Biggins, J. Alan (2011), ‘A Geophysical Survey of St. Leonard’s Medieval Hospital,...

A Geophysical Survey of

St. Leonard’s Medieval Hospital,

Northumberland Park, North Tyneside May 2011

A Geophysical Survey of St. Leonard’s Medieval Hospital,

Northumberland Park, North Tyneside

May 2011

Prepared for the North Tyneside Council

& supported by the Heritage Lottery Fund

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Compiled by:

J. Alan Biggins TD MA MSc MRSC MIBiol FSA FSA Scot

In Association With




Northumberland Park, North Tyneside: May 2011

INDEX

SUMMARY

1. INTRODUCTION 1

2. ARCHAEOLOGICAL BACKGROUND 2

3. LOCATION OF THE SURVEY AND GEOLOGY 9

4. GEOPHYSICAL SURVEY TECHNIQUES 9

4.1 Survey Parameters 9

4.2 Mapping and Set-out 10

4.3 Magnetic Survey 12

4.4 Resistance Survey 14

5. THE SURVEY RESULTS 15

5.1 Introduction 15

5.2 The Magnetometer and Resistance Survey Figures 16

5.3 Magnetic Survey Results 18

5.4 Resistance Survey 27

6. DISCUSSION 32

7. CONCLUSIONS 35

8. PROJECT ARCHIVE 35

10. PUBLICITY, CONFIDENTIALITY AND COPYRIGHT 35

11. STATEMENT OF INDEMNITY 36

12. ACKNOWLEDGEMENTS 36

BIBLIOGRAPHY 36

LIST OF FIGURES (APPENDIX 1) 39-65

1. O.S Location Map 1:50,000 39

2. Wood’s Plan of 1826 40

3. 1856 O.S. Map, Showing Spital Cottage I 41

4. 1861 O.S. Map; 1:2500 Scale 42

5. 1896 O.S. Map 1:500 Scale, Showing St. Leonard’s Foundations 43

6. 1898 O.S. Map Second Edition, Showing St. Leonard’s Foundations 44

7. Modern Map Showing Plan of Northumberland Park 45

8. Digital Map Showing 20m Survey Grid 46

9. Map Showing Archaeological Features 47

Magnetic Survey Figures

10. Magnetic Survey: Grey Scale Plot Overview; Scale 1:1000 48

11. Magnetic Survey: Grey Scale Plot; Scale 1:1000 49

12. Magnetic Survey: Relief Plots; Scale 1:2000 50

13. Magnetic Survey: Trace Plot; Scale 1:1000 51

14. Magnetic Anomaly Plan & Grey Scale Plot; Scale 1:1000 52

15. Magnetic Anomaly Plan - Annotated; Scale 1:500 53

16. Magnetic Anomaly Plan - Annotated Overview; Scale 1:1000 54

Resistance Survey Figures

17. Resistance Survey: Grey Scale Plot Overview; Scale 1:1000 55

18. Resistance Survey: Grey Scale Plot; Scale 1:1000 56

19. Resistance Survey: Relief Plots; Scale 1:2000 57

20. Resistance Survey: Trace Plot; Scale 1:1000 58

21. Resistance Anomaly Plan & Grey Scale Plot; Scale 1:1000 59

22. Resistance Anomaly Plan - Annotated; Scale 1:500 60

23. Resistance Anomaly Plan - Annotated Overview; Scale 1:1000 61

Survey Overviews

24. Magnetic & Resistance Anomaly Plans – Comparative 62

25. Magnetic & Resistance Anomaly Plans – Superimposed 63

26. Magnetic & Resistance Grey Scale Plot – Suggested Trench Locations 64

27. Suggested Trench Locations; Digital Map 65

APPENDIX 2

Geophysical Survey Theory 67-75



SUMMARY

A geophysical survey was undertaken at the location of St. Leonard’s Medieval Hospital,

Tynemouth, North Tyneside. This survey project was co-ordinated by North Tyneside

Council (Client) supported by Heritage Lottery Funding. The survey is part of a much

wider ranging conservation management and heritage strategy for Northumberland Park

forming part of the ‘Parks for People’ project. TimeScape Surveys supervised the

investigation which was conducted by a volunteer group comprised of the Friends of

Northumberland Park.

The objectives of the survey were set out in the County Archaeologist’s project design,

and included the use of local volunteers who were to be taught how to undertake

geophysical survey by an experienced geophysicist. The volunteers were offered on site

training in three survey techniques; EDM mapping, magnetic survey and earth resistance

survey. The geophysical survey was the second stage in a process to evaluate the

medieval hospital of St. Leonard’s and any associated archaeological features. The initial

stage comprised a desktop assessment, followed by the geophysical survey, then the

excavation phase.

The geophysical survey aimed to map subsurface anomalies which might indicate the

presence of features of archaeological significance, particularly those suggesting the

location of St. Leonard’s Hospital. Later excavation of the anomalies allowed a more

comprehensive understanding of the survey results. At the outset it was realised that

with one exception, an area towards the west, the results indicated a poor state of

preservation of the hospital’s foundations. It was obvious that stone robbing and later

landscaping had degraded the immediate area of the hospital. Subsequent excavation

confirmed this analysis.

Other areas of potential archaeological interest were identified which may warrant further

investigation. These are perhaps less spectacular than foundations, but may include

drainage systems and channels. More modern features such as the two temporal phases

of the Spital Cottages were recognised, but the response at the later cottage (1860-1960)

was subject to considerable modern interference.

The geophysical survey succeeded in its objectives, namely to train volunteers and also

determine the degree of preservation of St. Leonard’s Hospital. From this survey data,

appropriate areas to excavate were suggested and the volunteers themselves laid out the

excavation trenches. This project has perhaps laid the foundations for future communal

work in the area.

Geophysical Survey of St. Leonard’s Medieval Hospital Prepared for North Tyneside Council

TimeScape Surveys Report Number 123-11 1 Copyright Reserved



by

J. Alan Biggins - June 2011

with contributions by

Russell Anderson, Sheila Bell, Nina Brown, Martha Brummer, John Caulfield,

Susan Clements, Mike Coates, Isabel Cook, Jessica Currie, Christine Czarnowska,

Sheila Day, Elaine Dermody, Andy Graham, Paul Greenway, Jackie Henderson,

Liz Johnson, Lynn Johnson, Nick Johnson, Cath Mackley, David Mason,

Sarah Matthews, Linda McCann, Mike Morrison, Laura Reid, Jean Richardson,

Rob Shippey, Adrian Smiles, Alex Stevens, Dave Thompson, Mark Thompson,

Sophie Thompson, Susan Thompson & Eilis Weldon

1. INTRODUCTION

A geophysical survey was undertaken at the location of St. Leonard’s Medieval Hospital,

Tynemouth, North Tyneside during May 2011 (fig. 1). This survey project was co-

ordinated by North Tyneside Council (Client) supported by Heritage Lottery Funding. This

survey was part of a much wider ranging conservation management and heritage strategy

for Northumberland Park forming part of the ‘Parks for People’ project. TimeScape

Surveys supervised the geophysical investigation which was conducted by a volunteer

group from the Friends of Northumberland Park. The volunteers were offered on site

training in three survey techniques; EDM mapping, magnetic survey and earth resistance

survey.

The objectives of the survey were set out in the County Archaeologist’s project design

dated 20th December 2010 and include the following objectives:

Volunteers will be taught how to undertake geophysical survey by an experienced

geophysicist.

The geophysical survey aims to map subsurface anomalies which might indicate

the presence of features of archaeological significance, particularly those

suggesting the location of St. Leonard’s Hospital.

A survey grid will be established which will be geo-referenced to an O.S. base map

within 100mm accuracy.



The geophysicist will analyse and interpret the results and produce the report

which will identify anomalies and make recommendations for the emplacement of

excavation trenches.

2. ARCHAEOLOGICAL BACKGROUND

St. Leonard’s Hospital

The evidence both documentary and cartographic relating to St. Leonard’s Hospital is

incomplete, and, in some instances, speculative. A summary of the evidence has been

produced in the project design (Morrison 2010), which is reproduced in this section, and

also in the desk based assessment report which is being compiled in conjunction with

volunteers (Carlton 2011 forthcoming). It is thought that the hospital was founded about

1220 (Knowles and Hadcock 1971), with indirect mention made in the Assize Roll of 1293.

Lands belonging to the Hospital of St Leonard are first mentioned as being annexed to the

Benedictine Priory of Tynemouth in 1320.

In January 1539 the prior and convent of Tynemouth surrendered all their possessions,

including ‘Spytel House’ and ‘Spytel Close’ to the Crown. The land was then leased by

Henry VIII to Sir Thomas Hilton for a period of 21 years. The hospital had a considerable

amount of land attached to it. In a terrier1 of 1649, the extent of the hospital demesne was

13 acres, 3 roods and 5 perches lying in 46 rigs in various corners of land in Tynemouth,

Preston and Chirton (Morrison 2010).

The hospital was probably an alternative burial ground to Tynemouth Priory. In 1603,

William Milbank of North Shields left his body to be buried at either Tynemouth Priory or

the Spital. Many people were buried here during the Civil War when access to the priory

church was restricted. The site is listed as a burial place in the parish records in 1645. In

1656 the son of Gabriel Coulson, parish clerk, was buried there. In 1662 Ralph Pearson of

North Shields was buried at Spittle. In the Tynemouth burial registers for 1662, 19 were at

the Spittal. The last recorded burial at the Spittal was on 6th February 1708. This was

Jane, daughter of Anthony Elsdon of Whitley (Morrison 2010). Thompson’s map of

Tynemouth manor, 1757, shows the site of the hospital as ‘Spittle Yards’. By the 18th

century the hospital was in ruins and lying within pasture land.

1 This is a legal term derived from the Latin terrarius liber, i.e., a book belonging or pertaining to

land or landed estates and can be taken to mean;

(a) Formerly, a collection of acknowledgments of the vassals or tenants of a lordship, containing

the rents and services they owed to the lord.

(b) In modern usage, a book or roll in which the lands of private persons or corporations are

described by their site, boundaries, number of acres ([Webster 1913).



The ruins could still be traced in 1789 close to the road to Newcastle. In January 1885 the

Duke of Northumberland gave the land for a park and during the laying out of the park the

workmen rediscovered the hospital on the spot indicated by Brand and in Thompson's

Map of the Manor. In the course of the excavations they exhumed two stone coffins, one

or two medieval grave covers and a number of skeletons. They also uncovered a tiled

floor about 18 to 24 inches (0.46 x 0.61m) from the present surface. It measured about 20

feet by 12 feet (6.1 x 3.65m) and was covered up again to prevent it from damage while

the work was being carried out (Adamson 1889, 35). The building appears to have been

of considerable size. Its chambers were said to have been paved with stone. Mouldings

such as window tracery and the base of a cross were in Early English style. It was re-

covered to preserve it from damage. A few skeletons and grave covers were found. The

discovery of a ‘200 year old skeleton’ in Spital Burial Ground was reported in the Shields

Daily News on 5th January 1885 (Morrison 2010).

Plate 1. Relocated stone coffins and the grave slab together with a standing stone which may be

the base of a cross.

The surviving objects on the site include two stone coffins and a few worked stones

(Plates 1 & 2). The most interesting object is a limestone grave slab measuring 5 feet 9

inches x 2 feet 7 inches (0.61 x 1.75m). It depicts a man and his wife and their five

children at their feet. The grave slab is thought to be 15th century in date. The slab would

originally have been covered by brass sheeting but this has been stripped off. The rivets

are still visible. The grave slab was found face down having been reused as part of a floor.

It is now enclosed by modern railings (removed for the survey). The grave slab and coffins

are listed grade 2.



Plate 2. Detail of the matrix (grave cover), which may have had brass inlaid figures. The two larger

figures may be husband and wife, with the smaller inset indicating four sons and a daughter. A 15th

century date is implied on stylistic grounds. A cover like this suggests a high-status burial, possibly

a local dignitary or a benefactor.

The site was partially exposed in 1885 but for a long time has been overgrown. The main

buildings of the hospital are no longer discernable on the ground. Neither the site of the

hospital nor the associated grave slab and coffins are presently signposted or interpreted.

Wider Research Background (by Jennifer Morrison)

Hospitals were a fairly common feature of the medieval landscape, there being over 750

in England in the Middle Ages, but as a group of monuments they are not well

represented in the archaeological record for the period. Little research has been done on

the northern examples and across the country, only half a dozen sites have been

thoroughly researched, most notably St Bartholomew’s Hospital Bristol, which was

extensively excavated in the 1970s. Less extensively, and also in the south of the country

are St Mary Ospring, Kent and St Mary Spital, London, while in the north of England, the

only major excavation has been the hospital of St Giles, at Brompton Bridge, North

Yorkshire, where emergency excavations recorded remains threatened by river erosion.



St Giles, in common with a number of hospitals, was located at a river crossing point on a

major thoroughfare, a location also seen at St Leonard’s, which was on the main

Newcastle to Tynemouth road, on the bank of the Powburn stream. This reflects one of

the main functions of these semi-monastic institutions, to provide travellers with a hostel

for overnight accommodation, in addition to the role of providing alms and medical care to

the old and infirm. Run by religious orders or by clergy on behalf of the pious wealthy, they

were funded by grants of land and money in the same way as monasteries and chantry

chapels2, to say prayers for the souls of the benefactors, who would be credited with the

good works done in the institutions.

Plate 3. The hospital of St. Mary’s, Glastonbury viewed from the west, which appears to be of

similar dimensions to the foundations of St. Leonard’s. Although this is in the style of a later

almshouse, this illustration shows an eastern chapel, with a hall attached and separate cubicles

(after Clay 1909, Plate XI).

The specific medical connotation of the word “hospital” had no significance in the Middle

Ages, as these institutions provided little or no medical care, beyond providing

accommodation, food and spiritual guidance. The medical association has developed

primarily because a specific class of hospital cared for victims of leprosy, a disease that

was regarded as a punishment for sinful, usually carnal, behaviour which became

common in society in the 12th and 13th century, but which almost died out (for reasons not

well understood) by the 15th century, when lepers’ hospitals became homes for a wider

range of the long-term ill. The hospital of St Leonard’s has been assigned as a leper’s

2 A Chantry Chapel was established by a fund to pay for a priest to celebrate sung masses for a

specified purpose, generally for the soul of the deceased donor. Chantries were endowed with

lands given by donors, the income from which maintained the chantry priest. A chantry chapel is a

building on private land or a dedicated area within a greater church, set aside or built especially for

and dedicated to the performance of the chantry duties by the priest. A chantry may have only an

altar, rather than a chapel, within a larger church, generally dedicated to the donor's favourite saint.



hospital on uncertain evidence, possibly because St Leonard has been associated with

the care of lepers.

Plate 4. Sherburn Hospital, near Durham, was a much more expansive structure with a large range

of buildings connected to a chapel and outbuildings surrounded by a large courtyard. This was

founded in 1181 by Bishop Puisit. As with many other hospitals it was known that this was a lazar-

house which was appropriated for other uses (in 1434) such as care of the poor after leprosy

became less common. The original leper-hospital had as many as 65 lepers, which was reduced to

13 poor men around 1434 (after Clay 1909, 44, 118, fig. 21, 289).

The function of the hospital will determine the type and arrangement of building expected.

Hostels and alms-houses had a large central hall, an infirmary, which provided dormitory

accommodation and which was usually well-supplied with water. Ancillary buildings,

including kitchens and possibly a chapel, would be arranged around a small courtyard or

walled precinct. Leper hospitals often had groups of smaller cells for individual inmates,

again with central facilities under the supervision of a master, who had more lavish living

quarters.



Post-Medieval Features

Although earlier maps indicate the field systems associated with St. Leonard’s and

reference is made to spital, none indicate the location of upstanding masonry. Wood’s

map of 1826 (fig. 2) clearly shows the location of the House of Correction, Spital Bridge,

the Tynemouth road and a cottage located towards the west of the bridge. The 1856 O.S

map (fig. 3) shows two small steams which converge towards the south creating a triangle

of land within which the hospital is located. These natural features may well have

delineated the boundaries of the earlier ecclesiastical establishment. The 1861 1:2500

O.S map (fig. 4) shows a more complex group of buildings (hereafter referred to as Spital

Cottage I), together with Spital Dean Farm located north of the road in a position almost

opposite the present park gates. All these feature are seen on the 1856 map.

The section of the 1:500 scale 1896 map (fig. 5) shows the park shortly after opening, but

importantly indicates the presence of exposed masonry aligned east west for some 36m,

with a width of some 6.5m. The foundations are again seen on the 1898 O.S. map (fig. 6),

and a later map, although it is uncertain when these remains were recovered (or indeed

removed). On these two maps the cottage near Spital Dean Bridge (Spital Cottage I) has

been demolished a replacement Park Cottage (or Spital Cottage II) has replaced it. This in

turn was demolished some time in the 1960s.

Although the remains of St. Leonard’s Hospital would appear to be the most interesting

and important in archaeological terms, the presence of a cottage dating from at least 1826

(Wood’s map) and its mid-19th century replacement are themselves of interest. It would

appear that elements of the earlier cottage have been buried or lost in when the road was

straightened (see plate 5), although some elements may remain south of the embankment

of the Tynemouth road. Its replacement existed in living memory.

Earlier maps also indicate the presence of a market garden near to Spital Bridge. In the

case of the post-medieval features excavation may indicate invaluable evidence of these

earlier foundations. In addition, it is possible that masonry from the hospital may have

been used and may remain in situ. It should also be established whether or not any

remaining buildings were constructed on earlier foundations.



Plate 5. Cut Throat Lane renamed Spital Dene and then King Edward Road (after M. Coates).

Plate 6. Later map showing old Spital Cottage which was replaced in 1862 with New Spital Cottage

(II), later renamed Park Cottage which has since been demolished. Note the change in the

direction of the road and widening near Spital Dene Bridge (after M. Coates).



3. LOCATION OF THE SURVEY AND GEOLOGY

Northumberland Park (NZ 3612 6930), which extends over approximately 12 hectares, is

located some 300 metres to the west of Tynemouth Railway Station and 350 metres to the

north of the banks of the River Tyne (fig. 7). It is bounded by Park Avenue/Park Terrace to

the west, King Edward Road to the north, former railway sidings, allotment gardens and

Hazeldene Court to the east and Tynemouth Road to the south. The park is situated in

Spital Dene which cuts through the gently rising side of the estuarine Tyne valley. The

lowest point of the park is approximately 14 metres OD (the lake) and the highest

approximately 32 metres OD (King Edward Road entrance). The park dishes at its

southern end where 19th century engineering works raised the alignment of Tynemouth

Road and the adjacent railway, now metro, line to form a raised edge approximately 23

metres OD. (North of England Civic Trust 2009).

The drift geology of the area is dominated by boulder clay with deposits between 6-9m in

depth (Taylor et al 1971, 88). The solid geology of the area is generally not well exposed

due to the glacial drift and it is only in the exposed coastal regions, such as Tynemouth,

where good continuous sections of Westphalian strata can be seen (Jones 1967). Inland

exposures are restricted to quarries, mainly in sandstone and ganister (Johnson 1995,

267). The coast from Tynemouth to Seaton Sluice provides one of the best exposures of

rocks belonging to the Upper Carboniferous Coal Measures (approximately 300 million

years old) in Great Britain. It includes outcrops of numerous coal seams, and several

mudstone horizons yielding non-marine bivalves. Of particular importance are outcrops of

sandstones within the rock sequence, which have been interpreted as braided river

deposits. These contrast with the meandering river deposits which dominate the same

rocks in the Pennines Coalfields to the south. Geologists suggest from this evidence that

the Northumberland and Durham Coalfields formed in a more elevated area relative to the

Pennines Coalfield, and this area was thus probably rather further from the sea (Natural

England 2011).

4. GEOPHYSICAL SURVEY TECHNIQUES

4.1 Survey Parameters

The survey was located within the northern sector of the park adjacent to King Edward

Road (fig. 7). The survey was conducted according to those specifications indicated in the

project Design (Morrison 2010). The major requirements are outlined below.

The survey aims to map subsoil disturbances and locate anomaly-producing

structures or deposits which might indicate the presence of archaeological sites.

The survey grid is the network of control points used to locate the geophysical

survey measurements relative to base mapping and/or absolute position on the



Earth’s surface (dereferencing). These points were located to survey-grade

accuracy ± 100mm. The survey grid must be independently re-locatable on the

ground by a third party either by measurement to permanent features and/or by

the use of GPS coordinates.

The survey grid must be tied into known Ordnance Survey points with a total

station (EDM).

The magnetic investigation will be a 100% detailed survey using sample intervals

of 0.25m and the traverse intervals of 1m and a sensitivity of 0.1 nanoTesla.

The earth resistance survey should employ 1.0m sample intervals (0.5m were

employed in this survey) along 1.0m traverses.

20m survey grids to be employed.

The interpretation of magnetometer data must endeavour to distinguish between

anthropogenic and other causes of magnetic enhancement on the site.

A clear distinction must be made between interpretation that is scientifically

demonstrable and that which is based on informed speculation.

Any reference to negative evidence must be fully explained. Lack of geophysical

anomalies cannot be taken to imply a lack of archaeological features.

4.2 Mapping and Set-out

Survey areas were subdivided into 20m survey grids using a Total Station (Leica 403L)

which was used to provide cartographic information (fig. 8). A smaller grid size was

chosen because of the constrictions and obstructions within the survey area. The Leica

TC403L Total Station (EDM) fires a laser beam which is reflected from a prism (plate 8)

and the data recorded includes grid coordinate, height, slope angle, type of object (for

example; road, fence etc.). The digital mapping of the site is georeferenced in a DXF (or

dwg) format, and was superimposed upon an Ordnance Survey digital map.



Plate 7 Elaine Dermody and Paul Greenway setting out the survey grid and mapping using EDM

(total station). This instrument fires a laser beam towards a reflector (prism) which returns the

signal to the EDM which records mapping data.



Plate 8. David Mason can be seen with the prism which reflects back the laser beam pulse

converting the speed of light into a grid coordinate and height, which is recorded. David is seen

here recording firm map detail, stone coffins and the grave slab. The ashlar stone in the foreground

is probably a corbel bracket. This would have supported a structural element like a beam or roof

support.

4.3 Magnetic Survey

Magnetic prospection of soils is based on the measurement of differences in magnitudes

of the earth’s magnetic field at points over a specific area (down to 0.1 nanoTesla

sensitivity). The iron content of a soil provides the basis for its magnetic properties, with

the presence minerals such as magnetite, maghaemite and other iron oxides all affecting

the magnetic properties of soils. Variations in the earth’s magnetic field which are

associated with archaeological features are weak, however, especially considering the

overall strength of the earth’s magnetic field of around 48 Teslas (48x 109nT). It follows

that these instruments are very sensitive indeed.

Three basic types of magnetometer are available to the archaeologist; proton

magnetometers, fluxgate gradiometers, and alkali vapour magnetometers (also known as

caesium magnetometers, or optically pumped magnetometers). Fluxgate instruments are



based around a highly permeable nickel iron alloy core, which is magnetised by the

earth’s magnetic field, together with an alternating field applied via a primary winding. Due

to the fluxgate’s directional method of functioning, a single fluxgate cannot be utilised on

its own, as it cannot be held at a constant angle to the earth’s magnetic field.

Gradiometers therefore have two fluxgates positioned vertically to one another on a rigid

staff. This reduces the effects of instrument orientation on readings. Fluxgate

gradiometers are sensitive to 0.5nT or below depending on the instrument. However, they

can rarely detect features which are located deeper than 1m below the surface of the

ground.

A single Geoscan FM36 fluxgate gradiometer (plate 9) was used to carry out a

magnetometry survey employing 1m parallel traverses with 0.25m sample intervals. The

total area of the site surveyed is approximately 1.0ha (2.5 acres) comprising an irregular

area which were of particular interest to the Client. The distribution strategy and

emplacement of the survey grids across the field was discussed with the Client, but

follows general protocols found acceptable by English Heritage.

Plate 9. Magnetometer survey (Geoscan FM256 fluxgate gradiometer) being conducted by Rob

Shippey. This is a passive instrument which records changes in magnetic field across

archaeological features. Ditches and pits give a positive response whereas stone (but not brick)

gives a negative response.

Archaeological features such as brick walls, hearths, kilns and disturbed building material

will be represented in the results, as well as more ephemeral changes in soil, allowing the



location of foundation trenches, pits and ditches to be found. Results are however

dependent on the geology of the particular area, and whether the archaeological remains

are derived from the same materials. Around 1.0 hectares can be surveyed each day.

Data points taken per 20m grid totalled 1600 readings.

4.4 Resistivity Survey

Resistivity survey is based on the ability of sub-surface materials to conduct an electrical

current passed through them. All materials will allow the passing of an electrical current

through them to a greater or lesser extent. There are extreme cases of conductive and

non-conductive material, but differences in the structural and chemical make-up of soils

mean that there are varying degrees of resistance to an electrical current.

Plate 10. Geoscan RM15 resistance meter in use by students Sarah, Isabel and Laura, using a

dual probe configuration and 1.0m mobile probe separation.

The Geoscan Research RM15 Resistance Meter in twin electrode probe formation

represents the most popular configuration used in British archaeology (plate 10), usually

undertaken with a 0.5 or 1.0m separation between mobile probes. Details of survey

methodology are dealt with elsewhere but in this instance 1.0m probe intervals were

chosen, with 1m traverses and 0.5m sample intervals.



The technique relies on the passage of an electrical current from probes into the earth to

measure variations in resistance over a survey area. Resistance is measured in ohms (Ω),

whereas resistivity, the resistance in a given volume of earth, is measured in ohm-metres

(Ωm). Four probes are generally utilised for electrical profiling, two current and two

potential probes. Survey can be undertaken using a number of different probe arrays. The

twin probe array was used for this survey.

A number of factors may affect interpretation of twin probe survey results, including the

nature and depth of structures, soil type, terrain and localised climatic conditions.

Response to non-archaeological features may lead to misinterpretation of results, or the

masking of archaeological anomalies. A twin probe array of 0.5m will rarely recognise

features below a depth of 0.75m. More substantial features may register up to a depth of

1m, although the 1.0m array used in this survey may have increased that depth.

Although changes in the moisture content of the soil, as well as variations in temperature,

can affect the form of anomalies present in resistivity survey results, in general higher

resistance features are interpreted as structures which have a limited moisture content,

for example walls, mounds, voids, rubble filled pits, and paved or cobbled areas. Lower

resistance anomalies usually represent buried ditches, foundation trenches, pits and

gullies.

5.0 THE SURVEY RESULTS

5.1 Introduction

To simplify what is a complex report the maps, the survey figures and their derivative

anomaly plans are contained within Appendix 1, the figure numbers are prefixed (such as

fig. 1 etc.). Within the main body of the text, additional descriptive figures are included

where they will be useful to illustrate either equipment or specific items of text. The results

section endeavours to highlight as many anomalies as possible. This has been formulated

in such a way as to assist in the identification of archaeological features for future

excavation purposes. In many instances only the type, morphology and dimensions of the

feature are indicated, all of which can be georeferenced. The overall significance of the

results is brought together in the discussion section (section 6), where an overview of the

two survey methods is evaluated.

The known archaeological features are indicated superimposed upon an O.S. base map

(fig. 9). These include the remains (if any) of Spital Cottage I, which was seen on the 1860

O.S. map and demolished some time shortly afterwards (see plate 5). Elements of this

may now lie beneath the embankment of King Edward Road which has been widened and

straightened. Its replacement built in the 1860s originally as Spital Cottage (II) and more

latterly as Park Cottage was demolished in the 1960s although some residual boundary



walls remain (plate 6). Also shown are the foundations of St. Leonard’s Hospital and the

find spot of the stone coffins. At the time of survey there were few surface manifestations

of any foundations except a few exposed stones west of the display site of the coffins and

grave slab (plate 11).

Plate 11. Exposed stonework located west of the cross and coffins (right on photograph). The

quality of this masonry was not comparable to the surface corbel stone located near the coffins.

Also, the alignment of what was thought to be a sub-rectilinear arrangement was different to that

expected from the mapped foundations of St. Leonard’s.

5.2 The Magnetometer and Resistance Survey Figures

Magnetic Survey (Figures 10-16)

An overview of the magnetic survey (fig. 10) was superimposed upon the map of the

survey area. This has been complemented by a grey scale plot (fig. 11; 1:1000 scale) and

relief plots (fig. 12). This latter figure shows a shadow effect from four different angles and

can be invaluable in identifying some features, particularly linear and curvilinear features,

not immediately evident on the grey scale plot. The positive features, such as possible

ditches, are represented as hollows, whilst positive anomalies are highlighted. The third

type of data representation is the trace plot (fig. 13), which shows the magnitude of the

magnetic response (in nanoTeslas/cm). Positive features, such as ditches, are indicated

as raised ridges. In addition very strong dipolar response caused by burning, ferrous



material such as pipelines and brickwork give massive response, often beyond the upper

sensitivity of the instrument. In this case the tops of the peaks are flattened.

After analysis of the data, a derivative anomaly was produced (fig. 14) which, for

comparative purposes, showed the relationship between the grey scale plot and the

anomalies derived from all three data representation methods (grey scale, trace and relief)

at a scale of 1:1000. The enumerated anomaly plan (fig. 15) is shown at scale of 1:500.

Finally the plan is shown georeferenced to the O.S. base map (fig. 16).

It should be noted that the anomaly plan is not an interpretive plan in the sense that

features are characterised morphologically; it is probably best described as a schematic

representation of anomalies. Although this can be a succeeding derivative stage on some

sites, in general it is technically unsafe to do so at many others. A site like

Northumberland Park is not isolated and is therefore likely to bear the imprint of centuries

of land use. This will be represented in the geophysical record as a palimpsest of features,

some of which will be truncated, or even totally degraded. It is also not practicable to

indicate all anomalies so some will be grouped together as amalgamated areas.

Earth Resistance Survey (Figures 17-25)

The resistance survey results were represented in a similar manner to the magnetic

survey data, except that in most instances a reverse palette was used so that features

such as ditches (low resistance) appeared dark as in the magnetic survey. An overview of

the resistance survey (fig. 17) was superimposed upon the map of the survey area. In

addition a grey scale (fig. 18; 1:1000 scale) and relief (fig. 19), and trace plots (fig. 20)

were produced. The trace plot shows the magnitude of the response in ohms/metre. The

grey scale plots show low resistance features with black colouration, whilst high

resistance is indicated by grey.

A derivative anomaly was produced from the resistivity survey data (fig. 21) which, for

comparative purposes, showed the relationship between the grey scale plot (reverse

palette) and the anomalies derived from all three data representation methods (grey scale,

trace and relief) at a scale of 1:1000. The enumerated anomaly plan (fig. 15) is shown at

scale of 1:500. Finally the plan is shown georeferenced to the O.S. base map (fig. 16).

Low resistance features can indicate features such as wells, ditches and pits, but high soil

moisture conditions, often caused by biological or geological factors can significantly

influence the magnitude of the readings. Likewise, high resistance features can indicate

masonry, brickwork paving and roads, but also areas of naturally high resistance such as

rock outcrops, very dry conditions caused by well-drained surfaces or biological factors

such as trees which remove massive amounts of water from the soil. A large oak tree can

remove 40,000 gallons (151,000 litres) per year, and this effect can be relatively localised

(as a rule of thumb the tree canopy). Changes of slope also affect drainage and the net



soil moisture deficit, as well as proximity to water sources, can all affect resistance

readings. In these figures ridges or high points would almost always show as high

resistance values, regardless of the sub-surface composition. It follows that the

interpretation of results from non-planar sites is far from straightforward.

The two data set showing the magnetic and resistance anomaly overviews (figs. 16 and

23) are shown together for comparative purposes (fig. 24). The comparative data is

discussed within the results section where complementary data from each technique is

highlighted. The complex relationship between the two surveys is shown when the

resistance anomaly plan is superimposed directly over the magnetic plan (fig. 25)

The final figure (fig. 26) shows the suggested emplacement of excavation trenches over

those anomalies thought to be most promising. In doing so proximity, where possible, to

the extensive tree canopies (and associated roots) was avoided. A previous topographical

survey conducted by Landform Surveys Ltd. was used (not shown) to provide this

information. The final figure (27) was a digital map showing possible trench locations and

datum points

The excavation started before the completion of this report, and occasionally images of

aspects of the excavation are used to illustrate the nature of magnetic or resistance

responses. However, the excavation findings are not taken into account in the

assessment and analysis of the geophysical survey.

5.3 The Magnetometer Survey Results (Figures 10-16)

As a rule, positive linear anomalies (in magnetic survey) are generally ditches and circular

ones probably indicate pits. More intense positive anomalies may indicated burning, or

possibly fired clay (bricks, tiles kilns), but this group may at times have strongly dipolar

components. Negative anomalies indicate the absence of clay minerals, which is generally

taken to be masonry, cobbles, or some other non-clay (or soil) composition. Thus, the

term linear rectilinear anomaly would probably indicate building foundations. Naturally,

exceptions exist and are indicated. In should be noted that it is often the more subtle

anomalies which indicate older archaeological features.

The area of Spital Cottage I and buildings associated with it (see fig. 9) was to be

investigated, although it is understood that Trench 6 (see figs. 26 & 27) originally placed in

this area was moved a few metres to the south and east. Near the northern apex of the

survey a diffuse positive response (ditches or anaerobic conditions) is associated with

negative rectilinear anomalies which could indicate stone building foundations (figs. 15 &

16, anomaly m1). A few strongly dipolar anomalies were associated with these responses.

Within that same area a pair of large conjoined anomalies (fig.2), encompass an area

some 4m square. This large response (+164 to -194nTeslas) is entirely compatible with

burning and ferrous slag (see (plate 12), and is an effect often associated with older



buildings. Towards the east, a number of large positive anomalies, similar in morphology

to pits, are seen to be associated with the foundations (m2). The magnitude of the

responses (maximally 20-25nTeslas) suggests either very deep anaerobic pits or more

likely refuse pits containing slag or ceramic material.

Plate 12. This heavy iron rich slag was removed from trench 6 close to Spital Cottage I. It exhibited

a strongly magnetic response (54 nanoTeslas). This was only a small piece of the material; the

much larger complex would have produced a significantly greater response.

Closer to the park entrance is a massively magnetically disturbed area some 30m east-

west and 20m north-south (m3). This is close to the site of the former public toilet which

would probably have had ferrous pipe-work, reinforced concrete and brick associated with

it. Surface indications show cast iron covers and brick access hatches; all of which are

highly responsive and would tend to mask any archaeological features. The magnitude of

the response can be seen on the trace plot (see plate 13).

Plate 13. An example of a large ferrous pipe, probably a water main, is shown here in an arable

field. A truncated example is seen as part of the complex anomalous response near the park gates

(m3). Each section of steel pipe when welded together creates a bar magnet, hence to distinctive

‘zebra-crossing’ effect. This type of response swamps nearby lesser archaeological effects.



The area just north of the site of Spital Cottage II (c. 1860- 1960) also gave a very strong

response commensurate with burning, brick rubble, ferrous material, and iron pipes. The

excavation trench (Trench 2; 8m x 5m) was placed over a much ‘quieter’ area directly

south of this which showed linear negative and some stronger positive responses (see

plate 14). The positive responses (c. 20-35nTeslas) were much too intense to indicate a

trench, but did suggest elements of burning or ceramic material (bricks), possibly latterly

used as hardcore (m4).

Plate 14. This image of Trench 2 shows the eastern wall of the Spital Cottage II, the response from

which may have been swamped by non-specific interference. This showed as a very disturbed area

with the magnetic survey. The bricks used as hardcore and drains would have contributed to this

responses.

A cluster of highly responsive apparently discreet anomalies (m5) are spread over a area

of some 30m by 10m. However, these features when seen on the trace plot (fig. 13) may

indicate a linear feature some 30m in length, which is aligned almost east-west, extending

as far as the modern path. This is not typical of a modern ferrous pipe as seen in plate 13,

but may indicate fragmentary elements of an iron pipe or ceramic material, although the

spread of responses is quite wide for such linear structures. A small (5m) square

rectilinear positive anomaly may be present towards the western end of this complex.



Plate 15. The diagram shows the effect of a ditch or linear feature when it acts as a bar magnet. In

this instance (anomaly m6) the positive (black) response has a lesser return flux (white) giving the

impression of two features.

Near to the flanking northern footpath which delineated the edge of the survey was a

linear feature (see fig. 16) which ran parallel with it (m6). This feature was some 34m in

length and produced a ditch-like response, with a wall type response immediately

adjacent to it. In reality it is probably a ceramic drainage pipe, probably associated with

anaerobic (wet) conditions. The positive element of the response reaches +50nTeslas,

whilst the negative response is as high as -19nTeslas. Although this effect occurs all over

the site it is probably worthwhile explaining why this happens in the next paragraph. It is

not certain whether or not this feature continues across the path to connect with a number

of parallel linear anomalies (m7) which may indicate the location of drainage channels.

The age of these features is unknown.

The response discussed previously is shown in plate 15A & B, (c. 55°) where the feature

behaves as a bar magnet which gives a dipolar response (plate 16). The magnetic circuit’

is completed by the return flux (plate 15C), which is of opposite polarity to the magnet.

Every positive magnetic anomaly has a lesser negative anomaly beside it. Because of the

angle of dip, more of the return flux is exposed on the north side. Another effect of the

angle of dip is to displace the anomaly slightly to the south (see plate 16). This effect

disappears at the magnetic North Pole and is maximal at the Equator (Clark 1990, 82-3).



Plate 16. Diagrammatic

representation of the

magnetic field of a themo-

remanently (permanent

magnetism after firing)

magnetized ceramic pipe.

Note the angle of dip

(inclination), which will cause

the maximum response of the

anomaly to be displaced

towards the south. The

parallel lines represent the

Earth’s field whilst the curved

red lines force induced in the

field of the ceramic pipe are

curved (after Clark 1990, fig.

50).

The linear feature (m6 & m7) appears to cross a positive circular feature some 8m in

diameter (m8). It is tempting to ascribe a prehistoric origin to the response such an Iron

Age hut circle (700BC – 200AD); however there are no other corroborating features such

as a defensive rectilinear ditch. A linear positive anomaly (m9) runs east west for some

20m before changing direction and heading downslope, for a similar distance, north-east

towards the responsive linear anomaly (m6). The maximal response of this anomaly (c.

11nTeslas), may indicate a drainage ditch or channel, which is apparently less well

preserved or responsive after the turn north. Whether this complex of positive linear

anomalies (m6, m7 & m9) is linked is uncertain. They may however derive from a

perceptible rectilinear feature (m10), which appears to be some 16m long and 4m wide

(plate 17). It appears to be a series of interlinked pits or possible post-holes, perhaps

indicating a wooden building, with perhaps a hearth towards the northern end. In reality

the feature lies over the remains of a curvilinear footpath of which only part remains, so it

may just be a fortuitous alignment of (partially?) non-archaeological anomalies. This

anomaly will be discussed later in the context of the site of the remains of St. Leonard’s

hospital.



Plate 17. Schematic representation of anomaly m10 showing an apparent structure some 16m

long and 4m wide. This feature does however, in part, overlie the remains of vestigial footpath.

Towards the extreme western edge of the survey, few distinctive anomalies were

identified, although the faint sub-rectilinear positive anomalies (possible stone) were

detected (m12). These responses, although faint, may have potential for future

investigations as they are aligned on the known configuration of St. Leonard’s

foundations. The ground level in that area is higher than that towards the east and may

not have been landscaped.

Plate 18. The excavation viewed from the north (trench 1) across anomaly m12 revealed a paved

area at no great depth, flanked by boulders and apparently forming part of shallow foundations.

Some of these anomalies remain buried beneath the cross base and surface boulders.



Near to the display site of the stone coffins and grave slab were found some intense

magnetic anomalies which were rectilinear in shape (m12). The dimensions were some

9m east-west and 7.5m north-south with maximal responses of +75 to -62nTeslas. That

sort or response would tend to indicate burning (i.e. hearth), ferrous material or ironstone

impregnated material. Boulders were present as a surface collection, some of which may

have been glacial igneous erratics which also give a magnetic signature (plate 11). The

material uncovered (plate 18) shows a paved area which may have been disrupted by tree

roots, but the surface manifestations (see plate 11) appear to be all that remains of the

foundations. Some anomalies remain buried beneath the cross base and surface

boulders, although magnetic scanning of these rocks may be useful to determine their

contribution to the overall response.

South-east of this complex and beneath the coffins and grave slab was a positive

rectilinear anomaly (m13) which had a strongly positive ovoid feature in the centre

(18nTeslas). This diffuse anomaly, which is aligned south-east and may have some

residual stone components, has dimensions of 5m by 2.5m, which, if it were the

foundations of a building, would perhaps indicate a small, possibly wooden structure,

within a trench. This feature is not set along the accepted alignment seen on O.S. maps

for the foundations of the hospital.

Plates 19 & 20. Two views of the excavation near the approximate location of Trenches 4 and 5.

The foundations are both unsophisticated, close to the surface and fragmentary. This would explain

the response seen from anomaly m14, which is slight.

Continuing east some very indistinct anomalies (m14) suggest the faint outline of building

foundations as seen on the O.S. plan, but indicate an insubstantial response. Little in the

way of stonework in the form of negative anomalies appears to be visible. These

anomalies stretch for some 25m east-west and 8m north-south. They are interrupted at

the eastern end by a large dipolar anomaly (probably modern ferrous interference) which

overlies the remnants of a tarmac path. Further indistinct anomalies continue towards the

east for 20m (m15). Both these features are set upon the perceived alignment of St.

Leonard’s foundations but exhibit very little by way of negative anomalies; a general



indication of stone foundations. Trenches T4 and T5 (see plates 19 & 20) show how

insubstantial the remains are, with concomitant loss of response.

Plates 21. Human femur (left) and skull with associated finger bones (right). This formed a group of

burials. No pit is evident and such remains would be almost invisible to geophysical survey.

Running parallel with the modern tarmac footpath for much of its length), but in all for

some 80m, is a linear positive feature (m16). This feature is located some 3m south of the

footpath and appear to mirror its course. It seems to start on the level ground at one of the

highest points on the site at the location of a number of faint positive and dipolar

anomalies (m17). The eastern terminus of this linear feature (m16) lies towards the

bottom of the slope; possibly linking with what might be a drainage ditch or channel (m 7).

A number of magnetically active, but discrete areas were detected towards the southern

edge of the survey (m18). The aetiology is unknown, although they may indicate pits, but

are too large to indicate individual inhumations. This is the area where the coffins were

found in the 1870s and further human remains were found in this location, significantly at

no great depth below the surface3. Those seen would have been almost indistinguishable,

as a magnetic response, from the surrounding soil matrix.

3 Examination of the medieval cemetery at Barton-on-Humber suggest the modern practice of

burying bodies at depths of 6 feet did not occur in medieval or immediate post-medieval periods;

much shallower depths were used, nor were the graves spaced regularly in grids with headstone

markers (Rodwell and Rodwell 1982, 2843-315).



Two curvilinear positive anomalies (m19) may indicate the presence of ditches, although a

geological effect cannot be excluded. These are located towards the edge of an

embankment where differential drainage can occur. This may affect the REDOX potential

of the soil thereby affecting the anaerobic activity and resulting in changes in magnetism.

This effect is discussed in more detail in appendix 2.4 The final group of annotated

magnetic anomalies (m20) present as a group of amorphous ovoid and circular positive

anomalies, which show no distinguishing morphological characteristics.

Plate 22. Small drainage channel which, although it did not have any associated material to

confirm its origin, is probably medieval. This feature was identified by the survey, just south of

anomaly m14.

Many other anomalies are present on site such as faint linear anomalies which may

indicate small drainage ditches or channels (plate 22) and small pits. Some of these

features may be of archaeological origin and significance, but are too numerous to identify

individually.

4 Reduction-oxidation potential (REDOX), is a measure of the tendency of a chemical species to

acquire electrons and thereby be reduced. Reduction potential is measured in volts (V), or millivolts

(mV). In geological terms this is usually a seasonal effect caused by wetting and drying of soils. In

winter anaerobic (low oxygen) levels are common, causing fermentation reactions which enhance

magnetic susceptibility; hence the high positive readings of some waterlogged ditches and pits.



5.4 The Earth Resistance Survey Results (Figures 17- 24)

The earth resistance survey was conducted over essentially the same area as the

magnetic survey. The anomalies in this instance are prefixed by (r), although only in the

text, not the figures. Depending as it does upon soil moisture and the electrolytes within

the soil, this methodology gives no response at all (open circuit) to features such as paths

or exposed rock. In this section it is intended to follow the same ‘pathway of anomalies’ as

the magnetic survey, but to also illustrate where the methods produce complementary

data.

In the extreme north of the survey, near the location of Spital Cottage I and associated

buildings, two large high resistance anomalies (r1) were detected (figs. 22 & 23). Both had

very strong positive responses, maximally +800ohms and a rectilinear outline suggesting

some sub-surface features (plate 23). It should be noted that there were surface

exposures of rocks and decorative flower beds in this area. The diffuse low resistance

areas were seen on the grey scale plots (see fig. 17, black on plot), but for clarity these

have been omitted from the anomaly plans. These values were in the region of -210ohms

and indicate where some natural drainage channels may be present. It was from the

trench in this area that the iron-rich slag was removed (plate 12).

Plates 23. Trench 6 (relocated slightly south-east) shows significant stonework. It was from this

trench that the slag was removed and would have produced the exceptionally high magnetic

readings. This band of slag (dark feature) would have had different resistance properties to stone.

Other factors would have contributed to the high resistance reading such as near surface stone for

the rock feature.



A curvilinear medium-high resistance feature (r2) is located south of the Spital Cottage I

area. With the eye of faith this feature can be extrapolated further west to join a high

resistance curvilinear feature (r3). Neither presents a convincing case for the remains of a

curvilinear precinct wall, but their presence should be noted. The north-western sector of

the survey covered that area close to the demolished toilet block, iron pipes, inspection

covers etc. Not surprisingly some very high resistance values were detected (r4) in this

area (see also fig. 16, anomalies m3 & m4). Most of these, however, probably relate to

relatively modern interference. By contrast, the highly magnetically active area (m4) in the

vicinity of Spital Cottage II produced a very subdued resistance response (r5) which

probably corresponded to the wall foundations seen in plate 14.

Two narrow curvilinear linear features were detected (r6) which link with the linear positive

magnetic anomaly (m9). Taken together these features indicate interlinked drainage

channels or ditches (see comparative anomaly plans, fig 24). This relationship however

can be seen more readily when magnetic anomalies are superimposed upon the

resistance anomalies, (fig. 25), albeit providing a very complex overview. A second low

resistance linear anomaly (r7) may form part of the same system. Interestingly, two

drainage channels, if that is what they are, emanate from the putative building detected in

the magnetic survey (m10 and see plate 17). Directly east, and possibly adjacent to the

‘building’ (m10) is a sub-rectilinear high resistance anomaly (r8), with some evidence that

a fainter feature continues south across the modern path.

At the eastern edge of the survey a number of indeterminate high resistance anomalies

were detected (r9). Some linear characteristics were resolved, although a linear low

resistance feature (small ditch?) traversed the circular anomaly detected in the magnetic

survey (m9). There is insufficient evidence to conclusively suggest foundations.

At the extreme western edge of the survey area a number of high resistance anomalies

were detected (r10). Some of these anomalies had linear characteristics but were not

placed along the perceived alignment of St. Leonard’s foundations. Nevertheless, as

stated earlier, this is an area of ground which appears not to have been landscaped (or

levelled) and does have a number of non-specific magnetic anomalies including some

narrow ‘drainage channels’ (m11).

The area near the stone coffins proved to be a complex area (r11), in which small areas of

dummy readings (surface stonework) were taken. It is no surprise that the raised ridge

next to the relict footpath gave a high resistance reading (+205 ohms; good drainage and

dry). Subsequent excavation did not reveal any stonework in that location. A few metres

south-east of that ridge another area of moderately high resistance (+50 to +89 ohms)

indicated the location of the flagstones (see plate 18). Given the good state of

preservation this was a rather low resistance value; a point which should be noted for

future surveys. A T-shaped narrow linear low resistance anomaly is associated with these

anomalies, although this may extend towards the north-east as a sub-rectilinear feature.



The significance is unknown, but drainage, natural or anthropogenic is a possible

explanation.

Plate 24. Two photographs of the exposed cobbles and possible foundations in the area of the

western high resistance anomaly. A high resistance response is entirely compatible to such a

configuration, and has been seen on other sites where similar features have been exposed.

Moving directly east from the coffin area two high resistance areas were detected

associated with narrow low resistance linear anomalies (r12). Given that these anomalies

were located along the line of the foundations exposed in the Victorian period, a possible

area of flooring or path was suspected (plate 24). In one instance this assessment proved

to be correct, whilst at another sondage the exposition revealed a very dry compact clay

surface with no significant archaeological features. In this latter case the proximity

(transpiration and evaporation) of a large tree may have caused this effect.

North-east of the anomaly r12 are a group of small, indistinct high resistance rectilinear

features (r13). These are small in size with what appears to be a flanking ditch to their

south, with dimensions ranging from 2-4m. Their presence should be noted, as a ditch or

channel (m17), detected with the magnetic survey, appears to run past them. When these

features are combined (see plate 25) the resistance anomalies may take on a new

significance; two at least appear to have positive magnetic interiors, which is often the

response associated with a stone building. It is probably now appropriate to illustrate this

relationship on a site which has been well preserved. One such case, illustrated in the

Discussion section, is the Roman fort and settlement at Maryport, Cumbria, where

extensive survey was conducted between 2000-2010.



Plates 25. Comparison of the magnetic and resistance responses across anomaly r12. As

separate responses they look unremarkable but, when combined, the rectilinear high resistance

responses appear to have a central positive rectilinear magnetic response; often the signature of

stone buildings. The ‘ditches’ or ‘channels’ also take on a new dimension as they appear to be

linked. That is not to say that they are necessarily significant archaeological anomalies, but their

presence must be noted.

Next to the southern footpath a number of linear low resistance anomalies were found

beside a wide high resistance anomaly (r14). The latter feature may be caused by a

simple geological effect; drying of the ground surface near a well-drained embankment.

This effect has been described earlier and is a common occurrence on other sites. That is

not to say that there are never archaeological features such as walls beneath earthworks,

there often are, but it is not likely in this case. A number of more discrete high resistance

anomalies were located just few metres north of this larger feature, possibly associated

with a curvilinear low resistance anomaly some 20m long.

Moving further east, the most notable feature appears to be a large rectangular low

resistance area some 6m by 10m (r15) which appears to have a number of ‘feeder’ low

resistance anomalies. From there a ‘channel’ may link with the sinuous low resistance

feature north of the public footpath (r6). It would be tempting to ascribe an archaeological

function to this feature, although it may only be a low spot on the plateau which is part of

the natural drainage system. That said, in the composite anomaly plan (fig. 25), the two

linear positive magnetic anomalies (m16 & m17), tentatively described as drainage

channels, also connect with this feature creating what might be a complex drainage

feature. A number of sub-rectilinear low resistance anomalies surround this feature, but

their significance is unknown.



A number of diffuse high resistance anomalies (r16), including a broad curvilinear band,

are probably caused by geological factors. The curvilinear band is located on the edge of

a steep bank leading down to the footpath some 3-4m below. The northern side is

similarly placed with a steep bank leading down to a footpath; again this would cause a

net soil moisture deficit (plate 26). It should be noted that the ‘drainage’ feature was also

seen with the magnetic survey (m16).

Plates 26. Trench 3 viewed from the north (left) and from the south (right). Very little has been

revealed which could be described as masonry, paving, cobbles or indeed anything relating to solid

geology. The ridge on which these anomalies were located and the presence of trees may have

dried the soil differentially.

Moving to the extreme east of the survey where two footpaths join, the land is raised

significantly above the surface of the path, suggesting a degree of landscaping. Certainly,

large, imported stone boulders have been use as an edge to the southern path to create a

sheltered dell, presumably to support a fernery. Regular blocks of stone have been

incorporated and appear to be ashlar masonry, some of which have lewis holes5. Although

there are some interesting high resistance features (r17) in this area, they may be a result

of ornamental stone work, differential dry conditions and drainage. Although this is the

final resistance anomaly to be discussed there are many slighter, less obvious features

which may well have archaeological potential. The implications of the survey are

5 A lewis is one of a category of lifting devices used by stonemasons to lift large stones into place

with a crane, chain block, or winch. It is inserted into a specially prepared hole, or seating, in the

top or side) of a stone, directly above its centre of mass. It works by applying principles of the lever

and uses the weight of the stone to act on the long lever-arms which in turn results in a very high

reaction force and friction where the short lever-arms make contact with the stone inside the hole

and thereby prevents slipping. The three-legged lewis or St Peter's were used in Roman and

medieval times. An alternative method working on a similar principle is to use two countersunk

holes and a curb lifter.



illustrated in the next section (Discussion) where the broader implications of the

investigation are examined.

6. DISCUSSION

At the outset of the field reconnaissance before the survey was conducted, it was

recognised that some landscaping had occurred. The major land clearance, and it must

have been substantial in places, would have occurred during the construction of the Park

in the late 1800s. This would have meant the creation of paths and the grotto-like walkway

towards the south of the survey area; all of these would have resulted in significant

removal of the land surface. It is not known what landscaping occurred on the plateau

where the hospital was located, but the surrounding flower beds all appear to be raised.

The modern interference in the park comprised the remains of iron benches, toilet blocks,

manhole covers, paths, demolished buildings, iron service pipes and brick inspection

chambers, together with physical obstructions such as trees, hedges and vegetation.

Those were just the visible objects. The surveyors (some 31 individuals), entirely

comprised of volunteers, most with little experience of the methodology, produced a

technically sound survey in difficult conditions. At first sight the results looked

disappointing, particularly over those areas where the foundations of St. Leonard’s

Hospital were thought to lie. That substantial remains were not present was evident

almost as soon as the data was downloaded.

It should be noted that previously excavated foundations and buildings do not provide a

spectacular contrast between foundations and their surroundings. This is particularly

evident with magnetic survey. The reason for this that when buildings have been

excavated, the interior material which generally exhibits a relatively strong magnetic

response, which may have developed anthropogenically (human interference) over

centuries or millennia, is returned in a mixed random manner from the spoil heap. Thus,

the magnetic contrast is lost and the negativity of stonework (caused by the lack of clay

minerals) does not stand out so clearly against a neutral background.

At this stage it might be useful to give an example of a well-preserved site where

complementary magnetic and resistance survey was conducted. Such a site is the Roman

fort and civil settlement at Maryport. Although the surface stonework has been robbed, the

survey indicated the well-preserved remains of large buildings (Biggins, J. A. and Taylor,

D. J. A., 2004b; Biggins et al 2011). This preservation has perhaps been an accident of

geography with the land being kept under pasture for many years. This in itself preserves

the features, whereas sustained deep ploughing will rapidly degrade such a site.



Plate 27. Magnetic and resistance grey scale plot showing a comparison of the magnetic and

resistance responses on a Roman archaeological site. This has been well-preserved under pasture

for many decades as a scheduled ancient monument. The composite grey scale plot shows the

magnetic survey superimposed over the resistance plot, emphasising the complementary nature of

the two techniques (Biggins et al 2011).

The magnetic survey, part of which is shown here (plate 27), illustrates a row of well

preserved buildings lining each side of a street leading from the fort, which is located

towards the south-west. The buildings are contained within their own burgage plots

defined by a ditch, with some having their own wells in these gardens. At least two phases

of construction are evident from the superseded field and ditch system. The resistance

survey proved to be complementary; whilst it did show buildings, the main feature was the

detection of a distinctive main road surface together with several minor roads branching

from it. A stone enclosure is thought to exist and is highlighted in the resistance survey,

with two low resistance features located within which are thought to be water storage

tanks. Excavation of this area in 1880 revealed two temples, since reburied, but detected

by the magnetic survey within the enclosure together with a an encircling ditch.

This brief snapshot of the individual methodologies shows how invaluable an integrated

approach to survey can be. For example, the Maryport resistance survey shows the main

road and the magnetic survey implies its presence by showing the bordering ditches.

What this does demonstrate is that the road is not as wide as the ditches, suggesting a

flanking footpath on each side. Given the urban setting of St. Leonard’, we were never

going to emulate such results; stone robbing here has been conducted on an industrial

scale. That and a Victorian penchant for ‘improvement’ militated against survival to any

meaningful degree.



When these two methods are superimposed, as we have done for the Northumberland

Park survey, the results still provide valuable insights in to the remaining archaeological

features. The survey report was not completed when the excavation was conducted,

although the results did suggest where the excavation trenches should be placed. These

were, by and large, positioned according those seen in figs. 26 and 27, with pragmatic

modifications made to avoid tree roots etc. This report was able to illustrate the

geophysical survey analysis with real-time observation of the excavation trenches.

Photographs of some of these trenches have been shown not to demonstrate the

significance of the archaeological features found per se, but to show how the survey

response was influenced by sub-surface features. It is not very often that this procedure is

conducted. Very often a geophysicist has no idea if the theoretical assessment of the

anomalies has any basis in fact.

The temptation to claim total prescience has been avoided, although most assessments of

the anomalies were decided before the excavation commenced. It should be stressed that

geophysical survey always works, but not in the way that one would always want (i.e. the

revelation of fantastic archaeological features), but all results are explainable scientifically.

The comparative analysis of the two methods has enabled some of the less obvious

relationships to be established such as the ‘interlinked drainage system’. This may be

entirely natural, caused by human intervention, or more likely a combination of both.

Some of these more subtle features, and they are subtle compared to the massive

responses of iron pipes, massive depositions of industrial slag and modern building

foundations, have been highlighted in this report. If there is to be an additional phase of

excavation, this geophysical survey has at least provided a template upon which to site

those trenches.

An important aspect of the project was the opportunity to observe the excavation, which

provides coincident data, photographs of which have been included in the geophysical

survey report. For instance, it was obvious that the Victorians had created a footpath

which encompassed the fabric of the foundations (fig. 5). Uncertainty however, existed

over precisely what was used in its construction. The excavation was able to show that

the A-horizon of the soil was comprised in part of cindery material. This would have

affected drainage and ultimately the geophysical response; this subject is discussed in

more detail in Appendix 2. Information like this provides a database of responses for

future survey assisting with the analytical procedure. Likewise the high resistance areas

near to a steep slope (less surprisingly) were shown to be physical and geological

responses, not archaeological features.



7. CONCLUSIONS

This investigation was funded by a Heritage Lottery Grant administered and implemented

by North Tyneside Council, and was always intended as a community led project

supervised by professional archaeologists. The geophysical survey was the second stage

in a process to evaluate the medieval hospital of St. Leonard’s. The initial stage

comprised a desktop assessment, followed by the geophysical survey, then the

excavation phase. Necessarily, the evaluations were sequential with little time between

phases. This was no bad thing for the volunteers as it allowed continuity between these

phases.

The survey objectives were twofold, namely to teach the basics of geophysical survey

fieldwork and basic interpretation and secondly to acquire survey data which would

indicate the location and degree of preservation of St. Leonard’s hospital and suggest

appropriate areas to excavate. In that respect, the survey achieved its objectives, even

though it became obvious that the degree of preservation was not as good as had been

hoped. Nevertheless, some meaningful data was collected and the more subtle and less

obvious anomalies may indicate where archaeological features have yet to be

investigated.

8. PROJECT ARCHIVE

8.1 Four appropriately bound hard (paper) copies of the complete document archive

together with copy disks of any electromagnetically stored or processed data will

be deposited with the Client. The receiving archive is Tyne and Wear County

Archaeologist.

8.2 As part of the OASIS project (Online Access to Index of Archaeological

Investigations) Tyne and Wear Archaeological Services would prefer that

information from developer-led projects is assimilated into a national online

database (http://ads.ahds.ac.uk/project/oasis). TimeScape and will comply with

this requirement after written confirmation is received from the Client. Upon

occasion there are very good reasons why information should not be available,

not least data covered by the Copyright Designs and Patent Act, 1988, or if

Client confidentiality is to be maintained. In this instance, the Client must give

specific permission to make the data public.

9. PUBLICITY, CONFIDENTIALITY AND COPYRIGHT

9.1 Any publicity will be handled by the Client (North Tyneside Council).



9.2 TimeScape will retain copyright of all documentary and photographic material

under the Copyright Designs and Patent Act, 1988, although they will grant an

exclusive licence to reproduce the data and images to the Client.

9.3 The maps are reproduced under TimeScape Ordnance Survey licence number

100030361, except the digital map (fig. 8) which is produced under North

Tyneside Council O.S licence number 100016801. Landform Surveys Ltd

(Newcastle) provided elements of additional survey, such as datum points and

tree canopy cover. Crown copyright is reserved.

10. STATEMENT OF INDEMNITY

All statements and opinions presented in this report arising from the programme of

investigation are offered in good faith and compiled according to professional standards.

No responsibility can be accepted by the authors of the report for any errors of fact or

opinion resulting from data supplied by any third party, or for loss or other consequence

arising from decisions or actions made upon the basis of facts or opinions expressed in

any such report(s), howsoever such facts and opinions may have been derived.

11. ACKNOWLEDGEMENTS

The authors are grateful for the funding offered by North Tyneside Council and their co-

operation in facilitating the clearing of vegetation and park furniture; this help was

invaluable. The co-operation of colleagues Jennifer Morrison and David Heslop from Tyne

and Wear Historic Environment Section was appreciated, including permission to use

material from the project design. Discussion with Richard Carlton and Alan Rushworth of

The Archaeological Practice Ltd. over aspects of the desktop assessment and their

generous co-operation during the excavation was invaluable. Thanks are extended to Mr

Eric Hinds of Landform Surveys Ltd for provision of their digital data of the landscape

survey from Northumberland Park.

Finally, without the splendid efforts of the volunteers the survey could not have taken

place; this is their survey and their report.

BIBLIOGRAPHY

Adamson, H. A., 1889, The Hospital of St. Leonard, Proceedings of the Society of Antiquaries

of Newcastle, Series 2, III, 35-6 (note).

Aitkin 1974, Physics and Archaeology, 2nd Edition. Clarendon Press, Oxford. 24-6,

Biggins, A., Robinson, J., Blau, S., and Denham, T., 1999, Report on Geophysical Survey

Muweilah, Sharjah, U.A.E. 1998, TimeScape Archaeological Surveys, England & The

Australian National University Canberra, Australia. TimeScape Unpublished Report.



Biggins, J. A., Robinson, J., Taylor, D. J. A., 1999, ‘Report on Geophysical Survey of the Fort

and Vicus at Carvoran Roman Fort, Northumberland’, Unpublished report for English Heritage.

Biggins, J. A, and Taylor, D. J. A., 2001a, ‘Report on Recent geophysical Survey at Carvoran

Roman Fort, Northumberland’ Britannia. 32, 330-2.

Biggins, J. A. and Taylor, D. J. A., 2004b, ‘The Roman Fort and vicus at Maryport: geophysical

survey, 2000 – 2004’, in R. J. A. Wilson and I, Caruana (eds.), Romans on the Solway,

CWAAS for the Trustees of the Senhouse Museum, Maryport, 102-133.

Biggins, J. A. and Taylor, D. J. A., 2004c, ‘A Geophysical Survey of Housesteads Roman Fort,

April 2003’, Archaeologia Aeliana, 33 51- 60.

Biggins, J. A., Strutt, K. D. and David J. A. Taylor, 2011, A Geophysical Survey of the

Extramural Settlement at Maryport – 2010, Hadrian’s Wall Archaeology, 2, 27-39.

Blau, S., Robinson, J., Denham, T. and Biggins J. A., 2000, 'Seeing through the Dunes:

Geophysical Investigations at Muweilah, United Arab Emirates', J. Field Arch. 27, No. 2, 117-

129.

Clark, A. J. C., 1990. ‘Seeing Beneath the Soil.’ Batsford, London.

Clay, M.R. 1909, The Medieval Hospitals of England, The Antiquaries Books.

Craster. H. E., 1907, History of Northumberland, vol. VIII. 259-260.

David, A., 1995, ‘Geophysical survey in archaeological field evaluation’. Ancient Monuments

Laboratory, English Heritage Society.

Knowles, D. and Hadcock, R.N., 1953, Medieval Religious Houses : England and Wales.

London. Longmans Green and Co.

Johnson G.A.L. (ed.), 1995, Robson's Geology of North East England, 2nd ed., Natural History

Society of Northumbria.

Jones, J. M., 1967, The geology of the coast section from Tynemouth to Seaton Sluice, Trans.

Nat. Hist. Soc. Northumb. Special Publication 79-86.

Landform Surveys Ltd, 2010, Northumberland Park: Topographical Survey, Report No.

LS6332.

Morrison, J., 2010. Northumberland Park, King Edward Road, Tynemouth: Project Design For

Archaeological Work, Tyne and Wear Archaeology Officer. (Unpublished).

Natural England, 2011, Tyne and Wear Geology,

http://www.naturalengland.org.uk/ourwork/conservation/geodiversity/englands/counties/area_I

D36.aspx

Rodwell, W. and Rodwell, K., 1982, St. Peter’s Church, Barton-upon-Humber, Antiquaries

Journal, 62, 283-315.

Taylor, B.J., Burgess, I.C., Land, D.H., Mills, D.A.C., Smith, D.B. & Warren, P.T., 1971, British

Regional Geology: Northern England, 4th Edition, NERC, HMSO.



APPENDIX 1

LIST OF FIGURES 39-65

1. O.S Location Map 1:50,000 39

2. Wood’s Plan of 1826 40

3. 1856 O.S. Map, Showing Spital Cottage I 41

4. 1861 O.S. Map; 1:2500 Scale 42

5. 1896 O.S. Map 1:500 Scale, Showing St. Leonard’s Foundations 43

6. 1898 O.S. Map Second Edition, Showing St. Leonard’s Foundations 44

7. Modern Map Showing Plan of Northumberland Park 45

8. Digital Map Showing 20m Survey Grid 46

9. Map Showing Archaeological Features 47

Magnetic Survey Figures

10. Magnetic Survey: Grey Scale Plot Overview; Scale 1:1000 48

11. Magnetic Survey: Grey Scale Plot; Scale 1:1000 49

12. Magnetic Survey: Relief Plots; Scale 1:2000 50

13. Magnetic Survey: Trace Plot; Scale 1:1000 51

14. Magnetic Anomaly Plan & Grey Scale Plot; Scale 1:1000 52

15. Magnetic Anomaly Plan - Annotated; Scale 1:500 53

16. Magnetic Anomaly Plan - Annotated Overview; Scale 1:1000 54

Resistance Survey Figures

17. Resistance Survey: Grey Scale Plot Overview; Scale 1:1000 55

18. Resistance Survey: Grey Scale Plot; Scale 1:1000 56

19. Resistance Survey: Relief Plots; Scale 1:2000 57

20. Resistance Survey: Trace Plot; Scale 1:1000 58

21. Resistance Anomaly Plan & Grey Scale Plot; Scale 1:1000 59

22. Resistance Anomaly Plan - Annotated; Scale 1:500 60

23. Resistance Anomaly Plan - Annotated Overview; Scale 1:1000 61

Survey Overviews

24. Magnetic & Resistance Anomaly Plans – Comparative 62

25. Magnetic & Resistance Anomaly Plans – Superimposed 63

26. Magnetic & Resistance Grey Scale Plot – Suggested Trench Locations 64

27. Suggested Trench Locations; Digital Map 65



APPENDIX 2

Geophysical Survey Theory

Introduction

The use of geophysical survey (Remote Sensing) techniques has now become well

established as a methodology with which to effectively evaluate large areas. The major

methodologies are magnetic and resistance surveys and these two techniques have been

used extensively by the authors of this report. Each technique is mutually supportive and

can provide essential corroborative evidence (e.g. Housesteads; Biggins & Taylor 2004c).

More recently ground penetrating radar (GPR) has been used to define much larger

survey areas than has been usual in the past, such as the latest Maryport survey

conducted in 2010 when some 7ha of survey was conducted (Biggins et al 2011). The

theoretical aspects are considered here in outline, although what the majority of

archaeologists need to know to use the equipment effectively are the capabilities and

limitations of the two main methods, rather than a complex geophysical theory. The

complexity of relationships between archaeological structures can be highlighted when

magnetometer data is overlaid by resistivity results, and this will be discussed later in the

chapter.

Magnetic Susceptibility

In order to understand the basic principles of magnetic prospection it is first necessary to

discuss the basis of the effects of magnetic susceptibility. All materials, at an atomic level,

have magnetic properties, which are manifest when they are placed within a magnetic

field. That material acquires an induced magnetisation (Ji).

Ji =κκκκH

Where ; κκκκ = volume magnetic susceptibility (specific in strength to that material),

H = applied magnetic field (e.g. the Earth’s)

In addition to induced magnetisation, a buried feature may also possess remanent

magnetisation, (Jr). Thus, the total resultant magnetisation will produce a sub-surface

anomaly in that applied field detectable, in varying strengths, at ground level.

J Total = Jr + Ji

The production of magnetic anomalies relies on a contrast between the magnetic

susceptibility of the target feature and the surrounding subsoil. Many complex pedogenic

(soil formation) and anthropogenic (human) mechanisms may lead to the enrichment or

depletion of magnetic susceptibility and may be summarised in terms of common soil iron

oxides:



Fermentation (REDOX reactions)

Soils subject to a seasonal cycle of reducing (winter) followed by oxidising (summer)

conditions (REDOX reactions) demonstrate an enhanced topsoil magnetic susceptibility.

This process may well be (micro)-biologically moderated.

Bacterial Magnetism

Abundant soil bacteria may lead to the enhancement of magnetic susceptibility through a

number of mechanisms:

Altering the pH/Eh of the soil environment

Anaerobic Respiration utilising Fe2+ as an electron acceptor.

Presence of highly ordered magnetosome chains of magnetite (or greigite) crystals

may be a source (possibly used for navigation purposes, e.g. birds).

Note; pH is the negative logarithm of the effective hydrogen ion concentration used to

express both acidity and alkalinity. Eh is a measure of the oxidation or reduction potential

of the system. Low Eh values correspond to reducing conditions and high Eh values to

oxidizing conditions.

Enhancement through Fire

Both natural and deliberate burning leads to the considerable magnetic enhancement of

the immediate topsoil. This converts magnetite and haematite to the much more magnetic

ferrous oxide - maghaemite. This is one of the reasons that ditches for instance, are

generally much more magnetically responsive near settlements.

Site formation processes

Following habitation of a site, various anthropogenic processes including burning, and

creation of cut features accumulates and enhances the susceptibility topsoil and semi-

industrial activities (e.g. kilns) may occur. This results in the presence of local

concentrations of magnetic material that will generally produce detectable magnetic

anomalies. The resultant anomalies may be strengthened further through the

development of:

Thermo (burnt features)

Detrital (waterborne sediments)

Chemical Remanence (in situ formation of magnetic minerals).



In more general terms, the magnetic susceptibility of a soil is determined by the quantity of

ferrimagnetic minerals, usually magnetite or maghaemite. The earth’s crust contains 6%

iron in various mineral forms; ferro (Fe2+, II) or ferri-magnetic (Fe3+, III) particles exist in

two oxidation states (Co and Ni are also magnetic), but iron retains its magnetism after

being in a field and the ore is also easily magnetised. As early as 1957 Aitkin predicted

that a kiln should theoretically show magnetic anomalies (Aitkin 1974, 24-6). When the

Curie Point is reached a preferential alignment is retained on cooling. There is a

phenomenon of depositional remanent magnetism when particles are very slowly

precipitated in a lake (or ditch). Thermoremanent magnetism is associated with in situ

structures and these magnetic properties are:

a) Diamagnetic – which describes material which when placed in a magnetic field

exhibits weak repulsion of that magnetic field. This is a general, variable weak

phenomenon for all materials.

b) Paramagnetic - The direction of the field is the same as the direction of the

applied field.

This is the capacity to be magnetised; some compounds have higher capabilities than do

others. In the soil, one of the commonest substances is haematite (Fe2O3). Fires may

convert haematite into magnetite (Fe3O4 – the lodestone of old), another oxide of iron.

Upon oxidation, this is converted to maghaemite (Fe2O3) a much more magnetic

compound, which is metastable, and, when heated, transforms back into haematite. Both

natural and deliberate burning leads to the considerable enhancement of the immediate

topsoil or clay minerals, which converts magnetite and haematite to the much more

magnetic ferrous oxide, maghaemite. This enhancement occurs when the Curie point

675ºC for haematite and 565ºC for magnetite is exceeded and the oxides are

demagnetised. Upon cooling, the molecules are re-magnetised en masse and assume a

permanent magnetisation aligned to the Earth’s field at that time. The earth’s field and

magnetic force can be compared to a magnet (or more specifically a ball of iron) which is

in the region of 48-49 Teslas in Britain (a factor of 109 greater than can be measured with

an FM256 magnetometer). Diurnal changes, due to the effects of electromagnetic

radiation present a strong general field.

The measurement of magnetic susceptibility is quantified in Teslas, and typical values

found over archaeological features generally do not exceed ±20ηTeslas. Topsoil may

have very weak magnetic susceptibility, and a ditch may have a response of 1-10 ηT, but

compared to a potential with an instrument sensitivity limit of 0.1 ηT; this is quite a strong

response. Bricks, but more particularly in situ kilns made of fired clay can give relatively

massive signals of 500ηTeslas, which is more than the instrument’s maximum reading of

204.75ηTeslas (at 0.1 ηTeslas sensitivity). Magnetometry relies, in part, upon

anthropogenic changes in clay mineral caused when haematite and magnetite are

transformed to the superparamagnetic maghaemite form of the mineral. The slight but



significant rearrangement of these dipoles along the axis of the earth’s magnetic field

results in an alteration in the characteristics of the soil in relation to background levels.

Magnetic Survey

A number of magnetometers are available commercially, some used primarily for

geological survey, and whilst other types such as the Bartington 601-2 have only been

used on one site in this present study (Maryport). The robust Geoscan FM36, or its

upgraded version the FM256 fluxgate gradiometer were the instruments of choice for

these projects. These instruments have seen technological improvements developed over

the years, resulting in ease of use and greater memory capacity, which allows greater

intervals between downloads. The present author has retained a single instrument

methodology, not necessarily on costs grounds, but because the method adopted, and

discussed later, always captures superior data.

Fluxgate Gradiometer (e.g. Geoscan FM36 or FM256)

Magnetometry survey in this study was conducted using a Geoscan FM256 fluxgate

gradiometer; an upgraded version. Satisfactory results can be obtained using 0.5m

sample along 1m parallel traverse intervals, with the application for drift correction after

each grid. Drift (essentially instrument imbalance), is undesirable and is readily

recognised in surveys. The dominant cause of drift in magnetometer output was found to

be temperature change. Dull, cool, overcast, days provide the best environment for

survey. Increased sampling intervals of 4/metre (3,600 readings/30m grid) along the

direction of traverse can improve the quality of the results at little extra cost. Such a

sampling strategy provides a confident characterisation of sub-surface archaeological

features.

The application of zigzag traverses, whilst more rapid, is prone to striping effects which

may be difficult to remove. This is a sampling aberration which may create difficulties in

recognising faint archaeological features. Magnetic techniques routinely penetrate c.

1.0m, with some larger features being detected beyond that depth. Magnetometry can

detect pits, ditches, kilns, ovens and hearths, whereas resistivity is sometimes better

suited to the detection of sub-surface buildings walls and masonry foundations. A

simplified schematic view of the instrument is shown (plate 28), which shows how the

instrument passively measures magnetic signals.

A limitation of the method and a factor associated with uneven ground is the relative

distance from the land surface to the magnetometer. The strength of the magnetic

response is proportional to the reciprocal of the distance cubed (i.e. 1/D3). It follows that

any readings taken upslope with a magnetometer will give a stronger response than those

taken downslope. The differential effects of ploughing striations or more particularly

vehicle tracks can create the illusionary effect of linear features.



An additional factor, which may give erroneous results, is the presence of outcrops of

igneous rock. A summary of the suitability of various techniques (magnetometry and

resistivity) over differing geological conditions is summarised in the English Heritage

Guidelines (David, 1995 Table 9). The effect upon magnetometry of the Cheviot Massif is

well documented and similar effects have also been observed close to the Whin Sill

(Biggins et al, 1999; Biggins and Taylor 2001a). What are less easy to distinguish are the

effects of relatively small glacial erratic boulder, such as andesite. The response from a

football-sized magnetic boulder of the igneous rock, andesite, is identical to that measured

from a large pit 1-2m in diameter.

Plate 28. This class of instrument uses a pair of solid state fluxgates to measure the gradient of the

vertical component of the earth's magnetic field. Commonly used for archaeological applications

these instruments currently offer a moderate sensitivity (~0.l nTeslas) suitable for most sites. This

is the instrument in common usage, with a depth penetration of c. 1.0m. The fluxgate gradiometer

has become the standard instrument of British archaeological prospecting (after Clark 1990, 69).



Earth Resistance Survey (Geoscan RM15 Resistivity Meter)

In this method an electric current is passed through the ground between a pair of mobile

electrodes. The current passes more easily through soil having a low resistance, but is

impeded by buried walls or road surfaces of higher resistance. The electrical resistance of

the ground is primarily dependent upon net soil moisture and its distribution (see plate 29).

Stone or brick wall foundations act as insulators being in general impervious to moisture,

therefore giving high resistance readings. Natural geological variations can make

interpretation difficult and very often linear features are more readily observed with this

method. Resistance survey would not materially distinguish between brick and stone.

Plate 29. (A) The electrical resistance (R) of an object can be calculated by applying Ohms Law.

(B) The longer the current path through the object (δδδδL), or the smaller its cross-sectional area

perpendicular to the direction of current flow (δδδδA), the higher will be its resistance.

Electrical prospecting relies upon applying a known DC or low frequency AC current to the

ground. The potential difference (or voltage) can then be measured and the electrical

resistance (R) of an object can be calculated by applying Ohms Law:

R =V/I

Where; R =Resistance, V = voltage; I = current

Electrical resistance is measured in units of Ohms (ΩΩΩΩ ). The electrical resistance of a

medium does not depend solely on the material of which it is made, but also on its shape.

The longer the current path through the object, δδδδ L, or the smaller its cross-sectional area

perpendicular to the direction of current flow, δδδδA, the higher will be its resistance (plate

29). It is thus useful to define resistivity, ρρρρ as the resistance of a standard-sized piece of



material with unit length and unit cross-sectional area. Resistance and resistivity are

related by the equation:

ρρρρ = R. δ δ δ δA

δδδδL

Thus, resistivity is measured in Ohm-metres (ΩΩΩΩ m)

Plate 30. The typical range of resistivity of some materials commonly encountered in field surveys

is described above. In archaeological situations higher resistance can be equated to sub-surface

building foundations and paved roads.

The conduction of current does not rely upon water itself, but the level of electrolytes

contained within it. Dissolved atmospheric carbon dioxide and carbonic acid produce

acidic conditions, and reaction with soil minerals form a weak electrolytic solution. Humic

acids also play a role in the process of soil conduction (plate 30). Clay sub-soils or infilled

ditches give low resistance readings, due to the presence of dissolved electrolytes.

Resistivity is limited by soil moisture content, and best used during the summer months,

when soil moisture deficits are at a maximum. This can result in much better contrasts

between different buried features. Some particularly arid sandy regions are not amenable

to resistance survey at all, even when contact resistance is improved by watering the

electrodes (such as Muweilah, Sharjah, see Biggins and Robinson 1999a; Blau et al

2000) Resistivity survey can produce complementary data when used in conjunction with

magnetometry, and this is often a key factor in analysis. Natural geological variations can

make interpretation difficult and most rock acts as an insulator.

Contact Resistance

The electrodes have a very small contact area in relation to the overall volume of ground

traversed by the current; and, compared with metal, the soil is a poor conductor,



especially near the surface where it tends to be relatively dry. In combination, these

effects create much higher resistance immediately around the electrodes than is

encountered in the deeper ground that one wants to measure. When repeatedly inserting

the electrodes, variations in this high contact resistance are inevitable which would swamp

the significant changes in deeper soil resistance. A simple way to measure the electrical

resistance of the ground would be to insert two electrodes into the surface, pass a known

current through them and measure their potential difference (plate 30A) However, there

are two problems with this system:

Polarization

Plate 31. Diagrammatic view of a simple earth resistance system and the more effective 4-electode

system. This simple modification significantly reduces polarization (effectively electrolysis) at the

electrodes, but does result in having a static pair of electrodes connected by wire to the mobile

probes (after Clark 1990, 28).

Electrical current flows through soil mainly via ions dissolved in water contained in its pore

space. These ions will migrate towards the current electrode of opposite polarity to their

own electric charge and cluster around it (plate 31A). As they accumulate, the resistance

between the electrodes will change owing to the change in potential across the soil-

electrode interface.

The problems of contact resistance and polarisation can be avoided if a four electrode

system is employed, with the potential difference (voltage) being measured across a

second separate pair of electrodes. In addition, a low frequency AC or switched DC

system is often used as an additional measure against polarization. The continual reversal

of the direction of current flow does not allow the relatively slow moving ions time to

accumulate around the electrodes (see plate 31B).



Four Electrodes System

There are many ways in which 4 electrodes can be arranged (see Clark 1990, 27-48), but

for routine archaeological prospecting the Twin Electrode Array is most suited to regular

use over large areas. A number of other different electrode configurations are available

(such as Wenner, Double Dipole, Square Array, Schlumberger and Palmer). This enables

the instrument to simultaneously take readings from a series of twin electrode

configurations of different probe widths (and hence depth penetration). This method under

the right conditions can provide pseudo-depth analysis and as such has been used

effectively at Wroxeter providing definition between foundations of different depths and

dimensions. In general, this type of configuration (MPX15 Multiplexer module using

0.25m, 0.5m, 0.75m, 1m, 1.25m, and 1.5m separations) is only useful on ultra-planar

surfaces with short vegetation, and under ideal conditions (pers. comm. Dr Roger

Walker).

Plate 32. Multiplexer application at Wroxeter. Wider electrode separation permits pseudo-depth

visualisation of variable depth and dimensions of sub-surface foundations to be analysed, although

some time must be spent in obtaining the best results. The ground must also be flat to allow

simultaneous contact of all electrodes (Geoscan Research).

The normal method of choice is to conduct surveys using 1.0 m sample and 1.0 m zigzag

traverse intervals using a Geoscan RM15 resistivity meter, which has a twin electrode

configuration and a probe separation of 0.5 m. Deeper penetration can be attained by

extending the width of the mobile probes to 1.0m, but the instrument is more unwieldy and

must be used on clear surfaces. Such a sampling strategy provides a general

characterisation of sub-surface archaeological features and is appropriate for sub-surface

remains of variable depth. Better characterisation can be obtained with 0.5m sample

intervals, but this strategy has time and associated financial considerations.





CONTACTS

Director - TimeScape Surveys J. Alan Biggins TD MA MSc MRSC MIBiol FSA FSA Scot

Northumbria, Main Road, Dinnington,

Newcastle upon Tyne. NE13 7JW.

Tel: +44 (0)1661 823 135 Mobile 07889 071 654 E-mail: [email protected]

TimeScape Consultant – Roman & Building Studies David J. A. Taylor Dip Arch PhD RIBA FSA MIFA

2A Higham Road, Padiham, Burnley

BB12 9AP Tel. 01282 773 666

E-mail: [email protected]

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