A new survey at Stanton Drew in Somerset, UK, has revealed a previously unknown 5,000-year-old henge ditch surrounding the Great Circle (the second largest stone circle in Britain, after Avebury), and inside that a series of concentric rings, probably the remains of the largest wooden henge yet discovered. This has been done without any excavation — instead, the archaeologists measured the magnetic susceptibility of the soil.
Magnetic susceptibility is the ability of a material to become magnetized by a magnetic field (such as the Earth's). Archaeological features may have a higher or lower magnetic susceptibility than the surrounding soil, and such variations can be detected using a magnetometer. Many archaeological sites have been surveyed in this way.
Anticipating that Stanton Drew would show only weak magnetic anomalies, the team from the Ancient Monuments Laboratory of English Heritage adapted their survey to make it as sensitive as possible. A narrow sampling interval was used, and their magnetometer was modified so that the lower sensor was as close to the ground as possible.
Their most remarkable discovery was the presence of nine concentric rings of weak magnetic anomalies within the Great Circle. These are not additional ditches, but a series of closely spaced yet isolated anomalies, probably the remnants of rings of post holes. Although wood only possesses a very small magnetic susceptibility (and the wooden posts have long since disappeared), there are reasons why post holes might give positive magnetic anomalies. Magnetotactic bacteria that thrive on rotting wood could lead to concentrations of biogenic magnetite in the soil, which remain long after the wood has decayed1. Experiments have shown that the remains of wooden posts can indeed be detected by magnetic surveying2. Alternatively, burnt deposits (timber or otherwise) in the post holes would also produce a variation in magnetic susceptibility, as would a back-fill of topsoil (which tends to have a greater magnetic susceptibility than the underlying soil layers).
There have been no excavations at Stanton Drew, and so there is no secure evidence that the magnetic anomalies are post holes — but based on evidence from analogous sites such as Durrington Walls3 and Woodhenge4, it seems a safe assumption. Stanton Drew bears some resemblance to these sites, but it is twice as large as any of the seven other known timber complexes (which are peculiar to Britain), and so it is an important find. Its use might have been ceremonial, perhaps as the focal point for a certain tribe; there is no evidence yet of any astronomical purpose.
English Heritage is currently experimenting with a new form of magnetometer. Their caesium alkali vapour magnetometer has a sensitivity of 0.01 nanotesla (less than a millionth of the strength of the Earth's field) and it has been used to repeat parts of the Stanton Drew survey. Preliminary results show that this instrument has potential for ‘seeing’ archaeological features in even more detail.
The Stanton Drew survey is a fine example of the use of magnetic surveying in archaeology, and the power of the technique to detect features that would otherwise go unnoticed.