Geophysical (fluxgate magnetometry) survey of Tuama Na Spainneach, Breaffy South townland, Spanish Point, Co. Clare.

Transcription

1 Project code: SMPC15 Date: Geophysical (fluxgate magnetometry) survey of Tuama Na Spainneach, Breaffy South townland, Spanish Point, Co. Clare. Report Author: Enda O Flaherty Report Status: Draft

2 TABLE OF CONTENTS 1 INTRODUCTION AIMS AND OBJECTIVES THE SITE SITE TOPOGRAPHY SITE GEOLOGY CARTOGRAPHIC RECORD FLUXGATE MAGNETOMETRY METHODOLOGY THE RESULTS FLUXGATE MAGNETOMETRY SURVEY INTERPRETATION CONCLUSIONS SUMMARY OF ARCHAEOLOGICAL FINDINGS TECHNICAL INFORMATION DATA ARCHIVE DATA PROCESSING AND PRESENTATION REFERENCES

3 LIST OF FIGURES Figure 1 Spanish Point, Clare: Site location and RMP extract Figure 2 First and second edition 6 inch to the mile Ordnance Survey maps Figure 3 Tuama Na Spainneach, Spanish Point, Co. Clare: Survey plot Figure 4 Tuama Na Spainneach, Spanish Point, Co. Clare: Interpretation PLATES Plate 1 General view of Tuama Na Spainneach, Facing west Plate 2 Geophysical Survey at Tuama Na Spainneach Cart-mounted fluxgate magnetometer LIST OF TABLES Table 1 - Geophysical Classifications Table 2 - Geophysical Survey Results Table 2

4 EXECUTIVE SUMMARY This report details the results of an archaeo-geophysical (fluxgate magnetometry) survey undertaken on lands covering approximately 2 Ha located at Tuama Na Spainneach, Spanish Point, Co. Clare (ITM ). The aim of the survey was to identify any sub-surface features which may indicate the location of the buried remains of those lost on the San Marco and San Esteban after both ships were wrecked off-shore on the 20th of September This survey was grant funded by The Heritage Council. It was undertaken by Rubicon Heritage Services Ltd on behalf of The San Marcos Project between 30th July and 31st July The fluxgate magnetometry survey identified geophysical anomalies that may be indicative of anthropogenic ground disturbance and possibly indicate the attested burial spot of those lost. 3

5 1 INTRODUCTION This report details the results of an archaeo-geophysical (fluxgate magnetometry) survey undertaken on lands covering approximately 2 Ha located at Tuama Na Spainneach, Spanish Point, Co. Clare. The aim of the survey was to identify any sub-surface features which may indicate the location of the buried remains of those lost on the San Marcos and San Esteban after both ships were wrecked offshore on the 20th of September Local folklore suggests that a low mound situated at Tuama Na Spainneach is the burial place of those who lost their lives following the wrecking of the two ships. Suitable geological conditions allowed for a fluxgate magnetometry survey to be undertaken at the site. Fluxgate magnetometry survey was used because of its ability to identify a wide range of both negative (ditches, pits, etc.) and positive (structural, hard-pack, etc.) archaeological features. The archaeo-geophysical survey was undertaken by Enda O Flaherty and Damian Shiels of Rubicon Heritage Services Ltd. between the 30th July and 31st July The survey site comprised rough and uneven agricultural grazing land located at Spanish Point in Breaffy South townland in Co. Clare ITM The lands comprises a peninsular area with Tuama na Spainneach, the reputed burial spot of those lost on the San Marco and San Esteban located just to the west of the centre of the area to be surveyed. The lands are bounded to the north, west and south by the seashore, with further agricultural lands lying to the east. To the north of the site and included within the survey area is CL a coastal promontory fort. 4

6 2 AIMS AND OBJECTIVES The aims of the archaeo-geophysical survey were to: determine the extent and composition of any previously unknown archaeological features associated with the reputed burial place of those lost on the San Marco and San Esteban after both ships were wrecked off-shore on the 26th of October 1588 (Tuama na Spainneach). produce a fully illustrated report on the nature, extent and significance of any archaeological sites, features or deposits present with recommendations for further work. accurately locate these anomalies, define their extents and present the findings in map form. provide of a full report incorporating the results of the above work. 5

7 3 THE SITE 3.1 Site topography The site is located at Tuama na Spainneach in Breaffy South townland in Co. Clare ITM (Figure 1). The lands comprises a peninsular area of rough and uneven grazing lands with Tuama na Spainneach, located to the west of the centre of the area to be surveyed. The site is enclosed to the north and west by dry-stone walling with occasional post and wire fencing. To the north of the site and included within the survey area is CL a coastal promontory fort. 3.2 Site geology The underlying bedrock geology of the site comprises Central Clare Group Sandstone, siltstone & mudstone a sedimentary rock type which is amiable to Fluxgate magnetometry survey. 3.3 Cartographic record The cartographic record for the area surrounding Tuama na Spainneach was consulted for this report. These cartographic sources indicate that while much of the survey area has remained undeveloped through the early modern period, some field divisions are marked on early cartographic sources. 3.4 Fluxgate Magnetometry Methodology A cart-mounted Bartington Grad dual magnetic gradiometer (magnetometer) was used to conduct the magnetic gradiometer survey. The Bartington Grad dual magnetic gradiometer is capable of surveying to an accuracy of 0.1nT. This unit comprises two horizontally aligned sensors 1 m apart. Mounting the instrument on a cart reduces the incidence of operator induced errors caused by excessive jostling of the instrument when walking over rough terrain, or by the operator having a 'beat' to their walk. A Trimble R6 Differential Global Positioning System (dgps), capable of Real Time Kinematic (RTK) navigation is fixed to one end of a cart and the magnetic gradiometer is connected to the other end at a fixed distance. This allows each data point to be collected with sub-cm accurate GPS coordinates, thus negating the need to set out a nominal grid prior to the survey. This increases the accuracy and 6

8 efficiency of the survey. Both the GPS and magnetic gradiometer data is sent to an external mobile computer (Trimble Nomad) via a serial connection. The data is then downloaded from the unit on a daily basis, using a laptop PC and the data streams are subsequently combined. This 'irregular xy' data is then exported from the data collection software and imported into the data processing software, where it is converted to 'regular xy' data at user defined sample intervals (in this case 1 m by m). From there, it was processed as standard magnetometer data, such as that collected from a traditional pre-defined grid survey. The data was collected at a resolution of 1 m x m. This sample density is recommended for site evaluation (English Heritage 2008). 7

9 4 THE RESULTS 4.1 Fluxgate Magnetometry Survey For the purpose of this report, the following classifications are applied to geophysical anomalies of archaeological potential: Table 1 Geophysical Classifications High Archaeological Potential (RED) Identifiable, known archaeological features, often, (but not always), with additional evidence exists to support identification of High Archaeological Potential anomalies. This classification may also be applied to definable anomalies with no obvious alternative provenance, in an area that is known to be archaeologically rich. Moderate Archaeological Potential (GREEN) Geophysical anomalies that may represent possible archaeological features based on the appearance, pattern and strength of the magnetic response. Low Archaeological Potential (YELLOW) Known or possible archaeological feature types that are or are likely to be of low archaeological significance. Post-medieval field boundaries shown on historic Ordnance Survey mapping are marked blue. Table 2 Geophysical Survey Results Table ID Description Interpretation L: 01 Strong, positively magnetic irregular anomaly (c.11 m diameter) Unprovenanced disturbance L: 02 Strong, positively magnetic irregular anomaly (c.12 m diameter) Unprovenanced disturbance L: 03 Strong, positively magnetic irregular anomaly (c.18 m diameter) Unprovenanced disturbance L: 04 Strong, positively magnetic irregular anomaly (c.10 m diameter) Unprovenanced disturbance L: 05 Strong, positively magnetic irregular linear/anomaly (c.12 m) Unprovenanced disturbance L: 06 Strong, positively magnetic linear/irregular anomaly (c.6.5 m) Unprovenanced disturbance 8

10 ID Description Interpretation L: 07 Strong, positively magnetic linear/irregular anomaly (c.10 m) Unprovenanced disturbance L:08 Strong, positively magnetic irregular anomaly (c.9.5 m) Unprovenanced disturbance L:09 Strong, positively magnetic linear anomaly (c.25 m) Possible boundary L: 10 Strong, positively magnetic curving linear anomaly (c.33 m diameter) Possible boundary L:11 Strong, positively magnetic sub-circular anomaly (c.5 m diameter) Unprovenanced disturbance M: 01 Strong, positively magnetic linear anomaly (c.15 m) Possible boundary M: 02 Strong, positively magnetic broadly parallel linear anomalies (c.7 m) Unprovenanced disturbance M: 03 Strong, positively magnetic irregular anomaly (c.11 m diameter) Unprovenanced disturbance M: 04 Strong, positively magnetic irregular anomaly (c.5 m diameter) Unprovenanced disturbance M: 05 Strong, positively magnetic L-shaped linear anomaly (c.15 m) diameter) Unprovenanced disturbance M: 06 Strong, positively magnetic sub-circular anomaly (c.1.5 m) Unprovenanced disturbance M: 07 Strong, positively magnetic sub-circular anomaly (c.1.5 m) Unprovenanced disturbance M: 08 Strong, positively magnetic sub-circular anomaly (c.1.5 m) Unprovenanced disturbance M: 09 Strong, positively magnetic sub-circular anomaly (c.1.5 m) Unprovenanced disturbance The greyscale data plots (Figure 3) from which the results are derived is characterised by positive anomalies (black areas) and negative anomalies (white areas). Response Classifications Negative anomaly A negative anomaly possesses negative magnetism (as distinct from the archaeological meaning of a negative feature e.g. pit). Negative magnetic anomalies can be caused by walls, rubble, etc. These anomalies appear as white areas in the data plot. 9

11 Positive Anomaly A positive anomaly possesses positive magnetism (as distinct from the archaeological meanings of a positive feature e.g. bank). Positive magnetic anomalies can be caused by features, such as pits, ditches, etc. These anomalies appear as black areas in the data plot. 4.2 Interpretation The geophysical plot shows a magnetic homogeneity across the site. A number of former field boundaries that are known from the historical mapping sources can be identified to the east of the site. Adjacent to these are L:07 L:10 which are most likely associated vernacular settlement in this area. To the southwest of the survey area L:01 L:03 comprise four areas with a strong magnetic signature which may represent possible disturbance. The 1 st edition OS sheet for the area shows a structure marked in this area and L:01 L:03 may relate to demolition rubble associated with this structure. L:04 corresponds precisely with the locally known burial place of a small silver/fin whale in According to local tradition, a pit measuring approximately 10 m was excavated to bury the whale. In comparison with nearby L:01 L:03, L:04 has a weaker positive magnetic signature which would further suggest that L:01 L:03 may represent demolition rubble. Given that L:04 is a known large burial location that may be comparable in size and magnetic signature with the possible burial spot of those lost on the San Marco and San Esteban, it is possible that comparable responses L:05, L:06 or L:11 could represent further large pits or even burial locations, though this could only be confirmed through archaeological excavation. L:05, L: 06 and L:11 represent three areas of unprovenanced possible disturbance with a strong magnetic signature. L;05 and L:06 are irregular in shape. L:11 is sub-circular measuring c. 5 m in diameter. As discussed above, L:05, L: 06 and L:11 represent the responses with the greatest potential to represent the burial place of those lost on the San Marco and San Esteban. M:01 M:09 represent areas of possible disturbance located to the north of the survey site and in close proximity to RMP CL a coastal promontory fort. With the exception a faint linear feature (M:01), these responses are irregular in shape. However, given their proximity to RMP CL , there is a greater likelihood that they represent buried archaeological deposits and so have been marked with a medium archaeological potential.. 10

12 5 CONCLUSIONS 5.1 Summary of archaeological findings The results of the geophysical survey have identified a number of areas of possible disturbance within the survey area. L:01 L:04 may represent demolition debris associated with a structure marked at this location on early mapping. However, L:05, L:06 and L:11, areas of unprovenanced possible disturbance, are located on the southern side of Tuama Na Spainneach. L:05 and L:06 are irregular in shape with a strong magnetic signature suggesting possible stone packing or further debris. L:11 is sub-circular in shape with a weaker magnetic signature more characteristic of an earth-cut feature. It measures approximately 5 m in diameter and may represent a large pit. To the north, M:01 M:09 represent further disturbance, though lacking a discernible form. Nonetheless, given the proximity of these responses to RMP CL a coastal promontory fort, there is a greater likelihood that they may represent buried archaeological deposits. The results of the survey cannot conclusively identify a likely location of the buried remains of those lost on the San Marco and San Esteban on the Tuama Na Spainneach, though areas of disturbance have been identified just off the mound to the south which could potentially represent the burial place. 6 TECHNICAL INFORMATION 6.1 Data Archive The site archive will contain the following, as a minimum standard: Item Quantity Raw geophysical data files (xyz/txt) 6 Processed geophysical data files (asc) 6 Archaeological interpretation (shp) 1 Final report text (pdf) 1 Final report graphics (pdf) 1 Storage of the archive in a suitable format and location is required in order to provide for any future archaeological research. On completion of the data will form the basis of a full archive to professional standards, in accordance with current English Heritage guidelines (English Heritage, 2006), and compiled in line with ADS digital records (archaeologydataservice.ac.uk/advice). 11

13 6.2 Data processing and presentation Clip The clipping process will clip extreme values from the data set and increase the contrast in the data values closer to the mean. As most data within a data set is concentrated around the mean clipping can produce a better visualisation of standard data sets, particularly very weak signals that tend to be lost in a myriad of grey shades. De-slope The de-slope process uses a computational process to remove or minimise the distortion caused by large iron objects such as fences or pipelines within the data set. Often, data near highly magnetic sources is obscured by its proximity to the extreme values and this process can sometimes remove the extremes and recover this otherwise obscured data. De-spike The de-spike process will ONLY clip isolated extreme values from the data set. Extreme values are usually caused by surface iron of non-archaeological origin. This process will remove statistical outliers from the data set and consequently remove them from any further computational processes. De-stagger Sometimes data collected in zig-zag mode can show a staggered effect cause by slightly different walking speeds on the inward versus the outward traverse. De-stagger can adjust the positioning of traverses by a specified intervals to ensure that traverses match and detail is not obscured by the staircase effect stagger can sometimes produce. De-stripe The de-stripe process can reduce the occurrence of a striping effect sometimes known as heading error. This error occurs when working in zig-zag data collection mode and is caused by the different readings being obtained by the sensors when data is collected in opposite directions. This can also be caused by problems during the initial calibration of the instrument at the start of a survey. Some degree of heading error is inevitable when using a fluxgate gradiometer with such an acute sensitivity to the direction of travel in bi directional manner i.e. zigzag traverses. The error displays as a series of alternating lighter and darker stripes in the traverse direction and the function asses and corrects the mean for each line of data to bring them in to the same mean range and remove any visible artefacts. High/Low Pass Filters The low pass filter, when applied to a noisy data set i.e. a data set displaying too much variation that it obscures detail, will smooth the data, removing the noise and revealing 12

14 anomalies that were obscured. Conversely, high pass filtering accentuates differences in subtle data sets and can increase the visibility in data set, often uncovering very weakly magnetic anomalies. Two Dimensional Fast Fourier Transform (2D FFT) The 2D FFT algorithm examines the data set for repetitive signature that occupy the same wavelength. This is often used to mitigate the interference caused by plough furrows or other regular phenomenon. 13

15 7 REFERENCES Bonsall, J. Gaffney, C. and Armit, I A Sense of the Past Studies in current archaeological applications of remote sensing and non-invasive prospection methods. In Kamermans, H. Gojda, M. and Posluschny, A.G. (eds) BAR International Series 2588 Archaeopress Oxford. English Heritage 2008 Geophysical Survey in Archaeological Field Evaluation. English Heritage Research and Professional Services Guideline No. 1 (2nd ed). English Heritage, London. English Heritage MoRPHE Technical Guide 1 Digital Archiving and Dissemination. IfA, Pending DRAFT Standards and Guidance for Geophysical Survey, IfA Technical Paper (IfA, Pending). IfA, 2002 The Use of Geophysical Techniques in Archaeological Evaluations, Institute of Field Archaeologists Paper, No. 6. Schmidt, A Geophysical Data in Archaeology: A Guide to Good Practice. AHDS Guides to Good Practice. Oxbow Books, Oxford. Westropp, T.J., Notes on the Armada Ships Lost on the Coast of Clare, In The Journal of the Royal Historical and Archaeological Association of Ireland, Fourth Series, Vol. 9, No. 79 (Apr. - Jul., 1889), pp Published by: Royal Society of Antiquaries of Ireland Online/Electronic resources Geological Survey of Ireland, 2013 GSI Datasets Public Viewer [online]. Available [Accessed 10 August 2015]. Cartographic sources Ordnance Survey of Ireland, 1843, Map of Co. Clare, Sheet 30. First Edition. 6 inch: 1 mile. Ordnance Survey of Ireland, Dublin 14

16 Ordnance Survey of Ireland, 1912, Map of Co. Clare, Sheet 30. Second Edition. 25 inch series: 1 mile. Ordnance Survey of Ireland, Dublin 15

17 SITE Spanish Point Reproduced from 1997 Ordnance Survey of Ireland 1:50,000 Discovery Series no 57, C Ordnance Survey of Ireland, Government of Ireland. Licence No. EN Reproduced from 1920 Ordnance Survey of Ireland, Second Edition, Six Inch to One Mile map, Clare Sheet 30, C Ordnance Survey of Ireland and Government of Ireland. Licence No. EN SITE N RUBICON HERITAGE 0 1 km Figure 1 - Spanish Point, Clare: Site location and RMP extract.

18 Figure 2 - First and second edition 6 inch to the mile Ordnance Survey maps

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21 Plate 1 - General view of Tuama Na Spainneach, Facing west Plate 2 - Geophysical Survey at Tuama Na Spainneach Cart-mounted fluxgate magnetometer