Erddig Castle, Erddig, Wrexham, North Wales (NGR SJ 327 486)
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ERDDIG CASTLE, ERDDIG, WREXHAM, NORTH WALES Archaeological Services Ltd (NGR SJ 327 486) A GEOPHYSICAL SURVEY OF THE BAILEY M. Wilson & M. Planas October 2009 Commissioned by: L - P Archaeology Erddig Castle, Erddig, Wrexham, North Wales (NGR SJ 327 486) A Geophysical Survey of the Bailey by M Wilson & M Planas Souterrain Archaeological Services Ltd October 2009 © Copyright Souterrain Archaeological Services Ltd, 2009 Registered Office: 50 Rectory Drive, Exhall, Coventry, Warwickshire CV7 9PD Registered in England and Wales No. 03394485 e-mail: [email protected] www.souterrain.biz Affiliated to the Council for British Archaeology (CBA) (cover map: reproduced from the 1879 Wales - Denbighshire: 028, Ordnance Survey 1:10,560 © Crown Copyright. All rights reserved. Licence number AL 100015565) L-P Archaeology The National Trust Chester Enterprise Trinity Square Centre, Hoole Bridge, Llandudno, Chester, CH2 3NE Wales, LL30 2DE Erddig Castle, Erddig, Wrexham, North Wales (NGR SJ 327-486), A Geophysical Survey of the Bailey LIST OF FIGURES 3 Surveyors 4 1. INTRODUCTION 5 1.1 Subject of the Survey 5 1.2 Brief Site Description Archaeological and Historical Background 5 1.3 Geology 6 1.4 Ground Conditions 6 2. SURVEY OBJECTIVES 7 2.1 Fluxgate Gradiometer survey 7 2.2 Resistance Meter survey 7 3. GRADIOMETER SURVEY 8 3.1 Methodology 8 3.1.1 Survey Grid 8 3.1.2 Instrumentation and configuration 8 3.1.3 Data processing 8 3.2 General 8 3.3 Survey Results 9 3.3.1 Presentation of the data 9 3.3.2 Types of response 9 3.3.3 Discrete Anomalies 10 3.3.4 Negative responses and low frequency ‘noise’ 10 3.3.5 Linear Anomalies 10 3.3.6 Strong Positive Anomalies 11 3.3.7 Ferrous Objects 11 4. RESISTANCE METER SURVEY 12 4.1 Methodology 12 4.1.1 Survey grid 12 4.1.2 Instrumentation and set up 12 4.1.3 Data processing 12 4.2 Constraints and considerations 12 4.3 Results 12 4.3.1 Presentation of the data 12 4.3.2 Archaeological-type responses 13 Souterrain Archaeological Services Ltd 2 October 2009 Erddig Castle, Erddig, Wrexham, North Wales (NGR SJ 327-486), A Geophysical Survey of the Bailey 5. CONCLUSIONS 15 6. GENERAL 17 6.1.1 Statement of Indemnity 17 6.1.2 Acknowledgements 17 LIST OF FIGURES Figure 1 Location of Erddig Castle Figure 2 Location of the survey grid Figure 3 Gradiometer survey: grey scale plot Figure 4 Gradiometer survey: trace plot Figure 5 Gradiometer survey (grey scale plot): interpretative diagram Figure 6 Resistance survey: Grey scale plot and Trace plot Figure 7 Resistance survey: Grey scale plots with low pass filter Figure 8 Resistance survey: interpretative diagram Souterrain Archaeological Services Ltd 3 October 2009 Erddig Castle, Erddig, Wrexham, North Wales (NGR SJ 327-486), A Geophysical Survey of the Bailey SUMMARY In September 2009 an archaeo-geophysical survey was carried out at Erddig Castle, a motte and bailey, near Wrexham in Wales (NGR SJ 327-486) by Souterrain Archaeological Services Limited. The survey was undertaken for L-P Archaeology, on behalf of The National Trust. The geophysical survey, which focussed on the bailey, combined the techniques of Fluxgate Gradiometer and Resistance Meter. The aim of the survey was to attempt to identify buried archaeological remains which would enable and informed approach to a community excavation project, and provide information that would assist in development of conservation needs at the park. A quasi-rectangular area of medium to high resistance was located in the centre of the bailey, within which appear to be the foundations of least two buildings. A narrow linear feature, quite possibly a metalled pathway is seen to lead diagonally from the south corner of this central area to the foundations of another possible structure which was sited close to the southern perimeter of the bailey. It is uncertain as to whether these buried remains relate to the Norman garrison buildings or a mid-18th century bowling green which is known to have been built somewhere on the bailey. The survey demonstrated that the use of complementary geophysical techniques, may significantly contribute to our understanding of the extent of and nature of buried archaeological remains within Erddig Park. Surveyors Martin Wilson BA (Hons) MIfA MEAGE MInstLM FSA Scot Mercedes Planas BA MSc MIfA ICES Souterrain Archaeological Services Ltd 4 October 2009 Erddig Castle, Erddig, Wrexham, North Wales (NGR SJ 327-486), A Geophysical Survey of the Bailey 1. INTRODUCTION 1.1 Subject of the Survey On the 14th and 15th of September 2009, a geophysical survey (fluxgate gradiometer and resistance meter) was conducted by Souterrain at Erddig Castle, a motte and bailey earthwork which is situated within the grounds of Erddig Park, a National Trust property, 1.6km to the southwest of Wrexham at National Grid Reference SJ 327 486. The survey was carried out for L-P Archaeology, on behalf of The National Trust. The survey forms an initial part of a programme of archaeological research initiated by L-P Archaeology1 which aims to increase our understanding of the motte and bailey earthwork. The site is a Scheduled Ancient Monument (SAM DE 17, NPRN 307144). 1.2 Brief Site Description Archaeological and Historical Background The motte and bailey is situated within woodland to the north of Erddig House, which is located in the Community Council of Marchwiel, in the historic county of Denbighshire, the preserved county of Clwyd. The site occupies a promontory south of the confluence of the River Clywedog (a tributary of the Dee) and the Black Brook. The motte is entirely wooded, the bailey largely open, with the exception of an avenue of trees which crosses the centre of the site on an east-west alignment, and the encroachment of trees and shrubs on the periphery to the south, south-east and the north. The avenue was planted to ‘landscape’ the site in the 18th century. Previous archaeological study at the site comprised a detailed contour survey undertaken by the Clwyd-Powys Archaeological Trust (CPAT) in January 1999 as an aid to management and visitor interpretation. The following description is taken from the Summary Report: “Little is known of the history of the site, although it may be 'Wrislesham', referred to in the sheriff's annual accounts of 1161. The park and grounds were landscaped for John Mellor during the 1720s and 30s by Stephen Switzer, whose plans included a castellated tower on the motte, serving as a summer house, together with a series of tree-lined walks in part following the bailey ditch. A survey by Thomas Badeslade in 1739 shows the tower as well as a formal avenue within the bailey. However, no trace survives of the tower or any other landscape features. The motte and bailey appears to have been constructed in part by utilising a natural promontory which was carved by deep ditches to form a defensive site. The motte survives to a height of c. 5.75m, with a diameter of 44m at the base and 17m at the top. This is separated from the bailey by a broad ditch 36m wide. The bailey has two entrances, one opposite the motte and the other at the south-west corner. Along the southern side the bailey is defended by a substantial ditch 34m wide and up to 8.5m deep. Rounded projections at the corners of the bailey and along the southern side may 1 B Poole & J Young ‘Outline for Archaeological Research Project at Erddig Motte and Bailey Castle, Wrexham’ for L- P: Archaeology/National Trust, B Poole & J Young, Doc Ref: LP0741C-RPD-v2.3 Date: March 09 Souterrain Archaeological Services Ltd 5 October 2009 Erddig Castle, Erddig, Wrexham, North Wales (NGR SJ 327-486), A Geophysical Survey of the Bailey have been associated with towers. Wat's Dyke, which runs along the western side of the site, has been used as an additional defence forming a ditch up to 18m wide”2. The record sheet compiles by the Royal Commission on the Ancient and Historical Monuments of Wales3 records: “Winding earthen paths lead to the motte and bailey, and the centre of the top of the bailey has been cleared as a wide grass alley flanked by rows of beeches... …in a bird's eye view drawing by Thomas Badeslade, of 1740…formal walks and steps leading to the motte, on top of which is a circular battlemented building, and to the bailey, on top of which is a bowling green with ramps at the west end”. During the present survey an additional landscape feature, a part-sunken and mounded path, was observed, leading north-east from the steps (modern) on southern edge of bailey for a distance of about 20 metres. 1.3 Geology4 The local geology consists of some alluvial silt on top of Glacial Sand and Gravel (perhaps up to 5m thick). The Bedrock is Upper Carboniferous - red brown sandstones, with some calcareous conglomerate (of Halesowen Formation, Salop Formation and Clent Formation) with some silty mudstones. There is a lot or Iron Oxide in the cement, and quite a large fault runs north-south, just to the east of site. The glacial movement was from northwest to south-east and there are moraine mounds of Glacial Till just to the north of Wrexham. There is no mineralisation to speak of in the Upper Carboniferous. There may be some glacio-tectonically disturbed soil in the area, sand wedges in clay, or inversion or small folds in softer sediments - may result in soil creep on slopes. 1.4 Ground Conditions The ground cover at the time of survey was predominantly short grass or rough tussock- like grass with occasional scrub and tree stumps.