The long-term hydrological monitoring of relict landscapes at the Willingham Gravel Quarry, Cambridgeshire

C.A.I French. Department of Archaeology, University of Cambridge.

M.J. Davis. Hunting Land and Environment, Willingham.

The site of the Willingham gravel quarry lies close to the Cambridgeshire villages of Willingham, Over and Bluntisham on 350 hectares of alluviated river and fen-edge gravels centred on NGR TL 390 725. The site is delimited by the River Great Ouse to the west and north-west, by the artificial Old West River to the northeast and by river terraces to the south (See location map showing cropmarks/known archaeology. 73,Kbyte JPG).

The area was identified as one containing significant and exploitable gravel deposits in the county minerals plan and was therefore seen by archaeologists as an area under threat of destruction. A small scale archaeological evaluation, undertaken in the late 1980s, revealed the existence of a complex stratigraphy of alluvium, peat and freshwater marls with archaeological features and artefacts of Neolithic and early Bronze Age date. The water table was unusually high offering the possibility of the survival of waterlogged deposits, additionally protected by the alluvial silty clays. Across the area of the quarry as a whole five groups of archaeological features were recognised:

• a group of eight barrows, the southern (or Ouse Fen) group

• a group of six barrows, the northern (or Crane's Fen) group

• a cluster of multi-period artefact scatter sites dominated by Mesolithic and Iron Age material, although including some Neolithic, Bronze Age and Roman material

• parts of an extensive system of cropmarks occupying the southern and southeastern sectors of the application area

• Neolithic or Early Bronze Age settlement features on the Langdridge Spit in the central part of the application area

The whole area of the proposed quarry was under arable cultivation, but the considerable depth of alluvial silty clays had protected the northern two-thirds of the site from plough damage. Elsewhere plough damage had affected the crop mark complex and the tops of a number of the barrows were beginning to appear as the upper layers of the soil were removed by ploughing and erosion.

The Willingham quarry site offered the opportunity both to investigate the archaeology and to institute a scheme to monitor the changes in the ground water regime and the level of the water table as the gravel extraction proceeded. The immense significance of waterlogged sites for enhancing our knowledge of prehistoric society has been widely recognised in archaeology, both through the excavations of sites such as Flag Fen and Etton in Cambridgeshire and through studies of paleoenvironmental data from smaller scale projects. There have been few opportunities for the long-term monitoring of archaeological sites and landscapes during periods of fluctuations in the water table, whether caused by natural processes or the large scale environmental disruption resulting from mining. Limited scale projects associated with the Rose Theatre in London, the Iron Age and Romano-British site at Market Deeping in Lincolnshire together with more extensive European projects focusing on river marginal wetlands have highlighted the potential of such schemes to produce data of great significance in the planning and management of archaeological and paleoenvironmental landscapes. To date none of these projects have been able to examine water table changes within a buried relict and waterlogged archaeological landscape, beginning with the 'pristine' agricultural landscape and progressing through phases of quarrying and land reinstatement. Moreover none have yet been able to monitor the changing responses of archaeological contexts and materials to variations in the soil water and internal chemical systems of soils and sediments during such a process. The Willingham site offered an ideal opportunity to initiate a programme of monitoring which would cover the periods before, during and after gravel extraction, a process which might extend over a period of up to 25 years. The excavation of the waterlogged archaeological landscape during the progress of the quarrying operation, in tandem with the hydrological monitoring, presented an ideal opportunity to gather information of great value to the future management of archaeological resources in the fenland and fen edge regions.

The hydrological monitoring programme was designed with four principal aims:

1) The investigation of the preservation environment within the paleosol and the upper part of the gravel subsoil associated with a buried relict archaeological landscape in three states:

• buried and/or waterlogged conditions prior to ground intervention
• during ground intervention by archaeological excavation and mineral extraction
• after mineral extraction and/or land restoration

2) Investigation of the processes of change within the overburden / buried soil / subsoil / soil / water system

3) Identification of processes of decay and alteration of the archaeological record

4) Production of methodological guidelines for application to other sites with particular reference to the future management of buried and / or waterlogged archaeological sites and landscapes.

In addition the project will provide detailed data for comparison with the hydrological monitoring schemes at Market Deeping and the Rose Theatre. This will allow for the evaluation of the schemes and the redesign of specific components to take advantage of new knowledge and new problems highlighted by each of the projects.

The threatened area contains four discrete archaeological components which require monitoring:

• the southern barrow group
• the northern barrow group
• the relict river valley
• the cropmark / alluviated river valley interface zone

The initial stage of monitoring is intended to take place over a period of three years, after which a review and reassessment will take place. The project will be considered in the light of the information obtained from the Market Deeping scheme with the option of continuing the monitoring of the Willingham area for a further three years.

Three simultaneous phases and scales of hydrological monitoring work are suggested, taking into account the intended direction and scale of quarrying and the location of the areas of particular archaeological concern and the relict river valley system.

Phase 1 The southern barrow group

A pattern of five monitoring access tubes will be placed in a cross arrangement over the barrow group. Together with three boreholes and the possible reactivation of three of the quarry company's own boreholes these will provide a spread of monitoring points across the affected zone. The work will be complemented by the geophysical prospection of the whole of the southern barrow group and the excavation of an outlying barrow to the east of the main group. This work will take place during the removal of topsoil over the first take area and its storage to the south and southeast.

Phase 2 The alluviated relict river valley

Concurrently with phase 1, the second phase will monitor changes downstream of the southern barrow group within the relict river valley and to the south of the northern barrow group. Three monitoring access tubes will be placed straddling the barrows from north to south together with two new and two reactivated boreholes. Two further access tubes and two reactivated boreholes will be used to examine any changes to the land between the northern and southern barrow groups.

Phase 3 The cropmark zone and terrace/valley interface

While archaeological investigation of the cropmark zone is under way the hydrological monitoring of the dryland terrace / alluviated valley interface will be initiated. This will take place in the first year of quarrying prior to the construction of topsoil storage areas on the northern edge of the cropmark zone. The position of the cropmarks, and, most importantly, the points at which the linear features vanish beneath the alluvium, has been accurately plotted from aerial photographs. In order to understand the differences in preservation conditions existing within archaeological features on the alluviated terrace areas and the unalluviated / cropmark terrace areas four monitoring access points will be placed across this boundary. The modern hydrological data will be compared with the states of preservation found within the cropmark ditch systems during their excavation. This will allow the future prediction of the critical threshold at which waterlogging may occur within a cropmark/alluvial zone and will contribute to better targeted archaeological work during the quarrying programme.

Phase 3 will require four temporary hydrological monitoring positions combined with a reactivated borehole. These monitoring points, when combined with those placed as part of phase 2, will create a transect running north to south across the relict valley system and extraction area. Two of the access points will be placed flanking the soil storage areas with the intention of monitoring the localised effects of the dumping of topsoil on the soil water system.

The analytical methodology will focus on seven principal areas of interest:

1. The long term study of soil composition change. This will involve the analysis of the structure, components and chemical composition / alteration of the buried soil, peat and alluvial overburden using soil micromorphology, physical and chemical analyses. Samples will be taken along a transect at 100m intervals across and along the relict river valley.
2. The stratigraphic sequence and the history of soil formation. This will be based on field descriptions of the soil and on the micromorphological analysis of samples from on and off-site locations
3. The complementary geophysical prospection across the barrow groups using a variety of techniques which will include thermal image mapping and resistivity profiling together with magnetometer and resistivity surveys of the individual barrows.
4. The study of the sequence of environmental change associated with the developing archaeological landscape using palynological and hydrological data.
5. The long-term hydrological analysis involving the systematic monitoring of soil and subsoil moisture content using the neutron probe system, the groundwater table using conventional boreholes, pH, temperature, conductivity / redox potential and dissolved oxygen content.
6. The collection of long-term environmental and weather data. This will be based on information supplied by an appropriate meteorological monitoring station together with yearly land use and vegetation data.
7. The assessment of the conservation implications for the archaeological material recovered. This will involve the study of a sample of the artefact assemblage with particular attention being paid to the pottery and wood.

Results to Date

A series of 16 access tubes for monitoring with a neutron probe have been placed around and within two Bronze Age barrow groups, with two transects of monitoring points linking both barrow groups across the alluviated gravel terrace landscape. The intention is to monitor soil and groundwater changes throughout the buried soil and alluvial overburden profile, before, during and after archaeological disturbance and long-term gravel extraction.

Examples of Water Content Graphs

Water content graph for the access tube within the southern barrow group

Water content graph for the access tube within the I.20 barrow ditch

Particular attention will be paid to changes in soil structure, using micromorphological techniques, and the organic component through time. The overall aim is to provide base- line reference data on the effects of dewatering on the buried archaeological evidence, including palaeosols, and thereby provide data on how best to conserve such landscapes from the destruction of their archaeological resource.

Some of the main inferences drawn from the results to date are:

1. The alluvial overburden appears to be acting as an anaerobic 'cap' to this landscape if it is greater than about 60-80cm in thickness. The silty clay alluvial overburden is probably more important in contributing to good organic preservation than the absolute height of the groundwater table.

2. Crucial to the effectiveness of the alluvial 'cap' with respect to organic preservation is the rapidity of initial burial by river-borne flood-silts and clays.

3. Soil/sediment composition and structure may be just as important as the maintenance of high groundwater tables in preventing the ingress of oxygen and the onset of microbial decay on the organic record.

4. Even in the highest pads of this river valleyl fen-edge landscape. the buried soil becomes rewetted during the winter months, effectively its soil moisture content mirrors the expected seasonal fluctuations. This wetting/drying action may be more detrimental to good organic preservation than any other factor.

5. The primary fills of the barrow ditches remain effectively waterlogged and anaerobic throughout the year, despite being subject to some seasonal variability. \onetheless, the alluvial silty clays which act as the tertiary fills of these same ditches contain the highest soil moisture contents, and also act as an anaerobic or air-excluding zone, effectively sealing the underlying ditch fills.

Project participants

A notable aspect of the project is the part played by the different organisations involved. While the overall project management is the responsibility of Dr. C.A.I French of Cambridge University, the project asa whole represents a collaboration between the University, the University Archaeological Unit, Hunting Land and Environment Ltd, English Heritage and the developers, ARC Central. Suggested Illustrations: Plan of the site - no plans included in the project design but a location map and a detailed plan might be of advantage here (perhaps showing other fen edge sites)