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Beneath the sand В Эremote sensing archaeology aggregates and sustainabilitya case study from Heslerton the Vale of Pickering North Yorkshire UK.

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Archaeological Prospection
Archaeol. Prospect. 13, 291–299 (2006)
Published online 28 November 2006 in Wiley InterScience
(www.interscience.wiley.com) DOI: 10.1002/arp.297
Beneath the SandRemote Sensing,
Archaeology, Aggregates and
Sustainability: a Case Study from
Heslerton, theVale of Pickering,North
Yorkshire,UK
DOMINIC POWLESLAND*, JAMES LYALL, GUY HOPKINSON,
DANNY DONOGHUE, MARIA BECK, AIDAN HARTE and DAVID STOTT
The Landscape Research Centre,The Old Bridge Barn,Yedingham, NorthYorkshire,
YO17 8SL, UK
ABSTRACT
TheVale of Pickering, inYorkshire hasbeenthe setting forone of thelargest archaeologicalresearch
projects in Europe for more than 30 years.The accrued data includes over 1000 ha of fluxgate gradiometer measurements that can be interpreted with reference to 200 ha of subsurface mapping,
25 years of aerial photographic survey, two large-area multispectral and vertical photographic
surveys, a Laser Imaging Detection and Ranging (LIDAR) survey and area excavations in excess
of 22 ha. Crucially, this multifaceted approach has revealed flaws in our appreciation of the archaeological landscape based on isolated ‘sites’. This paper summarizes aspects of the approaches
and results using a variety of remote sensing techniques to characterize the nature and extent of
the archaeological resource in an aggregate rich environment. Copyright # 2006 John Wiley &
Sons, Ltd.
Key words: remote sensing: magnetometery; subsurface modelling; multispectral imaging
Introduction
The landscape centred on the village of West
Heslerton in the Vale of Pickering, UK (Figure 1),
has been the setting for one of the most ambitious
projects in landscape archaeology. Large-area
excavations covering more than 22 ha, 25 years of
aerial photographic survey, two large-area multi-
* Correspondence to: D. Powlesland, The Landscape
Research Centre, The Old Bridge Barn, Yedingham, North
Yorkshire, YO17 8SL, UK. E-mail: mamurphy@wiley.co.uk
Contract/grant sponsors: English Heritage (Aggregates Levy
Sustainability Fund); Natural Environment Research Council
(UK).
Copyright # 2006 John Wiley & Sons, Ltd.
spectral and vertical photographic surveys, a
Laser Imaging Detection and Ranging (LIDAR)
survey, over 1000 ha of contiguous gradiometer
survey and a 200 ha programme of subsurface
mapping have revealed the most comprehensive
body of archaeological landscape evidence for
any area of its size in Britain (Powlesland et al.,
1986, 1997; Powlesland, 2003a,b).
This paper provides a summary of the survey
and the various methodologies applied to elucidate the archaeological record of this landscape.
In addition to consideration of the near-surface
and surface record, a key task has been to
understand the subsurface (buried) resource,
since colluviation and especially aeolian activity
Received 31 August 2006
Accepted 25 September 2006
D. Powlesland et al.
292
Archaeological prospection
strategies used in the HPP
Magnetometer survey
Figure 1. Location of the Heslerton research area on the
southern side of theVale of Pickering,NorthYorkshire,England.
(see Radley and Simms, 1967) have played
important roles in landscape development in this
region.
Context
The Heslerton Parish Project (HPP) was established in 1980 within a rigorously designed
research framework (Powlesland, 1980, 2001,
2003a), although the strategy has constantly
evolved in response both to discovery and also
to changes in technology, particularly with
reference to remote sensing, field recording,
dating and geographical information systems
(GIS) technologies (Powlesland, 1986, 1991).
At the time of research design formulation,
evidence for archaeology of any period within the
Vale of Pickering was rare and the area was
largely considered an archaeological blank on
maps. The evidence there was for settlement was
restricted to discrete areas, for example, the major
late palaeolithic/early mesolithic sites at Star and
Seamer Carrs (Schadla-Hall, 1988), the late Bronze
Age/early Iron Age palisaded enclosures at
Staple Howe and Devil’s Hill (Brewster, 1963,
1981; Powlesland, 1987), the Roman centre at
Malton and medieval manorial elements in
Sherburn and Potter Brompton (Brewster, 1952)
and some Romano-British and early Anglo-Saxon
settlement at Seamer, Cross Gates (Pye, 1983).
Copyright # 2006 John Wiley & Sons, Ltd.
Initial trial surveys using gradiometery at Cook’s
Quarry in 1980 and again using resistivity near
Sherburn in 1984 failed to produce convincing
results. However, an immensely successful
magnetic gradiometer survey carried out by
English Heritage ahead of the Anglian settlement
excavation in 1989 proved that the technique
could work within the area.
Initially attention was focused on the application of gradiometry within the context of the
excavation, following the removal of the disturbed plough-soil (Figure 2). The survey
demonstrated that without the masking influence of the disturbed plough-soil, post-holes,
minor features and ditch complexes barely
visible on the surface could be seen (Lyall and
Powlesland, 1996).
Since 2000, four geophysical survey campaigns
covering around 900 ha have been undertaken in
the Vale hinterland using a pair of Geoscan FM36
fluxgate gradiometers in the early stages of
survey followed by a dual sensor, Bartington
Grad 601-2 fluxgate gradiometer in the latter
stages. Gradiometer data were collected using a
zigzag traverse method in 30 m grids, with all
machines set to detect magnetic variations of
0.1 nT. The resolution of all the surveys was 1 m
east–west and 25 cm north–south.
There is insufficient space here to discuss in
detail the results of the four large-area surveys
and it would also be premature given the limited
nature of the ground truthing so far undertaken.
However, with approximately 16 000 anomalies
derived from the dataset, the results have been
spectacular. For example, Anglo-Saxon Grubenhäuser features have recognizable and distinctive
geophysical characteristics and are amongst the
few anomalies that can be classified without
hesitation. The distribution of Grubenhäuser
indicated by the interpretation of the gradiometer data (Figure 3) suggests that the excavated settlement of West Heslerton is just one of a
number whose density within the Vale is similar
to that of the present villages (Powlesland, 1998,
2000; Haughton and Powlesland, 1999).
Archaeol. Prospect. 13, 291–299 (2006)
DOI: 10.1002/arp
Heslerton, the Vale of Pickering
293
Figure 2. Gradiometer survey results gathered before the removal of plough-soil at1 0.25 m resolution (left) and the same area
after the removal of the plough-soil (right), gathered at 0.25 m 0.25 m resolution.
Figure 3. The distribution of Anglo-Saxon Grubenhuser identified across all geophysical surveys.
Aerial photography
and multispectral imaging
An ad-hoc programme of aerial photography was
begun by the team in 1977 and has continued
with varying frequency on an annual basis,
augmented by the results of other aerial photographers. A number of fields are used as
‘reference fields’ where crop-marks form on a
regular basis and are visited during each flight to
check on crop-mark conditions. Despite repeated
survey, analysis of aerial photography showed
clear gaps in what appeared to be extensive and
coherent crop-mark complexes, such as the
ladder settlements. Therefore, it became clear
Copyright # 2006 John Wiley & Sons, Ltd.
that these gaps required investigation using
techniques that were more sensitive to crop
luminosity and reflectance than conventional
oblique photography, as well as techniques
capable of mapping the subsurface (buried)
resource.
In 1992, the West Heslerton area was flown by
the Natural Environment Research Council
(NERC) to capture high-resolution 12 band
multispectral data and high-resolution large
format vertical photography using the Daedalus
1268 Airborne Thematic Mapper (ATM) (Powlesland et al., 1997). This instrument collects
radiation from the Earth’s surface in 11 different
bandwidths. Bands 1–5 collect data in the visible
Archaeol. Prospect. 13, 291–299 (2006)
DOI: 10.1002/arp
294
D. Powlesland et al.
part of the spectrum, bands 6–8 in the nearinfrared, bands 9–10 in the shortwave infrared
and band 11 in the mid-infrared or thermal. The
ground resolution of multispectral data are both
altitude and airspeed dependent and these data
were collected at an altitude of 800 m providing a
nominal ground resolution of between 1.5–2 m
per pixel. Colour, near-vertical photographs that
were acquired at the same time used a Wild
RC-10. A second survey award from NERC in
2005 allowed the acquisition of multispectral,
high-resolution vertical digital air photography
and LIDAR data. This latter survey was undertaken using a Rollei medium format digital
camera (CCD resolution of 4080 by 4080; 16.64
megapixels with 48 bits per pixel). Again, flight
altitude was 800 m giving a nominal ground
resolution of around 14 cm per pixel. Work on
this second NERC dataset is in progress and will
be reported elsewhere.
Combining and comparing
multisensor remote sensing
The complementary return from three different
forms of remote sensing (i.e. aerial photography,
multispectral imaging and geophysics) is demonstrated by the analysis of two areas with different
underlying geologies. The first area is part of the
River Derwent floodplain and is underlain by
alluvial sands and gravels with zones of desiccating peat; an Iron Age Square Barrow cemetery
has been identified in this area. The second area
comprises alkaline sands and gravels, in part
sealed by aeolian sands, where parts of the late
Iron Age/Romano-British ‘ladder settlement’
and an early Anglo-Saxon or Anglian settlement
have been identified (Powlesland, 1998).
Figure 4 illustrates the results from area 1; the
interpretative drawings on the right are of
archaeological features only, with geological
features and field drains removed. Cursory
inspection reveals that all three remote sensing
techniques identify the main five barrows, but
also illustrates that each technique records
features which are not detected by the other
methods (Figure 5). Notably, the multispectral
data detected more barrows to the south of the
main cemetery area, where both crop-mark
Copyright # 2006 John Wiley & Sons, Ltd.
Figure 4. Case study area 1, with a colour rectified oblique
aerial photograph above, Band 11 (thermal) of the June 1992
multispectralimageinthe centreandthe fluxgate gradiometery
survey below (scale bar100 m).
formation and magnetic contrasts are very subtle.
The gradiometer was the most likely to find the
central grave pits of the barrows.
In the second study area, it is immediately
evident from Figure 6 that the returns from each
form of remote sensing vary considerably. The
crop-marks, which indicate the presence of a
ladder settlement in the north of the area, were
already well known before the 1992 multispectral
survey. The 1992 aerial-photography confirmed
the presence of a group of crop-marks to the
south of the ladder settlement interpreted as
Grubenhäuser relating to an Anglian settlement,
somewhat larger than the excavated example at
West Heslerton.
Subsequent fluxgate gradiometer survey of the
area, which was split into two sites (027 in the
east and 028 in the west) identified a total of 217
anomalies that were interpreted as Grubenhäuser.
Because this area responded particularly well to
the fluxgate gradiometer, the difference in
anomaly detection was not so marked in this
second case study area; all of the ladder
settlement in the northern part of the field that
was detected by the multispectral imagery or the
Archaeol. Prospect. 13, 291–299 (2006)
DOI: 10.1002/arp
Heslerton, the Vale of Pickering
295
Figure 5. The returns from three different forms of remote sensing in area1.
aerial photograph was also present in the
gradiometer data. By way of contrast, a total of
173 Grubenhäuser were discovered in site 27, of
which 67 were detected both by the fluxgate
gradiometer and the multispectral imagery. This
means that if only the gradiometer had been
used, then 22 Grubenhäuser would have gone
undetected, and if only the multispectral imagery
had been used, then 86 Grubenhäuser would
remain unknown (Table 1).
The buried landscape
Although remote sensing techniques provide a
detailed assessment of the surface and nearsurface archaeology, colluvial and aeolian processes have played an important role in burying
the archaeological record of the HPP area.
Understanding this subsurface dataset is instrumental to developing a more archaeologically
sustainable approach to aggregate extraction in
the region; in particular, allowing the identification of areas that are best preserved and those
that are already under serious threat or damaged
by agriculture.
Copyright # 2006 John Wiley & Sons, Ltd.
To achieve this understanding, an auger
survey was undertaken to map the thickness of
aeolian (blown) sand and plough soil, which had
the potential to bury archaeology and affect the
response of remote sensing technologies applied
to the area. Samples were taken on an arbitrary
grid at approximately 50-m intervals in each field
and located using a Leica System 500 GPS.
Deposit thickness and other attributes including,
sediment type, colour and texture were recorded
for each context in a ‘Smartlist’ relational
database operating on a Handspring Palm OS
handheld computer. In total, nearly 2500 auger
cores were drilled across the area (Figure 7).
The combined thickness of plough-soil and
aeolian sands overlying the natural and potential
archaeological deposits was used to construct an
overburden model (Figure 8). The figure illustrates those areas where potential archaeology is
threatened or being actively damaged by the
plough (red), and those areas where sands have
accumulated along field boundaries (blue).
Figure 9 illustrates the model of the thickness
of aeolian sand deposits across the project area.
Unlike the overburden model, for which the data
were always taken on a conservative basis, the
data for the aeolian sand model have been
interpreted slightly more leniently. For example,
Archaeol. Prospect. 13, 291–299 (2006)
DOI: 10.1002/arp
D. Powlesland et al.
296
logical blanks, particularly within the ladder
settlements.
Conclusions
Figure 6. A comparison of three different forms of remote sensing in study area 2, with interpretative plots of Grubenhuser
only on the right. (Photographic data courtesy of Getmapping.com.)
where deposits of aeolian sand were found
separated by a thin layer of a different deposit,
the combined depth of the aeolian sands has been
used and the lens ignored, unless it could be
related to an archaeological horizon. Both models
clearly demonstrate considerable thickness of
aeolian sand across the area and when compared
with the archaeological record, demonstrate one
potential factor that could explain archaeoTable 1. Tabulation of the numbers of Grubenhuser identified in each of the three data sources either uniquely or
shared across different data sources
Data source
Fluxgate unique
Multispectral unique
Cropmark unique
Found by all three
Found by fluxgate and multispectral
Found by fluxgate and cropmark
Actual Grubenhuser total
Site 27
Site 28
94
22
0
29
28
0
173
57
0
3
2
5
2
69
Copyright # 2006 John Wiley & Sons, Ltd.
Thirty years ago the known archaeology of the
Vale of Pickering comprised relatively few sites
and was poorly understood. Following the
discovery in 1977 of a multiperiod settlement
and cemetery complex during quarrying close to
West Heslerton, a programme of large-scale
rescue excavations and landscape survey was
begun, which formed the foundations for the
HPP and have revealed the most detailed picture
of an archaeological landscape for its scale in
Britain.
During the first few years of excavation it
became clear that much of the archaeology of the
area lay sealed beneath deposits of blown sand.
Although creating issues for visibility, burial
does offer excellent conditions for preservation.
A programme of remote sensing (aerial photography and magnetometry) initiated during
the first season of excavation on an ad-hoc basis
has subsequently been supported by research
grants, which has allowed the systematic examination of this landscape. These latter surveys
have sought to use and develop new technologies
such as multispectral imaging and developing
geophysical methods, particularly to elucidate
the nature of areas traditionally considered as
archaeological blanks. Critically, this work has
revealed flaws in our appreciation of the archaeological landscape based on isolated ‘sites’,
mostly identified through accidental discovery.
Until recently, attention has been focussed
primarily on the examination of a 1.5 km wide
strip of land on the gravels extending from
the foot of the Yorkshire Wolds to the edge of the
River Derwent floodplain, but work is now
extending into this latter area, itself another
source of aggregates and almost certainly buried
archaeology.
Today there is a focus on the management of
landscapes and securing sustainability of the
archaeological resource. However, it is not
possible to manage what is unknown, and
impossible to sustain a resource without at least
Archaeol. Prospect. 13, 291–299 (2006)
DOI: 10.1002/arp
Heslerton, the Vale of Pickering
297
Figure 7. Distribution of auger locations within the project area in relation to the geophysical plot.
Figure 8. Plot of project-wide overburden model in relation to geophysical results. Thickness of overburden is shaded from red
(0.35 m or less) through orange, yellow and green (0.36^0.99 m) to blue (1.00 m and above).
Copyright # 2006 John Wiley & Sons, Ltd.
Archaeol. Prospect. 13, 291–299 (2006)
DOI: 10.1002/arp
D. Powlesland et al.
298
Figure 9. Thickness ofaeolian sand depositsin relation to geophysicalresults.The colour ramp applied shows grey (0.01^0.20 m),
yellow to orange (0.20^1.00 m) green (1.00^1.50 m) andlight blue to dark blue (1.50^3.25 m).White areas are eitherdevoid ofaeolian
sand deposits or have not yet been surveyed.
some level of understanding of the resource, and
probably some degree of monitoring. The HPP
has demonstrated a multitechnique approach to
understanding a landscape, which has generic
applications beyond the Vale of Pickering and
North Yorkshire.
Acknowledgements
This dataset has been derived from more than
25 years of rescue and research archaeology,
mostly funded by English Heritage, latterly
through the Aggregates Levy Sustainability
Fund (ALSF). NERC funded data collection
flights during 1992 and 2005. All funding for
this work is gratefully acknowledged. Geophysical campaigns have been undertaken under
the supervision of James Lyall, with assistance
from Maria Beck, David Stott, Glenn Paterson,
Chris Fern, Will Hinchliffe, Sam Taylor, Heather
Clemence, Ben Wilson, Guy Hopkinson, Katherine Day, Peter Wilson, Kay McManus, Louise
Copyright # 2006 John Wiley & Sons, Ltd.
Cooke and Jess Tipper. Local pilots Ray Rochester and Karl Wilkinson are also thanked for
their repeated hard work. Finally, this work
could not have been undertaken without the
co-operation of the landowners of the Vale of
Pickering.
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DOI: 10.1002/arp
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