chapter 1 archaeological reconnaissance -...

Archaeologists use a wide range of reconnais-sance techniques to locate archaeological sitesand to investigate sites without excavating them.Some archaeologists predict that future advances

in non-invasive, and non-destructive, methodswill see them emerge as an alternative toexcavation. Reconnaissance techniques are alsoused to map evidence of human activity in the

ArchaeologicalReconnaissance

Chapter 1

YOUR GOALS

You need to be able to

■ understand the key methods used by archaeologists to locate and define sites

■ identify some strengths and limitations of the most important techniques

■ suggest appropriate methods for locating and exploring sites in particular circumstances.

Questions asked

Factorsdetermining choiceof reconnaissance

method

Type of study and time available

Type of site

Resources availableEthical issues

Location and terrain

Degree of accuracy required

Figure 1.1 Factors influencing the choice of reconnaissance methods

landscape. The appropriate methods in each casewill relate to the time and resources available aswell as to the particular case being investigated.

Every year hundreds of new sites are located.Some are spotted from the air or even fromsatellites in space, others through the discoveryof artefacts by metal detectorists. Quarrying,dredging and peat cutting all regularly produceunexpected finds while some of the mostimportant have come about completely bychance.

The discoveries of the body of Ötzi the IceMan by skiers and of the Altamira cave art bychildren are classic examples. So too was thediscovery of the Neolithic tomb at Crantit inOrkney, which was found when a digger fellthrough the roof!

Equally, some sites were never ‘lost’ to beginwith. Stonehenge and the Pyramids were wellknown before the development of archaeo-logy and many of the buildings of the last 200years are still in use. Other named sites weredocumented by historians and located by usingwritten sources. Schliemann’s discovery of Troyis the classic example but many historicbattlefields also fall into this category. In addition,a considerable number of new discoveries aremade during the exploration of known sites.

To locate or explore sites through research orahead of development there are four broad andcomplementary categories of methods that arecommonly used:

■ desktop study■ surface survey■ geophysical or geochemical survey■ aerial survey

In addition there is a range of newer techniques,most of which can be labelled remote sensing.

Reconnaissance should not be seen simply asthe precursor to the real business of digging. In some cases it can provide all or most of the evidence needed to answer questions. Thepioneering Shapwick project which investigated

the development of an estate owned byGlastonbury Abbey used a battery of reconnais-sance methods alongside limited sampling ofdeposits through shovel pit testing, geochemicalsurvey and excavation. The results whencombined with evidence from maps, historicalsources, and environmental data enabled theproduction of regression maps showing thedevelopment of settlement in the area.

DESKTOP STUDY

As its name suggests, this is an office-basedinvestigation using existing records. Somearchaeologists, usually concerned with ship-wrecks, aircraft or the investigation of historicalindividuals, continue to use written sources totrack down or identify particular sites. Moregenerally, most excavations and all research inBritain today begins with a search of informationthat has already been recorded. The majority ofthese investigations are part of the planningprocess. Their purpose is to determine whetherthere are likely to be archaeological remainswhich might be threatened by development (� p. 341).

Desktop study involves researching maps andhistorical or archaeological documents includingaerial photographs about the area under investi-gation. If they are not in private hands, these aremost likely to be held in planning departments,county records offices, historic environmentrecord, local Sites and Monuments Record(SMRs) or the National Monuments Record(NMR) offices.

Historical documents

A diverse assortment of documents may be ofvalue to the archaeologist. These will vary bycounty, area and period. In much of the countryknown documents are archived or recorded inthe County Records Office. In many areas, usefulsources have also been catalogued in a volume

4 ARCHAEOLOGY COURSEBOOK

of the Victoria County History (VCR). This isoften the first resource researchers turn to. Onlya fraction of early records have survived andthose that have need translation and interpreta-tion. Amongst the potential range available, thefollowing categories are important.

Legal documents. Records of ownership such asAnglo-Saxon charters or court records of disputesoften included physical description of boundariesand occasionally land use. Wills and inventorieswhich can be linked to particular buildings mayprovide lists of contents. These can provide cluesto that building’s use.

Tax records. These are particularly valuable inhelping to identify landowning units and theireconomic uses. The Domesday Book is the bestknown but later tax surveys and tithe awardsare often of more direct use.

Economic records. Order and sales books areinvaluable to industrial archaeologists whilenineteenth-century directories are useful in

exploring functions of buildings. Estate agents’bills are increasingly being preserved to recordchanges in important buildings.

Pictorial records. Paintings, engravings andphotographs can be of value both in identificationand in tracing changes. They are particularlyvaluable when studying standing buildings.Archives of aerial photographs (APs) such asthe RAF ‘or Luftwaffe’ surveys of Britain in the1940s are key documents in tracing landscapechange in the last sixty years. They are often theonly record of many sites.

Written accounts. Descriptions of places in books,diaries and travelogues are of use in identifyingthe function, construction methods and identityof many sites. The work of early antiquarianssuch as Stukeley is particularly valuable fordescriptions of monuments as they were beforethe modern period.

Archaeological records. There are three mainsources here. If there are early excavation or

ARCHAEOLOGICAL RECONNAISSANCE 5

Maps and plans,geological mapsblueprints,photographs,paintings andengravings, filmarchives

Nineteenth- totwenty-first-centurydocuments: directories,newspapers, populationrecords, estate agentdetails, surveys

Historical sources:Domesday Book,charters, tax records,tithe awards, courtrecords, collections, e.g.Victoria County History,County Records Office

Archaeologicalrecords andreports in SMR,NMR and localmuseums

Sources fordesktop study

Figure 1.2 Sources commonly used for desktop study


NOTTINGHAMSHIRE SITES AND MONUMENTS RECORD Site No. 03055

Cross-refs. N75174 T4416 OS SW 38District Newark NGR SK 7350 5125Parish Fiskerton cum Morton

Site Name

Class. Type Round barrow Linear featurePeriod General BA Period SpecificForm cropmark excavation

Site Status Area Status

DescriptionCircular enclosures, linear features. (1)Ring ditch, through to be a barrow, excavated 1975 in advance of development. Situated on a slight knoll on the flood plain terrace, it survived only as a cropmark. The circle is 25.0m in diameter, the flat bottomed ditch 2.0m wide and 70cm deep. 12 sections were made. In the infill, there were layers of iron panning and traces of iron stain in the deposits of natural silts. The only finds were 4 flint waste flakes, and a small fragment of handmade pottery, possible a fragment of an early BA collared urn or food vessel. No burials were found (destroyed by ploughing?) Looks like a BA barrow (2) See 03055a for adjacent cropmark.

Descriptive Typecircular enclosure linear feature

Findsworked flint pottery

Location of finds

Archaeology History (Event, Name, Date, Source)Full excav, O’Brien C, 1975 (2)

SourcesNo. 1 Type AP Pickering J, 7351/1

No. 2 Type Desc Text TTS, 1979, vol 83, pp 80–2

No. Type

No. Type

No. Type

Visits

Compiled/Revised24/08/1987 VB

Precise locations onOS map with 8 figuregrid reference

What the site is andhow it appears

Key information onthe site

Other material from thesite and where it is heldor recorded

Written records oraccounts of the site

When the localarchaeological serviceinspected it

Figure 1.3 How to read an SMR printout

survey results they can often be accessed throughlibraries or local museums. Local collections of finds and reports will also be held in localmuseums. Details of previous archaeologicalwork and stray finds for much of Britain areheld in local SMRs. These records are increasinglycomputerised and a national version is being builtup at the various NMR offices. Printouts whichinclude lists of earlier research can be made byinputting grid references.

Oral accounts. While living people may provideclues to the use and location of recent buildings,farmers and others who work on the land or thebuilt environment may have valuable knowledgefor archaeologists. Farmers, for example, may beable to identify areas where building rubble hasbeen ploughed up or where dressed stones havebeen removed. Sometimes estate managementrecords may hold this information for earlierperiods.

Maps

Maps are amongst the most basic tools andsources used by archaeologists. They are used tolocate and explore sites and to answer questionsabout previous use of the landscape. They are ofparticular value in tracking changes through time(settlement shape and location, boundaries, landunits, fields and hedges). They can also be usedto relate sites to geology and topography.Medieval archaeologists are often able to producetheir own maps for periods before mappingbegan. They do this by working back from theoldest available map and cross-referencinghistorical sources and fieldnames. This techniqueis known as regression.

A wide variety of maps are used by archaeo-logists, including the following.

Early mapsMaps from the sixteenth century tend to showthe properties of the rich. They are not alwaysto scale but may provide visual information such

as illustrations of specific buildings. From theseventeenth century there are also route mapssuch as Ogilvy’s Road Book, which is a series oflinear strips. Maps were produced to show theproposed routes of turnpikes, canals and railwaysin order to gain permission from parliament forbuilding to take place.

Changes in rural landownership from theeighteenth century onwards were recorded onenclosure award maps, while taxes owed to thechurch by landowners were sometimes writtenon tithe award maps. Occasionally these can becross-referenced and both can provide informa-tion about fieldnames, routes and boundaries,which are vital for landscape archaeology. Othermaps show landscaped gardens, battlefields orprovide plans of factories and mines.

These early maps are often held in countyrecord offices. Some may be in private hands orbelong to churches.

Ordnance Survey (OS) mapsDuring the early nineteenth century the OSmapped each county at 1 inch to 1 mile. Fromthe 1880s OS 6 inch to 1 mile maps providedmore detail of individual buildings and evenhedge species. OS maps established a newstandard in accuracy and a comprehensivesystem of coding and keys for features. A gridsystem was used which covered the wholecountry and enabled precise references to begiven. By examining a succession of maps forany area, changes in land use and the builtenvironment can be easily seen.

• www.ordsvy.gov.uk

Maps used in archaeologicalThe OS 1:25000 Pathfinder or Leisure series showthe location of some archaeological sites butplanning maps that use the OS grid system areneeded for investigations. 1:10000 (old 6 inch)maps are sometimes the most detailed availablefor mountainous, remote and some rural areasbut 1:2500 (old 25 inch 1 mile) rural or 1:1250


urban planning maps are normally used. For field walking 1:10000 or 1:2500 is used and forexcavation the 1:2500 or 1:1250 provides a base.A 1:2500 map allows you to identify individualmetre squares with a 10-figure grid reference.These maps are held in county or districtplanning offices.

Other maps sometimes used include the Geo-logical Survey series, street maps, factory plans,vegetation and climatic maps, land use andclassification, soil surveys and specialist archaeo-logical maps. Increasingly archaeologists areusing computerised mapping systems basedaround Geographical Information Systems (GIS).

As an archaeology student you need somebasic map skills including the ability to:

■ identify and interpret common archaeologicalfeatures from maps

■ ‘read’ contours and hachures■ use scales and at least 6-figure grid references■ produce basic cross-sectional sketches from

maps

■ interpret simple archaeological plans anddiagrams

■ use other evidence such as photographs andwritten accounts to interpret maps and plans.

SURFACE SURVEYS

This term can be used to encompass field-walking, surveying and even planned aerialphotography. We will use it to describe non-destructive visual surveys at ground level. Thesecan range from slow, painstaking searches on footto quite rapid examinations of a landscape byLandrover, looking for upstanding earthworks.Since most sites lack visible features, the formeris more common. Fieldwalking is largely con-cerned with finding traces of unrecorded sites.Scatters of building rubble or artefacts or slightundulations in the surface can reveal where thereare buried walls or house platforms. Differencesin soil or vegetation may also be indicative ofpast human activity. For studies of the Mesolithicand Neolithic in Britain, scatters of flint and


KEY TERM

Geographic Information Systems (GIS)

This refers to powerful databases which canstore many layers of data against individualmap grid references. This can include detailsof topography, geology and vegetation as wellas archaeological data. GIS can integrate datafrom satellites with field recordings. It canproduce topographic maps and site plans inthree dimensions and perform complex statistical analysis. It is revolutionising thepresentation and interrogation of archaeo-logical data. It can even be used to predictsite locations based on known patterns.

• http://www.esri.com/industries/archaeology/index.html

KEY TASK

Sourcing information

Take each of the following examples and listthe types of source you might find useful ininvestigating it.

Next take a real example of one of themand find out what actual sources exist. Youmay be surprised.

A round barrow or cairn

A Roman villa or Saxon church

A deserted medieval village or abbey

An eighteenth-century farm or canal

A nineteenth-century railway line or factory

A twentieth-century pillbox or airfield


Figure 1.4 Rock art survey using GPS and GIS

The survey of rock carvings or petroglyphs illustrates many aspects ofreconnaissance and recording techniques. In addition to a detailed record beingmade by tracing and photograph, the position of each petroglyph is identifiedby GPS. Its height above sea level and orientation are also measured and theinformation entered into a GIS database. This enables 3D presentations topermit study of relationships between petroglyphs and topography or each other.

animal bone are often the only traces of humanactivity visible in the landscape. To study theactivities of these mobile populations, carefulidentification and plotting of these scatters isessential. A variation on this is the study of hedges and woodlands for traces of pasteconomic activities and to help locate settlementareas (� p. 202). Surface surveys can cover largeareas such as Webster and Sanders’ work in theCopan Valley of Mexico or woods such as theThetford Forest project in East Anglia.

Surface investigations of known sites includemicro-contour surveys of the topography. Theseinvolve detailed and precise use of surveyingtools to build up a picture of variations in heightand levels. Data is increasingly being entered ondatabases to enable computer enhancement of

the landscape. These surveys can often revealhidden features that could not be detected withthe naked eye.

In most studies, the areas to be surveyed aremeasured using surveying equipment or GlobalPositioning Systems (GPS) and set out with rowsor squares of pegs, cane or marker poles. This is to enable accurate sampling and recording.

Recording standing buildings

One specialised area of archaeological surveyingfocuses on the built environment and linksarchaeology to architectural science. Detailedstudies of the material and constructiontechniques of structures are made both to enhanceknowledge of the development of buildings and


KEY TERM

Sampling

Whatever is deposited is a fragment of pastmaterial culture. Dependent upon the material,a variable portion of these deposits will survive.Archaeologists will recover a sample of these.Not every site can be fieldwalked, let aloneexcavated. Choices have to be made. If thesechoices are arbitrary (non-probabilistic) theycould lead to bias in the archaeological recordwith certain types of evidence being neglectedand others over-represented. For example,if archaeologists chose only to study hill fortsfrom the Iron Age or, as often happens, ifdevelopment only permitted excavation in onepart of a town, it might create an unrepresenta-tive picture of life in the past.

When archaeologists design research strate-gies they use some form of probabilistic samplingto reduce bias in recovery. This means that thechance of anything being recovered is known.Rigorous sampling is used in most aspects ofarchaeological reconnaissance and excavation.

Firstly the plan of the total area or site tobe surveyed is divided up either into a grid patternof numbered squares or a series of equidistantparallel lines or transects (� p. 12). Both areusually aligned north–south to link into thenational survey grid although sometimes gridsin fields are aligned on a particular boundary.With large areas it is common to select a sampleof grids and then use transects within them. The scale varies according to the task. An initialsurface survey of a whole landscape might start with 100 metre or kilometre squares and thenhave transects between 10 and 50 metres apart depending on terrain and resources. For test pittingon a known site the initial grid might be 1 metre square. You need to understand four basicapproaches to sampling. Our illustration is for grids but the principles are the same for transects.

A simple random sample works like a lottery. The numbered units are selected by computer ornumber table. This is fair as each unit has an equal chance of being selected, but it can also leadto clustering and thus miss features.

Stratified sampling overcomes clustering bias by first dividing the sample universe into sections.For example, if the site has natural zones such as hills, valley and plain then numbers are selectedrandomly for each zone in proportion to its area.

10

9

8

7

6

5

4

3

2

1

01 2 3 4 5 6 7 8 9 10

UplandUpland

Upland

MarshStratifiedsample

A randomsample of eachsub-regionin proportion toits size

10

9

8

7

6

5

4

3

2

1

01 2 3 4 5 6 7 8 9 10

UplandUpland

Marsh Systematicsample

A predeterminedpattern

10

9

8

7

6

5

4

3

2

1

01 2 3 4 5 6 7 8 9 10

UplandUpland

Marsh Randomsample

A lottery

Flood plain Pasture Sample

Pasture

Pasture

Pasture

Figure 1.5 Models of different approaches tosampling

to provide a record against future destruction ordecay. For example laser scanning is used in somebuildings which are covered with lichen to seehow they are constructed. Records will rangefrom written description to CAD (computer-aided design) based recording of every brick or stone. Most recording of buildings occurs aspart of the planning process (� p. 347) or duringconservation work. A recent example is theDefence of Britain project, which collected recordson surviving defensive monuments of the SecondWorld War.

• www.britarch.ac.uk/projects/dob/index.html

Fieldwalking

Fieldwalking, or surface collection, involvessystematic collection of artefacts from theploughsoil which might be indicative of humansettlement. This is based on the reasoning thatmaterial on the surface reflects buried remains.Sometimes high density scatters of particularmaterials such as building rubble or brokenpottery enable specific sites such as buildings orkilns to be identified. More typically, the methodhelps identify areas of settlement or activitiessuch as hunting. Ceramics and worked stone arethe most commonly gathered but metal, bone andburnt stone are often also collected. The methodis destructive in that archaeological material is

removed, but as it has been disturbed byploughing, it is not in its original context anyway.

Decisions about sampling have to be madewhen planning fieldwalking. Not everything willbe collected, particularly when building rubbleis involved. For instance, will all ceramics becollected or just diagnostic pieces? Decisionsalso have to be taken about the width of transectsor size of grids.

Timing is important. Ideally ploughed soilshould have been broken down by weatheringand recent rain will have cleared dust from thesurface. Walkers either proceed along a transectin a series of stints or search a grid. These havebeen carefully set out with marker flags or poles.Grids tend to be used when total coverage of afield is required. The material collected is baggedand tagged with the number of the grid or stintfor processing and analysis.

Once washed and identified, finds are countedfor each grid or stint. This can then be plottedon a distribution map to show patterns andconcentrations. There are many ways of display-ing this information. Phase maps or a series ofclear plastic overlays for each period or type of find are commonly used. Computer displaysusing GIS have an edge here since severaltypes of data can be linked to any point andcomparisons easily made.

Fieldwalking is a well established methodbecause it has many strengths. It is a relatively


KEY TERM cont.

Sampling

Systematic sampling overcomes clustering by selecting at evenly spaced intervals, for example everythird grid or every 10 metres. This ensures a more even selection although it could miss thingsthat are regularly distributed. It usually requires a higher number of samples.

Stratified systematic sampling combines the last two methods and could be used to take moresamples in particular zones than others.

• http://archnet.asu.edu/archives/theory/sampling/sampling.html

cheap way of surveying large areas sincevolunteer labour can be used to collect and washfinds. It can help establish the function andperiod of a site without excavation and provideinsights into location and exchange.

Fieldwalking can indicate the spread and fociof evidence. It does, however, have importantlimitations too. It is only really useful on arableland where access has been granted and thenonly at certain times in the agricultural cycle. In addition, its results cannot be taken at facevalue. For example, medieval manuring practicestransferred much domestic refuse to the plough-soil, creating a doughnut shape pattern of pottery distribution. Chris Gerrard’s work onthe Shapwick Project has explored two othermajor limitations.

Different materials behave differently in thesame soil. In Shapwick, rewalking the same fields


Figure 1.6 A Level students fieldwalking

The experience and training of fieldwalkers and the conditions on the day arefactors affecting what is recovered (� p. 117).

KEY TERMS

Transects, traverses and stints.

A transect is a sampling line which could beacross a single site or an entire landscape. Itis usually aligned north–south and tied intothe national grid. In fieldwalking transects areusually divided up into manageable chunks orstints of 10 to 50 metres where one walkerwill use one collecting bag.‘Traverse’ is a termused largely in geophysics and sometimesaerial photography to describe the straight,parallel paths passed over by the surveyor. Soa magnetometer survey might use traversesset at 0.5 metres apart.


S h i pt o

n s Gr a

v e La n e

983

Scale 1:2,000

Grid North

982855

981855 981856981857

982858982857982856

983856 983857983858

981858

982

The Far Green, Barton EndFieldwalking – 15/10/99

N

2 4 6 8 10 12 14 16 18 20 2 4 6 8 10 12 14 16 18 202 4 6 8

2 4 6 8

10 12 14 16 18 20

1 3 5 7 9 11 13 15 17 191 3 5 7 9 11 13 15 17 19

2 4 6 8 10 12 14 16 18 202 4 6 8 10 12 14 16 18 20

1 3 5 7 9 11 13 15 17 191 3 5 7 9 11 13

2 4 6 8 10 12

1 3 5 7

Figure 1.7 A planned fieldwalk which has been linked to the national grid system. Transects are 10mapart with 50m stints

S h i pt o

n s Gr a

v e La n e

983

Scale 1:2,000

Grid North

982

The Far Green, Barton EndFieldwalking – 15/10/99

N

Roman Pottery Densities

0 1–15 16–30 31–45

gms

46–60 61–75 76+

Figure 1.8 The density of one category of finds plotted in relation to each fieldwalker’s stint

Amounts of selected materials can also be shown with shapes or dots where the size and colour or shading represent the numbers of finds.

and monitoring ceramics in them showed thatsome material migrates further than others.Patterns for pottery from different periods werealso very different. It was not always a goodindication of settlement. A second variable wasthe differential collection by different field-walkers. Analysis of their finds showed that somewere good at recognising and collecting onetype of material but poor with another. Thisapplied to experienced walkers as well asnovices. Their performance varied according toweather and slope. Taken together it means thatwhat is recovered is a sample of what was in thetopsoil and the topsoil holds a sample of whatlies below. In both cases the sample varies foreach type of find. Fieldwalking results thereforeneed to be cross-checked with other data beforeconclusions can be drawn.

There are a number of other prospectionmethods which provide alternatives to field-walking although all are more destructive.Shovel pit testing can take place in woods,pasture and gardens where fieldwalking isimpossible.

Coring and augering are also used to samplethe subsoil. This can provide a snapshot of the

stratigraphy and the sample can be examinedfor artefactual or environmental evidence.Probing, which involves driving a rod into theground, is more useful for tracing shallow buriedfeatures such as walls on known sites � p. 253.

GEOCHEMICAL PROSPECTION

These relatively new methods and expensivetechniques attempt to locate areas of past humanactivity by detecting differences in the chemicalproperties of the soil. All living things produceorganic phosphate as waste or through decay.Unlike phosphate in fertiliser, this remains inthe soil where it was deposited. Samples of soil


Figure 1.9 An excellent example of a flintarrowhead recovered during fieldwalking atThetford

Figure 1.10 A total station

The total station combines the functions oftheodolite, EDM and data logger. It is highlyaccurate in calculating heights, angles anddistances and can be used to rapidly set out gridsor to record the position of hundreds of points.These can later be downloaded and with theright software, used to produce 3D maps of thesurvey area.

are taken and levels of phosphate measured ina laboratory. Once plotted, concentrations oforganic phosphate may indicate settlements or animal enclosures. Similar principles apply toheavy minerals such as lead and cadmium andto lipids (fats). These may become increasinglyimportant in the future. One possibility is thatdifferent chemical combinations could identify‘signatures’ (� p. 125) for different activities.

GEOPHYSICAL SURVEYS

This term covers techniques that detect featuresthrough their physical differences with thesurrounding soil. The most common techniquesdetect magnetic and electrical anomalies and require considerable skill to interpret. Withthe increasing involvement of archaeology inplanning development and a shift in emphasis

amongst archaeologists in favour of preservationrather than excavation, these techniques are nowcommonplace. The manufacture of increasinglyreliable instruments for archaeology has seenmagnetometry become a standard technique.

• http://www.geoscan-research.co.uk

• www.brad.ac.uk/acad/archsci/sbject/archpros.htm

Resistivity survey

There are differences in the ability of differentsoils to conduct electricity. This can be detectedby passing an electric current through the groundand comparing readings. Electricity is conductedthrough the soil by mineral salts contained inwater. The more moisture there is the better theconductivity of the soil. A buried ditch or gravewill generally retain water better than thesurrounding soil. A buried wall or road willconduct poorly and therefore resist the currentmore than the surrounding soil. Electrical currentflows close to the surface so it can be measuredusing shallow probes. The method works betterwith some soils than others. Clay retains moisture


Figure 1.11 Shovel pit testing

This approach to sampling is very common in theUSA. Only the top few centimetres are sampled.In each sample a standard volume of soil issieved through a fine mesh for ecofacts andartefacts.

Figure 1.12 Auger sampling

An auger is driven or screwed into the ground.It extracts a sample of the subsoil in much thesame way as an apple corer.

well, so differences in resistance between thesoil and buried ditches or pits may be impossibleto detect. This also applies to many soils if theybecome waterlogged in wintertime. Plants, rocksand variations in the depths of soils can alsocreate misleading readings.

Resistivity can also be used to create pseudo-sections of buried features. This involves takinga series of readings from a line of probes placedacross a buried feature such as a ditch. Widerspacing produces data on deeper parts of thefeature than narrowly spaced probes. The depthto which this technology penetrates the soil islimited and readings require considerable inter-pretation, as the sensitivity of the meters is not great. At Hindwell in Wales, a 4-metre wideditch identified by resistivity turned out afterexcavation to be a series of massive postholeswith construction ramps.

Magnetometer surveying

The earth’s magnetic field is generally uniformin any one place. However, local magneticdistortions can be caused by past human activity.Topsoil contains haematite (Fe2O3), an iron


LOW

The wall does not conductcurrent well. Its resistance ishigher than the soil around it.

The hearth does not affectthe current much. It wouldn’tbe detected.

The ditch was a good conductorof current. It showed as an areaof low resistance.

HIGH

NORMAL

Buried wall

CURRENT

RESISTIVITY

The resistivity meter works by detecting anomalies (differences) in the ability of subsurface remains to conduct electricitycompared with the surrounding soil.

RESISTANCE

L H

N

Buried hearth

L H

N

Buried ditch

Figure 1.14 A simplified diagram illustrating the principles of resistivity

Figure 1.13 Resistivity surveying

Meters are usually mounted on a ‘zimmer-like’frame and have a data logger on board to recordresults. While relatively easy to use they are notfast and are best suited to detailed explorationof a site rather than initial prospecting.

oxide. In some forms its crystals are magnetic. A ditch which has filled up with topsoil willcontain more haematite than the surroundingarea. Its fill will therefore be slightly differentmagnetically and may be detected by sensitive,modern magnetometers.

A second type of distortion is caused wheretopsoil has been subject to considerable heat. Thiserases the magnetic properties of the iron oxides.For Haematite 675 °C is required. When the soil cools the iron oxides become permanentlymagnetised according to the polarity of theearth’s magnetic field at that time. Since this fieldchanges over time the sites of kilns and hearthsappear as magnetic anomalies.

The earliest magnetometers were cumbersomeand slow to use. The development of hand-heldfluxgate gradiometers has enabled the techniqueto be used to rapidly scan quite large areas tohighlight anomalies. Magnetometers are alsoused in detailed site investigations where theycan detect small features up to 1 metre downand provide images of some buried features. Forvery detailed work traverses are set 0.5 metresapart with samples every 0.5 metres. 1 metre gapsand sample intervals are more common.


Figure 1.15 Magnetometer survey

Magnetometers are either hand-held or as in thisexample, mounted on a frame. The twin fluxgategradiometer shown here has two verticallymounted sensors a metre apart to maximise speedand sensitivity on large searches.

MAGNETOMETER SURVEY RESISTIVITY SURVEY

N

Figure 1.16 Resistance and Magnetometer plots compared

The essential complementary nature of these techniques can be seen in these plots from English Heritage’ssurvey of White Barrow.

• http://www.archaeotechnics.co.uk/

To be able to detect anomalies, the magneticbackground of the soil has to be measured and magnetometers calibrated against it. Themeasuring of this magnetic susceptibility of thetopsoil can also be used as a crude but rapidsurvey technique in its own right. Magnetichotspots suggest areas of past settlement orindustrial activity, which could be surveyed usingother methods.

Sensitive magnetic instruments are easilydisturbed by iron, including nails, pipes and wirefences as well as the zips and piercings of thearchaeologist. A further limitation can be back-ground interference from magnetic bedrock orwhere a long period of occupation has left amagnetic layer over a wide area. Sandy and claysoils often do not provide sufficient contrast.Fluctuations in the earth’s magnetic field alsohave to be taken into account. It requiresconsiderable skill and experience to interpretthe results.

Caesium vapour (CV) magnetometers

These are many times more sensitive thanconventional magnetometers and are more com-monly used in Germany and Austria. Typicallyseveral machines are used close together on alarge wooden handcart. They work by pumpingcaesium vapour and taking rapid measurementsat around 25cm intervals. This alkali is sosensitive to minute variations in magnetism thatit can detect and define the edges of buriedfeatures formed by traces of magnetite. This ironoxide (Fe3O4) is concentrated in the remains ofthe bacteria which consumed the woodenstructures such as posts which once stood there.It is being used at a number of well-documentedsites to reveal more of their secrets. Recent workat Stanton Drew stone circle revealed the ‘ghosts’of hundreds of postholes in concentric circles.Caesium magnetometers suffer less from thebackground ‘noise’ which occurs with hand-held devices but at £40,000 per machine and

perhaps four machines on a cart, this techniqueis expensive.

Other methods

Metal detectors are useful for metal objects downto about 15 cm. Some archaeologists use themon site to provide information in advance ofdigging such as the position of burial deposits.Skill is required to avoid time being wastedexploring buried slag or modern metal debris.Similarly they can sweep areas in advance ofdetailed geophysics to identify concentrations ofmetal that might distort readings.

Ground penetrating radar (GPR) which wasdeveloped for defence and engineering is start-ing to be used in archaeology. Aerial versions canhighlight buried landscapes and rivers. GPRworks by transmitting pulses of energy into theground and recording the time taken andstrength of the return signal. This can indicatethe density and depth of buried deposits. Databased on different energy wavelengths can beplotted as a series of ‘time slices’ to build up a 3Dpicture of buried remains.

Sonar, which was developed to detect sub-marines is a form of acoustic sensing. Side scansonar measures sound waves as they ‘bounceback’ and can map the sea bed and reveal the


KEY TASK

Test your understanding of geophysics

Which of the two main geophysical methodswould normally be effective in detecting theseburied features? Answers on p. 429.

1 Hearths 6 Large pits2 Cobbled floors 7 Stakeholes3 Stone walls 8 Building platforms4 Graves 9 Small pits5 Kilns 10 Ditches

depth and form of sunken structures. Bosing isa crude form of acoustic sensing used on land.It involves hitting the ground with a mallet andlistening for variations in resonant sounds. Thesemay indicate buried ditches or walls.

Finally there is dowsing. This is a traditionalmethod by which skilled dowsers use woodenrods to detect water or archaeology underground.

All of these geophysical techniques are limitedin the type of work they can do and they shouldtherefore be seen as complementary. None ofthem are particularly useful on waterlogged sites.Their value is often in pinpointing or exploringfeatures rather than finding new sites.

AERIAL PHOTOGRAPHY

The first aerial photographs (APs) were takenfrom balloons. Today, most photographs aretaken from light aircraft although even kites orballoons are used on occasions. APs are used formapping, finding new sites rapidly over largeareas and illustrating and exploring known sites.Substantial archives of aerial photographs areavailable publicly and commercially so newpictures may not always be needed.

• http://aarg.univie.ac.at/

• http://www.nmia.com/~jaybird/AANewsletter/


Figure 1.17 Handcart mounted GPR

Ground versions of GPR are useful for detecting buried floors, voids and walls. It has been particularlyeffective in revealing the internal structures of buildings and exploring burials. It is the only effectivegeophysics technique in city centres where it can even penetrate tarmac. Due to its cost and the availabilityof quicker methods it has not been used widely outside urban areas in the UK although this is starting tochange. It also works poorly on clay soils.

Verticals and obliques

Aerial photographs used for mapping are takenwith the camera pointing straight down at theground (verticals) with the aircraft flying alonggrid lines. Often these are taken from highaltitude. This is the case with the RAF archivesdating from the 1940s which are now housed atthe NMR. These also provide an excellentdesktop source for initial study of landscapedevelopments. Overlapping vertical photographscan be viewed through a stereoscope to see thelandscape in 3D. Their main value is in planningand illustrating sites. Where some dimensionsin the photograph are known, reasonably

accurate plans can be drawn of sites, includingtheir contours. This is known as photogrammetricmapping.

Oblique photographs are the most widely usedin archaeology to locate sites and illustratefeatures. These are taken from low-flying aircraftwith the picture taken at an angle to the ground.Aerial reconaissance usually precedes fieldsurvey. While this is fast and gives good cover-age, it can be expensive and can miss features iftheir signatures are not visible from the air.Equally, there may be features which are invisibleat ground level and this provides the only meansof recording them. There are three main ways inwhich archaeological sites show up from the air.

Shadow sites

In low light, either at the start or end of the day,shadows are at their longest and even quite minorvariations in ground level cast shadows, forinstance ploughed out barrows or the remainsof early field systems.

APs taken from a low-flying aircraft andrecorded with a camera pointed into the sun have a distorted perspective which emphasisesshadows. The technique is best used for illus-trating existing sites and locating details withinthem, for example features inside a hill fort.However, shadows are also created where cropsare at different heights (� p. 23) and occasionallynew sites can be detected. Winter is the bestseason for photography as the sun is low andvegetation which might mask sites has often died down. Snowfall and flooding can accentuatethe appearance of hollows and earthworks andcreate some of the most dramatic images ofshadow sites.

Cropmarks

The ripening and growth rate of crops is relatedto the amount of moisture their root systems canaccess. Plants with better access to moisture willoften grow taller and turn a different tone or


Figure 1.18 Side scan sonar image of a sub-merged aircraft

This scan of a US Navy PB4Y-2 Bomber wasrecorded by sonar mounted in a towfish device.The aircraft is at the bottom of Lake Washingtonin the USA under 164 feet of water. http://www.marinesonic.com/

Other information on marine surveys is athttp://www.uri.edu/artsci/his/mua/MUA.htm


Figure 1.19 An excellent view of the deserted medieval village (DMV) of Bingham’s Melcombe, whichshows up because of shadows cast by low sunlight. Traces of houses, enclosures and trackways are allvisible

Note how features at right angles to the sun show up best.

SUN

Highlightedareas

Eroded houseplatforms

Shadows

Figure 1.20 Why earthworks are visible as ‘shadow sites’

colour than those plants around them. If thereare buried archaeological features under a fieldthis can result in patterns showing in the crop.A buried ditch with its infill of humus and topsoilwill often hold moisture, creating a dark greenline in the crop above. This ‘positive’ cropmarkis visible from the air. The opposite occurs inplants over a buried wall. They are likely to bestunted and produce a yellowish, ‘negative’cropmark. ‘Parch marks’ show on grass for thesame reason.

Cropmarks sometimes only show for a fewdays a year. Repeatedly flying over areas overtime can pick up new and different features.Some only show up in drought conditions whencrops with access to moisture have the greatestadvantage and colour contrast is exaggerated.The technique works best on quickly draining

soils such as river gravels but is less good onclay or areas of deeper topsoil, where the soilretains moisture well. Cropmarks show up bestin cereal crops such as wheat and particularlybarley. They do not show up in many crops, forexample peas and beans, and the effect ofdifferential moisture can be overcome or maskedby irrigation or fertiliser. Care has to be takenwith interpretation, as geological features suchas periglacial cracks and modern field drainageand underground pipelines also create crop-marks. Trial excavation is often the only way tofirmly identify many sites. Cropmarks are themost prolific source of new sites, particularly for the late Neolithic to early medieval periods,and are also used to investigate existing sites such as the extent of the harbour at FishbournePalace.


Figure 1.21 An Iron Age ‘banjo’ enclosure on Cranborne Chase showing as a dark cropmark

The crops growing over the ditches of the feature are darker because their roots have better access to moisture than the surrounding crops. Crown Copyright 1955 & 1959/MOD.

Soil marks

On soils where there is a marked contrastbetween the colour of the topsoil and subsoil,evidence of ploughed-out monuments can occur

as soil marks. On chalk, the dark brown of ditchinfill will contrast with the chalk rubble of abank and the lighter brown of the plough soil.At Flag Fen a Roman road appeared as anorangey stripe against the black peat soil. Soil


Crops darkerand taller Crops paler

and stunted

Top soil

Bedrock

Stone wallRing ditch

Figure 1.22 Three-dimensional cross section of cropmarks

Figure 1.23 Winterbourne Stoke round barrow cemetery showing as soil marks

The difference in tone between the top soil and the material used for the barrowprovides a clear contrast. The monuments would not be easily detected on theground.

marks are sharpest in winter when vegetation is low.

Remote sensing

This can be a rather confusing term. Usually itis used to distinguish between the imagingtechniques used from planes and satellites andthose of ground based prospection. Sometimesit is used to describe all techniques that don’tremove material. When you come across it, besure to check which sense it is being used in. Weare using it in the first sense. The results of allthese techniques need to be checked at groundlevel.

• http://www.arcl.ed.ac.uk/arch/remotesense/

A variety of airborne and satellite techniques,including thermal imaging and infrared photo-graphy, are able to record temperature, dew andfrost dispersal variations invisible to light-sensitive film. They all work on the principlethat anomalies such as disturbed earth or buriedwalls will absorb and retain heat or moisture at different rates to the surrounding ground.Commercial equipment is really only suitable forlarge features although military developments toincrease sensitivity will no doubt filter throughto archaeology. Currently such equipment is tooexpensive for most archaeological surveys.


Figure 1.24 Loughcrew

The area around Loughcrew in Ireland is packed with neolithic monuments, however, littleis known about other use of the landscape. Current research is using a technique calledairborne Lidar (Light Detection and Ranging) to map all the earthworks within 3km ofthe site. The Lidar device is mounted in a light aircraft and transmits a scanning laserbeam which is reflected back from the ground surface and recorded on sensors. The timetaken determines the precise distance from the aircraft. This method is more sensitivethan conventional photographs of shadows and is revealing early field systems within themodern landscape which survive as slight earthworks.

chapter 1 archaeological reconnaissance -...

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