Promoting awareness of the archaeology and history
of north Devon
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WEST YEO
Archaeology Projects (external)
Geochemical Survey: Geoprospection methods for detection of metalworking
foci
C. J. Carey Dpt. of Archaeology, University of Exeter (Newsletter No 6
2003)
Chris Carey is a PhD student at Exeter.
Having worked at Sherracombe Ford and having visited the Holworthy Farm
site, he has offered the following description of a valuable new technique.
All Geophysical work referred to in this article is
the work of Substrata Ltd, Archaeological Geoprospection. For further
details please contact Ross Dean, e-mail: archaeology@substrata.co.uk
The aim of this piece is to explain how geochemical survey can be applied
to find evidence of metalworking and how this can be applied within North
Devon, in a general sense.
Geochemical survey is a powerful archaeological prospection
tool, which has a huge potential in exploring the rich metallurgical heritage
of the South West. Geochemical survey is a much-underused tool within
archaeological investigation. This underutilisation of a potentially powerful
technique is the result of continual misapplication of geochemical survey
in previous archaeological work. There have been many suggested and applied
uses of geochemical survey, but these have invariably ended with ambiguous
results. The only proven and robust application of geochemical survey
has been to find field
evidence of metalworking, based on the development by Carey (2003 and
forthcoming) and Carey and Dean (forthcoming).
Geochemical survey directly measures the levels of heavy metals deposited into the environment. The technique can be used to identify areas over which metalworking was occurring and can also be used to identify, map and interpret discrete features/areas. Geochemical survey works in a spatial dimension (analogous to geophysics) and on an intra- site basis, not on a landscape scale. The technique requires the removal of soil samples (c. 5g of soil) from the soil profile and subsequent analysis of the concentrations of heavy metals. Soil samples should always be taken below the topsoil.
Detailed high resolution work requires small sample intervals mainly of 1m or 2m. By using small sample intervals the location and extent of features related to metalworking can be accurately mapped and defined. In addition larger sample intervals (up to 5m as a maximum) can be used to locate large scale metalworking remains such as Romano-British slagheaps. The results of the geochemical survey can also give information on the process that was occurring, for example, where copper impurities are found within an ironsmelting site.
The prospection of metal-working remains has been largely
ignored until recently when the pioneering work of Crew (1997; 2002) and
Dean (2001) developed the use of magnetometry survey to map and date iron-working
features. When magnetometer survey is combined with geochemical survey
a very powerful metalworking prospection tool is formed. For example,
the magnetometer survey can identify a heating event (e.g. a furnace)
and a structure (e.g. a house), whilst the geochemical survey can identify
what the heating event was used for (e.g. copper smelting) and what the
building was used for (e.g.
lead ore processing).
The potential for application in North Devon (and the
South-West) is almost limitless.Combined geophysical and geochemical surveys
have been carried out so far on three hillslope enclosures. A pattern
is starting to emerge of small-scale localised iron production on some
of these sites, representing a glimpse of the prehistoric manufacturing
base of such metals. Similarly, combined magnetometry and
geochemical survey has been used on the Romano-British iron-processing
site at Sherracombe Ford, with startling results. A furnace location and
an intact smithing floor within a workshop were accurately mapped.
The prospection for metal-working residues within the archaeological record has moved into a new era.We are now able to detect exploitation and use of metals from the Bronze Age through to the Industrial Period, through using sub-surface prospection. In summary, the application of geochemical survey to finding metal-working deposits has a large potential. The execution of a geochemical survey requires careful consideration, but when properly used provides a wealth of information that is not available through other techniques. However, in common with many other archaeological approaches, it is best used in conjunction with other geoprospection techniques. The combination of geochemical survey with geophysical (magnetometer) survey has been particularly fruitful.