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The Waterlogged Deposits of Nantwich

Investigating the ‘urban wetlands’ of a historic town.

Four metres of archaeological deposits of national importance, including well-preserved salt-ships, barrels, timbers and wicker structures, as well as plant and animal remains, are known to lie beneath the ancient salt town of Nantwich, Cheshire.

Archaeologists recovering a part of a wooden salt ship made from an oak.
Lifting a salt-ship at 2 Wood Street, Nantwich, in 2004. © Mark Leah, Courtesy of Cheshire West and Chester Council

The background

Recent discoveries at Must Farm -see Historic England Research magazine Issue 3- have reignited public interest in the extraordinary potential wetlands have for the preservation of biodegradable archaeological material. Unknown to most, however, were the eight years of monitoring the preservation environment that took place prior to excavation (Malim et al 2015a).

Must Farm is in a rural location on the edge of the Fens; yet comparable environments can exist in urban settings where the underlying conditions are waterlogged. Such environments allow the growth of organic-rich silty deposits which then act like a sponge, sucking up and retaining moisture, thereby excluding oxygen, and as a result preventing normal types of decay.

Such sites may also face threats that rural locations do not: extensive use of hard surfaces and drains, which remove rainfall that would otherwise percolate into the ground; and damage from building works, especially when basements and deep-piled foundations are included. At Nantwich local planning decisions depended largely on anecdotal evidence, and detailed knowledge was urgently needed if the nature and extent of the waterlogged deposits were to be understood before it became too late to protect them. There are more than 10ha of these deposits below the town.

Cross-section diagram showing Nantwich deposit sequence.
Cross-section showing Nantwich deposit sequence. © Caroline Malim

In 2007, SLR Consulting Ltd was appointed by English Heritage and Cheshire Council to undertake a desktop study, which would draw together current knowledge, and to model  sub-surface hydrological flow-paths. The multi-disciplinary team included SLR Consulting’s archaeologists, geological engineers, hydrogeologists and GIS analysts, supported by experts from Palaeoecology Research Services and by York Archaeological Trust’s Conservation Scientist.

30  boreholes were drilled to map the extent of the waterlogged zone and characterize the nature of the deposits. The sediment core samples were then recorded using the Norwegian Protocols (Riksantikvaren 2008), and sampled for geochemical analysis. Radiocarbon dating (from samples of plant remains and gasses generated by chemical change within the deposits) established that waterlogging started in Anglo-Saxon times, with major growth in the 10th to 14th centuries. Drains and impermeable surfaces added in the Victorian and modern periods have, however, meant that the deposits have been drying out for 150 years.

Borehole drilling using a windowless sampler rig
Drilling in 2007 using a windowless sampler rig, steel casing and Perspex tubes. © Tim Malim

The reason for the good preservation conditions is the saturation of the terrace sands on which the town is built, and the fact that they overlie impermeable glacial till. As a result the sands have become a water-retentive matrix above which organic and inorganic debris from human activity has accumulated.

In some locations capillary action has sucked water up into a zone above the water table; here, the sediment retains water in its pores, and again the result is conducive to the preservation of organic remains. The lack of decay in the organic remains was due to such anaerobic conditions, and was especially marked at increasing depth, indicated by a decline in sulphate concentration and increases in sulphide.

Phosphate concentration, a potential nutrient source for the kind of microbial activity which causes archaeological deposits to rot, also declined (Malim and Panter 2012).

It is critical to establish the ratios of various chemicals in such environments if their preservation potential is to be understood: particularly important here are the ratios of oxidised to reduced species (a relationship known as redox). Overall the conditions at Nantwich were generally favourable for the survival of organic remains.

These studies established baseline conditions for the waterlogged deposits. It was thus possible to move onto a second phase of the project: a five-year programme of monitoring, which took place from 2011‒15.

Gas sampling equipment used to measure C14
Gas sampling from one of the dipwells, using a GA 5000 analyser and Tedlar bags for the C14 gas sample. © Tim Malim

A five-year monitoring programme

This was a pioneering study. Conditions in Nantwich were monitored for over five years, providing scientifically robust data on how preservation conditions within the urban waterlogged deposits beneath the town changed over time.

As part of this programme, 18 groundwater dipwells have been monitored every three months and sampled annually (Malim et al 2015b), while rainfall was recorded daily. Water quality was assessed for changes in dissolved oxygen, conductivity, pH, temperature and redox potential. Gas meter readings were also taken quarterly, staggered with the groundwater testing.

Groundwater levels were measured using an audible dipmeter, whilst water quality was assessed by inserting a digital water meter into the dipwell. In addition the water level was measured automatically on a daily basis at six key locations, so as to provide more detailed data for comparison with the quarterly monitoring. Groundwater samples were taken annually so that they could be tested in a laboratory for what levels of specific chemicals were present. This provided comparative data and a good control, helping provide confidence in the quarterly results of groundwater sampling.

Monitoring and analysis of the results of these investigations help to decide whether the conditions are aerobic, in which case archaeological remains could be degrading rapidly; or whether they are anaerobic, in which case there will be little microbial activity and degradation will be slow.

map of two waterlogged archaeology preservation zones in Nantwich
Two zones of preservation were identified within the historic core of Nantwich. © Caroline Malim

Conclusions

This Historic England-funded study is a unique attempt to systematically characterize a specific urban environment in which organic archaeological remains have been well-preserved. In Nantwich, two zones of preservation were found: well-preserved organic remains, in areas bordering the river; and more variable preservation, with some active decay, in a higher part of the town.

The first zone is a pH-neutral environment with high sulphide and low nitrate content. It is thus conducive to preservation of organic remains such as wooden artefacts and plant material.

The second zone was once also waterlogged, and there is grave concern that the burial environment there is drying out more quickly as a result of modern changes in the town centre (Malim et al 2016). Historic England has recently drafted guidance on the characterization and monitoring of urban waterlogged deposits (Historic England 2016).

In the current socio-economic climate it will be difficult to establish an effective way of protecting these deposits. The best approach is to change behaviour so that future infrastructure, public realm and building projects in the town are designed in such a way as to encourage re-watering of the deposits.

The aim is to raise awareness of the issue among decision-makers in the local authority (including spatial planners and engineers), whilst also educating developers in the importance of the archaeological resource and its sensitivity to intrusive works.

Standing buildings are threatened if the drying-out of waterlogged deposits results in subsidence, a factor that might ultimately be more persuasive than concern for the buried archaeology itself.

Diagram of methods for rainwater capture and recharge in waterlogged deposits.
Diagram of methods for rainwater capture and recharge in waterlogged deposits. © de Beer and Seither 2015; reproduced by permission of Hans de Beer and Anna Seither

The implications of this research are of value far beyond the Nantwich Supplementary Planning Document (Malim 2016), which has been produced to guide future development in Nantwich, and which has been included as part of the evidence base for Cheshire East Council’s emerging local plan.

For example, the issues of ground stability, water management and sustainable development raised by the situation in Nantwich are equally applicable to all urban centres with comparable environments (these are generally those with poor drainage and that are prone to episodic flooding).

The success of the Nantwich project in characterizing conditions beneath historic towns makes it a valuable comparator for similar projects in Norway, the Netherlands and other European countries. Indeed, considerable amounts of information and advice have been exchanged at international conferences between these various projects.

However the work has also identified the difficulties involved in producing a coherent understanding of all the complex issues that help to preserve, or threaten, buried remains. Equally challenging is the problem of how to influence decisions at a sufficiently strategic level to provide effective long-term management.

Acknowledgements

Many thanks to Jennie Stopford and Sue Stallibrass (English Heritage/Historic England), and to Dr Jill Collens and Mark Leah (Cheshire Archaeology Planning Advisory Service Cheshire Shared Services), who masterminded the project, steering it throughout Phase 1 (2007–10) and during the five years of monitoring (2011–15). Thanks are also due to all those colleagues who contributed to the project’s success, especially John Carrott, Caroline Malim, Ian Panter and Mark Swain.

 

Timber framed buildings in Nantwich High Street
Nantwich High Street: timber-frame built heritage in the conservation area. © Tim Malim
Tim Malim

Author

Tim Malim FSA, MCIfA is Technical Director at SLR Consulting Ltd, a multidisciplinary environmental planning consultancy based in Shrewsbury. He graduated from the Institute of Archaeology in 1980 and has worked extensively throughout the UK and abroad. He was part of English Heritage’s Fenland Survey in the 1980s, formed and directed Cambridgeshire County Council’s Field Unit during the 1990s, became a consultant in 2002, and is chair of the Federation of Archaeological Managers and Employers.

Further reading

De Beer, H and Seither, A 2015 ‘Groundwater balance’, in Rytter, J and Schonhowd, I (eds) Monitoring, Mitigation, Management: The Groundwater Project – Safeguarding the World Heritage Site of Bryggen in Bergen. Oslo: Riksantikvaren

Historic England 2016 Preserving Archaeological Remains. Swindon: Historic England, 7–10

Malim, T and Panter, I 2012 ‘Is preservation in situ an unacceptable option for development control? Can monitoring prove the continued preservation of waterlogged deposits?’. Conservation and Management of Archaeological Sites 14 (1–4), 429–41.

Malim, T Morgan, D and Panter, I 2015a ‘Suspended preservation: particular preservation conditions within the Must Farm-Flag Fen Bronze Age landscape’. Quaternary International 368, 19‒30.

 Malim, T Panter, I and Swain M 2015b ‘The hidden heritage at Nantwich and York: groundwater and the urban cultural sequence’. Quaternary International 368, 5‒18.

 Malim, T Swain, M and Panter, I 2016 ‘Monitoring and management options in the preservation of urban waterlogged deposits, Nantwich, UK’. Conservation and Management of Archaeological Sites 18 (1–3), 139–55.

Malim, T 2016, Management Strategy for the Archaeological Deposits of Nantwich, available from Cheshire Archaeology.

Riksantikvaren and Norsk Institutt for Kulturminneforskning 2008 The Monitoring Manual: Procedures and Guidelines for Monitoring, Recording and Preservation Management of Urban Archaeological Deposits. Oslo: Riksantikvaren

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