GEOPHYSICAL SURVEY REPORT. Meliden Access Road, Prestatyn, Denbighshire, North Wales. Client Penrhyn Homes Ltd. Survey Report

Transcription

1 GEOPHYSICAL SURVEY REPORT Meliden Access Road, Prestatyn, Denbighshire, North Wales Client Penrhyn Homes Ltd Survey Report Date May 2018 SUMO Geophysics Ltd SUMO Geophysics Ltd Cowburn Farm Vineyard House Market Street Upper Hook Road Thornton Upton upon Severn Bradford Worcestershire BD13 3HW WR8 0SA T: T:

2 Project Name: Meliden Access Road, Prestatyn, Denbighshire, North Wales Job ref: Client: Penrhyn Homes Ltd Date: May 2018 GEOPHYSICAL SURVEY REPORT Project name: Meliden Access Road, Prestatyn, Denbighshire, North Wales SUMO Job reference: Client: Penrhyn Homes Ltd Survey date: 8 th May 2018 Field coordinator: Tom Cockcroft MSc Report written by: Tom Cockcroft MSc Project Manager: Jon Tanner BSc MSc PCIfA Report date: May 2018 Field Team: Andrew Edwards BSc MSc Cassie Hall BA MSc CAD illustrations by: Tom Cockcroft MSc Report approved by: Dr John Gater BSc DSc(Hon) MCIfA FSA SUMO Geophysics: for Archaeology and Engineering

3 Project Name: Meliden Access Road, Prestatyn, Denbighshire, North Wales Job ref: Client: Penrhyn Homes Ltd Date: May 2018 TABLE OF CONTENTS 1 SUMMARY OF RESULTS 1 2 INTRODUCTION 1 3 METHODS, PROCESSING & PRESENTATION 2 4 RESULTS 3 5 DATA APPRAISAL & CONFIDENCE ASSESSMENT 3 6 CONCLUSION 3 7 REFERENCES 4 Appendix A Appendix B Appendix C Technical Information: Magnetometer Survey Method Technical Information: Magnetic Theory Previous Geophysical Survey Report LIST OF FIGURES Figure 01 1: Site Location Diagram Figure 02 1:2000 Location of Survey Areas Figure 03 1:2000 Magnetometer Survey Greyscale Plots Figure 04 1:2000 Magnetometer Survey Interpretation Figure 05 1:2000 Minimally Processed Data Greyscale Plots SUMO Geophysics: for Archaeology and Engineering

4 Project Name: Meliden Access Road, Prestatyn, Denbighshire, North Wales Job ref: Client: Penrhyn Homes Ltd Date: May SUMMARY OF RESULTS 1.1 Most of the responses detected at the site relate to postmedieval mining and associated buildings. Several anomalies of uncertain origin were recorded in the data; a pipe was also located in the south east of the southern survey area. 2 INTRODUCTION 2.1 Background synopsis SUMO Geophysics Ltd were commissioned to undertake a geophysical survey of an area outlined for residential development. This survey forms part of an archaeological investigation being undertaken on behalf of Penrhyn Homes Ltd. 2.2 Site details NGR / Postcode Location HER/SMR District Parish Topography SJ / LL19 8PX The site is located 6.4km east of Rhyl and 2km south of the centre of Prestatyn. The survey area is bounded by the A547 to the southeast, Ffordd Ty Newydd to the northeast and the B5119 to the south. Denbighshire County Council Denbighshire Current Land Use Pasture Geology Soils Archaeology Survey Methods Study Area Dyserth, Sir Ddinbych The western area sloped down from the southeast to northwest. The eastern area sloped from both the north and east, down to the southwest. Solid: Pennine Coal Measures Group Sedimentary Rock Cycles, Coal Measure Type. Superficial: Till, Devensian Diamicton (BGS 2018). Salop Association (711m) slowly permeable seasonally waterlogged reddish fine loamy over clayey soils with slowly permeable subsoils and slight seasonal waterlogging (SSEW 1983). There are a number of archaeological remains present within the survey area which are recorded in the HER, which mostly date to the postmedieval period. The remains include a reservoir earthwork, engine house, shaft chimney and a number of mine shafts. The potential for preserved buried remains of the postmedieval period is expected to be high. The eastern boundary butts against the A547, which was once the location of a Roman road. As such the potential for buried remains of the Roman period associated with either the road, or ancillary features such as cemeteries or Roman road stations, is considered to be medium within this part of the site (AA 2018). Magnetometer survey (fluxgate gradiometer) 4.5 ha 2.3 Aims and Objectives To locate and characterise any anomalies of possible archaeological interest within the study area. SUMO Geophysics: for Archaeology and Engineering 1

5 Project Name: Meliden Access Road, Prestatyn, Denbighshire, North Wales Job ref: Client: Penrhyn Homes Ltd Date: May METHODS, PROCESSING & PRESENTATION 3.1 Standards & Guidance This report and all fieldwork have been conducted in accordance with the latest guidance documents issued by Historic England (EH 2008) (then English Heritage), the Chartered Institute for Archaeologists (CIfA 2014) and the European Archaeological Council (EAC 2016). 3.2 Survey methods Detailed magnetic survey was chosen as an efficient and effective method of locating archaeological anomalies. Technique Instrument Traverse Interval Sample Interval Magnetometer Bartington Grad m 0.25m More information regarding this technique is included in Appendix A and B. 3.3 Data Processing The following basic processing steps have been carried out on the data used in this report: Destripe; destagger; interpolate 3.4 Presentation of results and interpretation The presentation of the results includes a minimally processed data and a processed data greyscale plot. Magnetic anomalies are identified, interpreted and plotted onto the Interpretation drawings. When interpreting the results, several factors are taken into consideration, including the nature of archaeological features being investigated and the local conditions at the site (geology, pedology, topography etc.). Anomalies are categorised by their potential origin. Where responses can be related to other existing evidence, the anomalies will be given specific categories, such as: Abbey Wall or Roman Road. Where the interpretation is based largely on the geophysical data, levels of confidence are implied, for example: Probable, or Possible Archaeology. The former is used for a confident interpretation, based on anomaly definition and/or other corroborative data such as cropmarks. Poor anomaly definition, a lack of clear patterns to the responses and an absence of other supporting data reduces confidence, hence the classification Possible. SUMO Geophysics: for Archaeology and Engineering 2

6 Project Name: Meliden Access Road, Prestatyn, Denbighshire, North Wales Job ref: Client: Penrhyn Homes Ltd Date: May RESULTS The survey has been divided into two survey areas (Areas 12) and specific anomalies have been given numerical labels [1][6] which appear in the text below, as well as on the Interpretation Figure. 4.1 Probable Archaeology No magnetic responses have been recorded that could be interpreted as being of archaeological interest, apart from industrial responces noted below. 4.2 Uncertain Several trends traverse the site, on two or three alignments. These are uncertain in origin but are likely to be relatively modern, possibly drains or agricultural features. 4.3 Former Mining Responses A number of responses in the data from Area 1 correspond to HER records from postmedieval mining activity and they are also shown on historic mapping. A large ferrous response [1] is due to mining buildings, a mine shaft and shaft chimney. Curvilinear feature [2] corresponds to the remains of a reservoir bank. Response [3], characterised by magnetic disturbance marks the location of a former engine house while ferrous response [4] is from the remains of the magazine building. Anomalies [5] and [6] correspond to the locations of old mine shafts. 4.4 Pipe In the south east of Area 1 a pipe was located within the data set. This has been confirmed to be a water main (Andy Faragher pers.comm.). 4.5 Ferrous / Magnetic Disturbance Ferrous responses close to boundaries are due to adjacent fences and gates. Smaller scale ferrous anomalies ("iron spikes") are present throughout the data and are characteristic of small pieces of ferrous debris (or brick / tile) in the topsoil; they are commonly assigned a modern origin. Only the most prominent of these are highlighted on the interpretation diagram. 5 DATA APPRAISAL & CONFIDENCE ASSESSMENT 5.1 Historic England guidelines (EH 2008) Table 4 states that the average magnetic response on sedimentary geology is good. The results from this survey indicate the presence of uncertain trends and associated mining responses; as a consequence the technique is likely to have detected any archaeological features. 6 CONCLUSION 6.1 Although no anomalies of archaeological origin were detected in the survey, postmedieval mine shafts and buildings have resulted in prominent magnetic responses. A water mains pipe was located, several uncertain linear trends traversing both survey areas were also detected. SUMO Geophysics: for Archaeology and Engineering 3

7 Project Name: Meliden Access Road, Prestatyn, Denbighshire, North Wales Job ref: Client: Penrhyn Homes Ltd Date: May REFERENCES AA 2018 BGS 2018 Land at Ffordd Ty Newydd, Meliden, Prestatyn, Denbighshire V2. Archaeological Assessment. Aeon Archaeology, Chester. British Geological Survey, Geology of Britain viewer [accessed 10/05/2018] website: ( CIfA 2014 Standard and Guidance for Archaeological Geophysical Survey. Amended CIfA Guidance note. Chartered Institute for Archaeologists, Reading. EAC 2016 EH 2008 SSEW 1983 EAC Guidelines for the Use of Geophysics in Archaeology, European Archaeological Council, Guidelines 2. Geophysical Survey in Archaeological Field Evaluation. English Heritage, Swindon Soils of England and Wales. Sheet 2, Wales. Soil Survey of England and Wales, Harpenden. SUMO Geophysics: for Archaeology and Engineering 4

8 N Site Location Reproduced from Ordnance Survey's 1: map of 1998 with the permission of the controller of Her Majesty's Stationery Office. Crown Copyright reserved. Licence No: Title: Client: Site Location Diagram Penrhyn Homes Ltd. Project: Meliden Access Road, Prestatyn Scale: 0 metres 1250 A3 Fig No: 01

9 El Sub Sta Play Area N Ysgol Melyd Mindale Football Ground Area 2 Posts Garage Area 1 High House 46.1m Magnetometer Survey Area showing 30m grids Title: Client: Project: Location of Survey Areas Penrhyn Homes Ltd Meliden Access Road, Prestatyn Scale: 0 metres 100 A3 Fig No: 02

10 El Sub Sta Play Area N Ysgol Melyd Mindale Football Ground Area 2 Posts Garage 2 nt 1 Area 1 High House 46.1m Title: Client: Project: Magnetometer Survey Greyscale Plots Penrhyn Homes Ltd Meliden Access Road, Prestatyn Scale: 0 metres 100 A3 Fig No: 03

11 El Sub Sta Play Area N Ysgol Melyd Mindale Football Ground 6 5 Area 2 Posts KEY Garage Uncertain Trend 4 3 Pipe Reservoir Earthwork Area 1 2 High House 46.1m Ferrous / Magnetic Disturbance 1 Title: Client: Project: Magnetometer Survey Interpretation Penrhyn Homes Ltd Meliden Access Road, Prestatyn Scale: 0 metres 100 A3 Fig No: 04

12 El Sub Sta Play Area N Ysgol Melyd Mindale Football Ground Area 2 Posts Garage 2 nt 1 Area 1 High House 46.1m Title: Magnetometer Survey Greyscale Plots Minimally Processed Data Client: Project: Penrhyn Homes Ltd Meliden Access Road, Prestatyn Scale: 0 metres 100 A3 Fig No: 05

13 Appendix A Technical Information: Magnetometer Survey Method Grid Positioning For hand held gradiometers the location of the survey grids has been plotted together with the referencing information. Grids were set out using a Trimble R8 Real Time Kinematic (RTK) VRS Now GNSS GPS system. An RTK GPS (Realtime Kinematic Global Positioning System) can locate a point on the ground to a far greater accuracy than a standard GPS unit. A standard GPS suffers from errors created by satellite orbit errors, clock errors and atmospheric interference, resulting in an accuracy of 5m10m. An RTK system uses a single base station receiver and a number of mobile units. The base station rebroadcasts the phase of the carrier it measured, and the mobile units compare their own phase measurements with those they received from the base station. This results in an accuracy of around 0.01m. Technique Instrument Traverse Interval Sample Interval Magnetometer Bartington Grad m 0.25m Instrumentation: Bartington Grad 6012 Bartington instruments operate in a gradiometer configuration which comprises fluxgate sensors mounted vertically, set 1.0m apart. The fluxgate gradiometer suppresses any diurnal or regional effects. The instruments are carried, or cart mounted, with the bottom sensor approximately m from the ground surface. At each survey station, the difference in the magnetic field between the two fluxgates is measured in nanotesla (nt). The sensitivity of the instrument can be adjusted; for most archaeological surveys the most sensitive range (0.1nT) is used. Generally, features up to 1m deep may be detected by this method, though strongly magnetic objects may be visible at greater depths. The Bartington instrument can collect two lines of data per traverse with gradiometer units mounted laterally with a separation of 1.0m. The readings are logged consecutively into the data logger which in turn is daily downloaded into a portable computer whilst on site. At the end of each site survey, data is transferred to the office for processing and presentation. Data Processing Zero Mean Traverse Step Correction (Destagger) This process sets the background mean of each traverse within each grid to zero. The operation removes striping effects and edge discontinuities over the whole of the data set. When gradiometer data are collected in 'zigzag' fashion, stepping errors can sometimes arise. These occur because of a slight difference in the speed of walking on the forward and reverse traverses. The result is a staggered effect in the data, which is particularly noticeable on linear anomalies. This process corrects these errors. Display Greyscale/ Colourscale Plot This format divides a given range of readings into a set number of classes. Each class is represented by a specific shade of grey, the intensity increasing with value. All values above the given range are allocated the same shade (maximum intensity); similarly, all values below the given range are represented by the minimum intensity shade. Similar plots can be produced in colour, either using a wide range of colours or by selecting two or three colours to represent positive and negative values. The assigned range (plotting levels) can be adjusted to emphasise different anomalies in the dataset. SUMO Survey: Geophysics for Archaeology and Engineering

14 Interpretation Categories In certain circumstances (usually when there is corroborative evidence from deskbased or excavation data) very specific interpretations can be assigned to magnetic anomalies (for example, Roman Road, Wall, etc.) and where appropriate, such interpretations will be applied. The list below outlines the generic categories commonly used in the interpretation of the results. Archaeology / Probable Archaeology Possible Archaeology Industrial / BurntFired Former Field Boundary (probable & possible) Ridge & Furrow Agriculture (ploughing) Land Drain Natural Magnetic Disturbance Service Ferrous Uncertain Origin This term is used when the form, nature and pattern of the responses are clearly or very probably archaeological and /or if corroborative evidence is available. These anomalies, whilst considered anthropogenic, could be of any age. These anomalies exhibit either weak signal strength and / or poor definition, or form incomplete archaeological patterns, thereby reducing the level of confidence in the interpretation. Although the archaeological interpretation is favoured, they may be the result of variable soil depth, plough damage or even aliasing as a result of data collection orientation. Strong magnetic anomalies that, due to their shape and form or the context in which they are found, suggest the presence of kilns, ovens, corn dryers, metalworking areas or hearths. It should be noted that in many instances modern ferrous material can produce similar magnetic anomalies. Anomalies that correspond to former boundaries indicated on historic mapping, or which are clearly a continuation of existing land divisions. Possible denotes less confidence where the anomaly may not be shown on historic mapping but nevertheless the anomaly displays all the characteristics of a field boundary. Parallel linear anomalies whose broad spacing suggests ridge and furrow cultivation. In some cases, the response may be the result of more recent agricultural activity. Parallel linear anomalies or trends with a narrower spacing, sometimes aligned with existing boundaries, indicating more recent cultivation regimes. Weakly magnetic linear anomalies, quite often appearing in series forming parallel and herringbone patterns. Smaller drains may lead and empty into larger diameter pipes, which in turn usually lead to local streams and ponds. These are indicative of clay fired land drains. These responses form clear patterns in geographical zones where natural variations are known to produce significant magnetic distortions. Broad zones of strong dipolar anomalies, commonly found in places where modern ferrous or fired materials (e.g. brick rubble) are present. Magnetically strong anomalies, usually forming linear features are indicative of ferrous pipes/cables. Sometimes other materials (e.g. pvc) or the fill of the trench can cause weaker magnetic responses which can be identified from their uniform linearity. This type of response is associated with ferrous material and may result from small items in the topsoil, larger buried objects such as pipes, or above ground features such as fence lines or pylons. Ferrous responses are usually regarded as modern. Individual burnt stones, fired bricks or igneous rocks can produce responses similar to ferrous material. Anomalies which stand out from the background magnetic variation, yet whose form and lack of patterning gives little clue as to their origin. Often the characteristics and distribution of the responses straddle the categories of Possible Archaeology / Natural or (in the case of linear responses) Possible Archaeology / Agriculture; occasionally they are simply of an unusual form. Where appropriate some anomalies will be further classified according to their form (positive or negative) and relative strength and coherence (trend: weak and poorly defined). SUMO Survey: Geophysics for Archaeology and Engineering

15 Appendix B Technical Information: Magnetic Theory Detailed magnetic survey can be used to effectively define areas of past human activity by mapping spatial variation and contrast in the magnetic properties of soil, subsoil and bedrock. Although the changes in the magnetic field resulting from differing features in the soil are usually weak, changes as small as 0.1 nanoteslas (nt) in an overall field strength of 48,000 (nt), can be accurately detected. Weakly magnetic iron minerals are always present within the soil and areas of enhancement relate to increases in magnetic susceptibility and permanently magnetised thermoremanent material. Magnetic susceptibility relates to the induced magnetism of a material when in the presence of a magnetic field. This magnetism can be considered as effectively permanent as it exists within the Earth s magnetic field. Magnetic susceptibility can become enhanced due to burning and complex biological or fermentation processes. Thermoremanence is a permanent magnetism acquired by iron minerals that, after heating to a specific temperature known as the Curie Point, are effectively demagnetised followed by remagnetisation by the Earth s magnetic field on cooling. Thermoremanent archaeological features can include hearths and kilns; material such as brick and tile may be magnetised through the same process. Silting and deliberate infilling of ditches and pits with magnetically enhanced soil creates a relative contrast against the much lower levels of magnetism within the subsoil into which the feature is cut. Systematic mapping of magnetic anomalies will produce linear and discrete areas of enhancement allowing assessment and characterisation of subsurface features. Material such as subsoil and nonmagnetic bedrock used to create former earthworks and walls may be mapped as areas of lower enhancement compared to surrounding soils. Magnetic survey is carried out using a fluxgate gradiometer which is a passive instrument consisting of two sensors mounted vertically 1m apart. The instrument is carried about 30cm above the ground surface and the top sensor measures the Earth s magnetic field whilst the lower sensor measures the same field but is also more affected by any localised buried feature. The difference between the two sensors will relate to the strength of a magnetic field created by this feature, if no field is present the difference will be close to zero as the magnetic field measured by both sensors will be the same. Factors affecting the magnetic survey may include soil type, local geology, previous human activity and disturbance from modern services. SUMO Survey: Geophysics for Archaeology and Engineering

16 Project name: Land at Mindale Farm, Fford Ty Newydd, Meliden, Prestatyn Client: Penrhyn Homes Limited December 2015 Job ref: J9324 Report author: Rosie Everett BA (hons)

17 GEOPHYSICAL SURVEY REPORT Project name: Land at Mindale Farm, Fford Ty Newydd, Meliden, Prestatyn Client: Penrhyn Homes Limited Job ref: J9324 Techniques: Detailed magnetic survey Gradiometry Survey date: 3rd4th December 2015 Site centred at: SJ Post code: LL19 8PG Field team: Jay Griffiths Tom Hynd Project manager: Simon Haddrell BEng(Hons) AMBCS PCIfA Report written By: Rosie Everett BA (Hons) CAD illustrations by: Rebecca Davies BSc (Hons) Checked by: David Elks MSc ACIfA

18 Geophysical Survey Report Project Name: Land at Mindale Farm, Fford Ty Newydd, Meliden, Prestatyn Job ref: J9324 Client: Penrhyn Homes Limited Date: December 2015 TABLE OF CONTENTS LIST OF FIGURES SUMMARY OF RESULTS INTRODUCTION Background synopsis Site location Description of site Geology and soils Site history and archaeological potential Survey objectives Survey methods Processing, presentation and interpretation of results Processing Presentation of results and interpretation RESULTS Probable Archaeology Possible Archaeology Medieval/PostMedieval Agriculture Other Anomalies DATA APPRAISAL & CONFIDENCE ASSESSMENT CONCLUSION... 7 APPENDIX A METHODOLOGY & SURVEY EQUIPMENT... 9 APPENDIX B BASIC PRINCIPLES OF MAGNETIC SURVEY APPENDIX C GLOSSARY OF MAGNETIC ANOMALIES Page 1

19 Geophysical Survey Report Project Name: Land at Mindale Farm, Fford Ty Newydd, Meliden, Prestatyn Job ref: J9324 Client: Penrhyn Homes Limited Date: December 2015 LIST OF FIGURES Figure 01 Figure 02 Figure 03 Figure 04 Figure 05 1:25000 Location plan of survey area 1:1500 Location of survey grids and referencing 1:1250 Colour plot of gradiometer data showing extreme values 1:1250 Plot of minimally processed gradiometer data 1:1250 Abstraction and interpretation of gradiometer anomalies Page 2

20 Geophysical Survey Report Project Name: Land at Mindale Farm, Fford Ty Newydd, Meliden, Prestatyn Job ref: J9324 Client: Penrhyn Homes Limited Date: December SUMMARY OF RESULTS A detailed gradiometry survey was conducted over approximately 4.4 hectares of grassland. The survey has identified a small number of possible archaeological features, however it is not possible to determine their origin with a degree of confidence. Alternatively they may equally relate to agricultural or natural features. Further anomalies can be associated with postmedieval agricultural activity, including the identification of 19 th century former field boundaries and ploughing. The remaining anomalies are of modern or natural in origin, relating to ferrous objects and fencing. 2 INTRODUCTION 2.1 Background synopsis Stratascan were commissioned to undertake a geophysical survey of an area outlined for residential development. This survey forms part of an archaeological investigation being undertaken by Penrhyn Homes Limited. 2.2 Site location The site is located within Mindale Farm, Prestatyn, North Wales at OS ref. SJ Description of site The survey area is approximately 4.4 hectares of grassland. The site is divided across two areas, situated northeast and west of a fenced outdoor riding arena. The site lies on a slight north facing slope. 2.4 Geology and soils The underlying geology is Pennine Coal Measures Group Sedimentary Rock (British Geological Survey website). The drift geology is Till, Devensian Diamicton (British Geological Survey website). The overlying soils are known as Sandwich which are typical sandpararendzinas. These consist of mainly deep well drained calcareous and noncalcareous sandy soils (Soil Survey of England and Wales, Sheet 2 Wales). 2.5 Site history and archaeological potential Local historic records (Archwilio online) suggests that within the immediate survey area, there is very little or no archaeological potential. However, within a mile radius local records show occupational activity dating from the Neolithic period onwards (Archwilio online). Page 3

21 Geophysical Survey Report Project Name: Land at Mindale Farm, Fford Ty Newydd, Meliden, Prestatyn Job ref: J9324 Client: Penrhyn Homes Limited Date: December 2015 Recent history of the survey area includes number of horse burials, and the burial of hardcore and other solid waste by previous owners (before 1986; pers. comm: Warren Ward, 3/12/2015). 2.6 Survey objectives The objective of the survey was to locate any features of possible archaeological origin in order that they may be assessed prior to development. 2.7 Survey methods This report and all fieldwork have been conducted in accordance with both the English Heritage guidelines outlined in the document: Geophysical Survey in Archaeological Field Evaluation, 2008 and with the Chartered Institute for Archaeologists document Standard and Guidance for Archaeological Geophysical Survey. Detailed magnetic survey (gradiometry) was used as an efficient and effective method of locating archaeological anomalies. More information regarding this technique is included in Appendix A. 2.8 Processing, presentation and interpretation of results Processing Processing is performed using specialist software. This can emphasise various aspects contained within the data but which are often not easily seen in the raw data. Basic processing of the magnetic data involves 'flattening' the background levels with respect to adjacent traverses and adjacent grids. Once the basic processing has flattened the background it is then possible to carry out further processing which may include low pass filtering to reduce 'noise' in the data and hence emphasise the archaeological or manmade anomalies. The following schedule shows the basic processing carried out on all minimally processed gradiometer data used in this report: 1. Destripe (Removes striping effects caused by zeropoint discrepancies between different sensors and walking directions) 2. Destagger (Removes zigzag effects caused by inconsistent walking speeds on sloping, uneven or overgrown terrain) Presentation of results and interpretation The presentation of the data for each site involves a printout of the minimally processed data both as a greyscale plot and a colour plot showing extreme magnetic values. Magnetic anomalies have been identified and plotted onto the 'Abstraction and Interpretation of Anomalies' drawing for the site. Page 4

22 Geophysical Survey Report Project Name: Land at Mindale Farm, Fford Ty Newydd, Meliden, Prestatyn Job ref: J9324 Client: Penrhyn Homes Limited Date: December RESULTS The detailed magnetic gradiometer survey conducted at Mindale farn has identified a number of anomalies that have been characterised as being of a possible archaeological origin. Features of possible archaeological origin differ from those of probable archaeological origin, based on a confidence rating. Those of possible origin tend to be more amorphous anomalies, although may have similar magnetic attributes in terms of strength or polarity to probable archaeology, but are difficult to classify as being archaeological or potentially natural or agricultural. The following list of numbered anomalies refers to numerical labels on the interpretation plots. 3.1 Probable Archaeology No probable archaeology has been identified within the survey area. 3.2 Possible Archaeology 1 Two weakly magnetic linear anomalies within close proximity to one another, located in the southern extent of the survey area. The low level magnetic contrast displayed by these anomalies make the interpretation of a possible archaeological cut feature, tentative at best. They may equally relate to agricultural or natural features. 2 Several small pitlike anomalies within close proximity to one another and Anomaly 1. The low level magnetic contrast displayed by these anomalies make the interpretation of a possible archaeological cut feature, tentative at best. They may equally relate to agricultural or natural features. 3.3 Medieval/PostMedieval Agriculture Low amplitude anomaly relating to a former field boundary identified on Flintshire OS county series map, dating to 1872 (1:2,500). Low amplitude anomaly relating to a former field boundary identified on Flintshire OS county series map, dating to 1872 (1:2,500). Low amplitude linear anomalies relating to probable former field boundaries not present on mapping. Page 5

23 Geophysical Survey Report Project Name: Land at Mindale Farm, Fford Ty Newydd, Meliden, Prestatyn Job ref: J9324 Client: Penrhyn Homes Limited Date: December Parallel linear anomalies running northwest southeast orientation, situated in the southwest of the survey area. These are indicative of modern agricultural practices. 3.4 Other Anomalies & 11a 12 Areas of weakly magnetic variation across the site. These are likely to be natural in origin. Areas of strongly magnetic disturbance with an unknown origin, likely to be modern. These may relate to waste materials buried by the previous land owners before 1986, as noted by the present landowner. Low amplitude bipolar linear anomalies running northwestsoutheast parallel to each other. These are indicative of modern land drains. Areas of magnetic disturbance as the result of substantial nearby ferrous metal objects such as fences and underground services. These effects can mask weaker archaeological anomalies. A number of magnetic spikes (strong focussed values with associated antipolar response) indicate ferrous metal objects. These are likely to be modern rubbish. 11a are due to telegraph poles. A weak negative linear anomaly probably associated with a modern pipe or service. 4 DATA APPRAISAL & CONFIDENCE ASSESSMENT Sedimentary geology can generally be recommended for gradiometer survey. In this instance the gradiometer survey has demonstrated a good to moderate response, as indicated by the range of anomalies detected. The low archaeological potential of the site supports the lack of detected archaeology indicating that the survey has likely been effective. However, it should be noted that many of the anomalies detected have a low magnetic contrast and the presence of unidentified features, although unlikely, cannot be completely discounted. Page 6

24 Geophysical Survey Report Project Name: Land at Mindale Farm, Fford Ty Newydd, Meliden, Prestatyn Job ref: J9324 Client: Penrhyn Homes Limited Date: December CONCLUSION The survey at Mindale Farm, Prestatyn, has identified few anomalies of possible archaeological origin. Those identified remain tentative at best and may have alternative origins such as agricultural or natural. Anomalies relating to 19 th century field boundaries, and recent ploughing suggest that the area has been used primarily for agricultural purposes, since the medieval period. The remaining anomalies are modern or natural in origin, relating to underground services, land drains, ferrous objects, and fencing. Page 7

25 Geophysical Survey Report Project Name: Land at Mindale Farm, Fford Ty Newydd, Meliden, Prestatyn Job ref: J9324 Client: Penrhyn Homes Limited Date: December 2015 REFERENCES Archwilio online. Website: British Geological Survey South Sheet, Geological Survey Ten Mile Map, South Sheet First Edition (Quaternary). Institute of Geological Sciences. British Geological Survey, Geological Survey Ten Mile Map, South Sheet, Fourth Edition (Solid). British Geological Society. British Geological Survey, n.d., website: ( Geology of Britain viewer. Chartered Institute For Archaeologists. Standard and Guidance for Archaeological Geophysical Survey. English Heritage, Geophysical Survey in Archaeological Field Evaluation.# OS county series maps. Website: Soil Survey of England and Wales, Soils of England and Wales, Sheet 2 Wales. Page 8

26 Geophysical Survey Report Project Name: Land at Mindale Farm, Fford Ty Newydd, Meliden, Prestatyn Job ref: J9324 Client: Penrhyn Homes Limited Date: December 2015 APPENDIX A METHODOLOGY & SURVEY EQUIPMENT Grid locations The location of the survey grids has been plotted together with the referencing information. Grids were set out using a Leica 705auto Total Station and referenced to suitable topographic features around the perimeter of the site or a Leica Smart Rover RTK GPS. An RTK GPS (Realtime Kinematic Global Positioning System) can locate a point on the ground to a far greater accuracy than a standard GPS unit. A standard GPS suffers from errors created by satellite orbit errors, clock errors and atmospheric interference, resulting in an accuracy of 5m10m. An RTK system uses a single base station receiver and a number of mobile units. The base station rebroadcasts the phase of the carrier it measured, and the mobile units compare their own phase measurements with those they received from the base station. A SmartNet RTK GPS uses Ordnance Survey s network of over 100 fixed base stations to give an accuracy of around 0.01m. Survey equipment and gradiometer configuration Although the changes in the magnetic field resulting from differing features in the soil are usually weak, changes as small as 0.2 nanoteslas (nt) in an overall field strength of 48,000nT, can be accurately detected using an appropriate instrument. The mapping of the anomaly in a systematic manner will allow an estimate of the type of material present beneath the surface. Strong magnetic anomalies will be generated by buried ironbased objects or by kilns or hearths. More subtle anomalies such as pits and ditches can be seen if they contain more humic material which is normally rich in magnetic iron oxides when compared with the subsoil. To illustrate this point, the cutting and subsequent silting or backfilling of a ditch may result in a larger volume of weakly magnetic material being accumulated in the trench compared to the undisturbed subsoil. A weak magnetic anomaly should therefore appear in plan along the line of the ditch. The magnetic survey was carried out using a dual sensor Grad6012 Magnetic Gradiometer manufactured by Bartington Instruments Ltd. The instrument consists of two fluxgates very accurately aligned to nullify the effects of the Earth's magnetic field. Readings relate to the difference in localised magnetic anomalies compared with the general magnetic background. The Grad6012 consists of two high stability fluxgate gradiometers suspended on a single frame. Each gradiometer has a 1m separation between the sensing elements so enhancing the response to weak anomalies. Sampling interval Readings were taken at 0.25m centres along traverses 1m apart. This equates to 3600 sampling points in a full 30m x 30m grid. Depth of scan and resolution The Grad 6012 has a typical depth of penetration of 0.5m to 1.0m, though strongly magnetic objects may be visible at greater depths. The collection of data at 0.25m centres provides an optimum methodology for the task balancing cost and time with resolution. Data capture The readings are logged consecutively into the data logger which in turn is daily down loaded into a portable computer whilst on site. At the end of each site survey, data is transferred to the office for processing and presentation. Page 9

27 Geophysical Survey Report Project Name: Land at Mindale Farm, Fford Ty Newydd, Meliden, Prestatyn Job ref: J9324 Client: Penrhyn Homes Limited Date: December 2015 APPENDIX B BASIC PRINCIPLES OF MAGNETIC SURVEY Detailed magnetic survey can be used to effectively define areas of past human activity by mapping spatial variation and contrast in the magnetic properties of soil, subsoil and bedrock. Weakly magnetic iron minerals are always present within the soil and areas of enhancement relate to increases in magnetic susceptibility and permanently magnetised thermoremanent material. Magnetic susceptibility relates to the induced magnetism of a material when in the presence of a magnetic field. This magnetism can be considered as effectively permanent as it exists within the Earth s magnetic field. Magnetic susceptibility can become enhanced due to burning and complex biological or fermentation processes. Thermoremanence is a permanent magnetism acquired by iron minerals that, after heating to a specific temperature known as the Curie Point, are effectively demagnetised followed by remagnetisation by the Earth s magnetic field on cooling. Thermoremanent archaeological features can include hearths and kilns and material such as brick and tile may be magnetised through the same process. Silting and deliberate infilling of ditches and pits with magnetically enhanced soil creates a relative contrast against the much lower levels of magnetism within the subsoil into which the feature is cut. Systematic mapping of magnetic anomalies will produce linear and discrete areas of enhancement allowing assessment and characterisation of subsurface features. Material such as subsoil and nonmagnetic bedrock used to create former earthworks and walls may be mapped as areas of lower enhancement compared to surrounding soils. Magnetic survey is carried out using a fluxgate gradiometer which is a passive instrument consisting of two sensors mounted vertically 1m apart. The instrument is carried about 30cm above the ground surface and the top sensor measures the Earth s magnetic field whilst the lower sensor measures the same field but is also more affected by any localised buried field. The difference between the two sensors will relate to the strength of a magnetic field created by a buried feature, if no field is present the difference will be close to zero as the magnetic field measured by both sensors will be the same. Factors affecting the magnetic survey may include soil type, local geology, previous human activity, disturbance from modern services etc. Page 10

28 Geophysical Survey Report Project Name: Land at Mindale Farm, Fford Ty Newydd, Meliden, Prestatyn Job ref: J9324 Client: Penrhyn Homes Limited Date: December 2015 APPENDIX C GLOSSARY OF MAGNETIC ANOMALIES Bipolar A bipolar anomaly is one that is composed of both a positive response and a negative response. It can be made up of any number of positive responses and negative responses. For example a pipeline consisting of alternating positive and negative anomalies is said to be bipolar. See also dipolar which has only one area of each polarity. The interpretation of the anomaly will depend on the magnitude of the magnetic field strength. A weak response may be caused by a clay field drain while a strong response will probably be caused by a metallic service. Dipolar This consists of a single positive anomaly with an associated negative response. There should be no separation between the two polarities of response. These responses will be created by a single feature. The interpretation of the anomaly will depend on the magnitude of the magnetic measurements. A very strong anomaly is likely to be caused by a ferrous object. Positive anomaly with associated negative response See bipolar and dipolar. Positive linear A linear response which is entirely positive in polarity. These are usually related to infilled cut features where the fill material is magnetically enhanced compared to the surrounding matrix. They can be caused by ditches of an archaeological origin, but also former field boundaries, ploughing activity and some may even have a natural origin. Page 11

29 Geophysical Survey Report Project Name: Land at Mindale Farm, Fford Ty Newydd, Meliden, Prestatyn Job ref: J9324 Client: Penrhyn Homes Limited Date: December 2015 Positive linear anomaly with associated negative response A positive linear anomaly which has a negative anomaly located adjacently. This will be caused by a single feature. In the example shown this is likely to be a single length of wire/cable probably relating to a modern service. Magnetically weaker responses may relate to earthwork style features and field boundaries. Positive point/area These are generally spatially small responses, perhaps covering just 3 or 4 reading nodes. They are entirely positive in polarity. Similar to positive linear anomalies they are generally caused by infilled cut features. These include pits of an archaeological origin, possible tree bowls or other naturally occurring depressions in the ground. Magnetic debris Magnetic debris consists of numerous dipolar responses spread over an area. If the amplitude of response is low (+/3nT) then the origin is likely to represent general ground disturbance with no clear cause, it may be related to something as simple as an area of dug or mixed earth. A stronger anomaly (+/250nT) is more indicative of a spread of ferrous debris. Moderately strong anomalies may be the result of a spread of thermoremanent material such as bricks or ash. Magnetic disturbance Magnetic disturbance is high amplitude and can be composed of either a bipolar anomaly, or a single polarity response. It is essentially associated with magnetic interference from modern ferrous structures such as fencing, vehicles or buildings, and as a result is commonly found around the perimeter of a site near to boundary fences. Page 12

30 Geophysical Survey Report Project Name: Land at Mindale Farm, Fford Ty Newydd, Meliden, Prestatyn Job ref: J9324 Client: Penrhyn Homes Limited Date: December 2015 Negative linear A linear response which is entirely negative in polarity. These are generally caused by earthen banks where material with a lower magnetic magnitude relative to the background top soil is built up. See also ploughing activity. Negative point/area Opposite to positive point anomalies these responses may be caused by raised areas or earthen banks. These could be of an archaeological origin or may have a natural origin. Ploughing activity Ploughing activity can often be visualised by a series of parallel linear anomalies. These can be of either positive polarity or negative polarity depending on site specifics. It can be difficult to distinguish between ancient ploughing and more modern ploughing. Clues such as the separation of each linear, straightness, strength of response and cross cutting relationships can be used to aid this, although none of these can be guaranteed to differentiate between different phases of activity. Polarity Term used to describe the measurement of the magnetic response. An anomaly can have a positive polarity (values above 0nT) and/or a negative polarity (values below 0nT). Strength of response The amplitude of a magnetic response is an important factor in assigning an interpretation to a particular anomaly. For example a positive anomaly covering a 10m 2 area may have values up to around 3000nT, in which case it is likely to be caused by modern magnetic interference. However, the same size and shaped anomaly but with values up to only 4nT may have a natural origin. Colour plots are used to show the amplitude of response. Page 13

31 Geophysical Survey Report Project Name: Land at Mindale Farm, Fford Ty Newydd, Meliden, Prestatyn Job ref: J9324 Client: Penrhyn Homes Limited Date: December 2015 Thermoremanent response A feature which has been subject to heat may result in it acquiring a magnetic field. This can be anything up to approximately +/100 nt in value. These features include clay fired drains, brick, bonfires, kilns, hearths and even pottery. If the heat application has occurred in situ (e.g. a kiln) then the response is likely to be bipolar compared to if the heated objects have been disturbed and moved relative to each other, in which case they are more likely to take an irregular form and may display a debris style response (e.g. ash). Weak background variations Weakly magnetic wide scale variations within the data can sometimes be seen within sites. These usually have no specific structure but can often appear curvy and sinuous in form. They are likely to be the result of natural features, such as soil creep, dried up (or seasonal) streams. They can also be caused by changes in the underlying geology or soil type which may contain unpredictable distributions of magnetic minerals, and are usually apparent in several locations across a site. Page 14

32 Reproduced from Ordnance Survey's 1: map of 1998 with the permission of the controller of Her Majesty's Stationery Office. Crown Copyright reserved. Licence No: AL 50125A Licencee: Stratascan Ltd. Vineyard House Upper Hook Road Upton Upon Severn WR8 0SA N Issue No. Date Amendments Description Stratascan Ltd 2016 OS 100km square = SJ Survey Area Survey Area Site centred on NGR Client SJ PENRHYN HOMES LIMITED 80 Project Title Subject Job No LAND AT MINDALE FARM, FFORD TY NEWYDD, MELIDEN, PRESTATYN LOCATION PLAN OF SURVEY AREA 79 STRATASCANTM GEOPHYSICS FOR ARCHAEOLOGY AND ENGINEERING VINEYARD HOUSE UPTON UPON SEVERN WR8 0SA T: E: info@stratascan.co.uk SUMO GROUP MEMBER Scale Plot 1:25000 A3 Survey date DEC 15 0m m Checked by DGE Drawn by RD Issue No. 01 Figure No. 01

33 Issue No. Date Amendments Description Stratascan Ltd 2016 F Area unsurveyable horse arena/overgrown vegetation OS GRID REFERENCES E A B , , B C D , , E , A C F Client , PENRHYN HOMES LIMITED Project Title Subject Job No LAND AT MINDALE FARM, FFORD TY NEWYDD, MELIDEN, PRESTATYN LOCATION OF SURVEY GRIDS AND REFERENCING D STRATASCANTM GEOPHYSICS FOR ARCHAEOLOGY AND ENGINEERING VINEYARD HOUSE UPTON UPON SEVERN WR8 0SA T: E: info@stratascan.co.uk SUMO GROUP MEMBER Scale 1:1500 0m m Plot A3 Survey date DEC 15 Checked by DGE Drawn by RD Issue No. 01 Figure No. 02

34 Issue No. Date Amendments Description Stratascan Ltd 2016 Plotting parameters Maximum +100nT (red) Minimum 100nT (blue) +100nT +25nT +3nT 3nT 25nT 100nT Client PENRHYN HOMES LIMITED Project Title Subject Job No LAND AT MINDALE FARM, FFORD TY NEWYDD, MELIDEN, PRESTATYN COLOUR PLOT OF GRADIOMETER DATA SHOWING EXTREME VALUES STRATASCANTM GEOPHYSICS FOR ARCHAEOLOGY AND ENGINEERING VINEYARD HOUSE UPTON UPON SEVERN WR8 0SA T: E: SUMO GROUP MEMBER Scale 1:1250 0m m Plot A3 Survey date DEC 15 Checked by DGE Drawn by RD Issue No. 01 Figure No. 03

35 Issue No. Date Amendments Description Stratascan Ltd 2016 Plotting parameters +2nT Maximum +2nT (black) Minimum 2nT (white) 2nT Client Project Title Subject +2nT PENRHYN HOMES LIMITED Job No. 2nT 9324 LAND AT MINDALE FARM, FFORD TY NEWYDD, MELIDEN, PRESTATYN PLOT OF MINIMALLY PROCESSED GRADIOMETER DATA STRATASCANTM GEOPHYSICS FOR ARCHAEOLOGY AND ENGINEERING VINEYARD HOUSE UPTON UPON SEVERN WR8 0SA T: E: SUMO GROUP MEMBER Scale Plot 1:1250 A3 Survey date DEC 15 0m m Checked by DGE Drawn by RD Issue No. 01 Figure No. 04

36 Issue No. Date Amendments Description Stratascan Ltd 2016 PROBABLE ARCHAEOLOGY Positive anomaly / weak positive anomaly probable cut feature of archaeological origin Negative anomaly / weak negative anomaly probable bank or earthwork of archaeological origin a a Client Project Title Subject POSSIBLE ARCHAEOLOGY Positive anomaly / weak positive anomaly possible cut feature of archaeological origin Negative anomaly / weak negative anomaly possible bank or earthwork of archaeological origin MEDIEVAL/POSTMEDIEVAL AGRICULTURE Widely spaced curving parallel linear anomalies probably related to ridgeandfurrow Closely spaced parallel linear anomalies probably related to agricultural activity such as ploughing Linear anomaly probably related to a former field boundary not present on available mapping Linear anomaly related to a former field boundary present on available mapping PENRHYN HOMES LIMITED STRATASCANTM GEOPHYSICS FOR ARCHAEOLOGY AND ENGINEERING VINEYARD HOUSE UPTON UPON SEVERN WR8 0SA OTHER ANOMALIES Linear anomaly probably related to pipe, cable or other modern service Linear anomaly possibly related to land drain Magnetic disturbance associated with nearby metal object such as service or field boundary Magnetic disturbance unknown origin, likely to be modern Scattered magnetic debris Area of amorphous magnetic variation probable natural (e.g. geological or pedological) origin Magnetic spike probable ferrous object Telegraph pole Job No LAND AT MINDALE FARM, FFORD TY NEWYDD, MELIDEN, PRESTATYN ABSTRACTION AND INTERPRETATION OF GRADIOMETER ANOMALIES T: E: info@stratascan.co.uk 11 SUMO GROUP MEMBER Scale Plot 1:1250 A3 Survey date DEC 15 0m m Checked by DGE Drawn by RD Issue No. 01 Figure No. 05

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