Scientific tests have shown that Seahenge, the mysterious wooden circle which appeared out of the waves on the Norfolk shore last year, dates back to 2050 BC - and springtime.

The results of the tests, using a combination of modern science and formulae devised by an 18th century vicar, are as romantic as the monument itself, one of the most dramatic archaeological discoveries of the century. The great oak, whose splayed roots formed the "altar" at the circle's centre, was already 150 years old when dragged into position using ropes of plaited honeysuckle.

The dating places the monument, at Holme-next-the-Sea, in the very early Bronze Age, a period when the massive stones were being raised at Stonehenge.

The timbers were removed from the beach, for testing and conservation, amid unprecedented protests. Many felt the monument so beautiful and striking it should have been left, even if the consequence was its gradual erosion and destruction.

The first attempt at dating using the simplest method, counting tree rings, failed because of exceptional conditions at the exposed site. Then samples were given radio-carbon dating, at Queen's university in Belfast, which produced a date but with a possible error factor of several centuries.

Alex Bayliss, dating expert at English Heritage, said they had then turned to 250-year-old mathematical formulae devised by Thomas Bayes to combine the carbon dating with botanical and climate information: this showed the tree died between April and June 2050 BC. The posts of the circle, which retain their bark, were chopped down exactly a year later, in spring 2049.

The tests leave the central mystery intact: the purpose of the monument, and the nature of the rituals held there, can only be guessed at.



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