Archaeologists are "excavating" a Roman city the size of Pompeii in the West Midlands without turning a spade or disturbing a single brick. With a range of experimental remote-sensing techniques, they are delving under the rolling countryside of Shropshire and piecing together a detailed matrix of the streets and buildings of Wroxeter.
Not only can researchers now tell what Viroconium Cornoviorum looked like when it finally subsided under the soil in the Dark Ages, but through the new process of "time slicing", an underground application of radar, they are able to chart the city's rise and fall during its 400 year history by sampling buried remains at a range of different depths.
News of the highly promising advances at what is perhaps the world's first open air archaeology laboratory was broken to the British Association's Annual Festival of Science at Birmingham last week [Sept 9th]. Findings so far include a streetscape much bigger than previously thought, containing clear outlines of houses, shops, workshops and what appears to be an early church. There are also tantalising traces of what appears to have been a huge fire, which swept through the flourishing city. With the traditional strategy of the large-scale dig becoming both prohibitively expensive - it also damages the very remains the scientist wants to examine - such new methods of non-intrusive detection could revolutionise archaeology. Nor should the public feel deprived: the sort of data becoming available will be used to put together virtual reality tours on a computer screen to replace physical visits.
Wroxeter, the fourth largest Roman city in Britain, is the ideal location to test experimental techniques. Unlike most Roman towns and cities, whose stones were plundered for subsequent building after the collapse of the Roman Empire round 410 AD, then were built over and largely destroyed - London and Rome are classic cases - Wroxeter remained largely untouched. It decayed gradually over time and was covered by soil. Modern Shrewsbury was built on another site nearby.
There has been digging at Wroxeter at various times over the past 200 years. But the features unearthed so far, a massive baths complex and part of the forum, still represents only about five per cent of the total city area, which extends across 78 hectares.
Even if excavation of the entire site, owned by English Heritage and the National Trust, was ever an option, it would have been painfully slow. It can take years to excavate a single building; digging up an entire city could last hundreds of years. Pompeii has been under the spade for centuries and much remains to be uncovered.
The Wroxeter Project was launched two years ago by Birmingham University. Scientists and research bodies around the world were invited to apply the latest technology to investigate the site. What lay below the grass was largely a mystery. There was only baseline knowledge of its approximate shape from aerial surveys of cropmarks, a standard way to map the rough outline of buried structures.
Among the techniques in use are two computer-aided variants of well-tried procedures. One is resistivity, which archaeologists have applied since the 1950s. Probes containing electrodes are stuck in the ground and an electric current is passed between them. The resistance to this current varies according to the composition of buried remains. Walls, for example, cause a high resistance; while ditches, which may contain water, present a lower resistance. However the process can be laboriously slow, even with a computer to capture data, because the probes have to be re-inserted for each fresh reading.
At Wroxeter archeaologists are using an experimental resistivity device, developed by French research agency CNRS. The electrodes are fitted to the spiked wheels of a buggy, which is pulled across the ground at walking speed, considerably speeding up survey work. A second procedure is magnetometry, which exploits the fact that certain buried remains such as walls, ditches, pits and kilns cause subtle distortions in the earth's magnetic field.
Using a device called a fluxgrade gradiometer, researchers from English Heritage and Geophysical Surveys of Bradford walk up and down the site taking readings every half metre. Data are downloaded onto computer. 80% of the site has already been probed by magnetomety, in one of the biggest surveys of its type yet attempted.
Normally walls show up on instruments as black lines. One fascinating finding at Wroxeter was that walls of some buildings registered as white lines. This is thought to be the result of the magnetic effect of the walls being reversed, possibly due to an intense fire. This may have been the same blaze that swept through the market stalls in the forum and left stacks of pottery in a gutter, where they were detected by other instruments.
The drawback to both these procedures is that they record only the latest level of buried remains. This is the archaeological palimpsest, or one story written over another. Ground Penetrating Radar (GPR), or time slicing, takes them beyond that information barrier. GPR, developed by the Japanese NARA Cultural Institute and the University of Miami from technology used in mineral exploration, works by directing electro-magnetic pulses at varying depths. The signals bounce back from whatever they happen to strike to produce a stack of underground "time" profiles, every 10 -15 cm down to several metres, representing many phases of building and rebuilding.
This equipment has enabled researchers to recreate a detailed picture of the development of Wroxeter back to the time when it was founded around AD90 as a fortress against the wild lands of the west that became Wales.
"This is the most interesting and exciting thing we are doing," said Simon Buteux [correct, no "a"] director of Birmingham University's Field Archaeology Unit. "And the results are great."
The challenge to the researchers will be to unify the data and create one composite electronic illustration of Wroxeter. That in turn will form the base information for a separate project, funded with a #92,000 grant from BT, to recreate Wroxeter in virtual reality, fleshed out with the latest knowledge of Roman life. The computer package will be made available free to schools. The first "school trips" to ancient Wroxeter are planned for 1998.
"I wouldn't ever see these techniques entirely replacing excavation," said Mr Buteux. "You don't get the detail or recover the artefacts, as you would from a major dig. But remote-sensing enormously widens the archaeologist's options.
"Mapping remains in such detail does allow your target limited excavation very precisely at some interesting feature. In the past we have been wallowing about in the ground only finding what we happen to find.
"Now we can leave sites intact for the archaeologists of the future, who may come up with superior remote sensing devices quite unimaginable to us today."
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