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Suspended preservation: Particular preservation conditions within the Must Farm – Flag Fen Bronze Age landscape
Quaternary International 368 (2015) 19e30
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Suspended preservation: Particular preservation conditions within the Must Farm e Flag Fen Bronze Age landscape Tim Malim a, *, David Morgan a, Ian Panter b a b
SLR Consulting Limited, United Kingdom York Archaeological Trust, United Kingdom
a r t i c l e i n f o
a b s t r a c t
Article history: Available online 27 December 2014
This paper discusses the approaches to characterizing and monitoring two very similar sites in proximity. It explores the paucity of scientific evidence for understanding conditions of preservation, and techniques for monitoring it, that presently exist for sites that lie in superficial unconsolidated sediments within which a capillary fringe has developed above a groundwater table, rather than for a more conventional water-logged site. Challenge and review by peers is essential for a robust approach to ensuring management measures are appropriate, rather than relying on the knowledge and experience of a single individual or team. Since 2007, a monitoring programme for the internationally important timber platform at Hanson's Must Farm clay extraction site has been developed and enhanced, whilst excavations within the footprint of the quarry have revealed outstanding preservation of organic remains. The 3000 year inundation and sedimentary sequence, and the 100 years recent development in this landscape, are described along with the unusual preservation processes which have contributed to exceptional survival of archaeological remains. The results from Must Farm are contrasted to the paucity of monitoring data from Flag Fen, and the separate trajectories between a developer-funded project and a nationally protected monument are discussed. © 2014 Elsevier Ltd and INQUA. All rights reserved.
Keywords: Unsaturated Waterlogged Monitoring Preservation Archaeological Perched
1. Introduction The fenlands of eastern England cover an area of 46,000 km2, and are the largest area of wetland that once existed in England. Since the 17th century, however, the landscape has changed dramatically due to drainage and the development of intensive farming, with a loss of 5 m depth of peat at the western margin of the fen. Following the work of John and Bryony Coles investigating the Somerset Levels, English Heritage funded a 10 year programme of survey in the fens because of its great potential for waterlogged remains. This survey revealed how rapidly the peat was disappearing, and how little waterlogging existed (Hall and Coles, 1994). Within the Flag Fen basin, however, on the western fen edge adjacent to the industrial zone of Peterborough, a series of submerged landscapes dating from Neolithic to medieval times has been preserved, revealed by deep, opencast quarrying and other development. These buried landscapes are each at different depths beneath the present ground surface, and have enabled valid comparative study of interrelated components of a single ecosystem, such as settlement,
* Corresponding author. http://dx.doi.org/10.1016/j.quaint.2014.10.042 1040-6182/© 2014 Elsevier Ltd and INQUA. All rights reserved.
field systems, stock handling, woodland clearance and land-use, exploitation of riverine resources and communications, domestic and ritual activity. This archaeology can be directly related to contemporary environmental change, and provides a vivid record of human adaption to a dynamic environment over millennia. The two sites discussed in this paper have been selected because they are in proximity to one another (Flag Fen is 2 km from Must Farm), they are both dated by dendrochronology to exactly the same period, the two sites are both constructed using timber piles, they are located within the same landscape, and they face the same threats from modern change within that landscape.
2. Flag Fen 2.1. Site description In 1982, Francis Pryor discovered timbers at Fengate, adjacent to the fen edge at Peterborough, and for the next two decades a programme of investigation, with open access for the paying public, followed at Flag Fen. This site was variously described as a timber alignment, a crannog/island/platform, settlement, and
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ritual site. A series of timber causeways was constructed over several hundred years, associated with metalwork deposition, to connect the upland at the Fengate (Peterborough) side of the Flag Fen basin with Northey on the island of Whittlesey on the eastern side (Fig. 1) (Pryor, 2001). Five lines of piles have been revealed by excavation and on-going dendrochronological analysis, forming in total a width of 7 m. These rows were constructed and repaired over a period of 400 years from 1300 e 900 BC. The timbers were identified as coming from established, perhaps dense woodland, and were generally from trees of around 100 years old. A collection of 275 metal objects has been found along the alignment, most of which were of Iron Age date rather than being contemporary with the timbers, and almost all were found at the fen edge rather than in the deeper fen. Over the past 10 years, the visitor centre and reconstructions of Bronze Age life have struggled to pay their way through visitor interest, and excavation all but ceased years ago. In 2012 English Heritage added the site to the schedule of ancient monuments, so that it has now become a statutorily protected national monument. The reasons given for giving it protected status include “the survival of timbers and artefacts within the wet conditions of the Flag Fen basin is outstanding …. A class of monument where relatively few examples survive …. Amongst these it is unique for its scale, completeness, longevity and complexity. Waterlogged deposits and artefacts are vulnerable to changes in water levels and to the effects of encroaching industrial development” (English Heritage, 2012). The site lies in a landscape dominated by sewerage works and industry, with uncertain preservation conditions (Fig. 2). A major drainage channel adjacent to the platform, the Mustdyke, was lowered in 1972 and every decade since, so that water levels are now 2 m below the highest timbers (Pryor et al. 1992, 442).
2.2. Baseline conditions The sedimentary sequence for the Fengate fen edge and Flag Fen platform was established through micro-morphological analysis, which identified a thin sandy silty clay buried soil overlying the natural aquifer of the terrace gravels (French, 1992, 458e61). Above this basal layer, a 2 m thick accumulation of organic silts and peat from reed fen, including lenses of sandy silt, developed during the later 2nd and 1st millenniums BC, which was later capped by alluvium. The timber causeways and platform were created during this period of peat formation, with dendrochronological dates from 1300 e 900 BC (Pryor, 2001). 2.3. Site management strategy In 1987, an artificial lake was created to flood two-thirds of the timber platform. Although some short-term and sporadic studies have been undertaken to understand the burial environment, there is little baseline information and there is no monitoring regime, and so one could believe that the Flag Fen Trust and English Heritage has merely assumed that preservation in situ is a viable long-term management option for this nationally important site, rather than further excavation. 2.4. Monitoring techniques: effectiveness In 1999, an experiment to assess the rates of decay was undertaken over an 18 month period with modern timbers inserted to a depth of 1.5 m below ground level in two locations (Fig. 1) (Brittain, 2010). Site A was north of the timber alignment, and Site B south of it, near the sewer outwash. Not surprisingly, the latter was aerobic with a neutral pH and groundwater 0.57 m below the timbers, and
Fig. 1. Flag Fen timber alignment connecting Fengate to Northey Island, showing locations for timber platform, Mustdyke, piezometers, Sites A and B for assessing preservation of wood, and sewage works (after Brittain, 2010, Fig. 1.2).
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soil) sealed the peaty deposit above from draw-down into the sand and gravels, whilst the top of the peaty deposit was sealed by alluvium which allowed capillary action within the peat and prevented the ingress of atmospheric oxygen. The latter contributes to decay of organic remains, as most bacteria can only survive in aerobic conditions, so exclusion of oxygen is a major factor in establishing conditions conducive for preservation. 3. Must Farm In this same drained and heavily industrialised landscape (Fig. 3), another well-preserved site has more recently been investigated as part of a developer-funded mineral extraction scheme. Hanson Building Product's Must Farm pit at Whittlesey has revealed an astonishing array of organic remains and also preservation of entire Neolithic, Bronze Age, and Iron Age landscapes, to cattle hoof- and human foot-prints within the mud (Knight and Murrell, 2012). Of particular public interest has been the recent discovery by Cambridge Archaeological Unit of eight log boats (Fig. 4), as well as fish weirs and eel traps, within a 120 m stretch of a palaeochannel (Knight and Murrell, 2012). Together with swords, daggers and a spear, these artefacts demonstrate the wealth of material remains and usage over several hundred years of one small part of the Old Nene. 3.1. Timber platform site description
Fig. 2. Aerial Photograph of the Flag Fen artificial mere and visitor centre. Flooded fields from sewerage treatment works can be seen top left and the Fengate industrial zone at the top of the picture (Ben Robinson).
showed rapid decay. Site A was anaerobic, with acidic conditions and groundwater only 0.1 m below the timbers, but also demonstrated decay. The experiment suggested that the burial conditions would not be suitable for long-term preservation. Two groundwater systems operate, the lower one in the natural sand and gravel aquifer, and a higher water-level which is perched within the peat, and is highly susceptible to artificial drainage. Initial groundwater monitoring (Lillie and Cheetham, 2002) showed that the water-table lay between 0.2 m and þ1.2 m above sea level, but after just 10 weeks fresh spreading of sewage by the water company unfortunately brought a premature end to this monitoring. A fresh programme of monitoring consisting simply of water level measurements, has been undertaken by Atkins since 2008, but the data has not been published. The most recent study of the burial conditions existing at Flag Fen was from a programme of 10 hand-augered boreholes with one core being selected for sampling at 150 mm intervals through the sequence where the timber structures should have survived (Boreham, 2013). No timbers were encountered in the boreholes, ground-water strikes and pH were recorded, and the electrical conductivity and redox potential of the sediment was measured using calibrated probes. Results showed acidic conditions predominate, that redox measurements indicated a mildly reducing burial environment, and that the groundwater-level included only the basal 50 cm of a 2 m deep deposit sequence. Pollen preservation was variable, good towards the base of the sequence but poor at the top, The interpretation given for the good level of preservation above the groundwater-level was that the thin band of silt (buried
Along this palaeochannel ~200 m further east, the substantial timber piles for a platform (Figs. 3 and 5), with an enclosing palisade, was discovered in 2006. This contained a contemporary cultural horizon amidst the sedimentary sequence, comprising a rich assemblage of artefacts. Dendrochronology shows that this structure was contemporary with the various timber causeways at Flag Fen, 1300e900 BC and an exploratory trench has shown that the platform underwent two highly damaging events (Knight, 2009). The first resulted in it slumping into the water channel, and the second was a catastrophic fire which destroyed the super-structure, charring organic remains and allowing non-organic artefacts to fall into the soft mud below the structure, where they became sealed and protected by the subsequent sedimentary process. This deposit has been described as the “cultural horizon” within the sedimentary sequence. 3.2. Baseline conditions The baseline conditions were established from two monoliths taken in 2007 close to the northern edge of the timber platform (Boreham, 2007). A total of 87 samples were collected, and 14 different chemical and physical analyses were undertaken. The pH, electrical conductivity and redox potential of the sediment was measured using calibrated probes. Sub-samples of sediment for nitrate analysis were extracted in saturated calcium sulphate solution and analysed using a calibrated ion sensitive electrode. This suite of analyses demonstrated that fine silts and clays alternated with lenses of coarser components (shells, organics), the pH was near neutral, redox indicated moderately oxidising e moderately reducing conditions, with highly variable levels of nitrates and other chemical species. Boreham (2007) concluded that raising of the water level should halt or reverse the oxidation process. 3.3. Site management strategy This timber platform is outside of the area for mineral extraction, and will not be developed by Hanson, and so SLR proposed a
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Fig. 3. (continued).
management plan for preservation in situ and a monitoring regime was devised (SLR, 2007) and installed in 2007 by Ian Panter, who was employed by English Heritage as a leading authority on conservation and preservation in situ until 2006. Following established practice for monitoring of conventional waterlogged sites developed over many years (e.g Brunning et al., 2000; Cheetham, 2007; Williams et al., 2008), the scheme included a network of 11 locations for redox probe clusters, and clusters of narrow piezometers at multiple depths, locations which included control points from the surrounding roddon silts as well as from the palaeochannel clays and cultural horizon. A roddon is a relict water-course which, during its lifetime, deposited silts within the channel it cut through the peat. In later periods, peat wastage around the old creek led to the silt bed of the roddon becoming a broad ridge of higher ground. The latest channel within a roddon is often visible as a darker strip within the roddon silts, comprising organic mud/clays and peat infill. After 18 months monitoring (SLR interim reports 2008, May 2009a), however, it became apparent that the data generated by this suite of monitoring techniques was producing variable results due to the peculiar burial environment of this particular site. The project was therefore enhanced in 2009 by three Time Domain Reflectometry (TDR) probes and installation of three larger diameter and deeper dipwells in a north-south transect through the middle of the site, with TDR A located in the roddon silts south of the cultural horizon, TDR D in the centre of the timber platform site, and TDR C at its northern end where the cultural horizon dipped down following the contour of the palaeochannel (SLR interim report October 2009b) (Fig. 6).
Data gathered by the team includes water levels from the extensive groundwater monitoring network at the archaeological site since 2008 (27 monitoring piezometers grouped in clusters to various depths), and from the surrounding areas since 2002 (circa 58 monitoring dipwells); the approach, monitoring data, and data analysis have been subject to numerous challenge and review meetings held with the regulatory authorities including the Mineral Planning Authority and English Heritage, which has enabled a more rigorous analysis, with invaluable questioning of the data, leading to project enhancements. As more data was gathered, knowledge of the detail and variability of the sub-surface deposits, their moisture content and chemical characteristics, developed, which provided an iterative process for the challenge and review process to refine the methodology for monitoring the site. An Independent Opinion on the monitoring data and whether the burial conditions are conducive to continued preservation in situ has also been conducted (Matthiesen and Gregory, 2013). 3.4. Monitoring techniques: effectiveness and enhancement Many lessons have been learnt over the past six years, which are important to articulate in a publication such as this, so that others can learn what has been successful and what techniques and assumptions have proved less effective than anticipated. The first element was in characterizing the deposits to ensure reasonable baseline knowledge of their physical and chemical character. During installation of the suite of 11 redox probe clusters and piezometers, a hand-held power screw auger was used, which had to overcome problems with brick inclusions within the overburden
Fig. 3. a: Flag Fen basin, showing some of the major archaeological investigations and the fen wetland (dark grey) as opposed to upland and island (light grey), with River Nene along southern edge of the fen embayment (with kind permission of Cambridge Archaeological Unit). b Industrialized nature of surrounding landscape to Flag Fen and Must Farm; the housing shows the western edge of the town of Whittlesey in the foreground, and the eastern edge of the city of Peterborough in the distance, with the Flag Fen basin (previous fenland) in between. c: Must Farm archaeological excavation areas. The timber platform site is on the eastern side, labelled 2006 with the palaeochannel of the old Nene shown to the west (courtesy of Cambridge Archaeological Unit).
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Fig. 4. One of the Must Farm log boats under excavation.
Fig. 5. Schematic reconstruction of Must Farm timber platform (Caroline Malim).
above the sedimentary sequence and cultural horizon (SLR, 2008). As the archaeological trial trench and monoliths had documented the baseline, the sediment cores from the boreholes used for the 11 locations were not individually logged. In retrospect, this was an error, because the benefits of each monitoring point being able to directly correlate the variable stratigraphic sequence with the data collected from the multi-level sampling is self-evident, whereas without this detailed log, the data gathered by monitoring can only be applied in a general sense to the single stratigraphic sequence and depths recorded for the archaeological trench. A second lesson was delivered by the redox probes, which were designed for monitoring saturated deposits. However, although the sediments at Must Farm contain increased moisture contents associated with the capillary fringe, sorption and perched water levels within the sediment horizons above the general water table, they are not waterlogged in the same way as peat would be, and therefore the equipment was not suitable for the particular burial environment at Must Farm (SLR, 2010a). After 18 months, the fluctuating data from the redox probes also suggested that the seals
had deteriorated and so it was decided to seal them to prevent oxygen ingress around the broken seals (SLR, 2010b, 2012a). The limited life expectancy of redox probes has been noted elsewhere (Matthiesen et al. 2004), and the problems of collecting accurate data from soils (sediments) have also been discussed (Rabenhorst et al. 2009). The narrow diameter of the 11 original piezometers limited the techniques and type of equipment that could be used for monitoring water level and quality. In 2009, the installation of three TECANAT TDR plastic access tubes was accompanied by insertion of three dipwells (P12, P13 and P14 near TDR A, D and C respectively) to provide complementary data. The dipwells were of wider diameter so that in situ testing and sampling was made simpler. Full cores were extracted and a detailed stratigraphic sequence for each was logged, as well as samples extracted for additional geochemical analysis (SLR, 2009b). This ensured that the TDR monitoring results (Figs. 7 and 8) could be related confidently to the depth of the cultural horizon for each dipwell, and could also be extrapolated to the corresponding TDR probe for each of these three locations. In
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Fig. 6. Plan showing extent of the Must Farm timber platform and monitoring locations.
addition, the dipwells could be used to corroborate the data from three of the Phase 1 piezometers (P2, P8, and P10 respectively) which were located in close proximity to the Phase 2 dipwells. Where present, the cultural horizon is represented by a planer surface reflecting the depositional environment, so assisting crosscorrelation between monitoring locations within the main body of the archaeological site. Time Domain Reflectometry (TDR) has been used to assess the presence of moisture in the capilliary fringe which is developed within the fine grained clays and silt sediments of the roddon and palaeochannel, above the permanent and perched water tables at the archaeological site. The TDR soil moisture probes provide an approximate indication of this capillary fringe condition in terms of the ‘calculated volumetric water content’ of the superficial soils. Although the TDR monitoring equipment is calibrated to generic rather than site specific soil conditions, it was possible to establish an approximate calibration of the TDR results to the sediments at each of the TDR monitoring locations, following the development of saturated conditions within the palaeochannel when groundwater levels rose due to works in late 2011 to help promote the recharge of surface water runoff into the archaeological site (Fig. 7) (SLR, 2012a, 2012b). This calibration approach is based on the assumption that once saturated conditions had been established, as confirmed by the groundwater piezometers installed at the archaeological site, the previous and future TDR profile responses at that specific TDR monitoring location could then be compared against this ‘saturated’ TDR response for that specific TDR monitoring location. The TDR results confirm the influence on moisture contents resulting from seasonal recharge into the palaeochannel sediments. Residual moisture content within the capillary fringe is present even during dry parts of the year when there is little or no rainfall
recharge taking place into the palaeochannel sediments. This demonstrates the moisture retention capability of the fine-grained sediment matrix which dominates within the cultural horizon. In addition to providing a general indication of the calculated volumetric moisture content profile within the sediments, the TDR monitoring results also allow a direct comparison of the calculated volumetric moisture content variation at each monitoring location throughout the monitoring period. Notwithstanding the recognised limitations of the TDR results, they can also be compared in general terms to typical expected porosity values for silt (ranging between 35% and 50%), and clays (ranging between 40% and 70%), meaning fully saturated conditions would occur between 50%e70%. The monitoring programme for the piezometers within the direct vicinity of the timber platform site has been expanded to include pH, temperature and redox potential. This additional monitoring was initiated in February 2013, and is carried out at the same biweekly frequency as the groundwater level monitoring for these piezometers. It is noted that the monitored groundwater within each piezometer is in direct contact with the atmosphere present within these piezometers, and so these results do not accurately reflect the groundwater within the pores of the adjacent sediments, a potential problem also noted during monitoring at Fiskerton (Williams et al. 2008). During monitoring, there is also an inevitable mixing of the groundwater within each piezometer as the probe is inserted and lowered in order to take the readings. Redox potential measurements were also taken from the water samples (SLR, 2013) (Fig. 9) and these mostly fall within the central zone of measurements used by the Netherlands National Service for Archaeology for assessing preservation conditions on archaeological sites (Smit et al., 2006), i.e. bands 4e6, with band 10 being Good preservation and band 1 being Poor preservation.
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Fig. 7. Groundwater level fluctuations since start of monitoring.
3.5. Analysis of the baseline and monitoring data In order to better understand the exceptional preservation on site and the potential for continued preservation in situ, the team completed a detailed assessment of all the available monitoring information gathered for the site, taking into consideration the site's historical and hydrogeological setting. The vertical geological section and hydrogeological characteristics at the site consist of an accumulation of superficial deposits up to 8 m thick overlying the Oxford Clay. These include sand and gravel river terrace deposits approximately 2 m thick which act as an aquifer, above which marine and freshwater silts and organic-rich clays have been laid down, with peat formation at the top of the sequence. During this gradual accumulation of flood deposits various drainage channels have been cut, initially a salt creek formation which has survived as the marine silts of a roddon, and secondly into this a migrating freshwater palaeochannel which was the forerunner of the present River Nene. The low permeability and slow vertical drainage characteristics of the organic clays and silts cause them to act like a barrier to rainfall infiltration, with any infiltration effectively being ‘perched’ within these deposits. Seasonal recharge is typically limited to the months of November, December, January, February and March, when rainfall inputs exceed evapotranspiration losses. The underlying Sand and Gravel aquifer is fully saturated. Capillary rise of water within the fine grained organic clay and silt sediments above the saturated Sand and Gravel aquifer can be expected; this is typically referred to as the ‘capillary fringe’. The water movement occurs due to capillary action, where the intermolecular adhesion of water to the soil exceeds the cohesion of water molecules to each other. Moisture is also retained as films adsorbed between the negatively charged surfaces of the clay particles. The monitoring wells and the redox probes are unable to measure this capillary fringe, but the TDR results demonstrate the soil pores within this zone are saturated to varying degrees e soil moisture contents within the cultural horizon range seasonally from 20% to 90%. Capillary rise within these sediments should help to prevent oxygenated conditions developing, by preventing or restricting the ingress of atmospheric air from above, although the effectiveness
will be dependent on the degree of saturation of the sediments affected. The following historical anthropogenic events have also influenced the hydrogeological conditions in the immediate vicinity of the timber platform at Must Farm: when Must Farm Pit was originally opened up for extraction of the Oxford Clay in the early 20th Century, groundwater levels in the Sand and Gravel aquifer in the immediate vicinity of the clay pit are very likely to have been drawn down given the significant depth of the clay pit (approx. 16 m AOD), some 10 m below the base of the Sand and Gravel Aquifer, resulting in partially saturated conditions within the Sand and Gravel aquifer; more recently, seasonal extraction from Must Farm Pit was also carried out under an Environment Agency abstraction licence by a group of local farmers to meet their summer agricultural irrigation requirements. Under these conditions the water levels in Must Farm Pit lake fluctuated significantly between approx. 2.6 mOD and 0 mOD on a seasonal basis. The northern edge of the timber platform site is abrupt, because the previous clay extraction in the 1960e70s had resulted in a quarry that cut through the site, leaving an open void on the northern side of the platform. A clay bund was constructed in 2007 to prevent water flow in this direction, but was only raised to its full design height above the level of the cultural horizon in 2011, due to its utilization as an informal haul road. Since 2011, the bund has acted to enhance the recharge and water retention capabilities of the site. On the eastern side of the timber platform site a large pit excavated through the cultural sequence in the 1960se70s had been filled with bricks. The sand and gravel aquifer is approximately 3 m below the level of the surviving archaeology of the timber platform, and yet in spite of significant changes to the site over recent decades, and the depth of the water table below the cultural horizon, the archaeological remains have continued to be preserved. Such burial conditions would normally argue for rapid decay, and yet the timber platform has not only survived 3000 years, but has also survived after drainage of the fen over the past 300 years, and has even survived the effects of an open void during the period of clay extraction which extended over several decades.
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Fig. 8. Sediment moisture content profile at locations TDR D and TDR C (period 2012e13) (from SLR, 2013).
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Fig. 9. Redox potential in dipwells FebruaryeOctober 2013.
Therefore, it is clear that water levels in Must Farm Pit have been lowered in the recent (through seasonal abstraction) and more distant past (when the quarry was excavated), below the elevation of the archaeological organic remains. It is also evident that the archaeological organic remains have survived these periods, thereby confirming that the low permeability of the palaeochannel organic clays and roddon silts, within which the archaeological remains are located, have not been significantly dewatered as a result of these activities. 4. Conclusions Comparison between these two similar archaeological sites, the Late Bronze Age timber structures at Flag Fen and Must Farm, which have been subjected to the same pattern of environmental change since their construction, and are located within the same water catchment area, are of interest as they illustrate how exceptional preservation can occur within the capillary zone above the groundwater table (Fig. 10). The threats that each face from rapid change to the surrounding landscape (from intensive agriculture, irrigation and water extraction, industrial development and quarrying) are evident as changes to the water quality and water levels. Survival of organic remains over 3000 years was enabled through the stabilization of a burial environment that was conducive to preservation, with a very slow rate of decay, but, even though many of these industrial developments were initiated over a century ago, the changes that are occurring in the vicinity of these monuments today could lead to an accelerated rate of decay. Site management at Flag Fen and Must Farm have both included an element of monitoring to provide an indication on preservation conditions for buried remains. The approach adopted by each has been significantly different, with the former adopting piecemeal monitoring and experimental assessment, whilst the latter has implemented an intensive grid of monitoring points and a diverse
range of techniques to help in understanding the characteristics and change within the burial environment. As part of the Flag Fen management plan, part of the site was isolated twenty years ago, so that an artificial lake could be created to ensure stability for water levels within the area of the lake, but the quality of the water has not been systematically monitored. At Must Farm, a bund has been constructed to prevent water flow out of the site into the quarry void to the north. The expectation is that effective rainfall over several years will gradually recharge the water level. As part of this recharging process, careful monitoring of the chemical nature of the rainfall and surface water would be undertaken, and measures implemented to prevent the introduction of oxygen into the sediments, thus preserving the archaeological remains. The lack of monitoring data from Flag Fen has prevented scrutiny of whether conditions conducive for preservation continue. This also perhaps stems from a presumption that the management plan is effective, because the site has an artificial lake and is not directly threatened physically by development pressures. The amount of data available from Must Farm allows close scrutiny and refinement of the results, and their analysis is ambivalent as to whether or not continued preservation is possible. Both sites are considered nationally important, and Flag Fen was made a statutorily protected monument in 2012, but this status does not prevent decay occurring from the environmental change within the surrounding landscape. Must Farm is protected through the planning process, as it lies within the consented boundary of a mineral extraction site, and national policy is for conservation of nationally important archaeological remains. In spite of these protection measures, the monitoring techniques employed and data gathered from these sites cannot provide guarantees of their continued longevity as organic remains. Flag Fen and Must Farm are not waterlogged in the conventional understanding of organic remains surviving below the permanent groundwater table, but have been preserved partly as a result of reduced atmospheric oxygen ingress to the surrounding sediments
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Fig. 10. Comparative schematic profiles for Flag Fen and Must Farm (Caroline Malim).
as a direct result of increased moisture contents associated with the capillary fringe, sorption, and perched water levels within the sediment horizons above the general water table. Such conditions are not unique and have been identified elsewhere, as in Bryggen (Bergen) and Oslo (Matthiesen et al., 2008; Martens et al., 2012). The challenge now is to increase our understanding of such deposits, and to accept that their sustainable management and effective monitoring will require new models from those that have been developed for more conventional waterlogged deposits and wetlands. Acknowledgements Thanks are given to Hanson Building Products for funding the works outlined in this article. In particular Tim Darling (Land and Planning Manager), Ian Willis and Dominic Delich (Works Managers), and Henry Godfrey who has undertaken the monitoring over the past six years; David Gibson and Mark Knight (Cambridge Archaeological Unit); Steve Boreham (University of Cambridge); Malcolm Lillie (University of Hull); the review team Jen Heathcote and Helen Chappell (English Heritage scientific advisors), Will
Fletcher (English Heritage Inspector of Ancient Monuments), Kasia Gdaniec (Cambridgeshire County Council), Charley French (University of Cambridge), Rebecca Casa-Hatton (Peterborough City Council); SLR engineers Mark Swain and Peter Clarke, and illustrator Caroline Malim.
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English Heritage, 2012. Scheduling Description for Flag Fen NHLE List Number 1406460 and Name: a Bronze Age Post Alignment and Timber Platform at Flag Fen and Associated Bronze Age and Later Field Systems and Settlement to Either Side of the Northey Road. http://list.english-heritage.org.uk/results. aspx. French, C., 1992. Fengate and Flag Fen: summary of the soil and sediment analyses. Antiquity 66, 458e461. Hall, D., Coles, J., 1994. Fenland Survey: an Essay in Landscape and Persistence. English Heritage. Knight, M., 2009. Excavating a Bronze Age Timber Platform at Must Farm, Whittlesey, Near Peterborough. PAST Issue 63 (November 2009). The Newsletter of the Prehistoric Society. Knight, M., Murrell, K., 2012. Must Farm and Bradley Fen. British Archaeology 123, 14e21. Lillie, M., Cheetham, J., 2002. Water Table Monitoring at Flag Fen, Peterborough (TL 227989). Wetland Archaeology & Environments Research Centre University of Hull. SAS-FF/02-01. Martens, V.V., Amundsen, C.E., Bergersen, O., 2012. Research on conservation state and preservation conditions in unsaturated deposits in Oslo. Conservation and Management of Archaeological Sites 14 (1e4), 72e84. Matthiesen, H., Gregory, D., Jensen, P., Sorensen, B., 2004. Environmental monitoring at Nydam, a waterlogged site with weapon sacrifices from the Danish Iron Age. I: a comparison of methods used and results from undisturbed conditions. Journal of Wetland Archaeology 4, 55e74. Matthiesen, H., Dunlop, A.R., Jensen, J.A., de Beer, H., Christensson, A., 2008. Monitoring of preservation conditions and evaluation of decay rates of urban deposits e results from the first five years of monitoring at Bryggen in Bergen. In: Kars, H., van Heeringen, R.M. (Eds.), Preserving Archaeological Remains in situ. Proceedings of the 3rd Conference 7 e 9 December 2006, Amsterdam Vrije Universiteit, GBS 10, pp. 163e174. Matthiesen, H., Gregory, D., 2013. Independent Opinion on the Results of a Hydrological and Geochemical Monitoring Programme for the Timber Platform Site, Must Farm Quarry. Dept. of Conservation, National Museum of Denmark, Whittlesey.
Pryor, F., French, C.A.I., Scaife, R., Robinson, M., Neve, J., Taylor, M., Halstead, P., Cameron, E., Anderson, K.J., Barrett, J.C., 1992. Current research at Flag Fen. Antiquity 66. Pryor, F., 2001. The Flag Fen Basin: Archaeology and Environment of a Fenland Landscape. Archaeological Reports. English Heritage, London. Rabenhorst, M.C., Hively, W.D., James, B.R., 2009. Measurements of soil redox potential. Soil Science Society of America Journal 73, 668e674. SLR Consulting Limited, March 2007. Baseline Testing and Monitoring Regime, Method Statement. SLR Consulting Limited, November 2008. Must Farm Pit, Whittlesey, Near Peterborough. Archaeological Monitoring Interim Report. SLR Consulting Limited, May 2009a. Must Farm Pit, Whittlesey, Near Peterborough. Archaeological Monitoring Interim Report No.2. SLR Consulting Limited, October 2009b. Must Farm Pit, Whittlesey, Near Peterborough. Archaeological Monitoring Interim Report No.3. SLR Consulting Limited, June 2010a. Must Farm Pit, Whittlesey, Near Peterborough. Archaeological Monitoring Interim Report No.4. SLR Consulting Limited, November 2010b. Must Farm Pit, Whittlesey, Near Peterborough. Archaeological Monitoring Interim Report No.5. SLR Consulting Limited, February 2012a. Must Farm Pit, Whittlesey, Near Peterborough. Archaeological Monitoring Interim Report No.6, Version 2. SLR Consulting Limited, December 2012b. Must Farm Pit, Whittlesey, Near Peterborough. Archaeological Monitoring Interim Report No.7. SLR Consulting Limited, October 2013. Must Farm Pit, Whittlesey, Near Peterborough. Archaeological Monitoring Interim Report No.8. Smit, A., van Heeringen, R.M., Theunissen, E.M., 2006. Archaeological Monitoring Standard. Guidelines for the Non-destructive Recording and Monitoring of the Physical Quality of Archaeological Sites and Monuments. Nederlandse Archaeologische Rapporten 33. Williams, J., Fell, V., Graham, K., Simpson, P., Collins, M., Koon, H., Griffen, R., 2008. Re-watering of the Iron Age causeway at Fiskerton, England. In: Kars, H., van Heeringen, M. (Eds.), Preserving Archaeological Remains in situ, Proceedings of the 3rd Conference 7-9 December 2006, Amsterdam Geoarchaeological and Bioarchaeological Studies, vol. 10, pp. 181e198.
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Suspended preservation: Particular preservation conditions within the Must Farm e Flag Fen Bronze Age landscape Tim Malim a, *, David Morgan a, Ian Panter b a b
SLR Consulting Limited, United Kingdom York Archaeological Trust, United Kingdom
a r t i c l e i n f o
a b s t r a c t
Article history: Available online 27 December 2014
This paper discusses the approaches to characterizing and monitoring two very similar sites in proximity. It explores the paucity of scientific evidence for understanding conditions of preservation, and techniques for monitoring it, that presently exist for sites that lie in superficial unconsolidated sediments within which a capillary fringe has developed above a groundwater table, rather than for a more conventional water-logged site. Challenge and review by peers is essential for a robust approach to ensuring management measures are appropriate, rather than relying on the knowledge and experience of a single individual or team. Since 2007, a monitoring programme for the internationally important timber platform at Hanson's Must Farm clay extraction site has been developed and enhanced, whilst excavations within the footprint of the quarry have revealed outstanding preservation of organic remains. The 3000 year inundation and sedimentary sequence, and the 100 years recent development in this landscape, are described along with the unusual preservation processes which have contributed to exceptional survival of archaeological remains. The results from Must Farm are contrasted to the paucity of monitoring data from Flag Fen, and the separate trajectories between a developer-funded project and a nationally protected monument are discussed. © 2014 Elsevier Ltd and INQUA. All rights reserved.
Keywords: Unsaturated Waterlogged Monitoring Preservation Archaeological Perched
1. Introduction The fenlands of eastern England cover an area of 46,000 km2, and are the largest area of wetland that once existed in England. Since the 17th century, however, the landscape has changed dramatically due to drainage and the development of intensive farming, with a loss of 5 m depth of peat at the western margin of the fen. Following the work of John and Bryony Coles investigating the Somerset Levels, English Heritage funded a 10 year programme of survey in the fens because of its great potential for waterlogged remains. This survey revealed how rapidly the peat was disappearing, and how little waterlogging existed (Hall and Coles, 1994). Within the Flag Fen basin, however, on the western fen edge adjacent to the industrial zone of Peterborough, a series of submerged landscapes dating from Neolithic to medieval times has been preserved, revealed by deep, opencast quarrying and other development. These buried landscapes are each at different depths beneath the present ground surface, and have enabled valid comparative study of interrelated components of a single ecosystem, such as settlement,
* Corresponding author. http://dx.doi.org/10.1016/j.quaint.2014.10.042 1040-6182/© 2014 Elsevier Ltd and INQUA. All rights reserved.
field systems, stock handling, woodland clearance and land-use, exploitation of riverine resources and communications, domestic and ritual activity. This archaeology can be directly related to contemporary environmental change, and provides a vivid record of human adaption to a dynamic environment over millennia. The two sites discussed in this paper have been selected because they are in proximity to one another (Flag Fen is 2 km from Must Farm), they are both dated by dendrochronology to exactly the same period, the two sites are both constructed using timber piles, they are located within the same landscape, and they face the same threats from modern change within that landscape.
2. Flag Fen 2.1. Site description In 1982, Francis Pryor discovered timbers at Fengate, adjacent to the fen edge at Peterborough, and for the next two decades a programme of investigation, with open access for the paying public, followed at Flag Fen. This site was variously described as a timber alignment, a crannog/island/platform, settlement, and
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ritual site. A series of timber causeways was constructed over several hundred years, associated with metalwork deposition, to connect the upland at the Fengate (Peterborough) side of the Flag Fen basin with Northey on the island of Whittlesey on the eastern side (Fig. 1) (Pryor, 2001). Five lines of piles have been revealed by excavation and on-going dendrochronological analysis, forming in total a width of 7 m. These rows were constructed and repaired over a period of 400 years from 1300 e 900 BC. The timbers were identified as coming from established, perhaps dense woodland, and were generally from trees of around 100 years old. A collection of 275 metal objects has been found along the alignment, most of which were of Iron Age date rather than being contemporary with the timbers, and almost all were found at the fen edge rather than in the deeper fen. Over the past 10 years, the visitor centre and reconstructions of Bronze Age life have struggled to pay their way through visitor interest, and excavation all but ceased years ago. In 2012 English Heritage added the site to the schedule of ancient monuments, so that it has now become a statutorily protected national monument. The reasons given for giving it protected status include “the survival of timbers and artefacts within the wet conditions of the Flag Fen basin is outstanding …. A class of monument where relatively few examples survive …. Amongst these it is unique for its scale, completeness, longevity and complexity. Waterlogged deposits and artefacts are vulnerable to changes in water levels and to the effects of encroaching industrial development” (English Heritage, 2012). The site lies in a landscape dominated by sewerage works and industry, with uncertain preservation conditions (Fig. 2). A major drainage channel adjacent to the platform, the Mustdyke, was lowered in 1972 and every decade since, so that water levels are now 2 m below the highest timbers (Pryor et al. 1992, 442).
2.2. Baseline conditions The sedimentary sequence for the Fengate fen edge and Flag Fen platform was established through micro-morphological analysis, which identified a thin sandy silty clay buried soil overlying the natural aquifer of the terrace gravels (French, 1992, 458e61). Above this basal layer, a 2 m thick accumulation of organic silts and peat from reed fen, including lenses of sandy silt, developed during the later 2nd and 1st millenniums BC, which was later capped by alluvium. The timber causeways and platform were created during this period of peat formation, with dendrochronological dates from 1300 e 900 BC (Pryor, 2001). 2.3. Site management strategy In 1987, an artificial lake was created to flood two-thirds of the timber platform. Although some short-term and sporadic studies have been undertaken to understand the burial environment, there is little baseline information and there is no monitoring regime, and so one could believe that the Flag Fen Trust and English Heritage has merely assumed that preservation in situ is a viable long-term management option for this nationally important site, rather than further excavation. 2.4. Monitoring techniques: effectiveness In 1999, an experiment to assess the rates of decay was undertaken over an 18 month period with modern timbers inserted to a depth of 1.5 m below ground level in two locations (Fig. 1) (Brittain, 2010). Site A was north of the timber alignment, and Site B south of it, near the sewer outwash. Not surprisingly, the latter was aerobic with a neutral pH and groundwater 0.57 m below the timbers, and
Fig. 1. Flag Fen timber alignment connecting Fengate to Northey Island, showing locations for timber platform, Mustdyke, piezometers, Sites A and B for assessing preservation of wood, and sewage works (after Brittain, 2010, Fig. 1.2).
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soil) sealed the peaty deposit above from draw-down into the sand and gravels, whilst the top of the peaty deposit was sealed by alluvium which allowed capillary action within the peat and prevented the ingress of atmospheric oxygen. The latter contributes to decay of organic remains, as most bacteria can only survive in aerobic conditions, so exclusion of oxygen is a major factor in establishing conditions conducive for preservation. 3. Must Farm In this same drained and heavily industrialised landscape (Fig. 3), another well-preserved site has more recently been investigated as part of a developer-funded mineral extraction scheme. Hanson Building Product's Must Farm pit at Whittlesey has revealed an astonishing array of organic remains and also preservation of entire Neolithic, Bronze Age, and Iron Age landscapes, to cattle hoof- and human foot-prints within the mud (Knight and Murrell, 2012). Of particular public interest has been the recent discovery by Cambridge Archaeological Unit of eight log boats (Fig. 4), as well as fish weirs and eel traps, within a 120 m stretch of a palaeochannel (Knight and Murrell, 2012). Together with swords, daggers and a spear, these artefacts demonstrate the wealth of material remains and usage over several hundred years of one small part of the Old Nene. 3.1. Timber platform site description
Fig. 2. Aerial Photograph of the Flag Fen artificial mere and visitor centre. Flooded fields from sewerage treatment works can be seen top left and the Fengate industrial zone at the top of the picture (Ben Robinson).
showed rapid decay. Site A was anaerobic, with acidic conditions and groundwater only 0.1 m below the timbers, but also demonstrated decay. The experiment suggested that the burial conditions would not be suitable for long-term preservation. Two groundwater systems operate, the lower one in the natural sand and gravel aquifer, and a higher water-level which is perched within the peat, and is highly susceptible to artificial drainage. Initial groundwater monitoring (Lillie and Cheetham, 2002) showed that the water-table lay between 0.2 m and þ1.2 m above sea level, but after just 10 weeks fresh spreading of sewage by the water company unfortunately brought a premature end to this monitoring. A fresh programme of monitoring consisting simply of water level measurements, has been undertaken by Atkins since 2008, but the data has not been published. The most recent study of the burial conditions existing at Flag Fen was from a programme of 10 hand-augered boreholes with one core being selected for sampling at 150 mm intervals through the sequence where the timber structures should have survived (Boreham, 2013). No timbers were encountered in the boreholes, ground-water strikes and pH were recorded, and the electrical conductivity and redox potential of the sediment was measured using calibrated probes. Results showed acidic conditions predominate, that redox measurements indicated a mildly reducing burial environment, and that the groundwater-level included only the basal 50 cm of a 2 m deep deposit sequence. Pollen preservation was variable, good towards the base of the sequence but poor at the top, The interpretation given for the good level of preservation above the groundwater-level was that the thin band of silt (buried
Along this palaeochannel ~200 m further east, the substantial timber piles for a platform (Figs. 3 and 5), with an enclosing palisade, was discovered in 2006. This contained a contemporary cultural horizon amidst the sedimentary sequence, comprising a rich assemblage of artefacts. Dendrochronology shows that this structure was contemporary with the various timber causeways at Flag Fen, 1300e900 BC and an exploratory trench has shown that the platform underwent two highly damaging events (Knight, 2009). The first resulted in it slumping into the water channel, and the second was a catastrophic fire which destroyed the super-structure, charring organic remains and allowing non-organic artefacts to fall into the soft mud below the structure, where they became sealed and protected by the subsequent sedimentary process. This deposit has been described as the “cultural horizon” within the sedimentary sequence. 3.2. Baseline conditions The baseline conditions were established from two monoliths taken in 2007 close to the northern edge of the timber platform (Boreham, 2007). A total of 87 samples were collected, and 14 different chemical and physical analyses were undertaken. The pH, electrical conductivity and redox potential of the sediment was measured using calibrated probes. Sub-samples of sediment for nitrate analysis were extracted in saturated calcium sulphate solution and analysed using a calibrated ion sensitive electrode. This suite of analyses demonstrated that fine silts and clays alternated with lenses of coarser components (shells, organics), the pH was near neutral, redox indicated moderately oxidising e moderately reducing conditions, with highly variable levels of nitrates and other chemical species. Boreham (2007) concluded that raising of the water level should halt or reverse the oxidation process. 3.3. Site management strategy This timber platform is outside of the area for mineral extraction, and will not be developed by Hanson, and so SLR proposed a
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Fig. 3. (continued).
management plan for preservation in situ and a monitoring regime was devised (SLR, 2007) and installed in 2007 by Ian Panter, who was employed by English Heritage as a leading authority on conservation and preservation in situ until 2006. Following established practice for monitoring of conventional waterlogged sites developed over many years (e.g Brunning et al., 2000; Cheetham, 2007; Williams et al., 2008), the scheme included a network of 11 locations for redox probe clusters, and clusters of narrow piezometers at multiple depths, locations which included control points from the surrounding roddon silts as well as from the palaeochannel clays and cultural horizon. A roddon is a relict water-course which, during its lifetime, deposited silts within the channel it cut through the peat. In later periods, peat wastage around the old creek led to the silt bed of the roddon becoming a broad ridge of higher ground. The latest channel within a roddon is often visible as a darker strip within the roddon silts, comprising organic mud/clays and peat infill. After 18 months monitoring (SLR interim reports 2008, May 2009a), however, it became apparent that the data generated by this suite of monitoring techniques was producing variable results due to the peculiar burial environment of this particular site. The project was therefore enhanced in 2009 by three Time Domain Reflectometry (TDR) probes and installation of three larger diameter and deeper dipwells in a north-south transect through the middle of the site, with TDR A located in the roddon silts south of the cultural horizon, TDR D in the centre of the timber platform site, and TDR C at its northern end where the cultural horizon dipped down following the contour of the palaeochannel (SLR interim report October 2009b) (Fig. 6).
Data gathered by the team includes water levels from the extensive groundwater monitoring network at the archaeological site since 2008 (27 monitoring piezometers grouped in clusters to various depths), and from the surrounding areas since 2002 (circa 58 monitoring dipwells); the approach, monitoring data, and data analysis have been subject to numerous challenge and review meetings held with the regulatory authorities including the Mineral Planning Authority and English Heritage, which has enabled a more rigorous analysis, with invaluable questioning of the data, leading to project enhancements. As more data was gathered, knowledge of the detail and variability of the sub-surface deposits, their moisture content and chemical characteristics, developed, which provided an iterative process for the challenge and review process to refine the methodology for monitoring the site. An Independent Opinion on the monitoring data and whether the burial conditions are conducive to continued preservation in situ has also been conducted (Matthiesen and Gregory, 2013). 3.4. Monitoring techniques: effectiveness and enhancement Many lessons have been learnt over the past six years, which are important to articulate in a publication such as this, so that others can learn what has been successful and what techniques and assumptions have proved less effective than anticipated. The first element was in characterizing the deposits to ensure reasonable baseline knowledge of their physical and chemical character. During installation of the suite of 11 redox probe clusters and piezometers, a hand-held power screw auger was used, which had to overcome problems with brick inclusions within the overburden
Fig. 3. a: Flag Fen basin, showing some of the major archaeological investigations and the fen wetland (dark grey) as opposed to upland and island (light grey), with River Nene along southern edge of the fen embayment (with kind permission of Cambridge Archaeological Unit). b Industrialized nature of surrounding landscape to Flag Fen and Must Farm; the housing shows the western edge of the town of Whittlesey in the foreground, and the eastern edge of the city of Peterborough in the distance, with the Flag Fen basin (previous fenland) in between. c: Must Farm archaeological excavation areas. The timber platform site is on the eastern side, labelled 2006 with the palaeochannel of the old Nene shown to the west (courtesy of Cambridge Archaeological Unit).
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Fig. 4. One of the Must Farm log boats under excavation.
Fig. 5. Schematic reconstruction of Must Farm timber platform (Caroline Malim).
above the sedimentary sequence and cultural horizon (SLR, 2008). As the archaeological trial trench and monoliths had documented the baseline, the sediment cores from the boreholes used for the 11 locations were not individually logged. In retrospect, this was an error, because the benefits of each monitoring point being able to directly correlate the variable stratigraphic sequence with the data collected from the multi-level sampling is self-evident, whereas without this detailed log, the data gathered by monitoring can only be applied in a general sense to the single stratigraphic sequence and depths recorded for the archaeological trench. A second lesson was delivered by the redox probes, which were designed for monitoring saturated deposits. However, although the sediments at Must Farm contain increased moisture contents associated with the capillary fringe, sorption and perched water levels within the sediment horizons above the general water table, they are not waterlogged in the same way as peat would be, and therefore the equipment was not suitable for the particular burial environment at Must Farm (SLR, 2010a). After 18 months, the fluctuating data from the redox probes also suggested that the seals
had deteriorated and so it was decided to seal them to prevent oxygen ingress around the broken seals (SLR, 2010b, 2012a). The limited life expectancy of redox probes has been noted elsewhere (Matthiesen et al. 2004), and the problems of collecting accurate data from soils (sediments) have also been discussed (Rabenhorst et al. 2009). The narrow diameter of the 11 original piezometers limited the techniques and type of equipment that could be used for monitoring water level and quality. In 2009, the installation of three TECANAT TDR plastic access tubes was accompanied by insertion of three dipwells (P12, P13 and P14 near TDR A, D and C respectively) to provide complementary data. The dipwells were of wider diameter so that in situ testing and sampling was made simpler. Full cores were extracted and a detailed stratigraphic sequence for each was logged, as well as samples extracted for additional geochemical analysis (SLR, 2009b). This ensured that the TDR monitoring results (Figs. 7 and 8) could be related confidently to the depth of the cultural horizon for each dipwell, and could also be extrapolated to the corresponding TDR probe for each of these three locations. In
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Fig. 6. Plan showing extent of the Must Farm timber platform and monitoring locations.
addition, the dipwells could be used to corroborate the data from three of the Phase 1 piezometers (P2, P8, and P10 respectively) which were located in close proximity to the Phase 2 dipwells. Where present, the cultural horizon is represented by a planer surface reflecting the depositional environment, so assisting crosscorrelation between monitoring locations within the main body of the archaeological site. Time Domain Reflectometry (TDR) has been used to assess the presence of moisture in the capilliary fringe which is developed within the fine grained clays and silt sediments of the roddon and palaeochannel, above the permanent and perched water tables at the archaeological site. The TDR soil moisture probes provide an approximate indication of this capillary fringe condition in terms of the ‘calculated volumetric water content’ of the superficial soils. Although the TDR monitoring equipment is calibrated to generic rather than site specific soil conditions, it was possible to establish an approximate calibration of the TDR results to the sediments at each of the TDR monitoring locations, following the development of saturated conditions within the palaeochannel when groundwater levels rose due to works in late 2011 to help promote the recharge of surface water runoff into the archaeological site (Fig. 7) (SLR, 2012a, 2012b). This calibration approach is based on the assumption that once saturated conditions had been established, as confirmed by the groundwater piezometers installed at the archaeological site, the previous and future TDR profile responses at that specific TDR monitoring location could then be compared against this ‘saturated’ TDR response for that specific TDR monitoring location. The TDR results confirm the influence on moisture contents resulting from seasonal recharge into the palaeochannel sediments. Residual moisture content within the capillary fringe is present even during dry parts of the year when there is little or no rainfall
recharge taking place into the palaeochannel sediments. This demonstrates the moisture retention capability of the fine-grained sediment matrix which dominates within the cultural horizon. In addition to providing a general indication of the calculated volumetric moisture content profile within the sediments, the TDR monitoring results also allow a direct comparison of the calculated volumetric moisture content variation at each monitoring location throughout the monitoring period. Notwithstanding the recognised limitations of the TDR results, they can also be compared in general terms to typical expected porosity values for silt (ranging between 35% and 50%), and clays (ranging between 40% and 70%), meaning fully saturated conditions would occur between 50%e70%. The monitoring programme for the piezometers within the direct vicinity of the timber platform site has been expanded to include pH, temperature and redox potential. This additional monitoring was initiated in February 2013, and is carried out at the same biweekly frequency as the groundwater level monitoring for these piezometers. It is noted that the monitored groundwater within each piezometer is in direct contact with the atmosphere present within these piezometers, and so these results do not accurately reflect the groundwater within the pores of the adjacent sediments, a potential problem also noted during monitoring at Fiskerton (Williams et al. 2008). During monitoring, there is also an inevitable mixing of the groundwater within each piezometer as the probe is inserted and lowered in order to take the readings. Redox potential measurements were also taken from the water samples (SLR, 2013) (Fig. 9) and these mostly fall within the central zone of measurements used by the Netherlands National Service for Archaeology for assessing preservation conditions on archaeological sites (Smit et al., 2006), i.e. bands 4e6, with band 10 being Good preservation and band 1 being Poor preservation.
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Fig. 7. Groundwater level fluctuations since start of monitoring.
3.5. Analysis of the baseline and monitoring data In order to better understand the exceptional preservation on site and the potential for continued preservation in situ, the team completed a detailed assessment of all the available monitoring information gathered for the site, taking into consideration the site's historical and hydrogeological setting. The vertical geological section and hydrogeological characteristics at the site consist of an accumulation of superficial deposits up to 8 m thick overlying the Oxford Clay. These include sand and gravel river terrace deposits approximately 2 m thick which act as an aquifer, above which marine and freshwater silts and organic-rich clays have been laid down, with peat formation at the top of the sequence. During this gradual accumulation of flood deposits various drainage channels have been cut, initially a salt creek formation which has survived as the marine silts of a roddon, and secondly into this a migrating freshwater palaeochannel which was the forerunner of the present River Nene. The low permeability and slow vertical drainage characteristics of the organic clays and silts cause them to act like a barrier to rainfall infiltration, with any infiltration effectively being ‘perched’ within these deposits. Seasonal recharge is typically limited to the months of November, December, January, February and March, when rainfall inputs exceed evapotranspiration losses. The underlying Sand and Gravel aquifer is fully saturated. Capillary rise of water within the fine grained organic clay and silt sediments above the saturated Sand and Gravel aquifer can be expected; this is typically referred to as the ‘capillary fringe’. The water movement occurs due to capillary action, where the intermolecular adhesion of water to the soil exceeds the cohesion of water molecules to each other. Moisture is also retained as films adsorbed between the negatively charged surfaces of the clay particles. The monitoring wells and the redox probes are unable to measure this capillary fringe, but the TDR results demonstrate the soil pores within this zone are saturated to varying degrees e soil moisture contents within the cultural horizon range seasonally from 20% to 90%. Capillary rise within these sediments should help to prevent oxygenated conditions developing, by preventing or restricting the ingress of atmospheric air from above, although the effectiveness
will be dependent on the degree of saturation of the sediments affected. The following historical anthropogenic events have also influenced the hydrogeological conditions in the immediate vicinity of the timber platform at Must Farm: when Must Farm Pit was originally opened up for extraction of the Oxford Clay in the early 20th Century, groundwater levels in the Sand and Gravel aquifer in the immediate vicinity of the clay pit are very likely to have been drawn down given the significant depth of the clay pit (approx. 16 m AOD), some 10 m below the base of the Sand and Gravel Aquifer, resulting in partially saturated conditions within the Sand and Gravel aquifer; more recently, seasonal extraction from Must Farm Pit was also carried out under an Environment Agency abstraction licence by a group of local farmers to meet their summer agricultural irrigation requirements. Under these conditions the water levels in Must Farm Pit lake fluctuated significantly between approx. 2.6 mOD and 0 mOD on a seasonal basis. The northern edge of the timber platform site is abrupt, because the previous clay extraction in the 1960e70s had resulted in a quarry that cut through the site, leaving an open void on the northern side of the platform. A clay bund was constructed in 2007 to prevent water flow in this direction, but was only raised to its full design height above the level of the cultural horizon in 2011, due to its utilization as an informal haul road. Since 2011, the bund has acted to enhance the recharge and water retention capabilities of the site. On the eastern side of the timber platform site a large pit excavated through the cultural sequence in the 1960se70s had been filled with bricks. The sand and gravel aquifer is approximately 3 m below the level of the surviving archaeology of the timber platform, and yet in spite of significant changes to the site over recent decades, and the depth of the water table below the cultural horizon, the archaeological remains have continued to be preserved. Such burial conditions would normally argue for rapid decay, and yet the timber platform has not only survived 3000 years, but has also survived after drainage of the fen over the past 300 years, and has even survived the effects of an open void during the period of clay extraction which extended over several decades.
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Fig. 8. Sediment moisture content profile at locations TDR D and TDR C (period 2012e13) (from SLR, 2013).
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Fig. 9. Redox potential in dipwells FebruaryeOctober 2013.
Therefore, it is clear that water levels in Must Farm Pit have been lowered in the recent (through seasonal abstraction) and more distant past (when the quarry was excavated), below the elevation of the archaeological organic remains. It is also evident that the archaeological organic remains have survived these periods, thereby confirming that the low permeability of the palaeochannel organic clays and roddon silts, within which the archaeological remains are located, have not been significantly dewatered as a result of these activities. 4. Conclusions Comparison between these two similar archaeological sites, the Late Bronze Age timber structures at Flag Fen and Must Farm, which have been subjected to the same pattern of environmental change since their construction, and are located within the same water catchment area, are of interest as they illustrate how exceptional preservation can occur within the capillary zone above the groundwater table (Fig. 10). The threats that each face from rapid change to the surrounding landscape (from intensive agriculture, irrigation and water extraction, industrial development and quarrying) are evident as changes to the water quality and water levels. Survival of organic remains over 3000 years was enabled through the stabilization of a burial environment that was conducive to preservation, with a very slow rate of decay, but, even though many of these industrial developments were initiated over a century ago, the changes that are occurring in the vicinity of these monuments today could lead to an accelerated rate of decay. Site management at Flag Fen and Must Farm have both included an element of monitoring to provide an indication on preservation conditions for buried remains. The approach adopted by each has been significantly different, with the former adopting piecemeal monitoring and experimental assessment, whilst the latter has implemented an intensive grid of monitoring points and a diverse
range of techniques to help in understanding the characteristics and change within the burial environment. As part of the Flag Fen management plan, part of the site was isolated twenty years ago, so that an artificial lake could be created to ensure stability for water levels within the area of the lake, but the quality of the water has not been systematically monitored. At Must Farm, a bund has been constructed to prevent water flow out of the site into the quarry void to the north. The expectation is that effective rainfall over several years will gradually recharge the water level. As part of this recharging process, careful monitoring of the chemical nature of the rainfall and surface water would be undertaken, and measures implemented to prevent the introduction of oxygen into the sediments, thus preserving the archaeological remains. The lack of monitoring data from Flag Fen has prevented scrutiny of whether conditions conducive for preservation continue. This also perhaps stems from a presumption that the management plan is effective, because the site has an artificial lake and is not directly threatened physically by development pressures. The amount of data available from Must Farm allows close scrutiny and refinement of the results, and their analysis is ambivalent as to whether or not continued preservation is possible. Both sites are considered nationally important, and Flag Fen was made a statutorily protected monument in 2012, but this status does not prevent decay occurring from the environmental change within the surrounding landscape. Must Farm is protected through the planning process, as it lies within the consented boundary of a mineral extraction site, and national policy is for conservation of nationally important archaeological remains. In spite of these protection measures, the monitoring techniques employed and data gathered from these sites cannot provide guarantees of their continued longevity as organic remains. Flag Fen and Must Farm are not waterlogged in the conventional understanding of organic remains surviving below the permanent groundwater table, but have been preserved partly as a result of reduced atmospheric oxygen ingress to the surrounding sediments
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Fig. 10. Comparative schematic profiles for Flag Fen and Must Farm (Caroline Malim).
as a direct result of increased moisture contents associated with the capillary fringe, sorption, and perched water levels within the sediment horizons above the general water table. Such conditions are not unique and have been identified elsewhere, as in Bryggen (Bergen) and Oslo (Matthiesen et al., 2008; Martens et al., 2012). The challenge now is to increase our understanding of such deposits, and to accept that their sustainable management and effective monitoring will require new models from those that have been developed for more conventional waterlogged deposits and wetlands. Acknowledgements Thanks are given to Hanson Building Products for funding the works outlined in this article. In particular Tim Darling (Land and Planning Manager), Ian Willis and Dominic Delich (Works Managers), and Henry Godfrey who has undertaken the monitoring over the past six years; David Gibson and Mark Knight (Cambridge Archaeological Unit); Steve Boreham (University of Cambridge); Malcolm Lillie (University of Hull); the review team Jen Heathcote and Helen Chappell (English Heritage scientific advisors), Will
Fletcher (English Heritage Inspector of Ancient Monuments), Kasia Gdaniec (Cambridgeshire County Council), Charley French (University of Cambridge), Rebecca Casa-Hatton (Peterborough City Council); SLR engineers Mark Swain and Peter Clarke, and illustrator Caroline Malim.
References Boreham, S., 2007. Stratigraphy, physical and chemical analysis of sediments. In: Gibson, D., Knight, M., Allen, M. (Eds.), The Must Farm Timber Alignments: Environmental and Cultural Evidence, Post-excavation Assessment, Volume 2: Specialist Reports. Cambridge Archaeological Unit, University of Cambridge, pp. 123e142. Boreham, S., 2013. Pollen Analysis of Sediments from Flag Fen (Hudson Building), Peterborough, Cambridgeshire (Unpublished report for Cambridge Archaeological Unit). Brittain, M., 2010. Preservation and degradation of the Wood. In: Pryor, F., Bamforth, M. (Eds.), Flag Fen, Peterborough: Excavation and Research 19952007. Oxbow, Oxford ch. 1. Brunning, R., Hogan, D., Jones, J., Jones, M., Maltby, E., Robinson, M., Straker, V., 2000. Saving the sweet track. The in situ preservation of a Neolithic wooden trackway, Somerset, UK. Conservation and Management of Archaeological Sites 4, 3e20. Cheetham, J., 2007. Hydrological studies and monitoring. In: Van de Noort, R., Chapman, H., Collis, J. (Eds.), Sutton Common: the Excavation of an Iron Age “marsh-fort”, Council for British Archaeology Research Report 154, pp. 25e31.
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