Geophysical Survey Report

CHERHILL
New Village Hall Site
JANUARY 2016
REPORT No. 1017.205
[email protected]
01793 731974
Cherhill new village hall: geophysical survey
January 2016
Summary
TALITS Archaeological Services was commissioned by Project Manager, Simon Tomlinson, to
undertake an earth resistance and a magnetometer survey of an area of land within the
field to the north of the A4 at the eastern end of Cherhill village. The land was earmarked
for a new village hall.
Maps from 1780 and 1843 indicated that two buildings had existed within the bounds of the
survey area. The location of these was not confirmd by these surveys. The line of a known
pipeline was detected as was the area where visible damage by rabbits had occured. Other
features, interpreted as possibly structural in origin but without any clear pattern nor
relationship to known structures were also to be seen in the processed results. There may
be geological or other explanations for other features which geophysical survey alone
cannot deduce.
Date of Surveys
8TH January 2016. The surveys were carried out by Jim Gunter (BA, MA, CPIfA) and Mike
McQueen (BA, MA) on behalf of TALITS.
Authorship
This report was compiled and written by Jim Gunter on behalf of TALITS with contributions
from Mike McQueen.
Disclaimer
This report has been compiled with all reasonable skill, care and attention to detail within
the terms of the project as specified and within the general terms and conditions of Talits.
No responsibility is accepted whatsoever to third parties to whom this report or any part
thereof is made known. Any such party relies upon this report at their own risk.
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January 2016
Location
The site for the proposed new village hall is situated at the eastern end of Cherhill, centring
on Grid Reference SU041701 and borders the main A4 road along Labour In Vain Hill at the
junction with Park Lane (Figure 1). It is less than 1km north of the Lansdowne Monument,
White Horse and Oldbury Camp and is 4kms east of the centre of Calne. The survey area
covered an area of 4,400m² (0.44 hectares/1.1 acres).
Figure 1: location map of Village Hall site – boxed in red
Geology and Topography
Most of the settlement area of Cherhill lies over a narrow belt of Upper Greensand
Formation with Gualt to the western side. However, the field to be surveyed was at the
start of the Chalk that stretches to Cherhill Down to the south and the Marlborough Downs
to the east. The field sloped from 130m AOD at its northern edge to 125m AOD to the
south. The village continues to slope to the west where it is 110m AOD. Cherhill Down to
the south rises steeply to 207m AOD. The study area is part of a field set to pasture.
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Archaeology
The County’s Sites and Monuments Record lists 40 archaeologically sites/findspots within a
1km radius of the proposed development. These include records from all archaeological
periods from Mesolithic to Late Medieval and serve to indicate that the village area is itself
of some considerable archaeological significance. The village is dominated to the south by
Cherhill Hill, with its imposing hillfort, white horse and obelisk. The hillfort, Oldbury Castle,
is a 10ha, irregularly shaped, bivallate Iron Age enclosure; the 40m long White Horse was
carved in 1780; the obelisk, known as the Lansdowne Monument, was erected c.1845 to
commemorate the 17th century economist, Sir William Petty (Child 1984, 12).
Although no archaeological features have been recorded in the field, within the immediate
area, a complete Bronze Age Beaker was found in 1927 while laying a water pipe at Upper
Farm to the north and five ditches excavated at Bell Farm to the west in 1996 are thought to
be Romano-British boundaries and features of a field system associated with the villa 0.2km
to the north. A small assemblage of pottery sherds and a coin were also found on Bell Farm
in 1940. A book on the history of the village (Blackford 1941) contains a map reconstructing
the known buildings in 1780. This shows two buildings within the site area. Park Lane,
which borders it site to its west, was previously called Rubble Lane and the target field was
called The Park (Figure 2).
Figure 2: the site in 1780
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The Tithe Map for Cherhill map also shows that the field was once covered by small
holdings with two potential buildings, a house and a barn/stables. The field was crossed by
fence lines (Figure 3).
Figure 3: site (edged red) superimposed on Enclosure Map
Figure 4: Cherhill in 1843
Although still shown in 1843 (Figure 4), by the time the Blackford book was written in 1941
these buildings had been removed.. The 1894 Ordnance Survey map also showed that
there were once some trees in the field.
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Methodology
Given the background record of the site the objective of the surveys was to use non-intrusive
geophysical techniques to attempt to answer questions about the the location of the
buildings shown on the earlier maps. It was also intended to attempt to identify any
features that might relate to other historical or archaeolical events, particularly with regard
to the continuation of the Romano-British field system identified in the neighbouring Bell
Farm area.
A total of 11 survey grids were laid out as shown in the Figure 5 map using tapes. The
position of the girds was recorded using a MobileMapper 300 and Pocket GIS Software. The
MobileMapper 300 was fully RTK corrected to give accuracy to 0.002m – 0.0.
Figure 5: location of survey grids
Two surveys were undertaken. First was a magnetometer survey using a Bartington
Grad601-2 Dual Sensor Gradiometer and covered all 11 grids. The second was an resistivity
survey with a Geoscan Research Resistance Meter RM15-D with a mobile Twin Probe array
set at 0.5m electrode separation and covered 8 of the grids.
The survey and report follow the recommendations set out by English Heritage (2008)
Geophysical survey in archaeological field evaluation; and the Institute for Archaeologists
(2010) Standard and Guidance for Archaeological Geophysical Survey.
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Conditions
The survey was carried out in a field used for pasture with a short grass covering. The
weather was cool and damp and there had been a sustained period of wet weather in the
weeks preceding the survey.
The northern edge of the field slopes quite steeply. The grids were laid out along the
bottom part of this slope. Further across the field there was also a small ridge running from
east to west.
There were several areas which had been disturbed by rabbits.
It is possible that the land surface close to road was substantially modified when the main
road was built up to to level the surface to deal with what had been an accident black spot.
Magnetometry Survey Results.
The survey was carried out using a Bartington Grad601-2 Dual Sensor Gradiometer set up as
follows:
Units:
Surveyed:
Collection Method:
Sensors:
Readings / Sampling
Dummy Value
Direction of 1st traverse
Palette
nT
8th January 2016
Zigzag
2 @ 1.0m spacing.
1 per 1m / 4 per 1m
32000
South East
Greyscale
The data collected in the survey was processed using TerraSurveyor 3.0.25 software.
This depends on a contrast between absolute magnetic susceptibility of the topsoil to the
underlying subsoil. Iron minerals within the soil may become altered by human activity
such as burning in hearths or kilns, the presence of material that has been heat treated (e.g.
brick and iron) and the break down of organic material increasing the magnetic
susceptibility of the soil. Accumulations of magnetically enhanced soils within features, such
as pits and ditches, may produce magnetic anomalies that can be mapped.
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Magnetometer Survey Result – Unmodified:
Minimum / Maximum
Standard Deviation
Mean / Median
1100.00 nT / 100.00 nT
18.12 nT
-2.60 nT / -1.35 nT
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Magnetometer Survey Result – Modified:
The following modifications were then carried out to the raw data:
1
2
3
4
Destripe
Clip
De-stagger
Clip
Minimum / Maximum
Standard Deviation
Mean / Median
Median Traverse All Grids
1.00 standard deviation
In-Bound and Out-Bound – -1 intervals
2.00 standard deviation
-13.01 nT / 11.71 nT
4.82 nT
-0.54 nT / 0.00 nT
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Resistivity Survey Result – Raw:
The electrical resistance or resistivity of the soil depends upon the moisture content and
distribution within the soil. Buried solid features such as walls are usually more moisture
resistant than other features such as the infill of a ditch. A large stone will generally give a
high resistance response and the moisture retentive content of a ditch will give a low
resistance response. Localised variations in resistance are measured in ohms which is the SI
unit for electrical impedance or resistance. Measurements were recorded at 1.0m intervals
along 1m traverses across the site. Data logged was downloaded and processed within
Geoplot 3.0 software.
Units:
Collection Method:
Sensors:
X Interval:
Y Interval:
Background resistance:
Gain:
Current:
Palette
Ohm
ZigZag
@ 0.50 m spacing.
1.0 m
1.0 m
8.10 Ohms
x100
1mA
greyscale 55
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January 2016
Resistivity Survey Result – Processed:
Processes: 4
1. Base Layer
2.85 Ohms to 16.13 Ohms
2. Despike:
Threshold: 3.0
3. Low Pass Gaussian filter X radius = 1; Y radius = 1; block off
4. Interpolation:
4.1. Direction X
4.2. Direction Y
4.3. Direction X
4.4. Direction Y
5. Final
1.38 Ohms to 17.28 Ohms
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Interpretation of Processed results
Both sets of results were problematical.
The magnetomoeter results clearly detected the line of a pipeline, crossing the field from
northwest to southeast, and which dominated other features. A further pipeline joined this
at a 90-degree angle at it’s southeast end. Even when the magnetometer results were reprocessed to exclude grids influenced by the course of the pipeline, the only other main
recognisable feature was an area of ground that has been disturbed by rabbits (Figure 6 ).
Figure 6: interpretation of magnetometer survey results
The resistivity results were influenced by the level of moisture in the soil which significantly
reduced any resistance. Indeed, it was not possible to get a background reading higher than
10 Ohms which is normnally considered to be the minimum required. The results were reprocessed deleting the grids at the southern end of the site where water retention was
greatest. This allowed some features to be detected (Figure 7 ).
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Cherhill new village hall: geophysical survey
January 2016
Figure 7: revised resistivity survey result and interpretation.
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The features highlighted included the pipeline and rabbit (confirmed by ecology report: The
Bat Consultancy 2015) damage area seen on the magnetometer results. Areas of
disturbance which might be structural were also visible but do not seem to relate to the
position on known buildings. Figure 8 shows these features. Three additional lines are
apparent running east-west but it is not possible to explain these, other than they might
mark where tracks or paths crossed the site or fence lines.
Figure 8: interpretation of main features detected
Conclusion
No firm evidence was detected by this survey for any archaeological feature. Only known
features such as the pipeline were highlighted. There were no clear traces of the buildings
known to have existed on the site. It is possible that the ground conditions, being very wet,
hampered the detection of these and other features of archaeological significance.
Recommendation
Although no finds or features were identified by this intervention the area remains one of
considerable archaeological interest.
Archive location
All archive material (this report, plus digital photographs, plans, copy of this report on CD) is
deposited with the Wiltshire and Swindon History Centre.
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Bibliography
Blackford, J H, 1941. Manor and Village of Cherhill, p82 Illus
English Heritage (2008) Geophysical survey in archaeological field evaluation;
Institute for Archaeologists (2010) Standard and Guidance for Archaeological Geophysical
Survey.
The BAT Consultancy 2015 Phase 1 Survey, Land at Cherhill, Langport, Somerset.
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