16 December 1999
Seahenge dated to spring 2050 BCE
Researchers using tree ring and radiocarbon methods have dated a Stonehenge-like timber circle, called Seahenge, found in August 1998 poking out of the Norfolk sea. Government agency English Heritage documented the site, removed the timbers from the mud bank they sat on - to save it from the eroding tides that kept it hidden from us for the last 4000 years - and brought them to Flag Fen Laboratory for analysis and preservation.
The excavated ancient monument's large inverted oak tree, discovered with its roots reaching to the sky last year, was felled in April to June of 2050 BC, according to a report published in the scientific journal Nature. The central oak was more than 150 years old when it fell, perhaps as the result of a windstorm. Researchers picked up the traces of honeysuckle ropes used to pull the stump into place. The 55 oak posts that encircled the inverted oak tree were cut down in April to June of the following year, 2049 BCE.
Researchers came up with such accurate dates by combining dendochronology, which matches the growth rings in wood to known historical climate data, with radiocarbon data, a measurement of natural carbon isotope decay.
This is the first time these two methods have been joined, the researchers say. Alex Bayliss, lead author of the report and scientific dating coordinator at English Heritage, says the rings show the trees were cut down in early spring. The central tree was felled or died a year ahead of the rest. "Radiocarbon results then told us the monument dated from between 2,200 to 2,000 BC, but we wanted a more precise answer," she says.
Bayliss and her team next used what is known as a Bayesian model, a complex mathematical process devised by a London Presbyterian Reverend, Thomas Bayes (1702-1761). Involving probability calculations and distribution curves, the mathematical model combined the two sets of information, showing that the central tree had died sometime between April and June 2050 BCE. The same type of analysis was applied to the data for the surrounding posts.
Toby Fox, general manager of Flag Fen Excavations, where the timber circle is now available for public viewing, agrees with the report findings. "We actually suspected the monument dated back to the Bronze Age because we can still see marks on the trees where a broad-bladed bronze ax was used," says Fox.
Both Bayliss and Fox theorize that Seahenge - dating to the same time period as Stonehenge - was a type of mortuary constructed by pre-Celtic individuals known as the Beaker people. Fox thinks the body of an important individual was laid to rest on top of the central tree, cradled by its roots. "There's an amazing mysticism about this monument. When we reconstructed it after the excavation, everyone felt a peculiar power in its presence," he says. "Frankly, it's a bit spooky."
The dating confirms the authenticity of Seahenge. Some will also regard it as further justification for the decision to remove the circle from the sands where they were found. Although druid groups opposed the operation, subsequent examination of the posts has shown that the exposed wood was deteriorating much faster than anyone had suspected. If Seahenge had been left where it was built, the world would soon have lost what many now regard as the Bronze Age discovery of the decade. The timbers of Seahenge will be returned to Norfolk when a future for the wooden circle has been decided.
Sources: BBC News (1 December, 1999), Discovery News, MSNBCE (3 December, 1999)