Holme II
© Andrew Parsons/PA
The timber circle Holme II on the coast of Norfolk near Seahenge.
A second prehistoric circle on a Norfolk beach has been dated to the same summer more than 4,000 years ago as its famous neighbour, Seahenge.

Archaeologists believe the two circles, which originally stood inland in boggy freshwater but are now being eroded gradually by the tides, were part of the same monumental complex connected with rites to honour the dead.

Unlike the giant boulders of monuments such as Stonehenge, the only evidence for most prehistoric timber structures is post holes in the ground. However in Norfolk, because the salty silt preserved the wood, the two circles at Holme Beach are the only ones in Britain to have been dated precisely, to 2049BC.

In 1999, images went round the world of druids and other protesters chanting, weeping and trying to block the diggers from dragging the ancient timbers of Seahenge out of the silt and removing them from the beach.

It was the eerie beauty of Seahenge, with the posts half submerged in the waves surrounding the upended stump of a giant oak tree, which made it international news. But the protesters who demanded it be left on the beach missed the second Bronze Age circle, just visible at the lowest tides.

The circles were discovered at the same time, but while some of the conserved Seahenge timbers are now on display in the museum at Lynn, the other was never excavated.

Norfolk county archaeologists monitoring its decay have just released the dating evidence, obtained by dendrochronology - counting the growth rings in the timbers - which shows it was built from trees felled in the spring or summer of 2049BC, exactly the same time as the Seahenge timbers.

The tests were carried out over the last year, before decay would have made the dating impossible.

Both circles were exposed because of their location on one of the fastest eroding coastlines in Britain.

The 55 oak posts of Seahenge surrounded a gnarled oak stump, but the second ring, known as Holme II, was centred on two oak logs laid flat. When it was found these were surrounded by an oval of oak posts with smaller branches woven between them, then an outer ark of split oak timbers, and finally a fence of closely set split oak timbers. Within four years the woven branches were gone, and in storms in October 2003 and March 2004, both the logs were washed away. More timbers have been eroded or lost completely every winter since.

The Seahenge timbers revealed the oldest marks of metal axes ever found in Britain, showing that bronze tools were being used for complex woodwork within a century of the introduction of bronze smelting. The marks of at least 36 separate axes were found, suggesting that its construction was a major communal project.

David Robertson, historic environment officer at Norfolk county council who ran the Holme II dating project, said he believed the circle may originally have been covered by a burial mound, with the central logs supporting a coffin.

"As the timbers used in both timber circles were felled at the same time, the construction of the two monuments must have been directly linked. Seahenge is thought to have been a free-standing timber circle, possibly to mark the death of an individual, acting as a cenotaph symbolising death rather than a location for burial. If part of a burial mound, the second circle would have been the actual burial place."

The inevitable destruction of the monument will be monitored, but there are no plans for further excavation.