Trials to Identify Soil Cultivation Practices to Minimise the Impact on Archaeological Sites (Defra project number BD1705) Effects of Arable Cultivation on Archaeology (EH Project number 3874) Known collectively as: ‘Trials’
Appendix 4: Studying the effects of different cultivation systems on archaeological earthworks
December 2010
Client: English Heritage and Defra
Issue No: 2 OA Job No: 1879
Appendix 4 The effects of different cultivation systems on archaeological earthworks
By K Spandl, C Champness, M L Dresser, M J Hann, and R J Godwin
Edited by P Booth and K Spandl
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Appendix 4 The effects of different cultivation systems on archaeological earthworks
1 Introduction ...... 1
1.1 Background ...... 1 1.2 Introduction to the project...... 2 1.3 Aims of the project...... 2
2 Agriculture and earthworks - background ...... 4
2.1 Affects of cultivation on earthworks ...... 4 2.2 Changes in earthwork management ...... 9 2.3 Why do we need to protect cultivation-damaged earthworks?...... 10 2.4 Rates of destruction...... 11 2.5 Previous experimental earthwork studies...... 12
3 Methodology...... 16
3.1 Introduction...... 16 3.2 Construction of the earthworks ...... 16 3.3 Monitoring Stations...... 18 3.4 Cultivation of the earthworks ...... 19 3.5 Monitoring and recording ...... 20 3.6 Final recording ...... 20
4 Results...... 21
4.1 General...... 21 4.2 Barrows ...... 21 4.3 Ridge and furrow...... 24 4.4 Ridge ...... 25 4.5 Slope...... 26
5 Discussion ...... 28
5.1 Transponder and soil movement ...... 28 5.2 Glass and sand indicator pits...... 29 5.3 Comparison between earthwork types...... 30 5.4 Summary of comparison between cultivation types...... 31 5.5 Survival of features below earthworks ...... 32
6 Validity of Results...... 34
7 Summary ...... 36
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8 REFERENCES ...... 38
Figure list
Figure 4.1 Layout of earthworks in the field at Cranfield Figure 4.2 Photograph of ridge and Furrow, pre-cultivation Figure 4.3 Photographs of barrows, pre-cultivation Figure 4.4 Photographs of ridge, pre-cultivation Figure 4.5 Construction and reprofiling of the earthworks Figure 4.6 Position of all monitoring stations within the earthworks, pre-disturbance Figure 4.7: Photographs of glass and sand indicator pits and disturbance Figure 4.8 Earthwork tillage Figure 4.9 Contour survey over non-inversion barrow years 0-10 Figure 4.10 Contour survey over non-inversion barrow years 15-30 Figure 4.11 Below ground disturbance - non-inversion barrow - years 0-10 Figure 4.12 Below ground disturbance - non-inversin barrow - years 20-30 Figure 4.13 Contour survey over mouldboard plough barrow years 0-10 Figure 4.14 Contour survey over mouldboard barrow years 15-30 Figure 4.15 Below ground disturbance - mouldboard plough barrow Figure 4.16 Cross Sections through mouldboard plough and non-inversion barrows Figure 4.17 Contour survey, direct drill barrow years 0-10 Figure 4.18 Contour survey, direct drilling barrow years 15-20 Figure 4.19 Pre-and post contour survey fallow barrow Figure 4.20 Contour survey, ridge and furrow years 0-10 Figure 4.21 Contour survey, ridge and furrow years 15-30 Figure 4.22 Below ground disturbance - ridge and furrow - years 0-1 Figure 4.23 Below ground disturbance - ridge and furrow - years 5-10 Figure 4.24 Below ground disturbance - ridge and furrow - years 15-20 Figure 4.25 Below ground disturbance - ridge and furrow - years 25-30 Figure 4.26 Cross Sections through ridge and furrow earthwork Figure 4.27 Contour Survey over ridge years 0-10 Figure 4.28 Contour Survey over ridge years 15-30 Figure 4.29 Below ground disturbance - ridge - years 0-1 Figure 4.30 Below ground disturbance - ridge - years 5-10 Figure 4.31 Below ground disturbance - ridge - years 15-20 Figure 4.32 Below ground disturbance - ridge - years 25-30 Figure 4.33 Cross Sections through the ridge - non-inversion tillage Figure 4.34 Cross Section through the ridge - mouldboard plough Figure 4.35 Contour Survey and below ground disturbance on the slope years 0-10 Figure 4.36 Contour Survey and below ground disturbance on the slope years 15-30 Figure 4.37 Re-excavation of indicator pits Figure 4.38 Post-cultivaton condition of indicator pits
Table list
Table 4.1: Recorded rates of truncation on earthworks Table 4.2: Earthwork statistics Table 4.3: Summary of height changes in earthworks Table 4.4: Summary of height changes
Sub-appendix Appendix 4A Cosmic Risk assessments
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1 Introduction
1.1 Background 1.1.1 Britain has a diverse and rich cultural heritage that is still visible within the arable landscape. The 'humps and bumps' seen within agricultural fields can reflect a wide range of archaeological sites; prehistoric barrows (burial mounds), the ramparts of Iron Age hillforts, Roman forts, medieval castle mounds, moats, lanes and evidence of previous agriculture in the form of ridge and furrow, field boundaries and lynchets etc. These earthworks cover a considerable time span and provide a visual link to our past. They are essential elements of our cultural landscape, and represent important educational and heritage resources. The loss of earthworks, historical farm buildings and field boundaries - the visible components of heritage - from the landscape, constitutes a lessening of individual regional character and reflects the spread of a more homogeneous modern agricultural landscape. 1.1.2 Rimmington (2004 pp 3) provides a convenient summary of why earthworks are important: Like all archaeological and historical remains, archaeological earthworks are a finite resource and any damage is irreversible and results in the loss of archaeological deposits, information and form. But it is not just for their archaeological value that archaeological earthworks are important. They are also important (significant); • for their contribution to the landscape • for recreational use • for their associations with people, memories, beliefs and events • and often for their ecology 1.1.3 Earthworks are also important in offering protection to potentially well-preserved archaeological features and palaeoenvironmental remains beneath them. 1.1.4 With the development of mechanised ploughing over the last 50 years, there has been a significant reduction in the survival of earthworks within the rural landscape (English Heritage 2003). Many of the earthworks that were identified and scheduled during the post-war era are now found to be ploughed flat and are either only identifiable as cropmarks or survive as small upstanding undulations in the landscape. From the air it is apparent that earthworks of any kind are a diminishing resource. In many counties, earthworks are now a rarity. Some monument types are reasonably well-represented, others are poorly-represented by a few surviving examples. One recent study of bowl barrows in South Cambridgeshire showed that those which stood on average 1.7 m high in the 19th century, were reduced to 0.4 m between 1960 and 1979, and to 0.2 m between 1990 and 2002. One of the conclusions of the study stated that it ‘it is feared that in another 30 years none of these will survive as earthworks’ (Thoden van Velzen 2003). 1.1.5 The function of a large number of these monuments is poorly understood and detailed modern investigations with records are rare. Without excavation or remote survey it is difficult to know what archaeological features and potential are being lost through the truncation of earthworks and whether the currently held interpretations of the earthworks themselves are correct. Without this information the approach to earthworks hitherto has been to schedule them and to recommend that they be put
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down to pasture where possible, whatever their actual survival, form, rarity, or location, to ensure that important remains are not lost. Many, however, still remain in cultivation. 1.1.6 This study of earthworks reflects the growing need for scientific research to underpin policies of heritage protection, arising from developments in the Planning Policy Guidance and potentially in heritage legislation discussed in Appendix 3.
1.2 Introduction to the project 1.2.1 The earthwork study was designed to investigate the effect of different cultivation systems on the morphology, soil movement and archaeological features within and below earthworks, over a 30 year, accelerated timeframe. A total of 6 sets of 5 cultivations was carried out on each earthwork over a 2 year period. 1.2.2 The study examined the rates and types of damage to earthworks caused by differing types of cultivation, and possible mitigation measures which could be applied to prevent or reduce damage. One of the main issues to be considered was whether earthworks necessarily need to be removed from cultivation and placed under pasture to guarantee their ongoing protection, or whether in certain circumstance a reduced cultivation regime, such as non-inversion tillage or direct drilling, may offer alternative approaches to long term protection. The reversion of all scheduled monuments to pasture would have significant resource and financial implications, whereas the use of reduced cultivation techniques may be less of a financial and practical burden for farmers. The study attempts to consider all these factors in the light of the experimental results, and to develop management guidelines to help protect earthworks within arable fields and in a range of circumstances (Appendix 5). 1.2.3 No real archaeological earthworks were harmed during the course of this study. All tests were undertaken on replica earthworks that were constructed as part of the experiment. The design of these earthworks was based on excavated archaeological examples; their form represented that of earthworks that had been previously ploughed or eroded. As in the flat experimental sites (Appendix 3), different coloured glass chips and sand, and radio transponders were used in each earthwork to track the movement of soil, measure disturbance depth and to monitor earthwork degradation.
1.3 Aims of the project 1.3.1 The main aims of the earthwork studies were: • to assess the effects of different tillage operations on the survival of a range of earthwork forms • to test whether the adoption of non-inversion tillage or non tillage cultivation could be used as an alternative to conventional ploughing or pasture in order to offer greater protection to archaeological earthworks • to make a series of suggestions that would help to protect earthworks that are currently within arable fields (Appendix 5). 1.3.2 Detailed research aims were also identified. These were: • to monitor soil movement and cultivation truncation across a range of earthwork forms to see whether any of the original earthwork soil would survive in position • to assess the potential loss of information that can occur when an earthwork is placed under cultivation • to assess the survival and preservation of buried soils and other archaeological deposits within and below earthworks under cultivation
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• to try and define the critical height to which an earthwork must survive in order to preserve a buried soil or other deposits below it, and identify when the threshold effect is likely to be reached at which these deposits are disturbed and destroyed • to examine the potential movement (displacement) of earthworks through tillage
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2 Agriculture and earthworks - background
2.1 Affects of cultivation on earthworks 2.1.1 To provide some context in relation to the issues of damage and possible mitigation examined here, the following information has been summarised from OA 2002 and from additional, subsequent research. An understanding of these issues allows a clearer understanding of both the questions investigated here and the management solutions developed in Appendix 5. 2.1.2 Modern farming techniques inevitably degrade or destroy earthworks by: • the preparation (including bulldozing) of earthworks prior to cultivation • cultivation which can flatten earthworks and disturb buried deposits and features within them • lateral encroachment at the edges of earthworks which can help erode them • soil erosion on earthworks and slopes which reduces ploughsoil depth and enhances the risk of damage to underlying archaeological deposits from further cultivation
Preparation of earthworks prior to cultivation 2.1.3 The preparation of land prior to cultivation can be enormously damaging to earthworks. Cultivation of reclaimed areas for the first time often involves drainage operations and use of large earth-moving machinery. There are cases where burial mounds have been flattened with bulldozers prior to cultivation; for example, Sale’s Lot Long Barrow at Withington, Glos. (O’Neil 1966), originally stood 117 ft long, 60 ft wide and 3 ft high before being flattened by bulldozers. This fate also befell prehistoric earthworks near Badbury Rings in Dorset (Gingell et al. 1987) and a Roman settlement at Thelverton, Norfolk (Broughton 1998). A Neolithic long barrow at Uplowman Road, Tiverton, Devon, was levelled prior to cultivation, before it could be scheduled, with the result that only the below-ground remains were left for scheduling (Smith 1990). 2.1.4 Bulldozing of earthworks, whilst extremely damaging to the earthworks themselves, can in certain cases lead to the differential survival of any below-ground features, as the spoil from the earthworks is pushed over the undulating surface of the field to level it up, especially in lower lying areas. Features in these areas may be inadvertently preserved as a result of such levelling, leading to misconceptions of likely survival when assessed from the surface. Cultivation of the earthworks themselves 2.1.5 The cultivation of earthworks leads eventually to their destruction as discussed in detail below. Lateral truncation 2.1.6 Sites that exist as islands within cultivated fields are vulnerable to gradual encroachment by cultivation, sometimes only a furrow’s-width at a time, destroying upstanding monuments bit by bit. Encroachment by cultivation is known to alter the shape of upstanding monuments gradually, making their interpretation and identification difficult. For example, at Hazleton long barrow, Glos., ploughing gave the barrow an artificially rectangular shape (Saville 1990).
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2.1.7 Movement of earthwork soil can also cause misunderstanding as to its original location and centre point. For example at Sutton Hoo Bruce Mitford felt ‘that the western end of mound 1 had already been ploughed away by the time the attempted robbing took place, so confusing the robbers as to where the centre really was’ (Carver 2005 pp 465). 2.1.8 Gradual encroachment, if not halted, can eventually lead to the total loss of a monument. If the earthwork survives to a reasonable height the gradual chipping away can result in undercutting of an upstanding monument, causing mini-collapses every time the plough share engages with the edge of the monument. Encroachment can also occur accidentally around upstanding monuments, as the visible earthwork is often only a small part of the overall archaeological resource. A burial mound, for example, will often have an associated, but unseen ditch around it, or there might be associated settlements, field systems or other ritual monuments in the near vicinity, which may often be unrecognisable at ground level. 2.1.9 At the Rollright Stones, Oxon., ploughing has occurred to within 0.5 m of the railings protecting the Whispering Knights standing stones (which originally formed a chamber within a burial mound). This has resulted in the reduction in the ground level on the south side of the stones by about 0.5 m since 1920 (Lambrick 1986). More rapid truncation has occurred in the last 10 years, when ploughing has encroached even closer to the stones, resulting in up to 0.15 m of soil being washed away from around the base of the stones within the protective railings. This loss has been recorded over the years through periodic surveys (a graphical representation of these results can be seen in OA 2002, Appendix Jvii). Soil erosion through solution 2.1.10 Differences in the rate and character of erosion between areas of subsoil protected beneath earthworks and subsoil in surrounding unprotected areas, especially on chalk geology, can result in the survival of the subsoil under the earthwork at a higher level than that away from it. Even after the complete removal of the earthwork this can still give the (false) impression of a slight surviving upstanding monument, whereas all that survives is in fact the upstanding remnant subsoil, formerly protected by the earthwork. 2.1.11 For example, at the western end of the Stonehenge cursus a slight rise topped with richer vegetation was thought to be a barrow. The feature was subsequently totally excavated, but no mound material or buried soil remained. The slight rise was formed by the upstanding chalk bedrock which had originally been protected by a burial mound. Recent ploughing had removed what remained of the mound and only a few inches’ thickness of chalky topsoil existed on the surface of the rotted natural chalk (Christie 1960). Another ‘ghost mound’ was excavated at Park Farm on the Berkshire Downs; no stratigraphy survived and the modern topsoil lay immediately over the raised natural (Richards 1990a). This differential erosion may be associated with natural solution processes accelerated by modern agricultural techniques (for further discussion of this see Atkinson 1957; Proudfoot 1965; Groube and Bowden 1982; OA 2002). 2.1.12 This issue was explored by Lawson in The past under the plough. Lawson writes: Much information on the degree of survival of earthworks has come from excavation. The excavation in 1971 of a round barrow at Harpley showed that although the barrow was visible as an earthwork virtually no mound material survived in situ. The phenomenon of differential erosion beneath and surrounding an earthwork is well known. Usually some protection is afforded to an ancient soil sealed beneath a barrow, while the surrounding
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bedrock is more actively weathered and its overlying soil eroded. A modern ploughsoil can be as thick as 35 cm. Hence, it is possible that an old ground surface, with its wealth of environmental evidence, will not survive ploughing once the barrow mound has been reduced to less than 50 cm (Lawson 1980 pp 76).
2.1.13 This leads to problems in both interpretation and management. If archaeological remains do survive, these rather unimpressive monuments can be very vulnerable to destruction. Only one episode of slightly deeper cultivation could destroy any surviving earthwork material and/or buried soil, and therefore destroy any archaeological evidence that there had once been a feature at this point. This illustrates the problem in judging the ‘threshold effect’, in which a single season of attrition, as the culmination of an extended sequence of such events, can wipe out the remaining, and perhaps the most important, evidence of an archaeological site. Soil erosion and topography 2.1.14 The effect of slope on the survival of archaeological sites can be variable. Archaeological sites located at the tops of slopes are very vulnerable to erosion as the covering soil erodes downslope. This movement of soil downslope leads to the deposition of material (colluvium) at the base of the slope, or against any obstruction across the slope. Where colluvium has been deposited, the overburden may protect and mask archaeological sites underneath. The presence of alluvium on valley floors will have the same masking and protective effect. On the slopes themselves, soil cover is reduced through being eroded downwards; underlying deposits are then increasingly vulnerable to further attrition by the plough. 2.1.15 The same principles affect upstanding earthworks. Protective soil on the top and sides of an earthwork will erode downwards, causing loss of both the earthwork profile and any features within the eroded profile, such as satellite burials within round barrows. Conversely, however, eroded soil will congregate at the base of the earthwork slopes and in hollows, therefore helping to protect the archaeological resource at this level. 2.1.16 Rates of erosion can be affected by rainfall, and exceptional rainfall events are particularly important in the context of long-term erosion rates over slopes. In the South Downs ten-year monitoring scheme, erosion in one year (1987-88) accounted for 72% of the decadal total, and 89% of all soil loss occurred in three of the ten years (Boardman 1992a). Exceptional rainfall events may or may not result in serious erosion, since many summertime storms fall on agricultural landscapes which are well vegetated. Boardman et al. (1996) describe an extensive and severe thunderstorm over southern England in May 1993. Erosion was confined largely to recently drilled linseed and maize fields – a small proportion of the landscape. However, erosion rates were very high where over 100 mm of rain fell in four hours on bare, recently drilled maize at Faringdon. The high risk of erosion associated with winter cereals in southern Britain is largely related to the existence of bare ground at the wettest time of year. High frequency, low magnitude rainfall events lead to the initiation of rills. Moderate to high magnitude events give rise to much more serious erosion, such as that seen in October 1987 on the South Downs (Boardman 1988a). 2.1.17 Andrew Burke of Stirling University has undertaken a detailed study of cropmarks near Perth (funded by Historic Scotland), during which he examined the correlation of topography with the survival of cropmarks. As part of this process he undertook detailed contour surveys within his study area and related these to the results of auger transects and trial trenching across areas of known archaeological sites. He was therefore able to show the exact correlation between slope and depth of topsoil and to
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further tie down the link between slope, depth of ploughsoil and survival of archaeological features. 2.1.18 Burke concluded that: The second key objective of this section of the research is to identify cropmark sites or parts of cropmark sites at greatest risk of damage from ongoing agricultural use. The excavations have demonstrated that the condition of the subsoil surface is closely related to the depth of the topsoil above it, which in turn is closely related to topography. It is likely that cropmark sites situated at knoll summits or on slope shoulders are at risk, as this is where topsoil is likely to be shallowest, while cropmarks located at slope foots are likely to be protected by the increased depth of topsoil. Of course, the actual risk posed to the archaeological features will also be heavily dependant on the types of cultivation being undertaken (Burke 2002 pp 49).
2.1.19 This work clearly demonstrates the importance of studying micro-topography across fields, in that localised areas of convexity are at greater risk from cultivation damage than areas of concavity. 2.1.20 Slopes can be categorised as follows: • Low slope/flat - less than 2 degree angle • Gentle slope - 2-5 degrees • Medium/mid slope - 5-15 degrees • Steep slope - 15-45 degrees 2.1.21 Defining the degree of slope likely to be susceptible to soil erosion is important when trying to predict archaeological survival. Anything in the mid-slope range upwards is likely to trigger soil movement, therefore making archaeological sites on these slopes vulnerable to damage. The occurrence of erosion on less steep slopes is less easily predicted and depends much more on a complex combination of variables which may include the type of soil, drainage, cultivation regime, compaction, crop cover, location and nature of field boundaries, time of sowing and so on. 2.1.22 For example, at Icklingham Roman villa, Suffolk, substantial soil movement can be seen to be occurring despite the fact that this site is on a slope of exactly 2 degrees. The soil here is mainly sand and gravel and C Pendleton (pers. comm.) has observed that a c 1 m thick deposit of plough wash has moved downslope. Also in Suffolk on the sandy soils of the Brecklands, deep layers of hillwash (c 0.30+ m thick) have been deposited following a single thunderstorm in areas with very slight slopes. A 1 m deep gorge resulted after just two days of heavy rainfall at Bromswell in the Sandlings (Pendleton 1999 pp 44). 2.1.23 Excavations associated with the insertion of a pipeline in the Lower Welland Valley, Suffolk, revealed that ‘hillwash’ was occurring in a landscape considered by most to be flat. The field survey showed that wide flat lynchets have accumulated upslope of hedge lines, even on relatively gentle slopes. Visually this resulted in gaps in cropmark coverage at the base of the gentle valley slopes below the 20 m contour line. The rate of colluvial accumulation over the last 2000 years was estimated at 0.25-0.5 mm a year and the period of time required for the formation of lynchets c 300-800 mm in height was thought on this basis to be c 500 years (Pryor et al. 1985). 2.1.24 In the Milfield Basin on the Cheviot slopes (characterised by intensively farmed, small fields on steep profiles), the build up of lynchets at the base of many of the fields averages from 1.5-3 m, indicating a very high level of soil movement (Waddington 2001 pp 9). At Buckles, Frocester a rare, buried possible Mesolithic
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ground surface and feature were discovered c 1.6 m below colluvium formed at the base of a slope (Darvill and Timby 1985). 2.1.25 Many sites display these characteristics; at Cotton Henge, Raunds, it was concluded that: excavation revealed a close relationship between topography, the depth of ploughsoils and the severity of truncation to the archaeological features: on the higher contour areas, the overburden is as little as 0.2 m, ranging to over 1 m downslope (Humble 1994 pp 177).
2.1.26 Even small changes in topsoil depths caused by soil creep or erosion can dramatically affect the survival of archaeological deposits. At Liglok Field, Westhide, Herefordshire, the field had been mainly under cereals for some 30-40 years. Over the last 10 years of the 20th century farming intensified, resulting in the removal of field boundaries and a change in plough direction from ‘along slope’ to ‘cross slope’ coincident with the use of heavier machinery and more deeply-penetrating ploughs. At the base of the slope the ploughsoil was recorded as 0.35-0.40 m deep above the subsoil. Here, well-preserved features, including a Roman furnace, were discovered. A trench cut into the slope of the hill revealed the ploughsoil to be c 0.25-0.30 m thick. A further trench excavated on a flat spur on top of the hill slope found the ploughsoil to be only 0.2 m deep over the subsoil and the features recorded here were severely truncated. It was concluded that the archaeological features in these upper trenches could soon totally disappear with repeated soil movement and ploughing (White 2001, Appendix Jii). 2.1.27 However, the presence of a slope does not always lead to the formation of colluvium. As part of the Stonehenge Environs project, investigations revealed a total lack of substantial colluvial deposits in the dry valleys. It is thought that the relatively immobile land surface in the Stonehenge area has worked to inhibit hillwash and the formation of colluvial deposits (Richards 1985). 2.1.28 The effects of cultivating a shallow slope are examined here as part of the present study. The ‘ Threshold effect’ 2.1.29 This is relevant to archaeological sites of all types, including earthworks, and describes a situation in which a single season of plough truncation wipes out the remaining, and perhaps the most important, evidence of an archaeological site. With earthworks it can be particularly hard to judge when this stage is reached. With small, often scheduled, upstanding mounds it is often difficult to tell if the earthworks represent only raised areas of natural subsoil which have in the past been protected by a mound that no longer exists, or whether they contain archaeological evidence which it would be worthwhile to continue to preserve. If archaeological remains do survive, these rather unimpressive monuments are very vulnerable to destruction. Only one episode of slightly deeper-than-usual cultivation could destroy any surviving mound material or buried soil, and therefore destroy any archaeological evidence that there was once a feature at this point. This can be illustrated using Lawson’s observations at Harpley discussed above (Lawson 1980). 2.1.30 Another example relates to the discovery of a rare earlier Bronze Age gold vessel, the ‘Ringlemere Cup’, one of only two known from this country, found by a metal detectorists in an arable field in east Kent in 2001. The cup had been badly crushed, possibly by subsoiling which might have dragged it from its original burial spot. Later survey and subsequent excavation within the field at the find-spot confirmed that there was a low remnant mound surrounded by a substantial circular ditch. Pottery
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and flintwork from the mound material, and from small pits, showed that there had been Late Neolithic (Grooved Ware) activity on the site. The cup itself appears to have come from within the mound material. There was no evidence for either a cut feature or a buried soil; these had most likely been removed by ploughing (Needham et al. 2006). 2.1.31 This issue is examined here both with survey data and by examination of objects buried at known depths, so that actual depths of disturbance can be assessed.
2.2 Changes in earthwork management 2.2.1 After the method of ridge and furrow cultivation, mainly of medieval date, became obsolete, in many areas the land was put down or left to pasture with the result that these earthworks were preserved. They were often only ploughed again in the 18th/19th centuries, as a result in part of the development of technology enabling under-draining of the fields, a function performed by the furrows during the medieval period. The fact that many of these earthworks survived up until quite recently suggests that the early ploughing did not cause the damage that modern ploughing implements can do. Both in the 1940s, as part of the war effort, and more damagingly from the 1960s to the 1990s, as a result of the availability of subsidies and more effective machinery, further pasture and marginal land were ploughed. It is only therefore in the last 50 years or so that many earthworks have been destroyed or seriously eroded as a result of the introduction of large and powerful machinery. 2.2.2 Most recent changes within our agricultural landscape have often reflected economic pressure and changing governmental and European legislation. With falling meat prices, the spread of diseases like BSE and the ending of subsidies, large areas of pasture and set-aside have been cultivated in recent years. This has also coincided with an increase in the price of wheat and other produce that has encouraged farmers to plough up areas of ancient pasture. These areas have tended to be the last refugee of surviving earthworks like ridge and furrow and barrows. Previous agricultural policies of hedgerow removal have also helped accelerate the rate of destruction by increasing field sizes in order to make them workable by larger machines. 2.2.3 On the face of it, agricultural and archaeological interests seem to be at odds with each other. However, some farming practices can also help preserve archaeological sites. Earthworks are well-protected in pasture (as long as stock damage is controlled) and agri-environmental schemes are promoting responsible land management on archaeological sites. Higher fuel costs and competitive markets have made some farmers think more favourably about adopting less intensive cultivation systems such as direct drill and non-inversion tillage systems, as well as developing better soil management strategies. Financial incentives to farmers, through agri-environmental stewardship schemes with access to archaeological expertise, can help protect archaeological sites whilst still allowing business development. 2.2.4 Since the introduction of agri-environmental schemes in England in 1987, a number of key sites with preserved earthworks have been given protection through the scheme; some of these have remained in agricultural use and some not. 2.2.5 The scheduled Neolithic henge complex at Thornborough includes three henges and a variety of associated monuments and features, including several burial monuments. This area has been identified as one of the most important Neolithic sites in the country and is a target area for the Higher Level Stewardship scheme. Here non- inversion and direct drilling has been introduced to protect the most vulnerable archaeological sites in cultivation.
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2.2.6 Braunton, in North Devon, contains a rare survival of an intact medieval open field that covers 142 ha of Grade II arable land. It was never enclosed and is still divided into strips, which average 0.2 hectares, separated by thin strips of grass, known locally as ‘landsherds’, which are vulnerable to loss through ploughing. The scheme has ensured that the landsherds are not ploughed but cut every year, that no fertiliser is applied and that furlong boundaries are managed. Given the agricultural value and quality of this land, it has taken time to persuade farmers that the scheme is worthwhile and will not detrimentally affect their farming systems. 2.2.7 The Roman fort at Clayhanger, also in Devon, was discovered through aerial photography followed by ground survey that revealed excellent survival of the enclosing ramparts as earthworks standing about 0.5 m high. The field was under arable cultivation, and the ramparts, at the top of the slope surrounding the site, were considered under significant threat from cultivation. An application for a Countryside Stewardship agreement, received in 1999, included management works to boundaries and in the margins of arable fields and the reversion of the fort to permanent grass from arable which, by that time, was under ‘set-aside’. The earthworks of the 16th- 17th century fort at Cudmore, Essex, have now been reverted to permanent grass, to be managed under a sustainable stocking regime with no application of fertiliser. 2.2.8 The number of sites within such schemes has been steadily growing over the last five years and is expected to continue to grow in the future. The potential scope of the scheme has highlighted the need for scientific research like the Trials Project to underpin the management proposals suggested by these schemes (Appendix 5), especially now that the option of set-aside no longer exists.
2.3 Why do we need to protect cultivation-damaged earthworks? 2.3.1 One of the fundamental questions raised in this study is whether cultivation-damaged earthworks are worth protecting. Certainly well-preserved upstanding earthworks have been shown to provide considerable archaeological and palaeoenvironmental evidence. However, some archaeologists dismiss cultivation-damaged earthworks as worthless ‘lumps and bumps’ in the ploughsoil, with the majority of archaeological material seen either as completely destroyed or, at best, significantly damaged. 2.3.2 By definition, earthworks that are in cultivation have been damaged and some, if not all, of the archaeological evidence may have been lost. The key question is whether cultivation-damaged earthworks still retain sufficient archaeological and palaeoenvironmental potential to justify the additional resources required to ensure their protection. Issues of preservation can only be established on a site by site basis, and are determined by a number of local factors, some of which are considered here. 2.3.3 Many studies of cultivation-damaged barrows have revealed the presence of significant buried soils and negative features preserved below the earthworks, and finds and associated ploughsoils and features within the earthwork, for example satellite burials contemporary with, or later than, the earthwork. 2.3.4 A study by French of the truncation of cultivated barrows, commissioned by OA (OA 2002), drew together his work on earthworks at Wyke Down in Dorset. The Wyke Down barrow group consists of only seven upstanding mounds out of a total of 29 ring ditches, illustrating the extent of destruction of these monuments over the years. The ploughed-out barrow mounds are just visible under the right conditions as very slight rises in the field, but are otherwise undetectable on the ground. 2.3.5 One of these barrows least affected by plough damage demonstrates some of the features that can be found within an earthwork if it is not too denuded. Such features
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included evidence of the turf used to construct the mound, wooden revetments of stakes which would have held it in place and evidence that there were actually two mounds built at different periods. Finds included significant amounts of worked flint, possibly in situ on the original ground surface, which also still survived. One half of this mound had been ploughed for the last 45 years, the other half had been put to pasture for 15 years, resulting in a ‘less abrupt and gently sloping tail to this part of the barrow mound’. Ploughing had also truncated c 0.52 m of subsoil under the mound in a series of three steps. 2.3.6 As mentioned above, earthworks are not only important archaeological sites in their own right but even degraded/ploughed examples can preserve earlier archaeological features beneath them, including the buried soil on which they were constructed. Palmer has studied the importance of earthworks in the form of ridge and furrow, not just for their own sake but for what they may protect underneath. Protection is best in the area of the ridges where it has been shown that up to 50% of the old land surface may survive (Palmer 1996). By contrast, in the area of the furrows, which are cut into the subsoil, earlier archaeological deposits are likely to be truncated although, once truncated, movement of soil from the ridges into the furrows as a result of cultivation may preserve anything left within them. Palmer cites examples where cropmarks of earlier features below these medieval earthworks are only now coming to light as the ridges have become thin through tillage erosion (see Appendix 3). 2.3.7 Examples of where ridge and furrow and their associated headlands have caused differential survival of earlier features have also been seen in excavations at Barton, Watkins Farm, Northmoor and at Gravelly Guy (all Oxon). At Watkins Farm a zone of better preservation c 15 m wide was identified during the excavation of an Iron Age enclosure. This zone was interpreted as having been protected under a medieval ridge and furrow headland, whereas the rest of the site had been truncated by ploughing to the natural gravel. One further example of the protection offered to archaeological sites from medieval headland boundaries is graphically illustrated by work undertaken at Cotswold Community, Glos., where in part of the site the paucity of surviving archaeological features in the middle of the open fields is in stark contrast to the mass of features recorded under the headlands (OA 2006). 2.3.8 Finally, any remains of earthworks act as visual markers within a landscape. They indicate the presence of archaeological sites and activity within a particular area and they enhance the visible heritage setting. 2.3.9 The cultivation of earthworks not only damages the physical remains but can also have a significant affect on how the monument is perceived. Cultivation-damaged earthworks are much less likely to be subject to monitoring, research and further protection measures than monuments not in cultivation. Once earthworks are placed under cultivation and denuded their value is deemed to be diminished to the extent that such sites are often neglected, regardless of the fact that many continue to have considerable archaeological importance.
2.4 Rates of destruction 2.4.1 Ridge and furrow earthworks are disappearing rapidly. In a study by Hall (2001), summarised by Anderton and Went, it was shown that ridge and furrow, once ubiquitous in the East Midlands, is now becoming rarer year by year (Anderton and Went 2002 pp 53). By using aerial photograph evidence up to 1996, the report confirmed that of the 2000 townships identified within the study area, as few as 104 retained more than 18% of their original coverage of ridge and furrow (ibid., 54). Most of this destruction had occurred in recent decades. Hall’s (1993) extensive survey of ridge and furrow in Northamptonshire showed that in many parishes there
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had been a loss of 60-80% of ridge and furrow from the 1960s/70s up to 1990, with only a 12% survival rate in 8 selected parishes. 2.4.2 Hall predicted that in 50 years (ie from 1993) there will be no ridge and furrow left in Northamptonshire and that in some parishes it will disappear within 5-10 years, unless effective protection is provided (ibid., 24-25). In a similar study carried out in Warwickshire and Worcestershire, up to 67% of ridge and furrow that had survived up until the mid 20th century had been lost, mainly to arable, at an annual rate of loss of 2-3% per year during the last 40 years (Hurst 1997). In Ripping up History (English Heritage 2003) it was estimated that 94% of the ridge and furrow in the East Midlands has been destroyed. This threat has not gone away; surviving areas are still under threat and being lost. Other earthwork types at risk include the prehistoric barrow cemeteries that can be seen on ridges and upper slopes across the country. In Essex it is reported that fewer than 10 out of some 1200 burial mounds now survive as ploughed-damaged earthworks (ibid). 2.4.3 As mentioned above a study examining the condition and management of a series of funerary monuments in Cambridgeshire and Peterborough (Thoden van Velzen, 2003) showed how rapidly this resource was disappearing. In addition to the figures quoted in Section 1.1 and Section 2.5 the study showed that: • 122 monuments were selected for review for scheduling, but when reviewed 14 of these sites were found to have been destroyed by ploughing1 • out of 12 scheduled bowl barrows in Cambs. almost half only existed as cropmarks close to complete destruction in arable • 16% (6 out of 38) of all previously scheduled bowl barrows were descheduled because they were completely destroyed by arable cultivation. Another 13 survive under arable cultivation; three as earthworks and 10 as cropmarks only • 70% of barrows are situated in fields under arable cultivation, with 57% of scheduled barrows now preserved as cropmarks only
2.5 Previous experimental earthwork studies 2.5.1 Past experimental earthwork studies, of which there have been a significant number, have tended to concentrate on the important initial pre-and post-burial processes and on natural erosion processes. Such studies include those undertaken at Overton Down (Jewell and Dimbleby 1966; Bell et al. 1992; Crowther et al. 1996) and those at Wareham (Evans and Limbrey 1974; Lawson et al. 2000; Macphail et al. 2003) and at Butser (Reynolds 1978). None of these studies have addressed the question of how surviving earthworks have been affected by modern farming practices. Natural processes 2.5.2 Studies like the Experimental Earthwork Project (Bell et al. 1996) on burial environments within barrows have shown that changes in the morphology of earthworks and the preservation of archaeological material within them can occur rapidly following burial. This work has also revealed that changes due to land-use factors, such as the type/intensity of grazing, can have significant effects on soil chemistry and artefact preservation. 2.5.3 Of particular interest is the biological evidence preserved within the buried soil sealed by earthworks. These buried land surfaces have been found to preserve pollen, molluscs and seeds which have provided a means of investigating the landscape of the monument. On chalk soils, at Overton, the buried soil exhibited significant
1 It is not clear from data studies whether just the earthwork was destroyed or whether this refers to below ground remains too
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evidence of reworking by earthworms and other soil animals in the first 32 years of the experiment (Macphail 1996). The earthworms were found to have carried humic topsoil and other forms of environmental evidence up into the barrow mound. In contrast, the sandy soils at Wareham exhibited no evidence of mixing across the buried soil surface. 2.5.4 Work here shows that consideration must be given to the vulnerability of the buried soil to bioturbation, both just after burial and when the earthwork cover reaches a certain decreased height, particularly in pasture. The process of bioturbation can disturb or destroy the original soil profile and may eventually make identification of layers difficult, as previously stratified deposits become mixed. The level of this reworking depends on the level of earthworm populations. Ploughsoils tend to have very low biological activity and so reworking is kept to a minimum. However, reducing tillage depth or placing an earthwork under pasture could increase the level of biological activity and disturbance by earthworms, which of course is actually good for the fertility of the soil itself.
Rates of cultivation truncation 2.5.5 The only recent large-scale study looking into the rates of plough damage over a series of monuments was that undertaken between 1998-2002 of funerary monuments in Cambridgeshire (Thoden van Velzen 2003) as part of a wider study considering their condition and future management. Some of the statistics from this survey included: • bowl barrows were reported to stand on average 1.7 m high in the 19th century, 0.4 m high between 1960 and 1979, and 0.2 m high between 1990 and 2002 • some barrows surviving in woodland measure between 2.1 m and 3 m high and are considered close to their original height
2.5.6 Table 4.1 below summarises changes in individual earthwork height and overall average heights. 2.5.7 The only long term, detailed study specifically looking into the truncation of a particular earthwork by agriculture, in this case by minimal cultivation techniques (non-inversion tillage and then direct drill), was that carried out by English Heritage at Rockbourne and published as part of the OA (2002) study. The long barrow was scheduled in 1977 and subjected to a light cultivation regime. In order to monitor the effects of this ongoing cultivation a series of annual topographic surveys was undertaken between 1982 and 1991, and a further survey, as part of the OA/MAFF (2002) work, in 2001. A number of caveats/problems were raised but the results of this work were summarised as follows: The 1982-1991 surveys show a gradual erosion of the top of the mound, with a corresponding filling of the ditches at either side (Wilson 1991). Between 1982 and 1991 the maximum height (at the north end of the mound) dropped from 2.16 m above TBM to 2.10 m above TBM, with a corresponding loss of the 2.15 m contour, and the reduction of the 2.00 m contour from a single oval to two smaller ovals separated by a 2 metre gap. By 2001 this maximum height had dropped to 2.05 m, and the 2.00 m contour had been reduced to a single oval at the north end of the mound. The ditches on either side of the mound show a corresponding rise in contours, indicating the redistribution of material from the mound into the surrounding hollows. In 1991 Pete Wilson felt that there was more infilling to the south of the mound, representing general downhill movement of soil from the top of the mound. The amounts of deposition quoted from 1982-1991 (0.05-0.10 m) are
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consistent with a comparison of the 1982 and 2001 data, suggesting that the rate of deposition around the base of the mound slowed down between 1991 and 2001. The rate of erosion at the top, however, appears to have been consistent over the two decades, suggesting that it can be projected forward to determine the likely state of the mound at any point in the future if the management regime is not changed.
Overall, the data suggest that the top of the mound is still eroding steadily even under the direct drilling regime, while there is less soil accumulating around the base. This might represent the effects of soil compaction over the relatively soft ditches that masks any perceived accumulation of soil now that the field is no longer ploughed (MAFF 2002, Appendix Jviii, 2).
2.5.8 Apart from the above there are relatively few data relating to annual rates of truncation from ploughing and other agricultural activities, but this information has been recorded for some monuments. These records suggest typical rates of truncation of 0.02-0.05 m per year (OA 2002). 2.5.9 Other data on estimated rates of truncation of earthwork monuments, albeit not derived from systematic long term recording, are assembled in Table 4.1 below.
Site Rate of Damage Average Ref damage per year Neolithic Causewayed Banks being eroded at c 0.04 m per year initial 0.05 m Drewett 1975 Enclosure in Sussex with initial truncation being most rapid at then 0.04 m c 0.05 m per year Norfolk Mean annual truncation of earthworks 0.022 m Lawson 1980 from ploughing has been calculated at 0.022 m per year Walton Basin 6 barrows lowered by ploughing by 0.015-0.05 m Gibson 1998 between 0.3 m to 0.95 m over the last 20 years Alfriston Neolithic 2 m high in 1914 and 1934 but only 0.25 c 0.05 m Drewett burial mound (Sussex) m of mound left when excavated in 1975; 1980 1974. Suggested that annual rate was probably greater in first few years as the mound was steeper with more hill-slope truncation occurring Gosbecks, Essex Depth of mound recorded between 1948- 0.006 m Crummy and 1977. Showed 0.15-0.20 m lost from Smith 1979 profile Rockbourne, Hants See above Thornborough Henge, Was uncultivated and 1 m high in 1864 0.012 m from Harding and N Yorks. and 0.9 m in 1952 (still uncultivated) 1952-2003 Johnson 2004 Cultivated and reduced to 0.30 m in 2003. Lost 10% of its height in nearly 100 years, compared to 66% when under cultivation in just under half that time Bowl barrow 33369 Present height 1 m, height in 1978 1.8 m. 0.04m Thoden van (Cambs fens) Only ploughed since 1972 Velzen 2003 Bowl barrow 33370 Present height 1 m, height in 1978 1.5 m. 0.02m ibid (Cambs fens) Only ploughed since 1972 Bowl barrow 33372 Present height 0.5 m. 1954 - 1 m. 1968 - 0.01m ibid (Cambs fens) 0.5 m - situated on edge of field Bowl barrow 155368 Present height 0.6 m. 1949 - 1 m. 1984 - 0.03m ibid
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Site Rate of Damage Average Ref damage per year (Cambs fens) 1 m Bowl barrow 155369 Present height 0.3 m. 1949 - 1 m. 1984 - 0.04m ibid (Cambs fens) 1 m Bowl barrow 155370 Present height 0.8 m. 1949 - 1 m. 1984 - 0.01m ibid (Cambs fens) 1 m Bowl barrow 155371 Present height 0.8 m. 1949 - m. 1984 - 1 0.01m ibid (Cambs fens) m Bowl barrow 33394 Present height 0.5 m. 1982 - 1 m 0.03m ibid (Cambs fens) Bowl barrow 33379 Present height 0.8 m. 1982 - 1.2 m 0.02m ibid (Peterborough fens) Bowl barrow 33381 Present height 0.7 m. 1981 - 1 m 0.02m ibid (Peterborough fens) Bowl barrow 33387 Present height 0.3 m. 1982 - 0.5 m 0.01m ibid (Peterborough fens) Bowl barrow 33388 Present height 0.3 m. 1982 - 1 m 0.04m ibid (Peterborough fens) Bowl barrow 33389 Present height 0.3 m. 1982 - 0.5 m 0.01m ibid (Peterborough fens) Bowl barrow 33390 Present height 0.1 m. 1982 - 0.3 m 0.01m ibid (Peterborough fens) Bowl barrow 24423 1994 - 0.7 m. 1978 - 1.5 m 0.04m ibid (Cambs. fens) Bowl barrow 24426 1994 - 0.4 m. 1986 - 1 m 0.07m ibid (Peterborough fens) Average for the Cambs. and Peterborough earthworks showing plough erosion here = ibid 0.025 m Table 4.1: Recorded rates of truncation on earthworks
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3 Methodology
3.1 Introduction 3.1.1 A range of earthworks was constructed/used for the experiment (Figure 4.1):
Earthwork Original Dimensions as Details Dimensions as cultivated constructed after alternations and settling Ridge and 32 m wide at frequency of 10- Comprising 3 32 m wide at frequency of 10- furrow (Fig 20 m and amplitude of 0.45 m ridges and 3 20 m and amplitude of 0.40 m 4.2) furrows 4 Round 16 m diameter and 1 m high 4 round barrows 20 m diameter and 1 m high barrows (Fig 4.3) Ridge/bank 0.55 m high One ridge 0.50 high (Fig 4.4) Slope 2.5-3.2 degrees Natural field slope
Table 4.2: Earthwork statistics 3.1.2 It was not possible to replicate an exhaustive range of earthwork forms, but rather the earthworks were intended to represent a range of general types that are commonly found within English agricultural fields. The results derived from the present study of these typical earthwork forms can be applied to many other types of earthwork. They did not replicate the original form of the monuments, but rather their condition as it would have been after they had already been in cultivation for a while, but were still visible and archaeologically viable. Figure 4.5 shows the construction of some of the earthworks.
3.2 Construction of the earthworks 3.2.1 The earthworks were constructed by a tracked 360° excavator under archaeological supervision, using both ploughsoil and subsoil obtained from three borrow pits dug at the edges of the field. The earthworks were compacted using the excavator and its bucket. As constructed, all the earthworks were slightly higher than their target size so that they could weather, settle and regain soil structure throughout the first winter after construction, prior to the experiments taking place. After settling for a year all the earthworks and the fields around them were surveyed using a Leica global positional system (GPS; GX1230). 3.2.2 It would have been impossible to create the soil consistency and compaction of a real archaeological earthwork. However, the key issue was not to try to replicate the compaction/construction at the core of the earthwork, but to replicate conditions within the ploughsoil, as this was where the experimental effects/changes measured would be taking place.
Round barrows 3.2.3 Four barrows were constructed and used for direct drilling, non-inversion tillage, and mouldboard ploughing, with the fourth left fallow. The location of each barrow can be seen in Figure 4.1.
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3.2.4 The four round barrows were constructed using a uniform design in order to achieve a high degree of consistency of results. A circular area 16 m in diameter was stripped of ploughsoil down to the surface of the subsoil, to provide a stable base for each barrow. The question of whether or not to leave the ploughsoil in place and construct the barrow above it was discussed at the planning stage of the project, but it was thought that by removing the ploughsoil a more stable structure could be produced, ensuring a consistent construction throughout the core of the barrow. A subsoil base and core was built up to a height of c 0.70 m above the ploughsoil and heavily compacted using the bucket of the 360° excavator. Ploughsoil was then used to provide an even, 0.40 m deep cover, which was then only lightly compacted with the machine bucket. 3.2.5 The barrows were built slightly higher than required to allow them to settle for a year. They were originally built with a gradient of 30 degrees. This had to be modified slightly to accommodate the different capabilities of the different cultivation systems used. The barrow used for non-inversion tillage had its diameter increased and the profile reshaped to a gradient of 15 degrees. The barrow used for ploughing was reduced in height to 0.60 m, with a gradient of 10 degrees. The barrow used for direct drilling was reduced in height to 0.80 m and the gradient was kept the same.
Ridge and Furrow 3.2.6 The ridges were constructed by piling up ploughsoil to form the ridge and heavily compacted with the machine bucket to a height of 0.20 m. An even cover of ploughsoil was then laid over the ridges and was only lightly compacted, adding an extra 0.25 m to the height. This resulted in three upstanding ridges c 0.45 m high, which settled to 0.40 m prior to commencement of the experiments. The three furrows were formed by excavating to a depth of 0.10 m below the level of the base of the ridges. The wavelength used for the ridge and furrow consisted of three 4 m wide ridges and four 3 m wide furrows (dimensions typical of much ridge and furrow in midland England), covering a total area of 24 m by 32 m. The ridge and furrow was subject to non-inversion tillage. Ridge 3.2.7 The area of this earthwork was first stripped of ploughsoil and a heavily compacted subsoil ridge was constructed 0.20 m above ground level. A cover of 0.35 m of lightly compacted ploughsoil was then placed over the ridge so that it protruded 0.55 m above the ground surface. This was left to weather over winter to a height of 0.50 m. The final dimensions of the ridge earthwork were 34 m long, 10 m wide and 0.50 m high. The ridge was subject to non-inversion tillage (to a depth of 0.125 m) and ploughing (to a depth of 0.22 m).
Slope 3.2.8 The slope was located in an area of the field with a natural gradient of approximately 2.5-3.2°. The slope was cultivated using non-inversion tillage and mouldboard ploughing in turn up and down the slope. Uncultivated comparisons 3.2.9 An uncultivated area was established on each type of earthwork so that comparisons could be made between the effects of tillage and natural erosion. Uncultivated earthworks comprised: • a 4 m length at one end of the ridge • a 4 m length at one end of the ridge and furrow • a 4 m width of the slope
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• one of the round barrows Management 3.2.10 Weed growth was allowed on the earthworks throughout the year till they were sprayed off using Glyphosate (Round up) prior to the autumn cultivations in July 2006 and again in early August 2007. A fence was erected round the relevant part of the field in order to discourage rabbits or other burrowing creatures from disturbing the earthworks.
3.3 Monitoring Stations
General
3.3.1 Each earthwork had a series of monitoring stations inserted across its profile to provide data on depths of disturbance and soil movement. The layout of these monitoring stations is shown in Figure 4.6 and illustrated in Figure 4.7. They consisted of transponders and pits containing glass chips and sand. The glass and sand pits (0.50 m x 0.50 m) contained three different colours of glass/sand buried at different depths depending on the cultivation depth/type, to show the depth of cultivation disturbance. The transponders were buried at a depth of 0.10 m in all the earthworks and were used to track soil movement as a result of each tillage treatment. As they were used here to illustrate soil movement, rather than the depth of disturbance (see Appendix 3), single transponders were buried and their locations plotted and tracked after each 5 years worth of cultivation.
3.3.2 Analysis of the movement of the transponders and of the glass chips within the monitoring stations was intended to address the following questions: • whether a mound of soil represents the original structure in situ • when the buried soil beneath the mound is affected • how far does the soil move when cultivated • what are the differences in the extent of soil movement between different types of cultivation 3.3.3 Lines of coloured glass on the surface were also used to explicitly explore/demonstrate the movement of soil from the different cultivations. This movement was only tracked after the first 5 phases of cultivation as the glass soon became dispersed, buried and/or lost. Ridge and Furrow 3.3.4 Glass and sand monitoring stations were inserted at predetermined points within the earthwork, furrows and the natural ground surface. Transponders were placed within the top 0.10 m of the soil and their 3D positions accurately recorded. The precise locations of the monitoring stations are shown in Figure 4.6. The cultivated areas (non-inversion tillage) had layers of different coloured sand and glass buried at depths of 0.05-0.10 m, 0.10-0.15 m and 0.15-0.20 m within the indicator station pits. Barrow 3.3.5 Glass and sand monitoring stations were inserted at predetermined points within each barrow and in the natural ground surface at depths dependent upon the type of cultivation to be carried out. No monitoring stations were inserted into the uncultivated barrow. 3.3.6 Within the direct drill and non inversion tillage barrows, pits of 0.5 m² were inserted containing layers of differently coloured glass buried at 0.05-0.10 m, 0.10-0.15 m and 0.15-0.20 m. Transponders were buried at a depth of 0.10 m.
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3.3.7 Within the ploughed barrow, pits of 0.5m² containing glass chips were inserted at the interface of the earthwork and what would have been the original buried soil (Figure 4.16). The 3 layers of glass, each 0.10 m thick, were inserted from the surface of the barrow by excavating through the earthwork and buried so that the upper layer was at the same depth as the top of the buried soil beneath the barrow. The holes within the earthwork were then backfilled and allowed to settle. This slightly different configuration was used specifically to examine the survival of buried soils beneath cultivated barrows. Movement of the earthwork itself and issues of soil truncation were recorded through detailed topographical surveys. Ridge 3.3.8 Glass and sand were inserted in monitoring stations at various places within the earthwork and natural ground surface at depths dependant on the type of cultivation to be carried out. Within the top 0.10 m a series of transponders was placed at predetermined intervals. The precise locations of the monitoring stations are shown in Figure 4.6. Where the earthwork was to be ploughed the glass/sand layers in the indicator stations were buried at depths of 0.15-0.20 m, 0.20-0.25 m and 0.25-0.30 m. The area subject to non inversion tillage had the layers buried at 0.05-0.10 m, 0.10- 0.15 m and 0.15-0.20 m. Slope 3.3.9 A series of test pits was excavated at various points along the natural slope of the field on which the trials were undertaken. The pits were filled with three layers of differently coloured glass chips and sand. Transponders were placed in the top 0.10 m of soil along the slope. The precise location of the monitoring stations is shown in Figure 4.6. Again the glass/sand indicator stations were buried at depths of 0.15-0.20 m, 0.20-0.25 m and 0.25-0.30 m in the plough plot. The area subject to non inversion tillage had the comparable layers buried at 0.05-0.10 m, 0.10-0.15 m and 0.15-0.20 m.
3.4 Cultivation of the earthworks 3.4.1 The implement used for the non inversion tillage operations was a Simba Solo (one pass system) disc and tine implement. The Simba Solo, set to a non-inversion tillage depth of 0.10-0.15 m, was pulled with a Fendt 818 tractor (in ‘cultivation years’ 1-5) and a Fendt 930 for the following years. This system was selected as it represents the most practical non-inversion tillage cultivation field implement that would be used on this and most types of soil. Thirty years worth of non inversion tillage cultivations were undertaken on all the earthworks – barrow, ridge, slope and ridge and furrow (Figure 4.8), with the exception of the direct drill barrow which was cultivated for the equivalent of 20 years only. Both the ridge and furrow and the ridge were cultivated across the earthworks, ie perpendicular to their alignment. 3.4.2 Ploughing was undertaken on a section of each of the ridge and the slope and over one of the barrows, using a Massey Ferguson 6180 pulling a conventional 4 furrow reversible plough, set at a depth of 0.20-0.25 m (Figure 4.8). The barrow was cultivated back and forth with the tractor wheel sitting in the previous plough furrow, reflecting standard practice. 3.4.3 Direct drilling was only conducted on the barrow and used a Vaderstad Rapide and Moore Unidrill pulled by a Fendt 930 and Claas Ares 836 tractor respectively. The depth of work was approximately 0.05 m.
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3.4.4 A total of 30 sets of cultivations over each earthwork were undertaken over a 2 year period, as follows: • year 1 – April 2006 • years 2-5 – September 2006 • years 6-8 – October 2006 • years 9-15 – May 2007 • years 16-20 – October 2007 • years 21-25 - April 2008 • years 26-30 - May 2008
3.4.5 The weather and machine availability played a significant role in the timings of the cultivations; care was taken to avoid cultivation in very wet or very dry weather. 3.4.6 A COSMIC risk assessment was completed for each earthwork (see Appendix 4A below).
3.5 Monitoring and recording 3.5.1 Every pass of the tractor during the cultivation phases was monitored to check whether any of the glass chips or sand were being brought to the surface. This was done by walking the earthworks in 1 m grids. The intention was to provide notification that a particular operation had caused disturbance to the level represented by the chips/sand, rather than to record the detailed positions of each of these. These materials therefore acted as a presence/absence trigger indicating that disturbance had reached a certain depth. The movement of the lines of green glass placed on the surface of the earthworks was also recorded after the first 5 passes. After this the glass became too dispersed/buried to record. Movement of the glass and sand was recorded using a GPS or Total Station. 3.5.2 After the equivalent of every 5 years of cultivation any movement of the transponders was checked using the transponder receiver and plotted using the GPS. 3.5.3 A detailed contour survey was undertaken at the pre-cultivation stage, after year 1, and at every 5 year interval up to 30 years. The changes in profile were recorded using a GPS (Leica 2000) with readings taken on a 0.20 m grid across the earthworks.
3.6 Final recording 3.6.1 After the final GPS recording of the altered earthwork profiles, sections across the area of the earthworks were excavated by machine (JCB with ditching bucket) and recorded at a scale of 1:50. This enabled the pre- and post-cultivation profiles to be compared and the remains of the monitoring stations to be analysed in relation to what was seen during the course of the trials. 3.6.2 Prior to all excavation and recording the position of all visible glass and buried transponders was recorded and photographs taken. All monitoring stations were cross sectioned to check presence/absence and levels of disturbance (Figure 4.37-4.38).
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4 Results
4.1 General 4.1.1 Cultivation smoothed out the original earthwork profiles and gradually reduced the gradients and heights. Tillage was observed first to attack the upper slopes and edges of the earthworks. Soil displacement was generally recorded on the top and upper slopes of the earthworks with the lower areas less affected. The data used to illustrate the changes in each earthwork include detailed 5 yearly contour surveys, and records of movement of transponders and glass and sand superimposed on the contour surveys. Cross sections through each earthwork showing the position and degree of disturbance of the monitoring stations were also recorded. 4.1.2 The key summary results have been included within Tables 4.3 and 4.4 in Section 5.4.
4.2 Barrows Non-inversion tillage (Figures 4.9-4.12) 4.2.1 The barrow as constructed was 1 m high, 16.2 m long and c 15.8 m wide. The first cultivation pass resulted in little change to the barrow’s profile, although 0.06 m was removed from its height. By year 5 it had become clear that cultivation was beginning to change the profile with material being moved in the direction of tillage, with the barrow becoming broader as soil moved to the middle and bottom of the slope. By year 5, a depth of 0.1 m of soil had been removed from the top of the barrow and the sides displayed smoother breaks of slope and less well-defined edges. By year 10 the height of the barrow was considerably reduced, with a further 0.1 m being removed from the top (giving a total loss of 0.2 m) and a 1.7 m gain in circumference in the direction of cultivation with a loss of c 2.7 m in circumference in the other direction (Figure 4.9) as the tillage planed off the lower edges of the mound. 4.2.2 By Year 5 a clear edge was seen in the direction of cultivation along the edge of the barrow. This lip/step developed further in Year 10, after which it was removed along with the gradual truncation of the rest of the barrow. This lip is likely to have been formed by the slippage of the tractor at the edge of the mound where the soil could not support its weight, therefore causing a step. 4.2.3 By year 15 the barrow was 0.70 m high (loss of 0.1 m since Year 10) and gained 0.8 m in length. Years 15-20 saw a further 0.2 m loss in height. The barrow had now lost half (0.5 m) of its total height and had grown in length by a further 1.6 m in the direction of cultivation. The barrow had changed from a prominent recognisable landscape feature into a gentle undulation in the landscape. Year 25 saw no further soil movement from the top of the barrow, but by Year 30 a further 0.11 m had been lost. Figure 4.10 shows the shape of the barrow after 30 years of non inversion tillage and clearly shows a fan tail effect in the predominant direction of tillage (clearer on year 30 Figure 4.12). Comparison between the pre-cultivation and post-cultivation cross sections clearly shows that the visible monument has spread across the landscape. Whilst the mound itself has been reduced in size the actual soil from the mound has extended c 6 m in the direction of cultivation and c 1.25 m laterally. This soil has protected the monitoring pits originally placed just outside the barrow from disturbance. By year 30 the height of the barrow was just 0.39 m and the mound was barely recognisable on the ground. The reduction in the rate of soil removal from the mound in the last 10 years may be due to the fact that the barrow had reached a level
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of equilibrium at which there was little further significant movement of soil caused by the cultivation equipment planing off the top of the mound (Figure 4.10). 4.2.4 Despite little change to the profile after year 1, the sand and glass stations showed damage occurring on the top and middle/upper slopes. Here glass/sand from both the middle layers (depth 0.10-0.15 m from the surface) at the top of the earthwork and mid-slope and top layers (0.05-0.10 m) from the lower slopes were disturbed (Figure 4.11). Some of the transponders were moved on the top (distances from 0.88-8.80 m) and upper slopes (movement from 0.10-0.20 m) with movement also seen all down the edge of the barrow in the direction of cultivation. 4.2.5 In Year 5 one of the indicator stations at the top was disturbed to a depth of 0.15-0.20 m with the mid and lower slopes showing disturbance to 0.10-0.15 m. In Year 10 there was a significant increase in disturbance reflecting the considerable reduction in height seen in the mound in that year. In the top of the mound disturbance was seen in four indicator pits to 0.15-0.20 m with three stations in the middle and lower slopes showing disturbance to 0.10-0.15 m. 4.2.6 Year 15 saw 0.15-0.20 m of disturbance mid slope and 0.10-0.15 on the lower slopes. By Year 20 most of the glass stations in the mound had been destroyed, with only one on the mid/lower slope still showing damage. The post-excavation sections excavated through the barrow showed that all stations examined in the mid and upper slopes were destroyed, while some of those in the lower slopes were partially affected with some glass still remaining (Figures 4.12 and 4.37). Away from the earthwork the indicator stations were buried and protected by the relocated mound material. 4.2.7 The presence and absence of disturbance shown by the glass indicator stations must have been caused at least in part by the Simba Solo bringing (ie lifting) some material to the surface (mainly glass). However, the main action causing both the glass and sand to appear on the surface would have been the Simba Solo machine physically exposing the glass and sand by removing/planing off the deposits above them. 4.2.8 By year 5 many of the transponders from the top and upper slopes had been moved. By year 10 virtually all had been moved from their original positions, even those on the lower slopes. The transponders continued to be moved by each phase of cultivation, those on the top and upper slopes generally more so than those on the lower slopes. The maximum movement per cultivation phase of a single transponder was c 14 m, although this was the exception, not the norm. Significantly less movement was recorded after Year 10, with most of the transponders being clustered near to the base of the slope in the later years. The significant exception to this occurred in Year 15 where two were removed distances of c 6 m and 14 m from the top of the mound to the lower slopes. Similar occasional large movements were also seen sporadically in Years 20 and 25, along with more subtle movements nearer the base of the slopes and away from the mound. In general, once the transponders had been removed from the upper parts of the earthworks their movements were more limited. 4.2.9 The non inversion tillage barrow lost 0.61 m in height over the 30 year period, leaving it 0.39 m high. The rate of truncation was thus on average 0.10 m every five years. The surviving barrow/cultivation soil was mostly high enough to protect any buried soil/features beneath it, especially in the centre of the surviving mound. Buried soil at the edge of the barrow would have been affected, as shown by the disturbance to depths of 0.10-0.15 m of the indicator stations buried on the lower edges of the mound.
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Mouldboard plough (Figures 4.13-4.16) 4.2.10 As constructed, the mouldboard plough barrow was 0.6 m high. It actually gained slightly in height, by c 0.05 m following the first set of cultivation and by a further 0.01 m after year 5 as a result of the plough ‘fluffing up’ the ploughsoil. The length of the barrow remained stable through years 1 and 5. In year 10 the height was reduced by 0.16 m and the length increased by 3.1 m. The barrow appeared to stabilise by years 15 and 20, with years 11-15 showing no loss and years 16-20 showing only a 0.04 m loss in height, although the length was found to have increased to 24 m in year 15 and 25.6 m in year 20 in the direction of ploughing. By years 25 and 30 the rate of truncation had picked up to 0.10 m and 0.16 m respectively and the length increased to 26.5 and finally to 27 m. During the later parts of the experiment it became very difficult to identify the extent of the earthwork. 4.2.11 The contour survey (Figures 4.13 and 4.14) and section (Figure 4.16) show these changes in length and height clearly. The barrow increased in size all round as it flattened; its profile sloped gradually down in the longitudinal axis/direction of cultivation but was more stepped in profile on the two opposing edges (Figure 4.16). 4.2.12 The indicator stations within the plough barrow were buried at the depth of the buried soil so that any damage to the latter could be clearly seen. These stations were located in the edges of the barrow. Damage to the buried soil was seen after year 15 in one of the stations buried mid slope, and again to the same one in year 25. This was despite the fact that the mound was 0.50 m high in year 15 and 0.36 m high in year 25, and shows the vulnerability of the edges of earthworks. 4.2.13 The plough barrow lost 0.46 m in overall height (including the 0.06 m ‘added’ in the first 2 years) over the 30 year period, leaving it 0.20 m in height with the soil extending c 4-5 m in the direction of ploughing. The rate of truncation was on average 0.06 m every five years. No actual mound was visible in the landscape after 30 years of ploughing; its height was reflected only by the depth of the ploughsoil. 4.2.14 The excavation of the earthwork revealed that ploughsoil over the buried soil was 0.20-030 m deep, representing a virtually flat profile. Given the depth of ploughing, the rates of plough attrition seen on flat sites (Appendix 3) and from examination of the post-excavation profile (Figure 4.16), it is clear that whilst damage to buried soil was only seen at the edges of the mound in years 15 and 25, further wholesale damage would have occurred if ploughing had continued for a further 5 years. Most of the original barrow soil was dispersed in the direction of ploughing. Direct drill (Figures 4.17-4.18) 4.2.15 As constructed the direct drill barrow was 18 m in diameter and 0.8 m high. Very little change occurred in terms of the shape of the barrow and movement of soil recorded. The first change seen was in years 10 and 15 where both years saw a 0.02 m loss in height and a gain of a few centimetres in diameter. A further 0.01 m was lost in the final year, year 20. The contour surveys revealed that the barrow kept its shape (Figures 4.17-4.18). No disturbance to the indicator stations was seen and excavation confirmed that these were undisturbed. Given the action of the direct drill, the changes in height seen here are likely to reflect compaction of the barrow due to the machine weight, rather than through tillage erosion. The total 0.05 m height loss was the same as seen on the control, uncultivated barrow (see below).
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Control barrow (Figure 4.19) 4.2.16 This showed no change in profile or height until year 25 when 0.03 m was lost through natural erosion and in year 30 when a further 0.02 m was lost. The barrow had weed cover for most of each year but this was sprayed off, along with weeds on the other earthworks, prior to the autumn cultivations.
4.3 Ridge and furrow 4.3.1 The ridge and furrow earthworks (see Figures 4.20-4.26) were constructed to a height of c 0.4 m. The first 4 years of cultivations over the ridge and furrow caused flattening and planing-off of the soil profile, with soil moving from the tops of the ridges into the furrows in the direction of cultivation. This led to a slight zig zag effect being observed (Figures 4.20-4.21) as the Simba Solo moved backwards and forwards across the earthwork. The transponders were relatively unaffected in year 1, with only small movements detected across both the ridges and furrows. The glass/sand indicator stations were affected to a maximum depth of 0.20 m at the top and upper part of the ridges, 0.15 m in the middle parts of the slopes and 0.05-0.10 m on lower slopes. The first year cultivation led to the loss of 0.05 m of soil from the tops of the ridges. 4.3.2 By year 5 it was seen that the ridges had become much broader and lower (0.10 m lower), with the furrows becoming more filled with the soil from the ridges (a 0.05 m gain). The indicator stations showed that disturbance to a depth of 0.15-0.20 m occurred mostly on the upper slopes. Some of the mid-slope and lower slope stations were disturbed to 0.10-0.15 m, but there was minimal disturbance within the furrows. The movement of the transponders also reflected this pattern, with some dragged off the slopes into the furrows. Most of the transponders did not move far. Those that did, typically moved 0.10-0.20 m and were mostly located in the top and upper slopes of the earthwork, and ended up in the furrows. 4.3.3 By year 10 the soil from the ridges had almost entirely filled the furrows, protecting the indicator stations in these features. The waved profile of the earthwork had virtually disappeared, with only slight undulations visible on the ground surface. The loss of 0.20 m of soil from the top of the ridges and the corresponding deposition of 0.15 m of soil in the furrows between years 5 and 10 suggested an acceleration of destruction at this stage. Only two of the indicator stations showed disturbance (at 0.20 m). No further disturbance was seen from year 10 onwards. The indicator stations had either therefore been totally destroyed or covered and protected (Figures 4.24 and 4.25). Most of the transponders still in circulation within the cultivation depth at this stage were moving on average c 1-2 m per 5 years. 4.3.4 Very little change occurred between years 15 and 30. Movement of those transponders not yet buried or dispersed on the now flat surface continued to be on average c 1-2 m every 5 years. The earthwork continued to be tilled for the remaining 15 years, although little change to its now flattened profile was recorded. 4.3.5 The cross section shows well the destruction of the indicator stations in the upstanding earthwork and the burial of those within the furrows, to a depth of 0.40 m in places. In some cases where the indicator stations on the mid and lower slopes had layers removed or disturbed (reflecting in some instances disturbance of the buried soil) they were subsequently buried and protected. The section shows that in some places where the lower layers of the indicator pits are now protected the earlier disturbance had penetrated to the level at which the buried soil would have lain.
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4.3.6 The ridge and furrow lost all its height very rapidly, from 0.40 m to 0 m. Prior to year 10 this happened at a rate of c 0.025 m a year. Once it reached a height of 0.25 m it was virtually destroyed in 5 years, a rate of truncation of c 0.04 m a year.
4.4 Ridge 4.4.1 The ridge was constructed to a height of 0.5 and was 15.5 m long and 10.5 m wide (Figures 4.27-4.34). Part of the ridge was subject to non inversion tillage and the other part to ploughing with the mouldboard plough. 4.4.2 Years 1-5 saw the non-inversion tilled part of the ridge reduced in height by 0.05 m in year 1 and by 0.04 m a year by year 5. The ploughed part lost slightly more, with a 0.07 m loss in year 1 and 0.06 m a year by year 5. Non-inversion tillage led to much less damage at depth, with only one indicator station showing any disturbance after a year and disturbance only at depths of 0.05-0.10 in year 5 on the lower slope. The mouldboard plough area showed significant disturbance throughout the profile in year 1, with disturbance reaching a depth of 0.25-0.30 m, mainly on the top and middle slopes, with disturbance to a depth of 0.10-0.15 m on the lower slopes. The extent of disturbance of the indicator stations reduced between years 5 and 10 with more variation in the depths of disturbance recorded in the ploughed part. Disturbance at a depth of of 0.05-0.10 m was seen on the top and lower slopes in the non-inversion tillage part. The transponders in the non-inversion tilled part of the ridge moved slightly in places throughout the profile in years 1 and 5, up to 1 m distance. More movement was seen across the profile amongst more of the transponders in the plough plot. In Year 10 further transponder movement was seen in the non-inversion tillage part, over distances of up to 2 m. 4.4.3 In year 10 the ridge had been diminished under both sets of cultivation, with a general spread of earthwork soil in the direction of cultivation. Year 10 saw a considerable flattening of the ridge in the ploughed area with a further 0.1 m removed from its top. The part under non-inversion tillage suffered less, with only a further 0.05 m of tillage truncation occurring. Only one indicator station at the base of the slope was affected (to a depth of 0.05-0.10 m) in the non inversion tillage ridge, while more widespread disturbance was seen in the ploughed part, with the top of the ridge and the lower slopes showing disturbance to 0.25-0.30 m and 0.10-0.15 m respectively. However, as for year 5, the disturbance was less than seen in year 1. By year 10, therefore, a depth of 0.14 m of soil had been lost from the non-inversion tillage stretch of the ridge and at a relatively gradual pace. By year 10 in the ploughed part a depth of 0.23 m had been lost, with a significant loss occurring in year 10. The transponders in both the non-inversion tillage and ploughed parts showed slightly more movement than previously, mostly on the edges of the ridge. As in the barrow much of the disturbance to the indicator stations in the non-inversion tillage areas showed direct exposure of the glass/sand caused by the planing off of the soil, rather than showing the depth of disturbance below the ground as seen with the plough. 4.4.4 From years 15-30 similar rates of soil truncation were seen in the two halves of the ridge, with the non-inversion tillage part losing a total of 0.22 m and the plough part 0.19 m, and evidence of extensive disturbance on the upper and middle slopes. The indicator stations on the non-inversion tillage ridge started to be significantly affected with four indicator stations on the top of the ridge showing disturbance to 0.15-0.20 m and five on the slopes disturbed to 01.0-0.15 m. By year 20 disturbance was seen to depths of 0.15-0.20 m mid slope, where previously disturbance to 0.10-0.15 m had been shown. No further damage was seen to the indicator stations after year 20, suggesting that the indicator stations had either been destroyed (ie those most vulnerable, at the top and mid slope) or protected (Figure 4.38). Both the non- inversion tillage and ploughed ridge show a similar pattern, with the indicator stations
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being totally removed on the upper and mid slopes and varying degrees of disturbance on the mid and lower slopes. Years 25 and 30 saw the non-inversion tilled ridge reduced to heights of 0.19 m and 0.14 m respectively and the ploughed ridge reduced to 0.15 m and finally to 0.08 m. 4.4.5 In year 15 transponder movements were mainly small, both in the non-inversion and ploughed plots, with a few exceptions across the slope profile. In year 20 slightly more movement was seen in a small number of transponders, again across the profile. This pattern was replicated in Years 25-30 as the earthwork became flatter, allowing unimpeded movement (on average from 2-3 m) to those transponders still within cultivation depth. 4.4.6 The truncation of the mouldboard plough part of the ridge totalled 0.42 m (average rate of truncation per 5 years 0.07 m). Truncation progressed in three stages: a relatively rapid phase in the first 10 years; a slower rate between years 15 and 20; and finally a more rapid rate again in years 25-30. The truncation of the non-inversion tillage part of the ridge totalled 0.36 m (average truncation every 5 years 0.06 m). The truncation rate was much more uniform throughout the 30 year period. The cross sections show that under the non-inversion tilled ridge the predicted buried soil survived in the centre of the ridge, with disturbance and destruction occurring at the edges. Further soil movement then protected the indicator stations and buried soil from further damage. This is in contrast to the ploughed part of the ridge, where the buried soil would have been totally destroyed (Figures 4.33-4.34). Non-inversion tillage was therefore just as destructive as ploughing in removing soil from the earthwork, through the planing off of material, but was less destructive below ground, reflecting both the differing depths and types of disturbance from the two implements below ground.
4.5 Slope 4.5.1 The ploughing of the slope in the first year caused minimal disturbance to the transponders, but damage was caused to the middle layer of glass/sand (at a depth of 0.20-0.25 m below the soil surface) in the upper part of the slope and to the lowest layer of glass/sand (at a depth of 0.25–0.30 m below the soil surface) in the lower part of the slope (Figures 4.35-4.36). Non-inversion tillage caused some movement of the transponders and disturbed the upper layers of the glass pits to a depth of 0.05-0.10 m below the soil surface consistently across the slope profile. 4.5.2 Throughout the trials only the glass was brought to the surface in the half of the plot subject to non-inversion tillage. This reflects the fact that where sand and glass stations are disturbed on the slopes of an earthwork this is mainly a result of the planing off of the earthwork to reveal the sand and glass beneath. On a slope this does not happen, and only small amounts of glass are brought to the surface by the action of the cultivation equipment (Figures 4.35-4.36). The smaller particles represented by the sand are not brought to the surface, or if they are they are in such small quantities that they are not distinguishable once there. The subsequent excavations of the sand pits within the slopes showed various disturbance to all three levels of material within these indicator stations, but this was not seen on the surface. 4.5.3 More transponders were disturbed in year 5 across the slope in both the ploughed and non-inversion tillage areas. Glass and sand continued to be disturbed across the slope in the plough plot to a depth of 0.25-0.30 m in the lower slope and 0.20-0.25 m across the rest of the slope. Disturbance to depths of 0.10 m in the glass pits occurred once more across the whole slope in the non-inversion tilled part. At year 10, virtually all of the sand/glass pits were disturbed to the deepest layer (0.25-0.30 m below the soil surface) in the ploughed area, and most of the transponders had been moved from
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their original locations. These moved in the direction of cultivation, although mostly slightly offset, and on average over distances of 1-2 m. Within the non-inversion tillage area the topmost layers of glass continued to be disturbed mid/lower-slope (to a depth of 0.10 m), with transponder movements recorded mainly across the slope in the direction of cultivation. 4.5.4 Year 15 saw continued disturbance to depths of 0.25-0.30 m in the majority of the indicator stations in the ploughed area. The transponders moved consistently more in year 15 than previously, across the slope and on average c 1 m per 5 years. The non- inversion tillage area continued to show disturbance to 0.05-0.10 m across the slope, but in the upper slope disturbance was also recorded to 0.10-0.15 m. Transponder movement was more variable; it was greatest on the lower slopes where the transponders moved on average slightly more than those on the ploughed area (ie c 1.5 m). In Year 20 the plough plot disturbance continued to be recorded at 0.25-0.30 m in the upper slopes and reduced movement of the transponders was observed. In the non-inversion tillage plot disturbance was reduced to two indicator stations at 0.05-0.10 m on the upper slopes and showing damage to 0.10-0.15 on the lower slopes. There was also less transponder movement here. 4.5.5 By year 25 disturbance was seen at 0.25-0.30 m, mostly on the mid and lower slopes of the ploughed plot, with disturbance of 0.05-0.15 seen across the slope in the non- inversion areas. No further disturbance of the indicator stations was seen in year 30 in either plot. Only a few transponders were disturbed in each plot, with a maximum movement of c 2.5 m. 4.5.6 The results appear to show that in this case the slope was not steep enough to cause noticeable tillage erosion downslope leading to the thinning of the plough soil up slope and its thickening downslope. If this had been the case one would have expected to see an increase in disturbance top/mid slope and a corresponding decrease in damage in the lower slopes. This pattern was not seen on either plot.
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5 Discussion
5.1 Transponder and soil movement 5.1.1 The transponders were inserted mainly as a means of monitoring soil and artefact movement resulting from different cultivation practices. Transponder movements can be clearly seen in Figures 4.11, 4.12, 4.22, 4.23, 4.24, 4.25, 4.29, 4.30, 4.31 4.32, 4.35 and 4.36 (also see Figure 4.40 for final disturbance levels). The transponders appear sensitive to disturbance under both non-inversion tillage and ploughing regimes. 5.1.2 The largest movement of transponders was seen on the barrow subject to non- inversion tillage. Here some moved quite large distances as they were dragged with the soil by the cultivation implement. For example in year 1 movement of c 8 m was seen. In year 10, when the average movement was c 3 m, one of the transponders moved c 9 m, and in year 15 one was dragged c 14 m off the earthwork. Once the earthwork became flatter movement became more localised as the transponders clustered around the edge of where the barrow had been. In year 25 one transponder was again dragged 11.5 m as the remaining earthwork was finally flattened. The movements in the ploughed earthworks were less marked. These differences reflect the differing characteristics of the two different cultivation systems. 5.1.3 The other earthworks showed less dramatic movements. The transponders on the ridge and furrow moved on average c 1-2 m. Significant movement was mostly off the ridges into the furrows, where the transponders were then left along with the relocated soil and protected. Once the earthwork was flat any transponders remaining in circulation moved on average 1-2 m, with some up to 3 m every 5 years. 5.1.4 On the slope and ridge comparisons between the movement of the transponders subject to ploughing and those subject to non-inversion showed little difference in the distances travelled. Both implements moved the soil/transponders c 1-2 m per 5 years where the earthworks were flattened or on the slope. Slightly greater movement distances were recorded on the ridge subject to non-inversion tillage than on the ploughed part in a few of the years, but the difference in distances travelled was only c 0.4 m. Very similar transponder movement distances were also recorded for the two cultivation systems on the slope. 5.1.5 In all cases transponder movement must relate mostly to the last episode of cultivation of each 5 year cycle, but also reflects the backwards and forwards movement of the previous 4 years. Once off the earthworks themselves the transponders did not move large distances overall, but mainly moved backwards and forwards reflecting the direction of cultivation. If cultivation had been consistently in one direction the transponders would have been expected to move larger distances across the field, especially in the non inversion tillage areas. 5.1.6 In general the transponders moved with the direction of cultivation. Both forms of cultivation appeared to offset the transponders slightly from movement in a straight line. This was seen more regularly and to a greater extent in the ploughed plots, reflecting the sideways displacement of soil by ploughing. This can be seen most clearly in the data for years 10 and 30 on the ridge (Figures 4.30 and 4.32).
5.1.7 A series of lines of green glass were laid on the surface of the earthworks and slope as a visual indicator of soil movement. Where these were aligned on the ploughed earthworks no movement was recorded as the plough simply inverted and buried the
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glass. Over the non-inversion earthworks and slope the surface glass was moved by up to 1 m following the first pass of the non-inversion tillage implement and widely dispersed during the first set of five tillage passes. After 5 passes, where glass still remained on the surface the visible fragments had moved a maximum of: • 7 m in both directions within the slope according to direction of cultivation • 5.5 m in both directions within the ridge and furrow according to direction of cultivation • 4.5 m in both directions on the ridge according to direction of cultivation • 6 m in both directions on the barrow according to direction of cultivation
5.1.8 This glass shows more movement than seen in the transponders. This is probably due to the fact that the glass was situated on the surface of the soil with no impediments to movement. After year 5 the glass was too dispersed to record. 5.1.9 Transponders appeared to broadly represent soil movement, although it was not always easy to track them year by year because of problems in locating those that moved over large distances and across slopes. In some cases they became too deeply- buried to be located or their signals cancelled each other out, especially when they clustered around the bottom of the slopes. Despite this, they appeared to offer a fair reflection of soil movement, reflecting differences where circumstances differed (for example, contrasting movement patterns in relation to the barrow and the ridge and furrow) and consistencies where similar circumstances prevailed (for example, once the earthworks became flat). Greater transponder movement off the barrow reflects the fact that soil was dragged off the top of the earthwork to where it was ‘dropped’ by the Simba Solo when flat ground was encountered. For the ridge and furrow this process happened within the earthwork, with the transponders showing less movement, travelling only from the ridge to the adjacent furrow where they would have been ‘dropped’ with the soil. Once the transponders (soil) had been removed from the earthwork and the sites became flatter the movement in general over all former earthworks remained on average 1-2 m per 5 years. This average appears reasonable given the observation by the implement manufacturers (P Wright pers. comm.) that the Simba Solo will move soil c 0.25-0.30 m for every pass on flat sites. 5.1.10 This pattern was reflected in both the ploughed and non-inversion tillage earthworks, although movement through non-inversion occurred principally when the earthworks were being planed off and the soil relocated. Soil could potentially be dragged quite a distance through non-inversion tillage, if the evidence from the barrow is a reliable guide.
5.2 Glass and sand indicator pits 5.2.1 Differences were seen between the patterns of disturbance of the earthworks that were ploughed and those subject to non-inversion tillage. The plough inverted the soil, thereby bringing material up to the surface. The Simba Solo appeared to reveal the glass and sand in two ways. Some glass was brought up by disturbance below the soil and the movement of the implement across the surface of the soil, but these movements did not usually bring sand to the surface. The main cause of the appearance of both glass and sand on the surface appears to have been their physical exposure as a result of the machine removing/planing off the deposits above them. For example, throughout the trials only glass was brought to the surface in the part of the slope subject to non-inversion tillage. The smaller sand particles were rarely brought up, or if they were they were in such small quantities as to be indistinguishable once on the surface. Where glass and sand stations were created within the upper slopes of the earthworks, for example on the barrow, the machine planed off the earthwork to reveal the sand and glass beneath. Across all earthworks
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by Years 25 and 30 no glass or sand was brought to the surface, showing that these indicator stations had either been destroyed/’worked out’ or were buried beyond disturbance.
5.3 Comparison between earthwork types 5.3.1 Whilst the general results are similar (ie both ploughing and non-inversion tillage on an earthwork will gradually erode it, with the ploughing doing so slightly more quickly), each earthwork was found to respond slightly differently to tillage as the shape, wavelength, and break of slope were all significant factors in determining their response. 5.3.2 The ridge and furrow very clearly went through a two stage process of truncation. Prior to year 10 truncation happened at a rate of 0.025 m a year. Once the earthwork was reduced to a height of 0.25 m, it was virtually destroyed in 5 years, with a truncation rate of 0.04 m per year. The rapid rate of truncation reflected the fact that once the earthwork height reached 0.25 m it fell within the range in which the Simba Solo was at its most effective in removing such an obstacle. Rapid destruction was also facilitated by the fact that the wavelength of the earthwork did not allow enough time/distance for the tractor and implement to ride over the soil. This meant that the upper slopes of the ridges were, in effect, subject to a more intense planing/levelling off due to the equipment’s lack of vertical articulation; the cultivation implement simply followed a relatively flat line through the earthwork. 5.3.3 A similar acceleration of truncation was also seen in the barrow which was subject to non-inversion tillage. Here the rate of truncation was 0.04-0.06 m over the first 6 years (barrow height between 0.9-1.0. m) which rose to 0.1 m between years 10 and 15 (barrow height 0.7-0.8 m) and to 0.2 m between years 15-20 (0.5 m high). No truncation was seen in the period of years 20-25, but truncation occurred again between years 25 and 30, with a loss of 0.11 m resulting in a final height of 0.39 m. Given the original height of the barrow, cultivation did not occur over a long enough period for it to be possible to see what would have happened once the apparently critical height of 0.25 m was reached (at which point very rapid truncation followed in the ridge and furrow) or indeed whether the isolated barrow feature would have responded in the same way as the ridge and furrow. 5.3.4 A similar phase of rapid destruction did not occur on the non-inversion tilled part of the ridge. Here the rate of truncation was more gradual and consistent throughout the 30 year accelerated trials period, being consistently c 0.04-0.07 m in every 5 year period. It appears, therefore, to be the wavelength of the ridge and furrow which has affected its rate of its destruction. 5.3.5 In the case of the ploughed earthworks (ridge and barrow) no patterns in the chronological trends of truncation were discernible, although both showed a slightly increased level of truncation at year 10. 5.3.6 The shapes of the barrows were rapidly changed. For example the non-inversion tillage barrow changed from a circular feature to a fan-shaped feature extending in the direction of tillage. The ploughed barrow not only showed an increase in length but also developed very distinctive steps, seen especially clearly in years 10, 25 and 30 (Figures 4.13 and 4.14). Similar steps were also seen briefly in the non-inversion tilled barrow, most clearly in years 5 and 10, before they were planed away. These steps indicate where the weight of the tractor caused a collapse of the vulnerable edges of the earthwork.
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5.3.7 The non-inversion tillage barrow remained visible because of its original substantial height. However, the edge of this barrow lies c 6 m distant from its original edge, as its length has grown in the direction of cultivation. This could lead to erroneous interpretations regarding its original position and the location of its centre point. 5.3.8 All earthworks showed relatively more truncation in year 1 than in all other individual years. This was presumably in part a result of the settling of the earthworks under the weight of the machinery used in the first cultivation pass.
5.4 Summary of comparison between cultivation types 5.4.1 A comparison of the rates of truncation in relation to the different types of earthwork from the different types of cultivation can be seen in Tables 4.3 and 4.4
Earthwork pre-cult height height height height height height height Total height after after after after after after after Loss year 1 year 5 year 10 year 15 year 20 year 25 year 30 Barrow (no cult.) 1.0 m 1.0 m 1.0 m 1.0 m 1.0 m 1.0 m 0.97 m 0.95 m 0.05 m Barrow (non inv.) 1.0 m 0.94 m 0.9 m 0.80 m 0.70 m 0.50 m - 0.39 m 0.61 m Barrow (direct drill) 0.80 m 0.80 m 0.80 m 0.78 m 0.76 m 0.75 m NR NR 0.05 m Barrow (plough) 0.60 m 0.65 m 0.66 m 0.50 m - 0.46 m 0.36 m 0.20 m 0.40 m Ridge and furrow 0.40 m 0.35 m 0.25 m 0.05 m - - - - 0.35 m (non inv.) Ridge (non inv.) 0.50 m 0.45 m 0.41 m 0.36 m 0.32 m 0.26 m 0.19 m 0.14 m 0.36 m Ridge (plough) 0.50 m 0.43 m 0.37 m 0.27 m 0.25 m 0.21 m 0.15 m 0.08 m 0.42 m Table 4.3: Height changes in earthworks over time
Type of earthwork Left Total height Average height Average standing loss loss per 5 years annual loss Barrow (no cult) 0.95 m 0.05 m Barrow (non inv.) 0.39 m 0.61 m 0.10 m 0.02 m Barrow (direct drill) 0.75 m 0.05 m 0.008 m 0.002 m Barrow (plough) 0.20 m 0.40 m 0.06 m 0.01 m Ridge and furrow (non inv.) 0.05 m 0.35 m 0.03 (over 10 years) Ridge (min inv.) 0.14 m 0.36 m 0.06 m 0.01 m Ridge (plough) 0.08 m 0.42 m 0.07 m 0.01 m
Table 4.4: Summary of height changes 5.4.2 The tables show that the average rate of truncation per year for the ploughed earthworks was 0.01 m. Truncation of the earthworks subject to non-inversion tillage varied, depending on the type of feature cultivated, but ranged from 0.01-0.03 m per year, with the ridge and furrow most severely affected at 0.03 m a year. For the two directly comparable earthworks, the barrow and the ridge, the results show that for the ridge the average overall rates of truncation are the same, for the barrow the rate of truncation was higher using non-inversion tillage. 5.4.3 One of the reasons for testing the non-inversion tillage technique was to see if it would be an effective way of minimising damage to earthworks if they remained in cultivation. These trials have proved that non-inversion tillage does not offer a sustainable way of keeping earthworks in cultivation, and in some cases can actually cause faster rates of truncation than the plough. Direct drilling is the only sustainable method by which earthworks can be kept under cultivation. 5.4.4 The non-inversion tillage equipment causes more truncation than the mouldboard plough, because while the plough tends to cleanly cut and rotate the furrow slice with relatively little longitudinal soil movement, the non-inversion tillage tool with the
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combination of two sets of shallow discs tend to drag the topsoil a greater distance. Nichols and Reed (1934) showed that the soil movement generated by a typical mouldboard plough was 0.1-0.15 m for the soil close to the existing furrow wall, whilst that removed to form the new furrow wall could move forward some 0.9 m. Experience here has shown that displacement of transponders off an earthwork can result in movement distances of 10 m plus, but that on average on flat sites this is reduced to c 1-2 m every 5 years. One of the Simba Solo designers states that an average yearly distance for soil movement is nearer 0.25-0.30 m. The Simba Solo is designed to flatten irregularities and the flattening of earthworks seen here is typical. This was exacerbated in the present instance by the geometry of the cultivator-tractor combination in relation to the size of the earthworks. The trailed design of this cultivator forces the discs to penetrate more deeply into the earthworks when the relative motion of the tractor drawbar pitches downwards as the tractor leaves the earthworks, pulling forward more soil than in the steady state condition. A similar effect could also occur upon entry to the earthwork as again the drawbar hitch point pitches downwards. 5.4.5 Once the earthworks were flat the non-inversion tillage equipment would not cause any further truncation to the now below-ground archaeological sites than is seen in the flat sites examined in Appendix 3, as long as the depth of soil above the archaeological remains exceeded the depth of the tines. If the tines were to make contact with archaeological deposits/buried soil they would cause damage. Any archaeological deposits under those former earthworks which continued to be ploughed would be at risk from damage below ground as seen in Appendix 3.
5.5 Survival of features below earthworks 5.5.1 The glass and sand indicator stations showed the depths of disturbance reached by each form of cultivation and clearly showed where most of the disturbance and soil movement occurred. In the early stages of cultivation discernible disturbance was concentrated on the top and upper slopes of the earthworks. The lower slopes were affected later, but in some cases these then became protected from further disturbance by the redeposition of the soil from the upper parts of the earthworks. In the case of a real life round barrow, for example, this redeposition would have helped protect any ring ditch or other features around its edge. 5.5.2 The question of the depth of earthwork soil that may be required to guarantee protection of buried soil or features beneath earthworks, above and beyond normal ploughsoil depths, can be considered using the evidence from the ploughed barrow. Here the mound virtually disappeared, leaving ploughsoil over a layer of barrow mound soil that was very thin, especially at the edges. The only buried soil disturbance was seen on the edge of the barrow at years 15 and 25, when the barrow was still upstanding to heights of 0.5 m and 0.36 m respectively. The earthwork material protected the ground surface on which it was built across the rest of its area, and provided protection over an increased area where the mound material was spread beyond its original extent. This ploughsoil was pretty much all that was left of the earthwork and the results of the trials on the flat sites indicate that these depths of deposit could not be expected to provide protection of the buried soil resource for very long. The gradual erosion by the plough seen on the flat sites would be replicated here; this would therefore soon leave the soil beneath the base of the ploughed -out earthwork vulnerable to direct attrition from continued ploughing. 5.5.3 The non-inversion till barrow was built higher and therefore survived to a higher level than the ploughed barrow at the end of the trials. However, continued non-inversion tillage over this earthwork would continue to erode the mound itself, with the edges and slopes of the mound being particularly vulnerable. Once it had been totally
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flattened, non-inversion cultivation could disturb the buried soil if it was undertaken at the same depth as the soil covering the buried soil, but if a buffer existed then the situation seen on the flat sites would be applicable here, ie the buried soil would be unlikely to be further affected if non-inversion techniques continued to be used. A similar situation would be true of the non-inversion ridge. The ploughed ridge showed disturbance of the buried soil when the earthwork stood 0.25 m high and destruction of the buried soil when the earthwork still stood at a height of 0.15 m. All the buried soil would have been destroyed here (Figure 4.34). 5.5.4 The ridge and furrow showed disturbance on the edges of the earthwork ridges (resulting from the levelling of the earthworks) when it was still 0.20-0.30 m high. Buried soil here would have been disturbed in places early in the process, but the movement of soil from the ridges into the furrows, resulting in the levelling over of what were once the lower slopes of the ridges, subsequently protected what would have been left of the buried soil. Once the earthworks are flat, these remains are likely to be protected as long as the depth of soil covering them exceeds the depth of the implement tines, as above. 5.5.5 Therefore, once these experimental earthworks were destroyed to the point where cultivation penetrated the depth of the remaining earthwork material then the buried soil was vulnerable to destruction. Buried soils are likely to be better protected under non-inversion tillage once the site is flat and if the soil cover is greater than the depth of the tines. Any buried soil under ploughed earthworks stands less chance of survival once the earthwork is reduced to no more than the depth of ploughsoil; the rates of truncation seen on the flat sites (Appendix 3) will apply. Earlier partial destruction could also occur at the edges of the earthworks as cultivation equipment digs into their sides, as described in 5.4.4.
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6 Validity of Results 6.1.1 No significant slope truncation or soil creep was recorded as a result of this work. It is likely that this slope was either not really steep enough for erosion to happen within this soil type, and/or that cultivation was not prolonged enough to show significant changes of this kind. Colluvial soil was observed elsewhere on the site, however, so there has been some soil erosion in the past. Nevertheless, no significant erosion incidents occurred in the life of the project and it is quite possible that any changes of this nature would have occurred at a rate too gradual to be recorded within the timescale of the project. Be that as it may, assumptions based on the understanding of soil erosion discussed in Section 2.1 and the results of the trails carried out here, have allowed suggestions for arable agriculture on slopes to be made. It is thought that these are realistic based on the variables stated (see sections 7.1.5 and 7.1.6 below). 6.1.2 The earthworks constructed and tested here were built from the surrounding subsoil and sandy loam topsoil. It is possible that the rates of truncation and behaviours recorded here are not representative of other soil types or of barrows built in a different way. Earthworks made up of material from their associated surrounding quarry ditches will survive better in some areas than others, depending on the resilience of the underlying quarried material. In a series of excavations on the West Berkshire Downs the relatively well preserved barrow, Lambourn 19, was thought to owe its survival to its large diameter and the fact that it was built of a ‘plough resistant clay compound’. In contrast the loam inner core of the Hodcott barrow proved very vulnerable to destruction after its protective chalk ‘envelope’ had been removed by ploughing (Richards 1990a). 6.1.3 Whilst the authors cannot be categorical about extrapolating these findings to other soil types it is reasonable to assume that because of the physical effects of the tillage tools used the soil movement would be independent of soil type and the results would apply equally to other soils. One major factor would be the effects of in situ soil strength which could restrict the operational depth of tillage on higher clay content and drier soils. 6.1.4 In the rare cases where the rates of truncation of earthworks under cultivation in the ‘real world’ have been recorded they appear to be similar to those recorded here. Table 4.1 in section 2.4 shows that average annual truncation rates vary from 0.01 m at Thornborough Henge, to 0.02 m for barrows in Norfolk, 0.04 m for a Neolithic causeway in West Sussex and 0.025 m for barrows in Cambridgeshire and Peterborough. Whilst the circumstances and type of cultivation are not necessarily stated, the assumption is that these sites have been ploughed rather than subject to other types of cultivation. Given the possible variables the rates seen in these examples and in the present study are comparable. 6.1.5 It is interesting to consider the rates of truncation recorded here in relation to upstanding earthwork monuments seen in the field today. Whilst it would be difficult to prove, given the lack of detailed records, we can assume that many earthworks were brought into cultivation using modern machinery in the 1940s as part of the war effort, and have been under cultivation ever since. If it is assumed that these were c 1 m high as a result of natural truncation over time, at an average rate of 0.01 m of truncation a year, 60 years worth of truncation would result in reduction to a height of c 0.4 m. If this equation is broadly correct, under continuing cultivation many of these monuments will be lost in another 20-30 years. However, the vulnerability of some earthworks to total and rapid destruction once they have been reduced to this height,
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combined with the effect of some of the machines used in these trials, could cause the rate of destruction to become more rapid. 6.1.6 One of the key issues facing those involved in making management decisions is how to prioritise the protection of sites, especially earthworks. How low does an earthwork have to be before buried soils and shallow features beneath it are destroyed by cultivation? The present work has considerably enhanced our knowledge of what happens to an earthwork during cultivation and has provided some indication of the extent of survival beneath a cultivation-truncated earthwork. However, the work of trials, while giving a good impression of the effects of cultivation over a 30-year period, cannot totally recreate the effects of hundreds of years of cultivation and other ongoing erosion issues on earthworks - the effects of bioturbation, animal burrowing and long term damage and the consequent survival of buried soils.
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7 Summary 7.1.1 In terms of their significance as features within the historic landscape and their potential to contain important stratigraphic information (ie the earthwork materials’ relationship with earlier and later features), the evidence which may be contained both within and protected underneath the earthwork (for phasing, composition, construction methods etc) suggests many reasons why it is worth preserving even remnant barrow mounds and other earthwork features. 7.1.2 Based on the results of this study the following conclusions can be drawn: • ploughing does not allow the preservation of earthworks • non inversion tillage of earthworks does not offer significant advantages in protection to earthworks over conventional ploughing • non-tillage (direct drilling) offers the only long-term sustainable protection for most earthworks if they remain in cultivation • managed pasture forms the only other sustainable protection for earthworks, although increased bioturbation may be an issue • the average height loss of the non-inversion tilled earthworks is c 0.01-0.03 m per year, the variation correlating significantly with earthwork type • the average height loss of the ploughed earthworks is c 0.01 m per year • truncation of the earthwork, especially leading to the redistribution of the soil over a wider area in the direction of cultivation may lead to miscalculation of the centre point (and thus, for example, of the position of a burial beneath a round barrow) and over a long period may lead to misconceptions as to the original position of the mound • buried soils and features below earthworks are likely to be affected at the edges of the earthwork first • once earthworks have been ploughed/cultivated flat then any features beneath them will survive better under non-inversion tillage regimes than ploughing (as seen in Appendix 3) • even severely plough-damaged earthworks can preserve buried soils, artefacts, burials and other features beneath
7.1.3 In addition, conclusions related to factors not considered in detail here include the following: • the rate of earthwork truncation will be determined by a number of key site intrinsic factors like soil type, mode of construction, shape and cultivation practice • earthworks located on a slope or on a lighter sandy or sandy loam soil may be prone to greater rates of truncation because of their natural erodibility. The effects of this could, however, be variable depending upon the geometry of the earthwork, with steeper slopes on the downhill face and shallower slopes on the uphill. Some earthworks, which tend to follow the contours, such as ridge and furrow, could act as soil conservation terraces and help to reduce gradual downslope drift
7.1.4 Ploughing does not offer protection to archaeological sites on slopes. The gradual increase in the depth of disturbance over time seen on the flat sites will be accelerated here by soil erosion downslope.
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7.1.5 Whilst non-inversion tillage is damaging to earthworks due to its capacity to move and level irregularities, it can be used on slopes with the following caveats: • Non-inversion tillage or indeed any tillage should not be undertaken on the upper and middle slopes where the slope is more than 5/6 degrees (direct drilling may be possible but should be assessed on a case by case basis). This is especially relevant on predominately fine sand and silty soils where the risk of soil erosion is greatest. It may be that soils with a higher clay content can be cultivated, but they would also need to be assessed on a case by case basis depending on the history of erosion and soil depth etc. 7.1.6 Where gentle slopes are assessed acceptable for cultivation: • Non-inversion tillage must be undertaken either along the contours of the slope or approximately perpendicular to the main field slope, and not up/down slope • Good slope tillage management should be adhered to; ie tillage should be practised in one direction one year and the other direction the year after. This will compensate for any small movements of the soil
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8 REFERENCES
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Model used for assessment Site Management factors LIKELIHOOD OF IMPACT Score + Serious Risk High Risk Medium Risk Low Risk Minimum Risk confidence Score 5 Score 4 Score 3 Score 2 Score 1 grade (CF)*
Buffer zones: previous Cultivation of areas or Present cultivation likely to be at Shallow buffer (eg. 0.10-0.20m); Consistent moderate Deeply buried (eg > A ...... cultivation depth/ extent in encroachment on parts of sites not interface with archaeology Previous cultivation has left undisturbed buffer (of 0.25m) B...... relation to archaeology previously in cultivation (or differential cut and fill old colluvium or C...... proposed in the future); alluvium eg. 0.20-0.25 Evidence of new disturbance or m) earthworks present. Cultivation method and depth New significantly deeper ploughing Regular deep ploughing, deep Normal ploughing, chisel Shallow minimum Continuous direct A...... with clear fresh disturbance eg. rotavating, stone cleaning (0.26-0.30m) ploughing cultivation methods drilling with no B...... presence of fresh subsoil (>0.30m) (or proposed in the future) (0.20-0.25m) (0.10-0.19m) subsoiling C...... (or proposed in the future) (<0.10m) Cropping regime Cropping includes sugar beet, Cropping includes cereals, non- Cropping includes A ...... potatoes, needing deep soils root crops long term grass ley (or B...... (or proposed in the future) set-aside) > 5 years C......
Compaction and drainage New regular subsoiling < 3 yrs old Regular or occasional subsoiling or pan Rare subsoiling required; No subsoiling A...... (or proposed in the future) busting required (3-6 years) moling and drains (7-15 years) B ...... wetland water table lowering C...... (or proposed in the future) Initial score (In box to Right) Intrinsic site factor. Any of above = Any of above = Any of above = Weighting Total score x 2.5 Total score x 1.5 Total score x 0.5 Probability of Occurrence score to be calculated in boxes a) and b) to right: a) Score above to multiply by Serious/High/Minimum b) Result = Final Score (may be Initial score multiplied by any weighting derived from ‘Serious’ or ‘High’ and/or ‘Minimum’ weightings: graded A,B,C*) columns as applicable (if Serious weighting applicable do not apply any High weighting as well). A ………. Do not weight scores at this stage if no ‘Serious’ or ‘Minimum’ risk issues arise. B …………. ………….. X ………… C ………….. *Scores to be given by quality of supporting evidence: A = Good evidence; B = Some evidence; C = Poor evidence, mainly assumption
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Future soil loss through erosion factors (site intrinsic factors) Susceptibility of cultivated soil to water erosion factors, Source : after Controlling Soil Erosion, MAFF 1999 Slope > Score + (CF)* Soil texture Steep slopes Moderate slopes Gentle slopes Level ground Light soils: Serious High Medium (Sand/loamy sand/sandy Score 5 Score 4 Score 3 Minimal loam/silty sand (High) (Medium) (Low) Score 1 loam/silt/silty loam) (Score 4) (Score 3) (Score 2) A...... Moderate soils: High B...... (Silty clay loams/sandy Score 4 Medium Low Minimal C...... clay/clay loam) Medium Score 3 Score 2 Score 1 (Score 3) Heavy soils: Low Minimal Minimal Minimal (Silty clay/clay) Score 2 Score 1 Score 1 Score 1 * Where average annual rainfall is less than 800mm, the likelihood of occurrence class in brackets applies Susceptibility to deeper cultivation through soil movement by wind erosion
Main soil Peats Silts/sands Loams Sand clay/silt clay Clay Score+ (CF) * group A...... Likelihood of Serious High Medium Low Minimal B...... occurrence Score 5 Score 4 Score 3 Score 2 Score 1 C...... Applied likelihood of occurrence result Susceptibility to deeper cultivation through soil loss during harvesting Crop type Roots/tubers Combinable crops Not under cultivation Score + (CF) * A...... Likelihood of Serious Medium Minimal B...... occurrence (score 5) (score 3) (score 1) C...... Any of above in highlighted grey columns = Any of above in highlighted grey columns = Total score x 2 Total score x 0.5 a) Score above to multiply by Serious/ Result = Final Score (may be graded A,B,C*) Applied likelihood of occurrence result Minimum weightings: ...... X ...... A ………. B...... C...... *Scores to be given by quality of supporting evidence: A = Good evidence; B = Some evidence; C = Poor evidence, mainly assumption
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Archaeological Weighting SCALE OF Serious Risk High Risk Medium Risk Low Risk Minimum Risk Score + ARCHAEOLOGICAL RISK Score 5 Score 4 Score 3 Score 2 Score 1 (CF)* Archaeological survival Clear upstanding earthworks and Settlement activity; Unknown archaeology or Site already Site largely destroyed A...... and vulnerability structures; Shallow negative features stratigraphy; substantially damaged; leaving very little B ...... Low earthworks coupled with with important contents (eg Shallow negative features; Only deep negative potential C...... buried ground surface; shallow graves) Surface finds not reflected in features likely to ‘Soft’ horizontal stratigraphy, underlying archaeology survive floor and occupation surfaces Archaeological SM/national importance Regional or County County or Regional importance Clear local importance No obvious A ...... significance importance importance B...... C......
Archaeological Risk Score Score
Weighting to be applied for For Archaeological Risk Score of 9-10 use weighting factor = 2; For score of 8-7 use weighting factor = 1.5; For score of 5 use weighting factor = Weighting archaeological risk 1.3; For score of 5-4 use weighting factor = 1; For score of 2-3 use weighting factor = 0.5.
Score above to multiply by weighting: Result = Final Score against Confidence grade* TOTAL WEIGHTED SCORE FOR ARCHAEOLOGICAL RISK: A …………. ………….. X ………… B …………. C ………..... *Scores to be given by quality of supporting evidence: A = Good evidence; B = Some evidence; C = Poor evidence, mainly assumption
Management factors (out of 50) Erosion factors (out of 30) Archaeological Weighting (out of 20) Total risk score (out of 100)
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Results of Model testing
Earthworks Non-inversions barrow Plough barrow Direct drill barrow Fallow barrow ridge non ridge plough ridge and furrow Slope non- Slope inversion inversion plough Management factors Buffer zones 5 5 1 1 5 5 5 4 4 Cultivation depth 3 3 1 1 3 3 3 3 3 Crop regime 3 3 3 1 3 3 3 3 3 Compaction and drainage 2 2 2 2 2 2 2 2 2
Site Intrinsic factors Water erosion 2 2 2 2 2 2 2 2 2 Wind erosion 3 3 3 3 3 3 3 3 3 soil loss 3 3 3 1 3 3 3 3 3
Archaeological weighting Vulnerability 5 5 5 5 5 5 5 5 5 Significance 5 5 5 5 5 5 5 4 4
Site management 32.5 32.5 3.5 2.5 32.5 32.5 32.5 30 30 Site Intrinsic 8 8 8 2.5 8 8 8 8 8 Archaeological weighting 20 20 20 20 20 20 20 18 18 total 60.5 60.5 29.5 24.5 60.5 60.5 60.5 56 56 Serious Serious Minimal Minimal Serious Serious Serious High High
Oxford Archaeology and Cranfield University 5 14/12/10 Figure 4.1: Layout of earthworks in the field at Cranfield Figure 4.2: Photograph of ridge and furrow, pre-cultivation Figure 4.3: Photographs of barrows, pre-cultivation Figure 4.4: Photographs of ridge, pre-cultivation Earthwork construction
Re-profiling the direct drill and mouldboard plough barrow Figure 4.5: Construction and re-profiling of the earthworks Non- Ridge inversion barrow Non- inversion
Mouldboard plough
Ridge and Un- furrow cultivated
Non-inversion Slope
Mould-board plough
Non-inversion
Uncultivated
Figure 4.6: Monitoring stations within earthworks, pre-cultivation Set-up of indicator pits Excavated indicator pits, post- Excavated indicator pits, post- disturbance with most of upper layer disturbance showing no disturbance of glass removed to sand layers
Glass seen on the surface from Excavated indicator pit, post- Excavated indicator pits, post- indicator pits after cultivation cultivation showing no disturbance cultivation showing half of upper of the glass layer of sand removed Figure 4.7: Photographs of glass and sand indicator pits and disturbance Ridge and furrow showing un-tilled furrows Year 1: Non-inversion tillage over the ridge and furrow Ploughing over ridge
Year 1: Non-inversion tillage Year 10-15: Direct drill over the Year 22: Mouldboard ploughing over the over the barrow barrow barrow Figure 4.8: Earthwork tillage Figure 4.9: Contour survey over non-inversion barrow: Years 0-10 Figure 4.10: Contour survey over non-inversion barrow: Years 15-30 Figure 4.11: Below ground disturbance, non-inversion barrow: Years 0-10 Figure 4.12: Below ground disturbance, non-inversion barrow: Years 15-30 Figure 4.13: Contour survey mouldboard plough barrow: Years 0-15 Figure 4.14: Contour survey mouldboard plough barrow: Years 20-30 Figure 4.15: Below ground disturbance mouldboard plough barrow Mouldboard plough barrow, pre-cultivation
MouldboardNon-inversion plough barrow, barrow, post-cultivation post-cultivation
Non-inversion barrow, pre-cultivation
Mouldboard plough barrow, post-cultivation
Figure 4.16: Cross sections through mouldboard plough and non-inversion barrows Figure 4.17: Contour survey direct drill barrow: Years 0-15 Figure 4.18: Contour survey direct drill barrow: Year 20 Figure 4.19: Pre- and post-cultivation contour survey of fallow barrow Figure 4.20: Contour survey of ridge and furrow - Years 0-10 Figure 4.21: Contour survey over ridge and furrow - Years 15-30 Figure 4.22: Below ground disturbance - ridge and furrow - Years 0-1 Figure 4.23: Below ground disturbance - ridge and furrow - Years 5-10 Figure 4.24: Below ground disturbance - ridge and furrow - Years 15-20 Figure 4.25: Below ground disturbance - ridge and furrow - Years 25-30 Ridge and furrow pre-cultivation
Ridge and furrow post-cultivation
Figure 4.26: Cross sections through the ridge and furrow earthwork Figure 4.27: Contour Survey - Ridge - Years 0-10 Figure 4.28: Contour Survey - Ridge - Years 15-30 Figure 4.29: Below ground disturbance - Ridge - Years 0-1 Figure 4.30: Below ground disturbance - Ridge - Years 5-10 Figure 4.31: Below ground disturbance - Ridge - Years 15-20 Figure 4.32: Below ground disturbance - Ridge - Years 25-30 V
Figure 4.33: Cross sections of Ridge - non-inversion tillage V
Figure 4.34: Cross sections of Ridge - mouldboard plough Figure 4.35 Contour survey and below-ground disturbance of the slope - years 0-10 Figure 4.36 Contour survey and below-ground disturbance of the slope - years 15-20 Direct drill barrow - undisturbed indicator pits
Non-inversion barrow - indicator pits away from barrow
Figure 4.37: Re-excavation of indicator pits Ridge Non- Non- inversion inversion barrow
Mouldboard plough
Ridge and Un- furrow cultivated
Slope
Mouldboard plough
Non-inversion
Uncultivated
Figure 4. 38: Post-cultivation state of the indicator pits
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