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Conclusions

As a result of the archaeological observations made at the Marks & Spencer site, 44-45 Parliament Street, York, a programme of deposit characterisation was undertaken by the EAU and of environmental monitoring by HTS for the period June 1995 to April 1998. As a result, the data from a total of 30 site visits have been collected and are presented in this report.

Information on the moisture content of in situ archaeological deposits (to a depth of 3.8m) and the depth and quality of a perched water table below the pavement of Parliament Street, and the moisture content and quality of water surrounding deposits below the Marks & Spencer store is available. The monitoring data have been obtained using a combination of an electrical dip-meter (water level), neutron probe (moisture content), portable dip-probes (water quality or chemistry) and moisture cells (moisture content and deposit temperature).

Following the initial settling period during which the monitoring devices established an equilibrium with the in situ archaeological deposits surrounding them, the monitoring dat can be summarised as:

• the perched water-table located within the dipwell in Parliament Street appears to be caused by water being prevented from draining down through the relatively impermeable clay soil deposits that underlie the highly organic archaeological deposits. The water-table is static. There is evidence of possible contamination from the nearby public toilets, also located in Parliament Street, or sewer pipes. This may require further investigation
• within the pavement area of Parliament Street the near-surface deposits are wet, but they experience moisture content changes which may be the result of seasonal weather changes (Tubes 1 and 3). At depth, however, the higher moisture content of the very wet, possibly saturated, deposits appear to be stable during the period for which data are available
• water samples recovered from the dipwell and suction samplers possess the characteristic smell of hydrogen sulphide, which is a good indicator of anaerobic burial conditions. The water chemistry data support this view because depressed levels of dissolved oxygen (mg/l and %sat) and low or negative redox potential values are reported
• data from the moisture cells indicate that, prior to the opening of the new store, the cells were being influenced by seasonal air temperature changes. This influence was lost once the store was open, and the data until recently indicate stability within the archaeological deposits. Slight increases in resistance of three of the cells is noted but further data are necessary to determine if this is significant. The moisture cells may have detected the impact on the ground from the construction of foundations for the new building on 44-45 Parliament Street; however, the results are not conclusive
• predominantly low and stable burial temperatures are recorded by the moisture cells and water samples recovered from the suction samplers
• the data obtained from both the suction samplers and moisture cells have not revealed any clearly demonstrable change within the deposits
• the various water samples analysed for electrical conductivity constantly record high values, which is consistent with the soil analysis data conducted following installation of the monitoring devices. The relatively high salt concentrations, and generally aggressive ground conditions, are thought to be responsible for the progressive failure of thermistors that are built into the moisture cells

The data presented in this article suggest, therefore, that the burial environment under the Marks & Spencer shopfloor and of the lower deposits in Parliament Street are stable, but the upper deposits in Parliament Street show considerable seasonal variations. The work undertaken by HTS and the EAU in characterising the state of preservation of the deposits has provided valuable baseline data for future studies on this site. There now exists a precise record of the deposits as they were in 1995. The EAU work has also suggested a methodology for analysing and documenting the state of preservation of the biota. In this respect this project has been a success. It has also established a robust set of methodologies for archaeological deposit monitoring in an urban environment.

However, the main problem with interpreting the data is the lack of any comparable data detailing the ground conditions for a similar period prior to the 1995/96 Marks & Spencer construction works or within Parliament Street. The project has established that the deposits which were observed in the upper 2m of the evaluation trench had undergone a period of rapid oxidation. However, it has not been possible to demonstrate the condition of the deposits before these changes either started or accelerated. It is at present unlikely that a comparable set of samples will be recovered from underneath the Marks & Spencer shopfloor. It is probable that it will only be possible to recover a comparable set of samples from Parliament Street for analysis in the future. Conclusions about the effect changes in ground conditions will have had on the preservational characteristics of the deposits will only be valid for the sub-street deposits. In this respect the project has been less successful.

The Marks & Spencer Deposit Monitoring Project was conceived as a response to a set of observations about the character of the deposits in the evaluation trench. The data refer to a single site. In order to draw wider conclusions, a range of sites need to be approached in this way. The project has been funded by the developer, and this context has meant that the project developed in a reactive manner. It has, however, demonstrated that future research projects of this type need to be conceived and executed in a proactive fashion.

A site or set of sites must be selected well in advance of any development threat and the condition of the deposits precisely defined. Monitoring must be undertaken for a significant period of time prior to construction works starting. Monitoring must be continuous throughout any construction works. It must continue for a significant period after construction works finish. There must be the capability built into the project of sample collection at defined interludes over an extended period of time. This will allow the collection of comparable sets of samples and facilitate the characterisation of the deposits so that any changes in the preservation of archaeological artefacts and ecofacts can be determined.

There is an urgent requirement for laboratory-based analogue studies of the effect of changes in the chemical, biological and physical aspects of the burial environment on the preservation of archaeological artefacts and ecofacts. This needs to be allied with detailed computer modelling and simulation of these complex environments. These field and laboratory requirements mean that such projects will be beyond the scope of developer-funding except in unusual and exceptional circumstances. Instead, it must be the role and responsibility of grant-awarding bodies and governmental organisations.

Preservation in situ?

The legislative and planning context for the protection and investigation of archaeological deposits is provided principally by the 1979 Ancient Monuments and Archaeological Areas Act and Planning Policy Guidance 16: Archaeology and Planning (Department of the Environment 1990). The concept of preservation of archaeological deposits of national importance is embedded within these two documents. It is vital, therefore, that when an archaeological curator offers advice to a planning authority (for planning applications) or to the Secretary of State (for Scheduled Monument Consent applications) they understand the total impact that the application will have on the archaeological deposits on a site.

It will be obvious from the previous section that at present it is very difficult to arrive at such a total assessment. Indeed, it may be argued that the concept of preservation in situ is itself fundamentally flawed. The concept arises in part from the assumption that buried archaeological deposits all stay in the same state of preservation if they are not disturbed and in part from the assumption that visible archaeological deposits (castles, henges, earthworks) can be physically preserved.

Current guidance and legislation does not define what is meant by preservation. Does preservation mean that one will accept no adverse change in the archaeological deposits? Does it mean that one will accept no change in the current rate of decay of the archaeological deposits? Or does it mean that we will be happy if 95% of the deposits are still in their current state of preservation in 50 years time, or 100 hundred years time? What do we mean when we say that we will preserve these deposits in situ?

The concept of in situ preservation also owes a great deal to the work pioneered by the Society for the Protection of Ancient Buildings (SPAB) with regard to the built environment.The SPAB have developed a firm set of principles about how old buildings should be repaired and have developed practical knowledge to show how these can be put into effect. The majority of repairs to buildings or monuments of national importance (scheduled ancient monuments and Grade I listed buildings) are undertaken in accordance with these principles. This process of in situ preservation, however, consists of the replacement of decayed or unsound material with the same materials: lime mortar for lime mortar; lime washes for lime washes; timber for timber. This is a process of active conservation. It has led to the successful preservation and restoration of structures and buildings. However, this is not a process that can be applied easily to the buried environment. If we do not fully understand the burial environment responsible for the preservation of archaeological deposits, and we do not have a clear definition of preservation in situ, is it possible to engage in a process of active conservation?

At the moment curators, planners, legislators, and archaeologists are engaged in a process of passive conservation. This is achieved through the acceptance of minimal disturbance to archaeological deposits. In York this is defined as the destruction of no more than 5% of the archaeological deposits on a site in order to allow development to take place (see http://www.york.gov.uk/environment/york/localplan/chapter4/04histen-HE10.html; Arup 1991, 40-57). This approach is based on an engineering and archaeological asssessment of what, given the archaeological and engineering qualities of the deposits in York, would be a reasonable compromise between archaeological preservation and new development.

The data from the Marks & Spencer Deposit Monitoring Project have given an indication that for deeply buried deposits this approach may be sustainable. However, it has also shown that the immediate sub-surface deposits (<2m deep) have highly dynamic characteristics and it was in these deposits that the initial observations of decay which triggered this project were made.

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