Overview
“Contexts are the essential ingredients of a sequence, and a sequence is constructed of progressively larger building blocks of these. Contexts are arranged firstly in ‘subgroups’ and then subgroups are arranged in ‘groups’, so that every context is in a particular subgroup and every subgroup is in a group. These terms are meant to be utilitarian and interpretatively neutral” (Westman & Shepherd 1992, 439).
Stratigraphic grouping and subgrouping are techniques that are employed to help make sense of the temporal and spatial relationships within archaeological sites. When examining layers of sediment or soil, archaeologists often encounter complex sequences of deposits that require systematic analysis. Subgrouping refers to the process of identifying and categorising distinct subsets or subdivisions within a stratigraphic sequence. By scrutinising the characteristics of these subgroups (such as material culture, architectural remains, or geological composition) it may be possible to infer variations in chronology, function, or cultural affiliation. Subgrouping (when applied) is generally conducted before grouping, as it allows for a more nuanced understanding of the site’s development at a relatively fine level of granularity (i.e. at the level of individual pits, postholes, ditches etc.) and helps to facilitate the construction of meaningful broader groups that reflect significant patterns and transformations over time.

Pearson and Williams (1993, 98-100) advocate that the process of (sub)grouping is based around identifying ‘context sequences’ (strands of stratigraphic contexts in the Harris Matrix) that extend from ‘nodal points’ (single stratigraphic contexts which pre- or post date blocks of stratigraphy on the site). They suggest that ‘context sequences’ should be examined to identify whether they represent inter-related activities (e.g. a pit cut or burial and its fill sequence). These activities need not be site-wide. Roskams notes, however that it is entirely possible that two stratigraphic contexts on different ‘strands’ of the matrix may also be closely associated and could be grouped together as well (Roskams 2001, 258). It should be noted that primary grouping across matrix strands has sometimes been frowned upon historically, perhaps most often on sites with ‘open area’ excavation strategies. But on sites with a number of separate trenches (as occurs commonly in ‘urban’ locations) there can often be contexts on separate strands of the matrix, deriving from separate trenches, that need grouping together. For example, a sequence of road surfaces that run across a site and through multiple excavation trenches, or when dating has shown for example that medieval pits are on different strands, or if two sequences are, in fact, the same, but completely truncated from higher up in the depositional sequence.
Thus the principles of grouping your matrix rest upon the association of meaningful stratigraphic contexts based upon a number of factors, such as:
- Stratigraphic level or relative date.
- Known relationships/like contexts.
- Similar Function within the wider landscape.
- Shared Space (ideally, where possible, supported by a relative date, although this may be difficult on broader, horizontally distributed ‘rural’ sites)
Stratigraphic (sub)groups might therefore include, a hearth or fireplace and its fills, or a midden pit and its fills (based on function), or for that matter a cluster of related pits (based on space and relative date). Alternatively it might include an occupation sequence (stratigraphic level), two sections of the same wall that has been bisected by a linear feature (such as a drain; colloquially: ‘same as’ contexts) or a series of walls which form part of the same structure (colloquially: ‘contemporary with’ contexts). Thus there is a lot of flexibility in what makes a stratigraphic group and therefore some variation in how the approach is applied conceptually. Whatever the case, the rationale for grouping should always be evidence based.

This two-step approach to subgrouping and grouping is a distinctly ‘deep-stratigraphic’ approach, designed to cope with the complexities of deep, and usually urban, stratigraphy. However, it is worth noting that not all organisations or projects use two tiers of grouping (i.e. ‘subgroup’ and ‘group’) but rather forgo the subgroup altogether. Ultimately this is usually a decision based upon the complexity of the stratigraphic sequence under analysis. In this case, where a sub-grouping step is not deemed necessary the ‘group’ will often be defined in a similar way to the subgroups outlined below, as a mechanism for pulling together related contexts into basic features (i.e. pits, postholes, linear cuts, architectural components etc.). It is important therefore for a project organisation to be transparent about the way in which grouping is deployed in their stratigraphic analysis as, despite a high degree of commonality in the actual structure of group data (contexts are assigned to groups essentially) there is quite a wide range of practice when it comes to how a group is conceived analytically.
To subgroup or not to subgroup
In certain instances, the relative complexity of an archaeological site may warrant a reconsideration of the traditional DUA/MoLA two-tiered approach to stratigraphic groups, consisting of ‘subgroups’ and ‘groups’. Instead, a single tier of groups, which may (confusingly) be referred to simply as “groups,” may suffice. In this alternative approach, the ‘group’ is the only primary grouping level, encompassing distinct episodes of activity and land use. By consolidating the stratigraphy into a single grouping level, the stratigraphic analysis of simple depositional sequences becomes more streamlined and easier to comprehend, particularly in cases where the site does not exhibit a high degree of complexity. This simplified approach ensures that the stratigraphic narrative remains coherent and accessible, while still capturing the essential chronological relationships and interpretive elements within the site’s archaeological record.
On groups and features (should grouping occur in the field or post excavation)
It is also worth noting that in some systems groups may be synonymous with the notions of a ‘feature’. This is a term often associated with Carver’s ‘feature-group’ approach to recording (Carver 1979, 1987) which is currently less commonly used in the UK commercial sector. In fact, Carver-style feature groups and a MoLA (or DUA)-style stratigraphic group represent two distinct approaches to organising and interpreting archaeological data.
A Carver-style feature group focuses on individual features within an archaeological site. Features in this case refer to discrete archaeological elements, such as pits, postholes, or walls. Carver-style grouping involves grouping these features based on shared characteristics, such as function, form, or spatial relationships. This approach emphasises the individual elements and their specific attributes and allows for a detailed analysis of the site’s components. Feature grouping typically occurs during the excavation stage of an archaeological project. As individual features are uncovered, archaeologists can begin to identify and group them based on their ongoing interpretation. This process arguably allows for a detailed understanding of specific elements within the site, and documentation of on-site ‘primary’ interpretations may serve to inform later (post-excavation) interpretations regarding activities, structures, or cultural practices associated with those features.
This relates to an often debated area in archaeological practice, which pertains to the stage of the process in which grouping should occur. In many instances, (particularly again in rural sites) it can be beneficial to employ a level of basic grouping in the field where circumstances clearly support it. For instance, grouping could be utilised in cases where multiple slots belong to a single ditch. Nevertheless, it’s essential not to over-group at this stage. The primary phase of grouping is typically best left to the post-excavation stage, which allows for a more comprehensive understanding and analysis of the archaeological context and interrelationships.
On the other hand, a MoLA-style stratigraphic group typically emphasises the vertical relationships between stratigraphic layers and the conventional wisdom suggests that this process should be done off-site, post-excavation when the stratigraphic sequences are analysed (although in reality many excavators will begin to draft groups during the excavation on the ‘running’ or draft matrix). Stratigraphic groups are created by grouping together the layers that share similar characteristics or depositional history. This approach prioritises the understanding of the site’s chronological and spatial sequence and the broader patterns and transformations over time.
It is important to note that these two types of grouping are not mutually exclusive and can serve to inform each other. However, MoLA-style of grouping is apparently the more wide-spread approach and forms the basis of the approach outlined below.
Primary Grouping: Subgroups (or ‘Groups’)
Introduction
As noted above, sub-groups emerged as the primary level of a two tiered approach to stratigraphic grouping originating from the DUA/MoLA school of urban stratigraphic analysis. In this approach ‘subgroups’ are always allocated before ‘groups’, which form a higher level of grouping. However, there is some ambiguity in the academic literature about when and how subgrouping should take place. The traditional DUA approach advocated by Westman and Shepherd (1992) and Hammer (2002) advocates that subgrouping comes first; with subgroups being subsequently placed into groups (thus the analysis reflects the hierarchy of interpretation).
Guidelines for work by Headland Archaeology on the A14 Infrastructure project suggest that It may only be necessary to subgroup stratigraphic contexts where there is a “need to do detailed work to understand specific features. This will be based on when and how individual contexts were created, and may separate contexts relating to a feature’s construction, contexts relating to a feature’s use, and contexts relating to a feature’s disuse, in order to allow a more nuanced understanding of the history of individual features and the overall landscape. This level of subgrouping will only be carried out where there is the information to do so, and where this level of detail will aid interpretation and understanding of the site (it will not be necessary for all groups)” (MoLA Headland Infrastructure, 2020 p16).
Hammer (2002, Section 3.3) suggests then that a subgroup represents the lowest level of an interpreted assemblage of stratigraphic units or contexts. In this sense a subgroup is the lowest order of interpretive grouping that might be applied to the basic stratigraphic sequence diagram. Subgroups are sets of closely related contexts of human activities, which will later be linked into groups” and should “contain stratigraphically closely related contexts that form part of the same ‘process’ or” a related interpretive feature, “in other words, of the same processual ‘small phase’ event such as construction, use (of the surface), or disuse and/or abandonment” (ibid.).
Hammer’s approach is also reflected in the Archaeology South-East Post Excavation Manual, which explicitly suggests that the concept of sub-grouping aims to achieve several objectives:
- Helping to reduce the amount of data by organising contexts into units based on construction, use, and disuse
- Identify which contexts are being used to date a feature
- Allowing for the provisional dating of features through association when there is no direct dating evidence available (Swift 2011, 12).
Stratigraphic subgroups can also serve as a narrative framework in archaeological analysis, offering a structured interpretive lens through which the site development can be presented. Subgroups can be used to create individual text sections, each telling the story of a distinct activity. This allows for an articulate reconstruction of how various interpretative elements at the site came together over time, an approach that extends beyond primary physical categorisation of deposits and processes.
Methods
Swift (2011) advocates that (henceforth) subgrouping (or primary grouping) in stratigraphic analysis involves organising contexts into distinct units based on their construction, use, and disuse (or CUD; a higher order terminology originally developed by the DUA) phases. The process starts by considering the context matrix, even if there are isolated strands of matrix or no inter-cutting features on the site. This is in line with Hammer’s (2001) assertion that the objective of subgrouping is to understand the contexts within the processual cycle of activity and land use.

In principle any sequence of stratigraphic contexts can be subgrouped, using the above criteria. However, to avoid complication, stratigraphic grouping should follow these 3 simple principles (expanded in the general rule for groups below):
- Where subgroups are used all stratigraphic contexts can be subgrouped, however it may be that not all contexts need to be subgrouped. For example an isolated floor surface or a ditch cut may just stand alone as a feature in its own right (but note: this has an implication on the construction and layout of a derived group matrix and there is a school of thought that argues that this should not be allowed). Very simple sites or areas may not need grouping at all (since the process is about simplifying the stratigraphy, why add another layer of interpretation if it is not necessary?).
- A stratigraphic context can only be allocated to one stratigraphic subgroup; it is either in or out, there can be no sharing of contexts or else groups will overlap.
- Subgroups should effectively respect the stratigraphy of your area. The relationship between subgroups should obey the same rules as your basic stratigraphic matrix (above, below or the same as). In this respect the stratigraphic subgroup matrix is an abstraction of your underlying stratigraphic matrix.
It should be noted that whilst the criteria for allocating subgroup numbers are meant to be flexible, one should always be explicit about which contexts are subgrouped and why. Whatever else, be mindful of Rule 2 above, stratigraphic subgrouping will not work if it does not respect your stratigraphic sequence.
Mechanically the simplest way to assign subgroups by this logic is to quite literally draw boxes around associated contexts in the stratigraphic matrix, or code up an open area plan (on horizontal sites where a matrix may be less relevant). Like in the basic stratigraphic matrix, if the contexts within a subgroup are split stratigraphically (i.e. by later truncation of the sequence, or across different excavation areas or trenches), then two subgroup numbers should be allocated and equated.

Bottom-up or top-down?
Subgrouping in the stratigraphic sequence can be approached from the bottom-up or the top-down. The choice of approach will likely depend upon the specific site and the complexity of the sequence under analysis. The bottom-up approach involves starting from the bottom of the sequence and progressively subgrouping contexts based on their construction, use, and disuse phases (common in the DUA/MoLA approach). The numbering of subgroups should therefore follow the chronological development of the site, allowing for a systematic understanding of the site’s history and the relationships between different activities. From a narrative perspective this may be easier from the bottom-up (i.e.numbered earliest to latest).
According to Hammer (2001) it is important to begin by examining the longest strand in the stratigraphic matrix. It is common to encounter side branches, or ‘floating’ strands, that terminate beneath a sealing context known as the nodal point (often extensive contexts which seal a large number of earlier stratigraphic contexts, or support a large number of later ones). These may indicate significant stratigraphic events, marking the need for a new subgroup. This bottom-up “critical path” approach to stratigraphic analysis, is also advocated by Roskams (2002). It is, according to Hammer (2001) important to comprehensively understand and incorporate all side branches into one or more subgroups before proceeding further up the sequence.
This approach appears to be primarily practical in nature, however, advocates of a bottom-up approach to subgrouping would argue that starting from the bottom of the sequence is also crucial to account for potential slumping or subsiding of earlier pit fills (for example), which can impact the levels of higher layers. This approach helps in understanding the relationship between contexts and identifying potential subgroups.
On the other hand, the top-down approach begins with the overall understanding of context spatial organisation (in a GIS for example) and subgroups based upon a holistic understanding of the way contexts are distributed and the interpretation of their function. This approach provides a broader perspective on site development and facilitates the identification of key chronological markers across a large area, being particularly effective in rural contexts where the stratigraphy is more horizontally distributed. Crucially, in the approach there is a reduced need to work with a formal Harris Matrix (except perhaps in more complex areas of the site), rather the subgrouping process is done spatially directly on the mapped outputs or in an intrasite GIS.
What makes a subgroup?
There is a lot of debate about what actually should constitute a subgroup (see resources below), but the general consensus is that the subgroup is a small (or the smallest) unit of grouping representing a discrete archaeological feature composed of closely related contexts. These are often defined as organisational, or project specific vocabularies, but can generally be seen as falling into the broad Construction/Use/Disuse (CUD) convention. This variation of definition means that it is critical to be explicit about what makes a subgroup from the outset of the grouping process. However commonly they might include:
- Natural Geology: The primary and untouched geological state of the site.
- Levelling for Construction: Thick deposits with an even surface containing cultural material, such as charcoal and artefacts, indicating preparation for construction.
- Construction: Features like trenches or linear cuts containing walls, timber or clay sills, rows of posts or stakeholes.
- Pits: The cut and fills related to the original construction and use form one subgroup.
- Quarry Pits: The cut represents the primary usage and forms one subgroup.
- Masonry-Lined Wells/Pits: The construction elements form one subgroup.
- Walls: The construction cut, any foundation, wall, and construction backfill form a single construction/use subgroup.
- Floors and External Surfaces, Routeways: Construction make-up layers, the surface itself and occupation debris should form one subgroup representing construction and use.
- Postholes, Stakeholes: The cut and the fill used to date the feature should be one construction/use subgroup, unless there is evidence that the fill is later than the construction.
- Ditches, Gullies, Drains: The cut and the primary and possibly the secondary fills should form one construction/use subgroup.
- Use/Occupation: Indicated by thin, dark spreads, patchy layers or middens of mid/dark grey silts containing charcoal, pottery, bone, etc.
- Floors: This refers to constructed flat surfaces within a built structure, such as a building or settlement. They are typically made of compacted earth, stone, brick, or other materials, and their presence can indicate the internal layout of a past structure. Subgroups within this category may include different construction phases and periods of use or disuse based on the dating of the surface and the debris associated with it.
- Occupation Accumulation: This term refers to deposits that result from human activities within a site over a period of time. Such deposits can include waste from domestic activities, such as food preparation, tool-making, or crafting, and can offer valuable insights into the daily life and practices of past inhabitants. They are usually characterised by dark layers or patches of soil enriched with organic matter and various artefacts.
- Structured Deposits: These are intentional deposits of objects or materials placed in a specific manner or pattern, often for a specific purpose. The purpose can vary widely from practical, like waste disposal or storage, to symbolic or ritualistic. These deposits may include a wide variety of materials, including pottery, animal bones, human remains, or other artefacts.
- Pyrotechnic Installations: These are structures or features associated with the use of fire or high temperatures, such as hearths, ovens, kilns, or furnaces. These installations can provide information about various human activities including cooking, metalworking, pottery firing, or other forms of manufacturing.
- External Area: Characterised by gravel/sandy makeup and occupation debris, middens, with pits, posts, stakeholes, drain or gully.
- Road or Path: Linear gravelled area with compacted trodden surface, sometimes raised at the centre and with side ditches.
- Alteration within Structure: Modifications made within an existing structure, such as new floors, partition/enlargement walls.
- Waterfront Construction: Timber or stone structures along river revetting or reclamation deposits.
- Disuse/Destruction: Deposits sealing wall lines or slots, extensive fire debris above internal and external areas, indicating a period of disuse or the site’s destruction.
- Pits: Upper fills form another subgroup(s) if the dating deems them to be later than the primary / secondary fills, or they are obviously later events.
- Quarry Pits: The fills should form other use and/or disuse subgroups.
- Masonry-Lined Wells/Pits: Backfills should form separate subgroups as these represent the disuse, or secondary use of the feature.
- Walls: Repairs, additions, doorways added etc. are secondary construction/use subgroups.
- Floors and External Surfaces, Routeways: If there is a variance between the dating of the surface and the debris, then these should form separate construction/use and use/disuse subgroups.
- Postholes, Stakeholes: If there is evidence that the fill is later than the construction, it forms a separate subgroup.
- Ditches, Gullies, Drains: Upper (silting, slumped-in etc) fills potentially with later dating evidence should form other use/disuse subgroup(s).
- Abandonment: Minimal changes, erosion and thin accumulations of weathering, possibly indicating the site’s abandonment.
Assigning contexts to subgroups
Following is a workflow summary on how to assign stratigraphic context to subgroups:
- Subgroup Numbering: Assign numbers to subgroups in a sequential manner, based upon your organisational convention. On vertical stratigraphy (urban sites) begin at the bottom of the sequence and move up. On horizontal stratigraphy (rural sites) systematically work through the plans or GIS.
- Grouping: Group elements of construction, use, and disuse together on the context matrix, or matrix strands for non-intercutting features. Number these according to the previously explained rationale to form sub-groups. If you encounter any challenges during this process, seek guidance from post-excavation managers.
- Sub-group Description: As you form each subgroup, concurrently add the descriptions to the Land Use Register (if appropriate).
- Creation of Date-Phased Subgroup Matrix: If appropriate redraw the date-phased subgroup matrix or strands (dating may more usually be appropriate at the group level in two tiered grouping systems), or annotate the groups (by circling or otherwise highlighting the stratigraphic matrix. This could be done manually or using a relevant tool. Altering the relationships of the underlying matrix based upon dating would only occur in exceptional circumstances. This subgroup matrix should be created with reference to:
- The stratigraphic relationship of the archaeology: Arrange the matrix in ascending/descending order of time, with the latest features at the top and earlier ones below (as you would the underlying Harris Matrix).
- Spot dates from datable finds: Organise these in the same way as the stratigraphic relationship, with later features at the top, and earlier features below. Sometimes, the evidence from dateable material may require you to reverse the original decision of ‘what cut what’ made on site.
- The spatial locations of the archaeology: Ensure proximate features are near each other on the matrix to the left or right, where possible and according to cardinal points.
Some further points to note:
- Consideration of primary observations and spot dates in subgrouping: This is an interpretative process and as such it is important to carefully consider, during the grouping process, any primary observations (colour, texture, inclusions, etc.,) as well as other descriptive, metric and interpretive observations made by the excavator in the field.
- Debate over context inclusion in groups: There is some debate as to whether every context needs to be in a group (as outlined in DUA/MoLA guidance. This will depend upon your organisational or project post-excavation protocols and should be decided in advance of stratigraphic analysis.
Groups
Introduction
If you have two tiers of stratigraphic grouping in your organisation or project then ‘Groups’ form a higher level and therefore is a higher order interpretative process above the primary grouping (subgroups). The aim of groups in this case is to further simplify particularly complex (often deeper urban) stratigraphic sequences by grouping subgroups. This assists in the broader interpretation of an area. From the perspective of structuring data, groups serve as useful ‘tags’ or conceptual containers for referring to clusters of related stratigraphic subgroups (and associated contexts) in the synthesis of the site. If it is done properly, tiered grouping can help assemble your stratigraphy into ‘bite-sized chunks’, which are easier to digest and write about. As such stratigraphic groups (and any subgroups nested within them) can be used to feed into understanding land use across a site (if this is being deployed) aiding in the organisation of the stratigraphic narrative and reporting (often forming subsections of a technical stratigraphic summary and phased stratigraphic discussion). Also, because like subgroups, they respect the stratigraphic sequence they can be used to highlight which features might be compared (or ‘lumped’ together) by specialists in their own analysis. Once grouped the stratigraphic matrix can be reduced down into a group matrix diagram.
Methods
In a two-tiered grouping system where subgroups have been deployed, the subgroups can be associated to form higher order ‘Groups’. In this case, the already subgrouped matrix serves as the central reference point for this process. This matrix should be formulated during post-excavation work by considering the aspects of construction, use, and disuse. In cases where such a matrix has not been previously established, it’s essential to create one before proceeding.
The subgroup matrix should have been devised with reference to three main aspects: the stratigraphic relationship of the archaeology, (where possible) spot dates from dateable finds, and the spatial locations of the archaeology. In terms of stratigraphic relationships and spot dates, the group should also reflect the underlying stratigraphic matrix and be organised in descending order of time, with later features at the top and earlier ones below, and sometimes the decision of ‘what cut what’ made on site might need to be reversed based on dateable material evidence. Spatial locations should be arranged such that proximate features are near each other on the matrix, left or right, according to cardinal points, as far as possible.
When preparing for the grouping, it’s important to anticipate the major land use units on the site, as groups should not be split across separate land uses. One must also ensure that the specialists are happy with the spot-dating at the time of reporting (accepting that future work might change the date range of materials). The process of grouping involves reviewing plans and the subgroup matrix and commencing to group together features of a common type and date into numbered groups, taking care not to conflict with land use issues. For instance, pits on one side of a trackway should be grouped separately from those on the other if a functional distinction can be made between two open areas. Similarly, walls of the same date should be grouped separately based on their ultimate land use, (e.g. if they are part of different buildings, or part of a later building modification/phase).
The use of provisional phase plans can be beneficial at this stage, allowing the drawing of conjectural extensions of ditches and buildings, which can present possibilities for what is happening in some of the ‘negative’ spaces on the site. For example, potential routeways or boundaries may suggest themselves as linear negative spaces, while gaps or pockets of ‘negative’ space may hint at possible building plots or areas that were disused due to factors like water-logging or because they overlay an old cemetery.
Once groups are defined and circled or coded on the subgroup matrix or plan (depending on the verticality/horizontality of the stratigraphic sequence) it should be allocated a number in line with the recording protocol for the project and subgroups can be annotated with the relevant group number. Group descriptions should also be added at this stage, and a date-phased group matrix should then be drawn. When group numbers have been allocated for an area a group list should be compiled for reference, which includes the group number, a detailed description and the phase allocation; this list should be appended to your stratigraphic and group narrative.
The group stage is a good time to eliminate all remaining null contexts, such as voided contexts, modern intrusions, repeated contexts, natural deposits, and any others that will not fit into the eventual land use entities.
The groups thus created are essentially the elements that, either individually or in combination, will constitute the land use entities. While simpler sites may bypass the group stage and place groups of subgroups directly into land use entities, it is generally beneficial to complete the group and group matrix stage, especially for complex sites, as it encourages structured thinking. For sites with several areas or any complexity, drawing up a new and date-phased group matrix can be a very helpful tool for the next stage, which is the land use.
Open and Closed Groups
Open and closed stratigraphic groupings is a concept that can play a crucial role in projects where areas of an archaeological site are consistently revisited over time, such as in long-term research projects. This consistent revisiting can span multiple seasons or even years, allowing the stratigraphic records to be refined and reinterpreted as more information is unearthed and understood.
In such scenarios, open stratigraphic groups are sometimes utilised. This approach allows for a more dynamic and evolving interpretation of the site’s stratigraphy. The groups remain ‘open’ to accommodate new findings or reinterpretations over time, thereby facilitating a more fluid understanding of the site’s historical context as the excavation progresses.
Whilst phasing may (and probably will) change subject to further work, stratigraphic groups are supposed to represent a stable tool for stratigraphic and other specialist analysis. To this end it is also important that it is noted when a group is closed. Unlike Phases, groups can often be ‘signed off’ as complete if a large enough area is excavated, although this obviously cannot happen if any of the features within a group extend beyond any limits of excavation. A group is considered closed when all of the possible stratigraphic contexts tied to that group have been excavated and fully recorded. If there is anything left to do in the field for any of these contexts then the group must be left open (the most obvious example being when features extend under the limit of excavation).
However, it’s important to note that the notion of open and closed groupings is not commonly encountered in commercial archaeological contexts. This is typically due to the project-based and time-bound nature of commercial archaeology where areas of the site are not consistently revisited in the same manner as in academic research projects. Instead, in commercial contexts,whilst grouping may remain open to editing at any stage in the stratigraphic analysis prior to publication and archiving, each excavation usually results in a unique, standalone set of records that are interpreted and ultimately grouped without the presumption of revisiting the same area for further work.
What makes a Group (in a two-tiered stratigraphic grouping system)
The definition and interpretation of these groups would often depend on the specific characteristics and context of the archaeological site under investigation. It may also be influenced by organisational and project based vocabularies and protocols. However, following are examples of common entities which might form discrete (second tier) stratigraphic groups, which might be formed by grouping relevant archaeological subgroups,broad examples might include:
- Domestic Structures Group: This group could be formed by subgroups representing different rooms within a house or different components of a single room (e.g., hearth, floor, wall).
- Cemetery Group: This group might be composed of subgroups representing individual graves or burial clusters within a cemetery.
- Fortification Group: This group might include subgroups representing different components of a defensive structure, such as a wall, a gate, or a tower
- Ceremonial Structures Group: This group could include subgroups representing different parts of a temple, shrine, or other religious or ceremonial structures.
- Agricultural Activities Group: This group might be formed by subgroups representing evidence of farming activities such as fields, irrigation systems, or crop storage structures.
- Industrial Activities Group: This group would comprise subgroups representing different components of an industrial site, such as a pottery kiln, a metalworking shop, or a mill.
- Road and Pathways Group: This group could include subgroups representing different parts of a transportation network, such as roads, bridges, or paths.
- Waterfront Structures Group: This group could be formed by subgroups representing various features of a waterfront, such as docks, warehouses, or shipyards.
- Destruction Horizon Group: This group might include subgroups representing widespread destruction debris that will typically seal several earlier groups.
- Open Space Group: This group could be formed by subgroups representing unoccupied or less structured areas within a settlement, such as courtyards, middens or activity areas.
Assigning subgroups to groups
If subgroups have been deployed then at some point they will need to be assigned to a higher level ‘Group’. Following is a workflow summary on how to assign subgroups to second-tier stratigraphic groups:
- Follow the Stratigraphic Sequence: Start at the bottom of the sequence, following the longest strand on the stratigraphic diagram, and then integrate the shorter, ‘floating’ strands before they are sealed.
- Group Related Subgroups: Place closely related subgroups, such as those representing the construction and use of one building, into one stratigraphic group. Do not include contemporary open areas in the same group, to maintain separate records of different types of land use.
- Numbering Groups: Establishing a clear numbering system for your stratigraphic groups is crucial, starting from the base of the sequence and progressing upward. This method reflects the proposed chronological order and can vary based on project or organisational protocols.
- Create a Group Matrix (Sequence Diagram): At this stage, your group matrix should take shape from your existing matrix, which is either your stratigraphic matrix or your subgroup matrix (if you created subgroups). This will provide a clear overview of the stratigraphic sequence and the connections between different archaeological elements.
- Link Groups to Land Use Synthesis and Diagrams: At this point, consider how the groups can be linked to broader understandings of land use. Develop land use synthesis diagrams that incorporate these stratigraphic groups to visually represent patterns of land use over time.
- Illustrate Groups: As a final step, produce visual representations or illustrations of these groups (these could be plans or sections). These visual aids will help to communicate the spatial relationships, sequences, and overall narrative of the site.
Some further points to note:
- Dealing with structural alterations through time: If alterations have occurred over a considerable timespan, they form part of different stratigraphic groups, even if they relate to the same structure.
- Handling ‘floating’ subgroups: Compare stratigraphically floating subgroups with events on the longest strand of subgroups. Determine where they could fit in as a group based on their description, surface height, and interpretation. If no clear association can be made, it may be necessary to assign these floating subgroups to a separate group of their own, before the long strand of subgroups is sealed.
- Considering cut features and sealing horizons: Ensure that if a subgroup contains cut features, these features remain visible at the surface of a group. The sealing horizon should be part of the following group and reflect a different kind of land use, to clearly display structural and occupational patterns and assign features a lifespan.
- Handling large-scale debris and demolition: If there is widespread destruction debris that seals several earlier groups, this can form a separate group as part of a destruction horizon. However, if the debris is small and localised, it could be part of the previous group.
- Grouping across discrete trenches or areas: The extent to which grouping should be confined within one trench or extend across multiple trenches, needs to be considered and may relate to factors such as project’s duration, team size, or excavation scale. Notably, in the case of the latter, grouping may well extend across separate strands of matrix.
- Remaining fluid and open to changes: Keep the grouping process as fluid as possible to allow for future phases of post-excavation where finds, samples, and dendrochronology dates may be combined. Avoid ‘locking’ features together in groups that may be difficult to reanalyse later.
Current Good Practice
Some general rules for Stratigraphic Subgroups
- Must contain at least one Context/SU (ideally more than one).
- They must be allocated to a Group/Feature,
- They must be allocated to a Phase and Period
- They will inherit stratigraphic relationships of component parts (i.e. Contexts); see illustration
- Therefore a Subgroup will either be above, below, or have no relationship with any other Subgroup,
- And may be correlated as: equal to… ,or same as…, or contiguous with… any other Subgroup,
- The subgroup will inherit the stratigraphic relationship (i.e. “Before”) that exists between the context with the latest end date in the subgroup and the context with the earliest start date that is stratigraphically above the subgroup.
- The subgroup will inherit the stratigraphic relationship (i.e. “After”) that exists between the context with the earliest start date in the subgroup and the context with the latest end date stratigraphically below the subgroup.
- If the Context stratigraphically above or below the subgroup is a member of a Group or subgroup then the inherited stratigraphic relation will also be inherited by the associated subgroup.
- It will also have a hierarchical relationship with component parts and its associated Group/Feature;
- as such all ‘lower order’ components (i.e. Context/SU) must nest inside their associated Subgroup,
- whilst a subgroup must nest inside its ‘higher order’ Group/Feature.
- Buildings, Structure Numbers, Spaces and Room Numbers may all form special stratigraphic subgroupings base upon spatial characteristics
- All contexts in a Subgroup should (unless exceptional) be on the same “stratigraphic strand” (stratigraphic chain) of the matrix.
Some general rules for Stratigraphic Groups
- Must contain at least one Context/SU (ideally more)
- They may include one or more Subgroups,
- They must be allocated to a Phase and subsequently to a Period
- Will inherit stratigraphic relationships of component parts (i.e. Subgroup and/or Context’s); see illustration
- Therefore a Group/Feature will either be above, below, or have no relationship with any other Group/Feature,
- And may be correlated as: equal to… ,or same as…, or contiguous with… any other Group/Feature,
- The Group will inherit the stratigraphic relationship (i.e. “Before”) that exists between the context with the latest end date in the Group (or subgroup) and the context with the earliest start date that is stratigraphically above the Group (or subgroup).
- The Group will inherit the stratigraphic relationship (i.e. “After”) that exists between the context with the earliest start date in the Group (or subgroup) and the context with the latest end date stratigraphically below the Group (or subgroup).
- If the Context stratigraphically above or below the Group (or subgroup) is a member of a Group or subgroup then the inherited stratigraphic relation will also be inherited for the associated group (subgroup).
- It will also have a hierarchical relationship with component parts;
- as such all ‘lower order’ (i.e. Stratigraphic Unit/Contexts & Subgroups) components must nest inside their associated Group/Feature. (e.g. all contexts contained in subgroups that are within a group are therefore also within that group)
- Buildings, Structure Numbers, Space and Room Numbers all form special stratigraphic groupings based upon spatial characteristics. Note that some systems (e.g. MoLA) may only assign these sort of high-order groupings during later ‘land-use’ stages of stratigraphic analysis.
- All contexts in a group should (unless exceptional) be on the same “stratigraphic strand” (stratigraphic chain) of the matrix.
Some further points to note:
- Consideration of spot dates in grouping: When grouping complex stratigraphy it is considered important, wherever possible, to begin with spot dates from the relevant specialists and have applied dates ranges to the context matrices. However, note possible issues of residuality or intrusiveness that may occur (see the Dating and Periodisation module of this handbook).
- The role of visual references in grouping: Visual references may prove very useful in the grouping process. Specifically with regard to vertical stratigraphic sequences (which have a Harris Matrix). Hammer ( 2002, Section 3.3) suggests that it may be important before making any hard decisions such as numbering subgroups to start at the bottom of the longest strand sequence diagram and may be useful to colour-highlight contexts according to their description. This colour coding of the Harris Matrix on vertical sites, reflects a similar practice of colour coding the plans/GIS on horizontally oriented sites and indicates that such visual tools are very useful for understanding both spatio-temporal and spatio-functional groupings of contexts. In fact colour coding of stratigraphic sequence diagrams is quite common and might be seen as a useful analytical tool. It is common to colour phases and to colour-code or symbolise specific contexts in the sequence according to date. Furthermore, some organisations colour code by Construction/Use/Disuse (CUD).
- The impact of digital technologies on grouping: As noted elsewhere the architecture and data structure of digital recording systems, databases and GIS is increasingly likely to affect the way in which grouping occurs, and this may vary across different projects and organisations. The mechanisms for storing and querying tiered grouping (i.e. subgroups and groups) would need to be built into the data structure of any system at the design stage. Conversely the way in which a database structures its groups may impact the the manner in which groups need to be constructed and documented. So it is important to consider all the principles that underlie grouping (and phasing, and land use – which may also be impacted in the same way) when designing digital systems for stratigraphic data management and analysis.
Recommended Outputs
Subgroup Outputs:
Output Category | Description | Example Output Format |
Subgroup Plans (Analogue/Digital) | Plans showing the spatial relationships between subgroups, including structural elements and significant finds or features. | Analogue or digital maps (e.g. .jpg, .pdf, .dxf or .dwg) |
Subgroup Matrix (Sequence Diagram) (Analogue/Digital) | Diagrammatic representation of the stratigraphic sequence of subgroups. | Analogue/Digital matrices (e.g. .pdf, .docx, .xslx, .dwg or .json) |
Group Outputs:
Output Category | Description | Example Output Format |
Group Plans (Analogue/Digital) | Spatial representation of group, highlighting contemporary group horizons and main features. | Analogue or digital maps (e.g. .jpg, .pdf, or .dwg) |
Group Matrix (Sequence Diagram) (Analogue/Digital) | Diagrammatic representation of the stratigraphic sequence of subgroups within each group. | Analogue/Digital matrices (e.g. .pdf, .docx, .xslx, .dwg or .json) |
Comprehensive Outputs:
Output Category | Description | Example Output Format |
Grouping Index or Index of Archaeological Association (Digital) | Comprehensive table documenting key stratigraphic, grouping, and phasing relationships between contexts on an archaeological site. | Flat file tables (e.g. .csv, .xlsx, or .ods) |
Overall GIS Plan (Digital) | Comprehensive spatial representation of all stratigraphic groupings within the archaeological site. | Digital maps in GIS compatible formats including 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).
Resources



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/10.5284/1117193. CC BY 4.0 Oxford Archaeology (South) (2023) Data from an Archaeological Evaluation, Excavation and Watching Brief at Wallingford Police Station, Wallingford, Oxfordshire, 2013-2019. https://doi.org/10.5284/1116908. CC BY 4.0 Cotswold Archaeology (2023) Data from Archaeological Works at St. Sidwell’s Point Leisure Centre and Exeter Bus Station, Exeter, 2018-2019 https://doi.org/10.5284/1116182