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Chapter 9 Threat to soil as a cultural r esource

This chapter reviews the role of soil as a cultural resource, its role in the preservation of archaeological evidence and of any threat to these roles.

9.1 Summary

• Human activities in the past and present can have a major effect on soil formation and thus soils need to be considered as cultural resources.
• The value of such resources is expressed in the record of human activity as evident in soils and also through the provision of a buried environment for the preservation of archaeological features.
• Much is known about cultural soils in general terms, but no detailed data exist on their extent and nature.
• Cultural soils are distinctive and valued, thus meriting protection.
• Soil erosion is a threat to cropmark sites.
• There is a lack of data on changes in soil conditions relevant to the preservation of archaeological evidence.

9.2 Introduction and description of threat

This chapter overviews the threats to the cultural value of soils in Scotland through considering issues at the landscape and site scales. Soils evolve in response to a variety of factors and there is increasing recognition that human influences can be considerable. This is particularly the case in a country such as Scotland where there has been substantial effort since prehistoric times to improve soils, primarily for food production. The consequences are that most Scottish soils today exhibit many properties which are a legacy from past land management and improvement (Davidson and Smout, 1996). Soils thus contain evidence of human use in the past giving them an important cultural value today. The other reason why soils are of cultural importance is that they contain and preserve features of anthropogenic origin, in particular archaeological sites and associated artefacts. The aim of this chapter is to evaluate the evidence for changes in soils of significance at the landscape and individual site scales. In a current project being undertaken by Davidson and Wilson on behalf of English Heritage, the most significant threats to archaeology and the historic environment were considered to be:

1. Erosion (including sediment redistribution) and plough damage which cause physical damage to cultural remains and degrade the stratigraphic context for artefacts.

2. Dewatering and associated changes in soil moisture regimes and redox conditions; changes to more oxidising conditions are a particular threat to the survival of fragile organic materials and pollen.

3. Changes in soil and groundwater quality, particularly pH, through atmospheric deposition, land use and land management practices, and changes in drainage which can lead to increased degradation of certain types of artefact.

4. Loss of soil organic matter. Soil organic matter can be an archaeological resource in its own right, preserving evidence, for example, of past land management activities. Hence, its degradation results in loss of information. Soil organic matter is also important in stabilising soils. A reduction in soil organic matter concentration can leave soils more susceptible to erosion.

Soils' role in cultural heritage has some effect on the other soil functions. Where they protect archaeological remains, they should not be ploughed and used for crop or woodland production (although in a number of instances these soils have been ploughed, often by accident) and they should not be used as a raw resource or be developed.

9.3 Policy

Because of the lack of data, consideration has not been given to assessing the effectiveness of current policies in terms of protecting cultural soils.

9.4 Evidence

9.4.1 Cultural soils

The former Soil Survey of Scotland when mapping soils from the 1950s to 80s considered soils as 'natural entities,' so there was little consideration given to anthropogenic influences. In the Soil Survey Memoirs which accompany some published maps there is reference to the effects of land drainage and stone clearance. It is only on the Soil Survey map for Orkney that a deep-top phase of the Bilbster Series is mapped and attributed to past land cultivation/manuring (Figs 9.1 and 9.2). Such a soil is best described as a Hortisol and was formed by the addition of organo-mineral materials as manure to fields over centuries. Such soils are thus valued for their cultural and agricultural qualities.

Current research is establishing the presence of similar soils on the south of Fair Isle (Pears, pers. comm.) and on the edge of post-Medieval burghs in Scotland as at Nairn (Davidson et al., 2006), Pittenweem, Lauder and Wigtown (Golding, pers. comm.) where disposal of town waste played an important role in soil deepening. These valued soils on the edge of old towns are at greatest risk of loss due to sealing. The presence and importance of such soils have not to date been appreciated in Scotland. Plaggen soils are perhaps the best known anthropogenic soils and occur extensively on the North European Plain. In Scotland, their occurrence is limited to small and very remote localities - St. Kilda (Meharg et al., 2006) and Papa Stour in the west of Shetland (Davidson et al., 1998). On St. Kilda the topsoil on the old arable land below the village was deepened to 1.5 m, primarily as a result of manuring using fuel residues from the hearths and waste from the byres. One consequence was the enhancement in such elements as lead and zinc in the old arable land (Davidson et al., 2006). The deepened soil on St. Kilda is integral to the cultural record and contributes to the World Heritage status of the site. Interestingly, exactly the same applies on the west Mainland of Orkney with the deepened soils as part of the international designation there. Deepening of soils in Scotland was also achieved in many marginal areas through the formation of rigs - the ridging of soil to improve depth and drainage; rig-and-furrow is the most extensive archaeological feature in Scotland (Halliday, 2003). Historic Scotland through the formation of the Medieval or Later Rural Settlement ( MoLRS) advisory group in 1991 (MacInnes, 2003) in combination with the Royal Commission of Ancient and Historic Monuments for Scotland ( RCAHMS) through their detailed field surveys have done much to provide data and increased awareness of cultural landscapes including soils. One consequence has been the development of 'Guidelines for the preservation of areas of rig and furrow in Scotland' which includes guidance on soil investigation (Barber, 2001).

Figure 9.1 Distribution of the Bilbster Series and its deepened phase on Mainland Orkney.

Figure 9.2. An example of a deepened soil on the west Mainland of Orkney

9.4.2 Soils and archaeological sites

Change at the landscape scale

The Monuments at Risk Survey 1995 ( MARS) outlined rates and causes of identified monument loss in England, showing that 16% of recorded monuments had been completely destroyed by 1995, 8% since 1945. In 2002 Oxford Archaeology undertook on behalf of DEFRA a study on The Management of Archaeological Sites in Arable Landscapes ( DEFRA project code BD1701). Emphasis was given to such processes as cultivation of previously uncultivated sites, lateral impingement on undisturbed archaeological deposits, deeper ploughing, soil compaction, physical damage to artefacts and the effect of soil conditions in increasing the corrosion of metal artefacts. This report proposes a simple method of assessing the risk of damage occurring to particular sites. Considerations include the nature of the archaeological remains and quality of their survival, depth of current ploughsoil and extent/ thickness of previous ploughsoils, colluvium and alluvium over archaeology, soil characteristics (erodibility, drainage requirements, susceptibility to compaction etc), slope characteristics relevant to the likelihood of erosion, cropping patterns and rotation, and cultivation methods, depths and timing. Results are presented in the form of site maps showing risk to damage as a result of these factors.

In Scotland Burke (2004) undertook a large desk-based study, using a wide range of data on monument condition change and environmental characteristics, and demonstrated a number of relationships between monument distributions, rates of monument loss and environmental characteristics. The results of the desk-based study were calibrated using data obtained from aerial photographs, field survey and condition reports on scheduled monuments obtained from Historic Scotland. These results suggest that among monuments extant in 1850, a minimum of 38% have been reduced in extent, with at least 5% destroyed. Monument loss has been greatest in lowland areas, particularly in arable land, while monuments in upland areas have been subject to fewer pressures and, with the exception of those monuments found in areas of forestry, have survived better than their lowland counterparts.

Change at the site scale

Archaeological features are preserved within soils, thus any soil changes have potential impacts on the nature and quality of the archaeological record. Such changes can arise from surface erosion processes or from subsurface processes of soil change.

(1) Erosion

Cropmarks are subsurface archaeological features evident on aerial photographs of arable land and are caused by variations in soil moisture or depth. Features such as ditches, walls or banks can be identified under favourable soil moisture and land use conditions. Cropmarks are extremely important archaeological features and their identification plays a key role in the location of new sites. They are most evident in areas with sandy soils such as on river terraces and areas of glacial outwash. Concern is that if soil erosion occurs with concomitant surface lowering, then ploughing will cause progressive damage and ultimate loss of the feature. As a landsurface is lowered through erosion, then cultivation will progressively damage and ultimately destroy any buried archaeological features. Of the c. 7,800 scheduled ancient monuments in Scotland, approximately 1,000 are cropmark sites (Burke, pers. comm). In addition, approximately 250-300 scheduled ancient monuments in Scotland contain the remnants of field systems and other cultivation remains including rig and furrow.

A recent cropmark survey in the Lunan Valley, Angus, has been undertaken by Dunwell and Ralston (2005); one aim was to provide guidance on the management of cropmarks which occurred dominantly on the sandy soils of the Boyndie and Corby Associations. Excavation at selected cropmark sites demonstrated damage by cultivation. Use was made of the DEFRA system to assess the risk of damage and most of the excavated sites were judged to be at moderate risk. There are few studies which have quantified rates of loss on cropmarks due to the difficulties of such investigations. ¹³⁷Cs measurements from four sites in the Quantock Hills, Somerset suggests that over 4 mm of soil are being removed from some archaeological sites each year, suggesting that archaeological sites in the area are under threat over a timescale of 140 - 160 years (Wilkinson et al., 2006). It is presently unclear how typical the erosion rates calculated for the Quantock sites are, but similar rates are likely for other intensively cultivated areas, implying a significant threat to sub-surface archaeological sites. An erosion rate of similar magnitude has been determined for a cropmark site in Perthshire (Davidson et al., 1998; Tyler et al., 2001). The very few studies which do exist thus highlight the potential impact of erosion from water and tillage processes on archaeological cropmark sites. Areas of high potential erosion on light textured soils often coincide with areas of archaeological importance.

(2) Subsurface change

Changes in soil conditions, especially in drainage, oxygen status and pH, can result in an acceleration or deceleration in the rate of decay of different archaeological materials (Table 9.1).

Table 9.1: Preferential conditions (pH, drainage and oxidation) for preservation of different archaeological material, based on English Heritage (2002).

Likelihood of survival refers to moderately acid, moderately drained soil conditions. * poorly resistant - rarely survive; ** moderately resistant; *** highly resistant - usually survive. NB all materials are susceptible to physical damage in the ploughzone.

9.5 Gaps in evidence

Cultural soils

Much is known about cultural soils in general terms, but no detailed data exist on their extent and nature except in a few areas such as Orkney and other parts of the northern isles. More recent studies are beginning to reveal the existence of distinctive cultural soils on the fringes of 18 ^th and 19 ^th century towns. Cultural soils are distinctive and valued, thus meriting protection. The value arises from the cultural information contained within such soils as well as from the enhanced fertility status. However, there is a dearth of information on the effectiveness of scheduling in terms of protecting cultural soils.

Soils and archaeological sites

The fundamental problem in Scotland is that there is a lack of monitoring of properties of direct relevance to the preservation of archaeological features. There is also a lack of data on changes in soil conditions relevant to the preservation of archaeological evidence. Because of this, it is difficult to address 'what if' type questions, for example, changes in drainage in an urban area where there are extensive deposits.

9.6 Conclusions

• there is need to collate existing data from a range of sources on cultural soils and to ensure that such information is incorporated into national soil datasets to guide policy.
• policy initiatives are necessary to provide protection to these soils, particularly in areas subject to soil sealing.
• datasets on cropmark sites and soil erosion risk need to be integrated in order to guide policy on the managements of such sites.
• consideration is given to monitoring soil conditions in selected and vulnerable archaeological contexts
• studies should be undertaken to assess the effectiveness of scheduling with particular reference to the impact of ploughing and tillage erosion.

9.7 References

Barber, J. 2001. Guidelines for the preservation of areas of rig and furrow in Scotland. Scottish Trust for Archaeological Research.

Burke, A.D. (2004) Patterns in archaeological monument loss in East Central Scotland since 1850. PhD thesis, University of Stirling

Darvill, T. and Fulton, A. 1998. The Monuments at Risk Survey of England 1995: Main Report. Bournemouth: School of Conservation Sciences, Bournemouth University.

Davidson, D.A. and Simpson, I.A. 1984. The formation of deep topsoils in Orkney. Earth Surface Processes and Landforms 9, 75-81.

Davidson, D A. and Carter, S.P (1998) Micromorphological evidence of past agricultural practices in cultivated soils: the impact of a traditional agricultural system on soils in Papa Stour, Shetland. Journal of Archaeological Science 25, 827-838.

Davidson, Donald and Smout, Christopher. (1996) Soil change in Scotland: the legacy of past land improvement processes. In (Taylor, A.G., Gordon, J.E. and Usher, M.B eds) Soil Sustainability in Scotland. HMSO, Edinburgh, pp 44 - 54.

Davidson, D.A., Grieve, I.C., Tyler, A.N., Barclay, G.J. and Maxwell, G.S. (1998) Archaeological sites: assessment of erosion risk. Journal of Archaeological Science 25, 857-860.

Davidson, D.A, Dercon, G., Stewart, M. and Watson, F. (2006). The legacy of past urban waste disposal on local soils. Journal of Archaeological Science 33, 778-783.

Davidson, D.A., Wilson, C.A., Meharg, A.A., Deacon, C. and Edwards, K.J. (in press, 2006). The legacy of past manuring practices on soil contamination in remote rural areas. Environment International.

Dunwell, A.J. and Ralston, I.B.M. (2005). The management of cropmark archaeology in the Lunan Valley, Angus. Report No. 904, CFA Archaeology Ltd., Musselburgh.

English Heritage (2002) Environmental archaeology: a guide to the theory and practise of methods, from sampling and recovery to post-excavation. Centre for Archaeology guidelines. Portsmouth, English Heritage.

Halliday, S. (2003), Rig-and-furrow in Scotland. In Medieval or later rural settlement in Scotland: 10 years on (edited by S. Govan), Historic Scotland, pp 69 - 81.

MacInnes, L. (2003) Medieval or later rural settlement: 10 years on. In Medieval or later rural settlement in Scotland: 10 years on (edited by S. Govan), Historic Scotland, pp 3 - 6.

Meharg, A, Deacon, C, Edwards, K.J., Donaldson, M., Davidson, D.A., Spring, C., Scrimgeour, C.M., FeldmanN, J. and Rabb A. (2006; in press) Ancient manuring practices pollute arable land. Chemosphere.

Tyler, A.N., Davidson, D.A. and Grieve, I.C. 2001. In situ radiometric mapping of soil erosion and field-moist bulk density on cultivated fields. Soil Use and Management 17, 88-96.

Wilkinson, K., Tyler, A., Davidson, D., Grieve, I. (2006) Quantifying the threat to archaeological sites from the erosion of cultivated soil. Antiquity 80, 658-670.

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