Quantifying the threat to archaeological
sites from the erosion of cultivated soil
Keith Wilkinson1 , Andrew Tyler2 , Donald Davidson2 & Ian Grieve2
Ploughing is probably the greatest agent of attrition to archaeological sites world-wide. In every
country, every year, a bit more is shaved off buried strata and a bit more of the past becomes
unreadable. On the other hand, people must eat and crops must be planted. How can the fields be
best managed to get the best of both worlds? Perhaps the most pressing need for resource managers
is to know how quickly a particular field is eroding: negotiation and protection is then possible.
Up to now that has been difficult to measure.
The new procedure presented here, which draws on the unexpected benefits of nuclear weapons
testing, shows how variation in the concentration of the radioisotope 137Cs can be used to monitor
soil movements over the last 40 years. The measurements allow a site’s ‘life expectancy’ to be
calculated, and there are some promising dividends for tracking site formation processes.
Keywords: CRM, site formation processes, 137 Cs inventory, soil redistribution, tillage, erosion, Quantock Hills
Introduction
It has long been recognised that modern cultivation techniques, particularly ploughing,
have an erosive impact on archaeological sites (e.g. Lambrick 1977). Nevertheless, it is
only in recent years that any attempt has been made to quantify such erosion. According
to the Monuments at Risk Survey for England (MARS), ‘cultivation is the single biggest
hazard facing monuments, accounting for 10 per cent of wholesale monument destruction and
30 per cent of piecemeal loss’ (Darvill & Fulton 1998: 236-7). The effect of ploughing
across an archaeological site is to reduce its surface expression, eventually removing all
upstanding elements. Roman and prehistoric sites in particular are more likely to be
known from cropmarks or soilmarks seen in aerial photographs, or as artefact scatters
in the plough soil, rather than as upstanding monuments. In the 65km2 area investigated
by the Southern Quantock Archaeological Survey (SQAS, a collaboration between the
Department of Archaeology, University of Winchester and Somerset County Council)
in south-west England, for example, 80 per cent of archaeological sites recorded in the
local Historic Environment Record (HER) (Somerset County Council 2005) are known
from cropmarks only. Clearly, buried sites are of no lesser archaeological importance than
upstanding monuments and their future survival is therefore an important consideration for
1
2
Department of Archaeology, University of Winchester, West Hill, Winchester SO22 4NR, UK (Email:
[email protected])
School of Biological and Environmental Sciences, University of Stirling, Stirling, FK9 4LA, UK
Received: 14 March 2005; Accepted: 31 October 2005; Revised: 3 January 2006
antiquity 80 (2006): 658–670
658
Keith Wilkinson et al.
local authority and national archaeological service archaeologists who have the responsibility
for managing and conserving the archaeological resource (Somerset County Council and
English Heritage respectively in the case of the SQAS study area). However, two factors
have particularly hindered efforts to protect buried archaeological sites in England: firstly
a legislative framework that is primarily designed to conserve upstanding monuments and
secondly the difficulty in quantifying the risk to sub-surface archaeological features. Here we
address the second by presenting the results of a pilot study which used soil 137 Cs inventories
to model soil redistribution over the last 40 years or so across four buried archaeological
sites. At the end of the paper we return to the first question and discuss the implications of
our work given forthcoming changes in both domestic and European Union (EU) policy
on rural land management.
The pilot study was carried out as part of SQAS between August 2002 and September
2004. SQAS itself comprises a five-year long programme of fieldwork and associated postfieldwork analysis which seeks to investigate the pre-medieval archaeology of the southern
slopes of the Quantock Hills. The Quantock Hills are a north-west to south-east trending
range of hills in the county of Somerset (Figure 1). They are largely comprised of Devonian
(slates and limestones) and Triassic (marl and sandstones) strata (Figure 1c), in which brown
earth soils of the Milford, Whimple and Bromsgrove Series have been mapped (Findlay
et al. 1984). The southern slopes have an average aspect of 7 per cent, but in practice the
topography is highly variable. We measured 137 Cs archives of soils at four sites with evidence
for sub-surface archaeological features located on contrasting geological substrates and in
varied slope positions. However, for reasons of space we only consider two in detail here
(Stoneage Barton and Yarford) (Figure 1). As is the case with 80 of the 100 HER entries in
the study area, the four sites were first recognised as cropmarks in aerial photographs taken
in the mid 1970s - early 1990s (Wilkinson & Thorpe 1999).
Prior to 1999 there were only two sites of Roman date known from the study area and
recorded in the HER, and it was therefore thought that the area had not been intensively
occupied or exploited at this time. However, Roman settlement, ranging from simple
farmsteads to a villa and dating from the second to fourth centuries AD, has been found
on five of the six sites excavated by SQAS. Where Roman settlement was found, it always
succeeded Iron Age and/or Bronze Age activity in the same location. Later prehistoric features
investigated include settlements, a metalworking site, barrows and field systems. Other than
a cemetery of sub-Roman date at Stoneage Barton Farm, no post-Roman activity has been
recognised on the sites so far investigated. Therefore, on the basis of present evidence, it seems
likely that certain discrete locations – evidenced by cropmarks – in the Quantock landscape
were used on a number of occasions during later prehistory and the early historic period.
Later settlement activity is probably restricted to the area occupied by the present villages.
Assessing soil loss rates using 137 Cs inventories
Although the 137 Cs technique has been employed for over 20 years in agricultural studies to
quantify soil erosion (e.g. DeJong et al. 1983), it has not been widely or systematically applied
659
Method
The Quantock Hills study area and its archaeology
Archaeological sites and the erosion of cultivated soil
Figure 1. Locations of the SQAS and REAS study area (a and b), and the sites sampled for 137 Cs measurement (c).
to archaeological situations (Quine & Walling 1992; Davidson et al. 1998). The radioisotope
137
Cs is a fission product and occurs in the environment as a result of atmospheric testing of
nuclear weapons or release into the environment from licensed discharges or from accidents.
Nuclear weapons testing commenced in the 1940s and peaked in 1963, while the Chernobyl
accident ejected further 137 Cs into the atmosphere in 1986. 137 Cs is attracted to clays and
silts and is thus retained in both soils and sediments (Cook et al. 1984; Cremers et al. 1988).
Therefore the nuclear ‘events’ previously mentioned can be recognised in the 137 Cs concentrations in certain sedimentary environments, for example lake deposits (e.g. Bonnett &
Cambray 1991). However, in soil erosion studies it is the total 137 Cs inventory of the soil
profile that is of importance (Quine 1995). In locations that have not been subject to
erosion/deposition or mixing, the 137 Cs content (inventory) of a soil exponentially decreases
with increasing depth in the profile (Figure 2). However, where mixing processes such as
ploughing occur, 137 Cs inventories are homogeneous throughout the mixed zone. Where soil
particles are lost due to active erosion processes, soils have relatively low 137 Cs inventories
whereas in soils where deposition is occurring 137 Cs concentrations are correspondingly
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Keith Wilkinson et al.
Figure 2. Vertical variations in 137Cs content of soils across an idealised slope and land-use transect. Modified from Walling
& Quine (1990: figure 1).
high, when compared to non-eroding sites (Figure 2). Such comparisons can be used to
calculate an erosion/deposition index for any sample point, so long as a local control point
can be established (for example from grassland or woodland that has been unaffected by
ploughing since the 1940s) (Zhang et al. 1990).
The aim of the present study was to carry out a systematic test of the 137 Cs technique in an
archaeological situation and thereby to assess its usefulness as an archaeological management
tool.
Methods
For each sample site, 10.5cm diameter cores were taken through the soil profile to a depth
of 45cm (where possible) at 50m intervals along transect lines (e.g. Figure 3). One core
from each transect was split into 5cm depth increments so that 137 Cs measurements could
be used to determine plough soil depth, while all remaining cores were divided into 15cm
segments (Figure 4). Following laboratory analysis, the thickness of the plough layer was
defined as the depth where 137 Cs activity falls to half its observed maximum value (Tyler
et al. 2001). Differential GPS (dGPS) was used to locate all sample points and to carry
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Archaeological sites and the erosion of cultivated soil
Figure 3. 137Cs derived soil redistribution data from the Stoneage Barton site plotted against a rectified aerial photograph
of the site and archaeological features. Contours were determined by Differential GPS topographic survey. Aerial photograph
taken by Frances Griffith and Bill Horner, Devon County Council.
out a topographic survey within each sampled field. Sample pits were also excavated along
each transect in order to describe representative soil profiles, while archaeological trenches
provided an opportunity for visual assessment to be made of the impact of soil redistribution
on archaeological features. These data also provided some control for the soil movement
models derived from 137 Cs measurements. Cores for control purposes were taken from areas
of permanent pasture in locations as close to the sampled fields as possible. Three such
cores were taken at each control site, one being cut into 3, 4 and 5cm depth segments for
137
Cs measurement and the others into 15cm divisions as previously described (Figure 4).
Once in the laboratory, samples were initially air-dried and sieved through a 2mm mesh. Subsamples were then taken for particle size distribution and gamma spectrometry measurement
to calculate 137 Cs concentrations. Particle size distribution was determined by sieving for
size fractions >212 µm and by using a Coulter LS100 laser granulometer for finer particles,
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Method
Keith Wilkinson et al.
Figure 4. Vertical variations in 137Cs from the sampled fields at Stoneage Barton and Yarford.
while the methodology outlined by Tyler (Tyler et al. 2001) was used to measure 137 Cs
activity.
In order to determine the land use history since nuclear testing commenced and
particularly since peak 137 Cs discharge in 1963, interviews were carried out with current and
previous farmers of each sampled site. Aerial photographs and the results of the (unpublished)
Second Land Utilisation Survey of Britain (undertaken during the early 1960s) were used to
check and supplement these oral data. Thus it was possible to determine cultivation history
accurately back to 1960, and as far back as the 1940s for the Volis Hill site.
Results
Particle size measurements of samples from all four Quantock sites indicate that soils subject
to erosion have similar grain size distributions to non-eroded soils. These data suggest soil
has been moved predominantly by the plough and that soil redistribution is therefore mainly
the result of tillage. Accepting this view, it is reasonable to suggest that total soil movement
is a function of the number of tillage events that have taken place. Given that land-use
history of the sampled fields is known for all years subsequent to peak 137 Cs discharge in
663
Archaeological sites and the erosion of cultivated soil
Table 1. Summary of 137 Cs calculated erosion rates for all four sampled sites
Stoneage Barton
Toulton
Volis Hill
Yarford
Sandstone
33
0.9 +
− 0.1
4.3 +
− 0.9
1.0 +
− 0.1
1.1 +
− 0.1
Solifluction
23
0.4 +
− 0.0
2.5 +
− 0.4
0.4 +
− 0.0
0.7 +
− 0.1
Slate
33
0.5 +
− 0.1
3.3 +
− 0.1
0.8 +
− 0.1
1.0 +
− 0.1
Slate
31
0.1 +
− 0.0
4.7 +
− 0.3
2.0 +
− 0.1
2.5 +
− 0.2
Geology
Number of years under arable crops since 1963
Minimum erosion (mm yr−1 )a
Maximum erosion (mm yr−1 )
Median erosion (mm yr−1 )b
Median erosion (mm/tillage episode)b
a
b
Excludes sample locations where deposition was evidenced.
Includes sample locations where deposition was evidenced.
1963, it is possible to calculate an erosion rate per tillage episode as well as a median annual
rate (Table 1).
Stoneage Barton
This site was excavated in June - July 2000 following a magnetometry survey carried out
in February 2000. The field is on a subtle promontory of other sandstone with gentle
slopes down to the east and south (Figure 3). Thick soils have developed in the weathered
sandstone, but despite this, Roman artefacts lie on the ground surface indicating that
archaeological features are within the plough zone. Several phases of cropmarks appear
on aerial photographs taken in 1995 and more clearly in magnetometry plots (Figure 3).
Excavation revealed that rectilinear features seen in these surveys are field systems of first
to mid-fourth century AD date, while the sub-circular enclosure is Bronze Age. The latter
feature partially enclosed an important ‘Dark Age’ cemetery dating to 595-675 cal. AD
(a 2σ calibration combining 1355 +
− 45 BP (AA-43026))
− 45 BP (AA-43025) and 1440 +
using the IntCal04 curve (Reimer et al. 2004) and OxCal 3.10 software (Bronk-Ramsey
2005), which could not be seen in either the aerial photographs or the geophysics (Webster
& Brunning 2004). Samples for the present study were taken in August 2002.
Soil redistribution rates at Stoneage Barton range from erosion of 4.3mm yr−1 to deposition of 2.9mm yr−1 (see Table 2 in Project Gallery, http://antiquity.ac.uk/ProjGall/wilkinson
et al, and Figures 3 and 4). Rates calculated from comparable samples in each of the three
transects are broadly similar, suggesting that the erosion pattern corresponds to topography.
The one exception is sample SB 1.4 which indicates deposition in the centre of the field. This
anomalous result may have been caused by accidentally sampling spoil from the adjacent
archaeological trench. Highest erosion rates occur between c . 75m and c . 105m from the
western boundary of the field (Figure 3). This location is significant as it lies on the convex
crest of the promontory, relatively steep downward slopes surrounding it to the northwest, west and south-west. It is also notable that the Dark Age cemetery and the Bronze Age
enclosure both lie along the crest, both presumably taking advantage of the visibility afforded
by such a pronounced position, but now preferentially suffering erosion as a result. On the
gently rising slopes to the east, erosion rates are generally lower, although an annual average
of 1.0mm of soil is still being lost over the area occupied by the Roman rectilinear field
664
Method
Keith Wilkinson et al.
Figure 5. 137Cs derived soil redistribution data for the Yarford site. These are plotted against archaeological features determined
from aerial photographs, magnetometry and excavation. Aerial photograph taken by Frances Griffith and Bill Horner, Devon
County Council.
systems. However, in the lea slopes to the west of the promontory and within 10m of the
western field boundary, soil thickness is increasing at between 1.0mm yr−1 and 2.9mm yr−1 .
Yarford
Samples for 137 Cs studies from the Yarford site were collected in July 2003. Aerial
photographs taken in 1995 and a magnetometer survey carried out in April 2003 indicate
that a series of enclosure systems exist on north-south slopes above the hamlet of Yarford
(Figure 5). Sample excavations were carried out of the larger enclosure systems in June - July
2003, which proved them to be of Middle and Late Iron Age date. A prominent doubleditched enclosure in the east of the field and a feature which had removed its north-east
corner were the subject of excavation between 2003 and 2005. The features are a defended
high-status Late Iron Age settlement and a fourth-century Roman villa respectively. The
latter was constructed from locally quarried sandstone and was built within an artificial
665
Archaeological sites and the erosion of cultivated soil
Figure 6. The north-easternmost room of the Yarford Roman villa showing the mosaic. The artificial terrace in which the
main villa range was built and which has had the effect of protecting the northern part of the villa, can be seen in the
background (photograph by Tony King).
terrace dug 0.7m deep into the south-facing slope (Figure 6). Although small (the main
villa range measures just 20m from east to west, and 12m from north to south), the villa
is well-appointed. Its north-easternmost room contains a decorated mosaic of fine tesserae,
while the corridor that provides access to all the rooms is floored with a coarse monochrome
mosaic. A courtyard to the south of the main villa building had been resurfaced on a
number of occasions and was surrounded by wooden structures to the west and a kiln
and bath house to the east. The Yarford villa is an extremely important archaeological site
given the fact that it is located in an area of south-west England where Roman villas were
previously unknown, and 20km west of the nearest known example. The main villa range
is well preserved thanks to its construction in a terrace, although the courtyard areas are
undergoing some disturbance by ploughing.
The pattern of soil redistribution seen at Yarford is more complex than that at Stoneage
Barton, largely as a result of the presence of a field boundary dividing the northern
third of the surveyed area from that to the south (see Table 3 in Project Gallery,
http://antiquity.ac.uk/ProjGall/wilkinson et al, and Figure 5). This hedge sits on top of
a pronounced lynchet-like feature. Soil is accumulating immediately behind the hedge,
albeit at a slow rate (0.2mm yr−1 ), but 16m further to the north soil is eroding at 2.43.9mm yr–1 . Indeed plough furrows could clearly be seen cutting into the slate bedrock in
two trenches excavated at this location. As with Stoneage Barton, this eroding zone coincides
with a convex crest, although at Yarford archaeological features have not been preferentially
666
Keith Wilkinson et al.
located at this point. North of the crest erosion decreases, although – excepting a single point
of accumulation – rates are still higher than those observed at Stoneage Barton. Erosion rates
vary between 0.1mm yr−1 and 4.7mm yr−1 south of the hedge, with a tendency for increased
erosion along a further convex crest stretching between sample points YAR 14 and YAR
27 (Figure 5). However, the highest erosion rates of all occur in the north-east of this field,
c. 50m to the north of the Roman villa. The villa is buried by about 0.8m of sediment and
soil, while the courtyard and outbuildings to the south lie beneath c . 0.3m of soil. Erosion
rates calculated from the 137 Cs measurement are 2.6mm yr−1 immediately to the north of
the villa and 3.1mm yr−1 adjacent to the courtyard. To the south of the crest at YAR 14 the
slope is steeper and erosion rates drop to 0.1-1.9mm yr−1 .
Soil redistribution data calculated from 137 Cs measurement of four sites from the Quantock
Hills confirm the obvious: hillcrests suffer erosion while soil accumulates in depressions.
Archaeological sites tend to occur in high-visibility areas such as on crests, and are
therefore impacted more severely than if they were randomly scattered across the landscape.
Nevertheless, the data are of much greater significance in that they quantify mean erosion
(and deposition) over the last 40 years. Assuming that 1) soil loss has followed a linear
trajectory since 1963 – which on the basis of an unchanging land use, appears to be the
case for all sites except Toulton – and 2) that agricultural practices of the last five decades
will continue into the future, then the data provide a time frame over which damage to
archaeological sites is likely to occur. For example erosion rates in the vicinity of the Dark
Age cemetery at Stoneage Barton average 1.1mm yr−1 , while archaeological features lie
400mm below the ground surface (Webster & Brunning 2004). Given that current plough
depth is 250mm (Figure 4), this suggests that the site will suffer significant erosion in around
140 years. At Yarford the situation is more problematic from an archaeological management
perspective. The present pattern of land use will impact the villa structure in about 75 years,
although it will not totally remove it before 2250. The courtyard is now being taken up into
the plough soil and, assuming that archaeological deposits in this area are around 500mm
thick, potentially important material will be incrementally removed over the next 160 years
or so.
The Yarford and Stoneage Barton maximum and median erosion rates are higher than
those from the other two sites examined (Toulton and Volis Hill) (Table 1). However,
this may be explained by the fact that Toulton has been pasture for a higher proportion
of the time since 1963, while slopes are on average less steep at both Toulton and Volis
Hill. Nevertheless, in exactly the same way that soil redistribution will significantly impact
archaeological sites at Stoneage Barton and Yarford, important archaeological remains (a
Bronze Age to Roman settlement and metalworking site) at Volis Hill will similarly be
removed over the next 150 years.
Erosion rates calculated from the Quantock sample sites are high in comparison to the few
other published 137 Cs erosion studies (e.g. Quine & Walling 1992; Davidson et al. 1998).
The studies that have previously been undertaken are mostly from Scotland and south-west
England, while very few fields have been sampled for 137 Cs measurement elsewhere in
667
Method
Discussion
Archaeological sites and the erosion of cultivated soil
England. Therefore, despite the fact that the geology and soil types that characterise the
Quantock study area are found only in south-west England and on the Welsh borders, the
results of this study cannot be dismissed as being exceptional without a great deal more
work being carried out. Other areas of England, for example the East Anglian Brecklands
and the chalk downlands of Wessex and Sussex, have since 1963 been cultivated with an
equal if not a greater intensity as the southern slopes of the Quantock Hills. Areas such
as these also contain important, and indeed better known, sub-surface archaeological sites,
but the redistribution rates of the overlying soils, and therefore the risk to the sites, have
not been quantified. Understanding erosion and hence the potential risk to archaeological
sites is nevertheless likely to become increasingly important with impending changes in
both British and European Union policy. These will see a move away from subsidising farm
production towards paying farmers to conserve the landscape – including archaeological
sites (DEFRA UK 2004a). Alongside realignments in farming policy there are likely to be
changes to heritage protection measures in England and Wales that will affect the way rural
archaeology is managed (Trow 2004). With the coming of this new paradigm it will be
important to prioritise according to immediacy of threat. Use of 137 Cs archives represents
a particularly useful way of quantifying risk for sites under the plough and will therefore
enable such prioritisations to be made.
In addition to supplying data for cultural resource management purposes, 137 Cs studies
also provide data for calibrating field survey (Quine & Walling 1992). By indicating which
areas of a ploughed field are eroding and which are accumulating soil particles it is possible
to predict from where in a field surface artefacts derive. From this information artefact
distribution patterns can be assessed in a more informed manner. For example a shovel pit
survey in the fields around the site of Toulton failed to find Roman artefacts despite the fact
that high concentrations of fourth-century finds were made from a number of features on
the site. 137 Cs studies have, however, demonstrated that erosion rates are very low over the
archaeological site (Table 1), and therefore that artefacts from the site are unlikely to have
been incorporated in the plough soil as a result of cultivation since the 1960s. A plough soil
survey carried out in isolation would therefore be unlikely to detect the Roman site.
Conclusion
The use of 137 Cs inventories to study erosion across four archaeological sites on the southern
flank of the Quantocks reaffirms its importance as an archaeological management tool
(see Quine & Walling 1992; Davidson et al. 1998). By reconstructing patterns of soil
redistribution since peak atmospheric nuclear weapon testing in 1963, it enables future risk
to sub-surface archaeological sites to be quantitatively assessed. Our results demonstrate
that both regionally and nationally important archaeological sites in the Quantock Hills are
currently being eroded and will be completely removed in the next 150 years or so, should
the present cultivation regimes be maintained. On the basis of the available data we are
at present uncertain how typical the results are for England or further afield; however, we
suspect they may not be exceptional.
The 137 Cs methodology has until now been relatively slow and expensive, but nevertheless
it still has the advantages of minimal intrusion into sub-surface archaeological strata and a
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Keith Wilkinson et al.
high spatial resolution. For many sites this makes it a better option than traditional methods
of soil erosion measurement such as monitoring sediment traps or the use of pits containing
glass balls (DEFRA UK 2004b). The first of these measures erosion by its product and is not
therefore spatially sensitive, while the second is both intrusive and expensive to carry out at
high spatial resolutions. However, newly available 137 Cs detector technology allows real-time
measurements to be made in situ (Tyler 1999; Tyler et al. 2001; Tyler 2004), meaning that
future surveys can be carried out at the same rate and at a similar cost as, for example, a
magnetometer survey. We therefore predict that the 137 Cs technique will in the near future
become an important tool for those attempting to conserve our archaeological heritage.
The study reported here formed part of the Recent Erosion of Archaeological Sites project funded by English
Heritage. We would like to thank our Centre for Archaeology monitor Tom Cromwell for his enthusiastic
input throughout the project. We are also very grateful to Michelle Collings who carried out most of the
background research and organised much of the logistics. The following tenant farmers and landowners are
gratefully acknowledged for giving us permission to work on their land: Brian Bartlett, the Crown Estate, John
Dill, Peter and Jane House, the Tetton Estate and Hugh Warmington. We would like to thank Prof Alice Colman
for allowing us access to data from the Second Land Utilisation Survey of Britain and Somerset County Council
for permitting us to use their aerial photographs. Thanks are also due to Robert Croft, Frances Griffith, Tony
King, Phil Marter, Nick Thorpe, Alex Turner, Chris Webster, Helen Ewen and Stuart Bradley for their help
during the course of the project. The text has been copy edited at all stages by Myra Wilkinson-van Hoek.
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