Oxford
Archaeotechnics
Topsoil
Magnetic Susceptibility Survey
Brief
overview
Human activity has constantly modified and restructured the landscape
through the agencies of clearance, settlement, agriculture and
industry; the construction of trackways, roads, fortifications and
ritual sites contribute to this tapestry. Elements of earlier patterns
survive in varying degrees as components of the modern landscape. Much
of the activity, which is also often associated with the dispersal into
the topsoil of artifactual material, will
have resulted in slight but persistent changes in the magnetic
character of the local soils which tend to develop varying degrees of
'magnetic contrast' both as horizons modified by differential use, and
within silted or backfilled features. The success of magnetic survey in
locating areas of archaeological potential is dependent upon a number
of factors, and is influenced by geology, the nature of the former
activity and subsequent land use. Soils, archaeological
horizons and features may, for example, become magnetically enhanced or
modified in
situ
directly or indirectly, as a result of burning or by local
incorporation of burnt or exotic materials. Topsoil magnetic
susceptibility patterns will then be generated by the admixing, mostly
by ploughing, of deposits derived from the underlying occupation and
industrial sites. Human habitation can also indirectly influence the
magnetic susceptibility of both soils and buried deposits through
alterations to local pedological and ecological systems (e.g. by
drainage). Recording these variations in topsoil magnetic
susceptibility can be extremely useful in both locating and defining
areas of archaeological potential. The magnetic contrasts
often present between buried infilled features or structures and the
substrate also create local anomalies in the earth's magnetic field
which can be detected by magnetometer
survey allowing relatively detailed plots to be made of
buried archaeological features.
Below are selected examples from over three hundred surveys, in which
topsoil magnetic susceptibility mapping has been used to predict the
location of former settlement and industrial activity, the maps also
reveal interfaces of differential land use, defining the patterns of
ancient boundaries and fields even where no tangible archaeological
features survive. Topsoils retaining contrasting magnetic properties as
a result of some former anthropogenic modification will often show
unique signatures on either side of former earthworks and hedgerows
(even intensive modern agricultural techniques does not usually totally
destroy this magnetic record). Used in combination with, and to inform
other archaeological investigation this technique can provide a
valuable additional level of information which would otherwise be lost
upon removal of the topsoil. The surveys below have been recorded at
10m grid intervals and represent information contained within the top
10 cm of plough soil or pasture. Many of the plots are the result of
one day's fieldwork
Values
are volume
susceptibility SI (x10-5). For an
explaination of susceptibility terms see J.Dearing on Bartingtington
Instruments webpage (PDF)
Industrial sites
Former metalworking,
pottery,
and tile and brick manufacturing sites usually
respond extremely well to topsoil magnetic susceptibility survey. the
presence of fired structures, quantities of assocaited burnt material
and the import of
exotic
material contribute to the formation of graphic plots which often
dramatically indicate the core activity areas.
Below: the site of a late
medieval brickworks, Spalding, Lincolnshire,
UK.
Survey area: 800 x 200 m. Topsoil volume magnetic susceptibility range: 10 - 200 SI (x10-5).
The map was produced from readings taken at10m intervals which revealed the location of a medieval kiln and other structures used in the manufacture
of glazed bricks, this site was found within a field which was to become clay extraction pit for road construction. A strong linear feature running
diagonally across the centre of the plot represents the course of a medieval ditch which contains magnetically enhanced deposits derived from
the brickworks in its backfill; subsequent ploughing has dispersed and incorporated a component of this material into the topsoil. The former
infilled ditch is actually only some 2m in width. The ability of 10m soil mapping to indicate its position and other subtle landscape elements
demonstrates the effects of agricultural dispersal.
Below: the site of a medieval Ironworking site
Industrial debris has been incorporated into the local topsoils by
centuries of ploughing and spread to the boundaries oflong abandoned
former strip fields. Excavation proved the example shown above to be an
important Medieval Ironworking site with numerous furnaces
beneath the focus of the extremely magnetic soils, it was surveyed
prior to opencast
coal mining. The topsoil magnetic readings taken here (locally
exceeding 3,000 SI) are the highest we have ever recorded
magnetically enhanced
material spread from former furnace sites.
Topsoil magnetic susceptibility contour plot: susceptibility range: 5 -
120 SI. Contour interval 5 SI. Geology: alluvial clays.
Part of a series of site
surveys undertaken in advance of a proposed pipeline. The example above
is the result of less than half a day's fieldwork including total
station survey to the National Grid and plotting of results. The focus
of magnetically enhanced soils, as with the other industrial examples
shown here, is so obvious that the first magnetometer grid sited to
investigate the pattern produced an almost complete plan of a structure
housing two large furnaces
salt making sites
The above survey of a coastal field was undertaken in advance of the construction of a sewage treatment works. The example shows both
the persistence and potential subtlety of topsoil magnetic information from ancient industrial sites. Magnetometer survey over
the site of the small topsoil magnetic susceptibility focus resulted in a plot showing the edge of a large salt pan. The site was
subsequently excavated and underlying this location (buried below a metre of marine silts) was found one of the one of the earliest
salt making sites in Britain
Below Essex sea defence: strip siurvey locting a Romano-British saltern
