other contexts, groups and phases accordingly.

Seriation

It’s important to note that while cross-dating provides a flexible methodology for temporal analysis, additional methods such as seriation could be employed for more depth and precision. Seriation, an archaeological technique that involves arranging artefacts in chronological order based on stylistic or typological changes, can be particularly useful. It is a relative dating method that relies on the premise that styles or types of artefacts change progressively over time and can, therefore, be used to place archaeological contexts into a sequence. However, it’s necessary to acknowledge that formal seriation may not be universally applicable, as its effectiveness largely relies on the site types and the range and scale of material culture present. 

For instance, sites with an extensive and varied collection of artefacts, where observable shifts in design or material use occur, are best suited for formal seriation. In other cases, an informal sequence could be more appropriate. This might be particularly useful when dealing with finds of similar character or when identifying groups of material that imply an earlier or later date – once issues of residuality and intrusion have been factored in. 

By approaching seriation with flexibility, it can be aligned more closely with the realities of the archaeological record. It’s about taking into account the artefacts’ similarity in character and the impact of temporal outliers, which could be embedded into a loose sequence. This sequence, though less rigid than a formal seriation, would nonetheless contribute to a more nuanced understanding of the site’s chronology.

Periodisation

Once the relative and absolute chronology of the Harris Matrix is established, it should be possible to divide the site’s occupation into phases based on significant changes or breaks in the sequence. These phases will likely be defined by cultural, technological, or environmental shifts observed in the archaeological record and may very well (although not a prerequisite) be allocated to a broader period. Periods may reflect (but definitely should not contradict) the stratigraphic phasing of the matrix. Periodisation has the advantage of making the phased excavation sequence broadly comparable to other sites across a region.

It is very helpful for increasing the interoperability of published data to use standard terminologies for referencing different periods that are most commonly associated with the geolocation from which the site data derive. One way of doing so is to use shared terminology lists such as the Perio.do Linked Open Data (LOD) terminologies that cover an international range of periods appropriate to the geolocation of the origin of the dataset.

It’s important to note that completing the dating and periodisation of a Harris Matrix diagram can be a complex and iterative process and may require more than one version, and repeated working through, to complete the final matrix.

Please note that the formats mentioned are examples, and the actual output formats can vary depending on project requirements and preferences.

Integration of Spot Dates and Period Assignment Outputs:

Output CategoryDescriptionExample Output Format
Integrated Spot Dates and Periods Matrix (Digital)Matrix integrating spot dates with period assignments.Spreadsheet or database file (e.g. .xlsx, .ods or .db)
Spot Date and Period Correlations Report (Digital)Report correlating spot dates with periods.PDF document or Word document (e.g. .pdf or .docx)

Stratigraphic Harris Matrix with Spot Dates and Period Assignments:

Output CategoryDescriptionExample Output Format
Updated Stratigraphic Harris Matrix (Digital)Harris matrix updated with spot dates and period assignments.Diagram or graphic representation (e.g. .jpg or .svg)
Stratigraphic Harris Matrix Summary Report (Digital)Summary report of the updated Harris matrix with spot dates and period assignments.PDF document or Word document (e.g. .pdf or .docx)

Period Plans and Overviews:

Output CategoryDescriptionExample Output Format
Period Plans (Analogue/Digital)Visual representations of archaeological periods.Hand-drawn or digitized maps/illustrations (e.g. .jpg or .pdf)
Period Overview Report (Digital)Comprehensive report overviewing archaeological periods.PDF document or Word document (e.g. .pdf or .docx)

Chronological Interpretation and Analysis:

Output CategoryDescriptionExample Output Format
Chronological Interpretation Report (Digital)Report interpreting chronological findings.PDF document or Word document (e.g. .pdf or .docx)
Chronological Analysis Tables (Digital)Tables analysing chronological data.Spreadsheet or database file (e.g. .xlsx, .ods or .db)

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

CIfA Toolkits Guidance

Scientific Dating & Chronological Modelling

An excellent guide to various scientific dating methodologies has been produced by the Historic England Scientific Dating team.

listing of things that a standard specialist report should contain

What should be in a spot dating report

See section 2.3.3 of the Pottery Standards document

Nomisma.org

Nomisma.org is an international, collaborative project to provide stable digital representations of numismatic concepts according to the principles of Linked Open Data (LOD).

Perio.do – Gazetteer of international Periods for Linking and Visualizing Data

Header Image Credits: left to right – CC BY 4.0 Network Archaeology Ltd (2024) Images from a Trial Trench Evaluation and Watching Brief at the Tirley Feeder Connector, 2011 https://doi.org/10.5284/1117194. 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/10.5284/1117190. Beechbrook Wood, prehistoric pottery from Stuart Foreman (2018) Channel Tunnel Rail Link Section 1 [data-set]. York: Archaeology Data Service [distributor] https://doi.org/10.5284/1000230. That work is licensed under the ADS Terms of Use and Access.