Thrislington Quarry eastern extension, West Cornforth, County Durham
geophysical surveys
on behalf of
for
Lafarge Aggregates
Report 1373 December 2005
Archaeological Services Durham University
South Road Durham DH1 3LE
Tel: 0191 334 1121 Fax: 0191 334 1126
archaeological.services@durham.ac.uk www.durham.ac.uk/archaeologicalservices
Thrislington Quarry eastern extension, West Cornforth, County Durham
geophysical surveys
Report 1373
December 2005
Archaeological Services Durham University on behalf of
Entec UK Ltd
Canon Court North, Abbey Lawn, Abbey Foregate, Shrewsbury, Shropshire SY2 6BZ
for
Lafarge Aggregates Ltd
Northern Service Centre, PO Box 36, Retford Road, Worksop,
Nottinghamshire S81 7YU
Contents
1. Summary . . . . . 1
2. Project background . . . 2
3. Archaeological and historical background . 3
4. Landuse, topography and geology . . 3
5. Geophysical survey . . . 3
6. Conclusions . . . 7
7. Sources . . . . . 8
Appendix 1: Trace plots of geophysical data . 9
� Archaeological Services 2005
1. Summary
The project
1.1 This report presents the results of geophysical surveys (implementing a 25% sampling strategy) conducted in advance of a proposed eastern extension to Thrislington Quarry at West Cornforth, County Durham.
1.2 The works were commissioned by Entec UK Ltd, on behalf of Lafarge Aggregates, and conducted by Archaeological Services in accordance with a project design provided by Archaeological Services.
Results
1.3 Evidence of ridge and furrow cultivation, which can date from the medieval period to the late 19th century, has been detected throughout the study area. Fast Fourier Transform processing has been used in an attempt to identify features present but obscured by the recurrent magnetic anomalies characteristic of ridge and furrow.
1.4 A group of possible ring-ditches discovered in Survey Area 1, which may be indicative of round-houses or barrows, could indicate activity dating from the early prehistoric period. Other features identified in Survey Area 1 include parts of possible enclosures and an alignment of pits.
1.5 Strong anomalies likely to reflect large ditches or similar soil-filled features have been detected in Survey Area 2.
1.6 A grouping of anomalies detected in Survey Area 2 may reflect a cluster of pits, or could be indicative of geological sink-holes, which are known to occur over Magnesian Limestone.
1.7 The results of the 25% survey sample indicate that there is potential for an extensive archaeological resource in these areas.
2. Project background
Location (Figure 1)
2.1 The proposed development area is located to the east of Thrislington Quarry, West Cornforth, County Durham (NGR centre: NZ 3275 3330). The study area for this report comprised two fields measuring 27ha in total. The northwestern field, in which Survey Area 1 was located, measures 12.5ha and is bounded to the north by Stobb Cross Lane, to the west by the A1 and by fields to the south and east. Survey Area 2 was located in the southeastern field, which measures 14.5ha, and is bounded to the north by a field, to the east by a road, and to the southwest by the Highlands Quarry waste disposal site.
Development proposal
2.2 The work has been carried out in advance of a proposal to extend Thrislington Quarry eastwards from its present location.
Objective
2.3 The principal aim of the survey was to determine the nature and extent of any sub-surface features of potential archaeological significance.
Brief
2.4 The project brief required a 25% sample of the study area to be surveyed.
Dates
2.5 Fieldwork was undertaken between 23rd November and 1st December 2005. This report was prepared between 2nd and 9th December 2005.
Personnel
2.6 Fieldwork was conducted by David Graham and Sam Roberts. This report was prepared by Sam Roberts, with illustrations by Martin Railton. The Project Manager was Duncan Hale.
Archive/OASIS
2.7 The site code is TQD05, for Thrislington Quarry, County Durham 2005. The paper and data archive is currently held by Archaeological Services University of Durham. It is anticipated that the data archive will be transferred to the Bowes Museum in due course. Archaeological Services University of Durham is registered with the Online AccesS to the Index of archaeological investigationS project (OASIS). The OASIS ID number for this project is archaeol3-11847.
3. Archaeological and historical background
3.1 No archaeological features are known in the study area; however cropmarks indicating prehistoric settlement have been identified in the field which lies to the east of Survey Area 1 and north of Survey Area 2. Previous work carried out by Archaeological Services, approximately 1km to the southwest of the present study area, revealed evidence for extensive activity including a ditched enclosure of probable Iron Age date and the possibility of an earlier phase of unenclosed settlement (Archaeological Services 2003a, 2003b). To the east of the study area the A177 follows the route of a presumed Roman road.
4. Landuse, topography and geology
4.1 At the time of survey the study area comprised 2 fields of recently sown arable crop.
4.2 The survey areas were located on undulating landscapes at elevations between 120m and 130m OD.
4.3 The underlying solid geology of the area comprises Magnesian Limestone, which is overlain by glacial till, sands and gravels.
5. Geophysical survey
Standards
5.1 The surveys and reporting were conducted in accordance with English Heritage (1995) Research and Professional Services Guideline No.1, Geophysical survey in archaeological field evaluation; the Institute of Field Archaeologists Technical Paper No.6, The use of geophysical techniques in archaeological evaluations (2002); and the Archaeology Data Service (2001) Geophysical Data in Archaeology: A Guide to Good Practice.
Technique selection
5.2 Geophysical surveying enables the relatively rapid and non-invasive identification of potential archaeological features within landscapes and can involve a variety of complementary techniques such as magnetometry, electrical resistivity, ground-penetrating radar and electromagnetic survey. Some techniques are more suitable than others in particular situations, depending on a variety of site-specific factors including the nature of likely targets; depth of likely targets; ground conditions; proximity of buildings, fences or services and the local geology and drift.
5.3 In this instance, based on existing aerial photographic cropmark evidence in adjacent fields and previous work in the surrounding area (Archaeological Services 2003), it was considered likely that cut features, such as ditches and pits, may be present on the site, and that other types of feature such as trackways, wall foundations and fired structures (for example kilns and hearths) might also be present.
5.4 Given the shallowness of the targets and the non-igneous geological environment of the study area a geomagnetic technique, fluxgate gradiometry, was considered appropriate for detecting each of the types of feature mentioned above. This technique involves the use of a hand-held magnetometer to detect and record minute perturbations, or �anomalies�, in the vertical component (i.e. gradient) of the Earth�s magnetic field caused by variations in soil magnetic susceptibility or permanent magnetisation; such anomalies can reflect archaeological features.
Field methods
5.5 Survey areas designed to achieve 25% coverage were laid out in each of the two fields constituting the study area, using a Leica GS50 global positioning system (GPS). Each of the survey areas utilised a series of 20m wide transects with staggered connections (Figure 2). Survey Area 1 measured 3ha and Survey Area 2 measured 4ha, making the total sample surveyed 26%.
5.6 Measurements of vertical geomagnetic field gradient were determined using Geoscan FM36, FM256 and Bartington Grad601 fluxgate gradiometers with automatic datalogging facilities. A zig-zag traverse scheme was employed and data were logged in 20m grid units. The instrument sensitivity was set to 0.1nT, the sample interval to 0.5m and the traverse interval to 1.0m, thus providing 1800 sample measurements per 30m grid unit.
5.7 Data were downloaded on-site into laptop computers for initial processing and storage and subsequently transferred to a desktop computer for processing, interpretation and archiving.
Data processing
5.8 ArcheoSurveyor v.1.3 software was used to process the geophysical data and to produce both continuous tone greyscale images and trace plots of the raw data. The greyscale images and interpretations are presented in Figures 2-9; the trace plots are provided in Appendix I. In the greyscale images, positive magnetic anomalies are displayed as dark grey and negative magnetic anomalies as light grey. A palette bar relates the greyscale intensities to anomaly values in nanoTesla.
5.9 The following basic processing functions have been applied to each dataset:
Clip � clips, or limits data to specified maximum or minimum values; to eliminate large noise spikes; also generally makes statistical calculations more realistic.
Zero mean traverse � sets the background mean of each traverse within a grid to zero; for removing striping effects in the traverse direction and removing grid edge discontinuities.
Destagger � corrects for displacement of anomalies caused by alternate zi g- zag traverses.
Despike � locates and suppresses random iron spikes in gradiometer data.
Interpolate � increases the number of data points in a survey; to match sample and traverse intervals and so create a smoother appearance to the data. In this instance the gradiometer data have been interpolated to 0.5 x 0.5m intervals.
5.10 The dataset in Survey Area 1 has also been processed using a 2D Fast Fourier Transform; the resulting greyscale is shown in Figure 3. This process converts the survey data into a spectrum diagram that displays frequency and direction, and can be used to recognize and remove regularly repeating anomalies caused by recurrent features such as ploughmarks, or by operator/terrain-induced error. In this instance the dataset has been processed to remove the anomalies associated with ridge and furrow, in order to identify any features that had been obscured.
5.11 Due to the irregular spacing and varying alignment of the ridge and furrow in Survey Area 2 the survey data proved to be unsuitable for further processing with the Fast Fourier Transform.
Interpretation: anomaly types
5.12 Colour-coded geophysical interpretation plans are provided for each survey area. Two types of geomagnetic anomaly have been distinguished in the data:
positive magnetic regions of anomalously high or positive magnetic field gradient, which may be associated with high magnetic susceptibility soil-filled structures such as pits and ditches.
dipolar magnetic paired positive-negative magnetic anomalies, which typically reflect ferrous or fired materials (including fences and service pipes) and/or fired structures such as kilns or hearths.
Interpretation: features
5.13 Colour-coded archaeological interpretation plans are provided for each survey area. Significant anomalies detected have been assigned feature [F] numbers, some of which represent feature groups, in the following discussion.
Survey Area 1 (Figures 3-6)
5.14 Linear positive magnetic anomalies reflecting ridge and furrow cultivation are present throughout the sample area surveyed and fall into two groups. The majority of the field is covered by ridge and furrow remains on a northwest- southeast alignment [F2], evenly spaced at intervals of 7m. Ridge and furrow remains in the northern end of the field [F1] are aligned northeast-southwest and at slightly wider intervals of approximately 9m.
5.15 A row of dipolar magnetic anomalies [F3] detected across the field on an east- west alignment corresponds to a line of electricity pylons crossing the field. A dipolar magnetic anomaly [F4] on the boundary of the westernmost survey transect reflects the presence of a steel-sleeved or steel-capped borehole.
5.16 A linear positive magnetic anomaly [F7] in the north of the survey area and on a different orientation to the groupings of ridge and furrow reflects a ditch feature that may form part of an enclosure or field system.
5.17 A curvilinear positive magnetic anomaly [F5] almost certainly reflects a ring- ditch, such as is often associated with round-houses. The ditch measures approximately 15m in diameter and has a possible entrance to the east. A break in the circuit to the south is also evident, although this may be an effect of ridge and furrow cultivation impacting upon the feature. A smaller circular positive magnetic anomaly [F6] is located close to the west of [F5]. This feature has a diameter of approximately 4m and may reflect another small ring ditch.
5.18 Further positive magnetic anomalies were identified after application of a Fourier Transform processing technique. These include a group of curvilinear anomalies [F8, F9, and F30] to the southwest of [F5]. [F19] has a similar diameter to [F5] and these anomalies may reflect the remains of further ring ditches.
5.19 A line of discrete positive magnetic anomalies [F16] detected in the eastern part of the survey area may reflect a series of pits. A discrete positive magnetic anomaly [F17] may also reflect a pit-like feature.
5.20 A series of linear [F10, F13, F14 and F18], curvilinear [F11] and rectilinear [F12 and F15] positive magnetic anomalies were detected in the southern end of the survey area, and further distinguished after processing using the Fourier Transform. These are likely to reflect ditches, and may form parts of enclosures or field systems.
5.21 The only other anomalies detected here are small, discrete dipolar magnetic anomalies. These almost certainly reflect items of near-surface ferrous or fired debris, such as horseshoes and brick fragments. A larger dipolar anomaly in the southwest corner of the survey area is likely to reflect a larger piece of ferrous debris.
Survey Area 2 (Figs 7-9)
5.22 Ridge and furrow cultivation has been found across the majority of the survey area. This is identified by a series of positive linear magnetic anomalies on an approximate northeast-southwest alignment and spaced at varying intervals between 4 to 8m [F20].
5.23 A linear positive magnetic anomaly [F21] in the southeast corner of the survey area is likely to reflect a ditch which may be a former field boundary or form part of an enclosure.
5.24 An intense positive linear magnetic anomaly [F22] aligned northwest- southeast may reflect a wide ditch. Two linear positive magnetic anomalies [F23 and F24] lead off this anomaly to the southwest, on a different alignment to the ridge and furrow. A third linear positive magnetic anomaly [F30] runs parallel to [F23]. These anomalies may reflect ditches or other soil-filled features. A small linear positive magnetic anomaly [F31] which crosses [F23] at its southern end is on a similar alignment to the majority of the ridge and furrow remains detected, and is likely to reflect a furrow.
5.25 To the northwest of [F22], the survey has identified a cluster of discrete positive magnetic anomalies which may reflect a group of pits or sinkholes; geological phenomena which are a common occurrence over Magnesian Limestone. Further discrete positive magnetic anomalies [F26 and F27] have been detected west of [F25], which may also correspond to pits or sinkholes.
5.26 An intense, discontinuous, linear positive magnetic anomaly [F28] situated in the northern part of the survey area is likely to reflect a ditch feature and may form part of an enclosure or boundary ditch.
5.27 Weak, linear positive magnetic anomalies [F29] on an east-west alignment in the northeast of the survey area reflect soil-filled features, possibly land drains.
5.28 The only other anomalies detected here are small, discrete dipolar magnetic anomalies. These almost certainly reflect items of near-surface ferrous and fired debris, such as horseshoes and brick fragments.
6. Conclusions
6.1 Geophysical surveys have been carried out over land to the east of Thrislington Quarry, West Cornforth, Co. Durham, completing a 25% sample of the study area.
6.2 The features detected are likely to be only a small proportion of the potential archaeological resource present in these two fields.
6.3 Evidence of ridge and furrow cultivation, which can date from the medieval period to the late 19th century, has been detected throughout the study area. Fourier Transform processing has been utilised in an attempt to identify features present but obscured by the recurrent magnetic anomalies characteristic of ridge and furrow.
6.4 A group of possible ring-ditches discovered in Survey Area 1, which may represent round-houses or barrows, could indicate settlement activity dating
from the early prehistoric period. Other features identified in Survey Area 1 include parts of possible enclosures and an alignment of pits.
6.5 Strong anomalies likely to reflect large ditches or similar soil-filled features have been detected in Survey Area 2.
6.6 A grouping of anomalies detected in Survey Area 2 may reflect a cluster of pits, or could reflect geological sink-holes, which are known to occur over Magnesian Limestone.
7. Sources
Archaeological Services 2003a Area 4 extension, Thrislington Quarry, Co.Durham; geophysical surveys unpublished report 978 for Entec UK Ltd on behalf of Lafarge Aggregates, Archaeological Services Durham University
Archaeological Services 2003b Area 4 extension, Thrislington Quarry, Co. Durham; archaeological evaluation unpublished report 1014, for Entec UK Ltd on behalf of Lafarge Aggregates, Archaeological Services Durham University
Archaeology Data Service 2001 Geophysical Data in Archaeology: A Guide to Good Practice Arts and Humanities Data Service.
English Heritage 1995 Geophysical survey in archaeological field evaluation, Research and Professional Services Guideline 1
Institute of Field Archaeologists 2002 The use of geophysical techniques in archaeological evaluations, IFA Technical Paper 6
on behalf of
Entec UK
for
Thrislington Quarry eastern extension, West Cornforth, County Durham Lafarge Aggregates
geophysical surveys Reproduced from Explorer 305 1:25 000 by permission of Ordnance Survey on behalf of
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Figure 1 Location plan The Controller of Her Majesty's Stationery Office. c
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Appendix 1: Trace plots of geophysical data
Thrislington Quarry eastern extension: geophysical surveys; Report 1373, December 2005
Thrislington Quarry eastern extension: geophysical surveys; Report 1373, December 2005