By David Barrett. Eastern Daily Press, Norwich. Tuesday, June 8 1999
[FROM: "The Times" - London. Thursday 2 December 1999]
BRONZE AGE OAK
WERE FELLED IN APRIL, 2050BC
By Nigel Hawkes, Science Editor
A
CEREMONIAL circle of oak posts found last year at Holme-next-the-Sea on the
Norfolk coast was begun between April and June in 2O50BC, precise dating
techniques have shown.
This is the first time an exact date has been put
on a Bronze Age monument, built around the same time as Stonehenge.
At
the centre of the circle was placed the stump of a huge oak, already 150 years
old. It was dragged into place by ropes made of honeysuckle and formed the
focus of the ceremonial site. The following year a circle of smaller oak posts
was arranged around it.
By combining two dating methods, and using a
statistical technique invented 200 years ago by Thomas Bayes, an obscure
clergyman, British archaeologists have been able to pin down with precision the
moment building began. "It is lovely to be able to get such an exact date for
something built more than 4,000 years ago," said Alex Bayliss, of English
Heritage, who worked with colleagues from Sheffield University and Queen's
University, Belfast.
Tree-ring and radiocarbon dating were used, their
results combined using the methods of Bayes, which have enjoyed a renaissance
in recent years after half a century of neglect.
Tree-ring dating, or
dendrchronology, relies on matching the varying widths of the seasonal growth
rings in wood with a master record going back thousands of years. Ms Bayliss
first compared the rings on the central stump with those on the outer posts and
found that the posts were felled a year later - they had one extra ring. By
pooling the results, she produced an average tree-ring profile for all the
timber. She compared this profile with the records. The closest matches were
for 2050BC, 2454BC and 2019BC.
Next samples of wood were taken from
rings 20 years apart and carbon-dated. This showed that the tree had died
between 2200BC and 2000BC. But the team already knew that the samples dated
differed in age by 20 years. This made it possible to narrow down the
possibilities.
Using a version of Baye's theorem, they combined the
tree-ring and carbon dates and found that 2050BC was most consistent with the
data.
The final touch was to examine the partial growth ring on the
outside of the stump. This showed that the tree had fallen, or had been felled,
between april and June.
- - - - - - - - -
THEORY OF CHANCE COMES
OF AGE
THOMAS Bayes lived and died in relative obscurity, but 200 years
later his method of statistical analysis is enjoying a new vogue (Nigel Hawkes
writes). Bayes was the son of a nonconformist clergyman, and he went into the
ministry, ending his life at the Presbyterian Chapel in Tunbridge Wells, Kent.
Born in 1702, he was educated privately, and in his lifetime gained a
reputation as a mathematician. Bayes was elected a Fellow of the Royal Society
in 1742, but he published little.
But after his death in 1761, Richard
Price, a friend, found among his papers "Essay towards solving a problem in the
doctrine of chances". It was published by the Royal Society in 1763 and became
the basis of a statistical technique, now called Bayesian estimation, for
calculating how true a proportion is likely to be. Unlike classical statistics,
it enables prior judgement to be factored into the equation.
His
conclusions fell into disuse earlier this century, and Bayesians have always
felt despised by orthodox statisticians because of their guesswork. However,
now they are growing in numbers and confidence. Bayesian methods have been used
in court cases, in analysing the results of drug trials, and in improving the
service of banks.