Overview
While the stratigraphic sequencing of 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 typically present on the sites in that region, but it should be noted that phases do not have to align specifically with common 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 chronological 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:
- 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.
- 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.
- 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).
A problem can arise if spot dating is used in this manner and then Bayesian chronological modelling uses the same finds dates. The key point is that one must not use the same piece of dating evidence more than once in the interpretive process. For example, an excavation supervisor/director might choose to use spot date estimates from several coins to help define the chronological order of phase boundary dates on a site. Those coin dates should not be used again when fitting a more formal Bayesian chronological model, during post excavation. Instead, other dating evidence (such as C14 dates or spot dates on other finds) should be sought.
Methods
Ultimately all context numbers in the stratigraphic sequence should be assigned a date, being sure to check for possible issues of residuality and contamination of material culture (e.g. the latest spot date from the fill of 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 (e.g. more recent animal burrows or unrecognised or unrecorded intrusions). 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 these processes special attention should be paid to questions that are posed by residuality of finds. 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.
Yet, on many open-area excavations, particularly those with relatively limited vertical stratigraphic complexity, stratigraphic analysis and dating evidence alone may not provide a sufficient basis for phasing or interpretation. In such cases, depositional and distributional analyses provide essential complementary evidence. These may include the character, composition and extent of deposits; the density and spatial distribution of artefacts, ecofacts and residues; patterns of absence as well as presence; relationships between finds, features, surfaces, structures and topography; and evidence for residuality, intrusion, truncation, reworking or structured deposition. These analyses can help identify activity areas, episodes of dumping or clearance, changing land use, depositional histories and the formation processes that shaped the archaeological record. GIS and other spatial tools now provide powerful ways to visualise and interrogate such patterning, but the analytical reasoning should still be made explicit: the spatial units used, the categories plotted, the assumptions made, and the relationship between distributional evidence, stratigraphic relationships and proposed phasing should all be documented and, where possible, archived.
Additionally, in open-area excavations that incorporate programmes of buried or topsoil test-pit sampling for surface-deposit finds densities, distributional analyses of residual material in later features provide evidence of earlier occupations, particularly those with no, or only a limited presence of cut features. In such cases, the distributional ‘halo’ of worked flint and/or pottery extending beyond, for example, small Neolithic–Bronze Age pit-cluster groupings reflects the wider extent of their activity-usage. Equally, residual artefact distributions identify entirely ‘open’ flint scatters, or spreads of finds, where there are no accompanying cut features whatsoever. To this end, where residual finds tallies are recorded during excavation, increasing the sample-intensity of their register in later linear features helps to further define the footprint of earlier ‘open’ occupations and achieve representative assemblages. Recent intensive sampling programmes in landscapes where buried soils survive demonstrate that upwards of half of their ‘occupation events’ have no, or very limited, accompanying cut features; only spreads of finds (Evans et al. 2014).
GIS-based spatial analyses (and more traditional Land Use diagrams when still used) may therefore be understood as related forms of interpretative synthesis, translating stratigraphic, depositional and distributional evidence into a reasoned account of how spaces were used, modified and reworked through time.
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.