Welcome to the Urban Archaeology blog. Freelance archaeologist Chiz Harward provides a range of on and offsite services to the archaeological profession, including running and working on excavations, post-excavation services, training and development work, and illustration work. This weblog will carry news of projects as and when they happen as well as wider thoughts on archaeological issues, especially recording, stratigraphy and training.



Post-excavation -going beyond the MoLAS Site Manual



Post-excavation work is well under way on the 100 Minories excavation dataset. The last of the finds are all being washed and the samples processed ready to go off to the specialists to prepare their reports, however I've been busy 'sub-grouping' the contexts, which is a job that involves being in the warm and dry. Perfect for this time of year.
The MoLAS Archaeological Site Manual -still the definitive work for on-site recording, but what do you do with the records?

We excavated and recorded several thousand individual contexts on the 100 Minories site, each context a separate 'event' in the archaeological sequence, each with a set of records and each potentially with associated artefacts and environmental material. Understanding all these thousands of contexts is a daunting task, so we aggregate the contexts into a series of larger groupings, working from context level through 'Sub-groups', 'Groups' and finally to the level of landscape 'Land-use' units like Buildings, Open Areas and Waterfronts. 
This process provides the structure behind a wider methodology that leads you through the post-excavation process, creating larger and larger units, working up through higher levels of understanding and interpretation, and eventually providing the basic structure for the final publication text. The system was developed at the Museum of London's Department of Urban Archaeology, and is designed to fit with the Single Context Recording system described in their manual -although it can be used for almost any logical recording system.

Subgrouping is the first step in this process, and is where we aggregate individual contexts into larger groupings (subgroups) based on their stratigraphic position and interpretation, working with the context matrix. Typically there may be 2 or 3 related contexts in a subgroup, but sometimes there is only one, and sometimes a lot more. Each subgroup consists of a context or contexts which are immediately related to each other and of the same date, so a subgroup might be the construction cut for a wall, the wall itself, and the construction backfill; a mortar bedding layer and its brick floor; or a row of posthole cuts. However the disuse backfill of a cellar cannot be in the same sub-group as the cellar wall as it is obviously not contemporary, this can make things tricky as generally the dating and other specialist data is not yet ready, so sub-groups are often kept small so we don't have to dismantle them later on.

The subgrouping process is carried out using a combination of tools and resources, at the centre is LP Archaeology's Ark browser based database: this contains all the context level information, as well as scanned copies of the context sheets, and is where we enter the new sub-group details and descriptions. Alongside this we use a QGIS project containing all the spatial data from the site -all the digitised plans, trench edges, section lines and other spatial elements; during sub-grouping we check all this spatial data and use the QGIS project to interrogate the contexts spatially. Finally there is the digital copy of the context level Harris Matrix, which is annotated with the subgroups as they are created, and a sub-group level matrix is then created.

LP Archaeology's ArkTools suite of plug-ins for QGIS allows querying, digitising, editing and manipulation of the dataset, including the site Harris matrix

The team at LP Archaeology have been busy building new Open Source tools to help in this process, building a set of tools to rapidly digitise the context plans and sections into our QGIS project, but also tools to check those drawings, and explore the spatial relationships between them and link to the database which contains bespoke forms for subgrouping allowing rapid inputting and querying. 

Alongside this work we are writing a work-flow based guide describing the sequence of tasks within the post-excavation process, using these new LP Archaeology QGIS tools and the Ark database, to guide users through the various interlinked tasks within a post-excavation programme. When completed this will cover post-excavation from on-site checking and data entry through the off-site processes and initial writing tasks; it will pick up where the MoLAS 3rd edition Archaeological Site Manual stops, and will provide a clear structure for post-excavation work on Single Context Recording sites.