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 Primary Grouping.
- 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 Contexts); see Primary Grouping.
- 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.