Introduction: Excavation initially involves the removal of any topsoil overburden by machine. This material may be examined by metal detector for stray finds but unless the site has remained untouched since its abandonment there is invariably a layer of modern material on the surface of limited archaeological interest. In rural areas, any features are often visible beneath the surface as opposed to urban areas where there may be thick layers of human deposits and only the uppermost contexts will be initially visible and definable through isolation from other contexts.
A strategy for sampling the contexts and features is formulated which may involve total excavation of each feature or only portions. It is preferred goal of excavation to remove all archaeological deposits and features in the reverse order they were created and construct a Harris matrix as a chronological record or “sequence” of the site. This Harris matrix is used for interpretation and combining contexts into ever larger units of understanding. This stratigraphic removal of the site is crucial for understanding the chronology of events on site. It is perhaps easier to think of this as “archaeological deposits should leave the site in the reverse order they arrived”. A grid is usually set up, dividing the site into 5 m squares to better aid the positioning of the features and contexts on the overall site plan. This grid is usually tied into a national geomatic database such as the Ordnance Survey in the UK. In urban archaeology this grid becomes invaluable for implementing single context recording.
The single context recording system: Single context recording was developed in the 1970s by the museum of London and has become the de facto recording system in many parts of the world and is especially suited to the complexities of deep urban archaeology and the process of Stratification. Each excavated context is given a unique “context number” and is recorded by type on a context sheet and perhaps being drawn on a plan and/or a section. Depending on time constraints and importance contexts may also be photographed, but in this case a grouping of contexts and their associations are the purpose of the photography. Finds from each context are bagged and labelled with their context number and site code for later cross reference work carried out post excavation. The height above sea level of pertinent points on a context, such as the top and bottom of a wall are taken and added to plans sections and context sheets. Heights are recorded with a dumpy level or total station by relation to the site temporary benchmark (abbr. T.B.M). Samples of deposits from contexts are sometimes also taken, for later environmental analysis or for scientific dating.
Stratigraphic excavation in practice: Best practice of stratigraphic excavation in its basic sense involves a cyclical process of cleaning or “troweling back” the surface of the site and isolating contexts and edges which are definable in their entirety or part as either
1. Discreet discernible “edges” that form an enclosed area completely visible in plan and therefore stratigraphically later than the surrounding surface or
2. Discrete, discernible “edges” that are formed by being completely separated from the surrounding surface as in 1 and have boundaries dictated by the limit of excavation.
Following this preliminary process of defining the context, the context is then assessed in relation to the wider understanding of the site, for considerations of reduction of the site in phases, and then removed and recorded by various methods. Often, owing to practical considerations or error, the process of defining the edges of contexts is not followed and contexts are removed out of sequence and un-stratigraphically. This is called “digging out of phase”. It is not good practice. After removing a context or if practical a set of contexts such as the case would be for features, the “isolate and dig” procedure is repeated until no man made remains are left on site and the site is reduced to natural.
Physical methodology of excavation: The process of excavation is achieved in many ways depending on the nature of the deposits to be removed and time constraints. In the main, deposits are lifted by Trowel and Mattock and shovelled or carried from the site by wheel barrow and bucket. The use of many other tools including fine trowels such as the plaster’s leaf trowel and brushes of various grades are used on delicate items such as human bone and decayed timber. When removing material from the archaeological record some basic guidelines are often observed.
- Work from the known to the unknown. This means that, if one is unsure of the stratigraphic boundaries of the material in question, the removal of material should start from an area where the sequence is better understood rather than less.
- Work from the top to the bottom. As well as working from the known to the unknown, also as far as possible, remove material at the physically highest level in the context and work towards the lowest. This is best practice because loose spoil will not then fall onto and contaminate the surface being worked on. In this way blurring detail that might have been instructive to the excavator is avoided.
- In archaeology, we use our eyes. Excavation of contexts correctly often relies on detailed observations of minute differences.
- If in doubt, bash it out. This rather cavalier-sounding maxim is a concise way of expressing the need to progress. There is always more to be done on a site, than there is time in which to do it. At times the next feature or context to be removed in the sequence is not clear even to an experienced archaeologist. When it is not possible to proceed in an ideal manner, the excavation must be continued in a more arbitrary way, with temporary sections, until discernible stratigraphy is again encountered. An area of the site is reduced leaving arbitrary, temporary sections as a form of stratigraphic control to provide early warning of “digging out of phase”. If the arbitrary area for excavation is wisely chosen, the sequence should be revealed and excavation can return to a truly stratigraphic method. It is important to realise that “bash it out” is not a totally random act but a best guess based on logical deductions, observation and experience.
Common errors in excavation: Common errors during excavation fall into two basic categories and one or the other is almost inevitable because excavation is a destructive process that removes the information it seeks to record in real time and mistakes cannot be rectified easily.
1. Under-cutting. Under cutting occurs where contexts are not excavated fully and some remainder of the context is left in situ masking the nature of the underlying contexts. This is especially common among inexperienced archaeologists who have a tendency to be timid. The consequences of undercutting are quite serious as the nature of the archaeological sequence is obscured and subsequent recording and excavation is based on a flawed reading of the deposits on site. Unchecked, what follows from under-cutting is the production of false data often from the failure to spot intrusive finds and in turn, serious ramifications for the ability to interpret the sequence post excavation. Entire sites can be “thrown out of phase” where relationships recorded in the Harris matrix bear no genuine association with any understandable phase of occupation. If a regime of under-cutting is allowed to progress its effects multiply as the site is reduced.
2. Over-cutting. Over-cutting occurs when contexts are unintentionally removed along with material from other deposits and contexts. Heavy over-cutting represents reckless removal of the sequence. However some degree of over-cutting is almost impossible to avoid and is certainly preferable to unchecked under-cutting even though over-cutting represents a loss of information.
Over-cutting represents the loss of information whereas undercutting represents false information. One role of an archaeologist is to avoid false information and minimize the loss of information.
Finds and artefacts retrieval: Finds and artefacts that survive in the archaeological record are retrieved in the main by hand and observation as the context they survive in is excavated. Several other techniques are available depending on suitability and time constraints. Sieving and flotation is used to maximize the recovery of small items such as small shards of pottery or flint flakes. The use of sieving is more common on research based excavations where more time is available. Some success has been achieved with the use of cement mixers and bulk sieving. This method allows the quick removal of context by shovel and mattock yet allows for a high retrieval rate. Spoil is shovelled into cement mixers and water added to form a slurry which is then poured through a large screen mesh.
Flotation is a process of retrieval that works by passing spoil onto the surface of water and separating finds that float from the spoil which sinks, this is especially suited to the recovery of environmental data such as seeds and small bones. Not all finds retrieval is done during excavation and some especially floatation may take place post excavation from samples taken during excavation. One important role of finds retrieval during excavation is the role of specialists to provide Spot dating information on the contexts being removed from the archaeological record which can provide advance warning of potential discoveries to come by virtue of residual finds redeposited in contexts higher in the sequence which should be coming offsite earlier than contexts from early eras and phases. This spot dating also forms part of a confirmation process of assessing the validity working hypothesis on the phasing of site during excavation. For example the presence of an anomalous medieval pottery sherd in what was thought to be an Iron Age ditch feature could radically alter onsite thinking on the correct strategy for digging a site and save a lot of information being lost due to incorrect assumptions about the nature of the deposits which will be destroyed by the excavation process and in turn, limit the sites potential for revealing information for Post excavation specialists. Or anomalous information could show up errors in excavation such as “undercutting”. Dating methodology in part relies on accurate excavation and in this sense the two activities become interdependent.
Mechanical diggers: There is an increasing use of machine diggers especially in developer lead excavation due to time pressures. This is an area of controversy as their use inevitably results in less discrimination in how the archaeological sequence on a site is recorded. Machines are used primarily to remove modern overburden and for the control of spoil. In British archaeology mechanical diggers are sometimes nicknamed “the big yellow trowel”.
Organisation of workforce: A group of archaeological excavators will generally work for a supervisor who reports to the site director or project manager. He or she will have ultimate responsibility for interpreting the site and writing the final report. Most excavations are eventually published in professional journals although this process can take years. This process takes place Post excavation and evolves a myriad of other specialists. [GFDL Article and Copyright]