The monument, which consists of two huge, circular enclosures — each outlined by tall, wooden posts — is about 5,300 years old, meaning the structure predates the first stones erected at nearby Stonehenge by about 800 years, the study found.
Though the exact purpose of the Avebury monument is still shrouded in mystery, archaeologists think the two wooden circles were used for only a short time for a ceremony or festival before burning to the ground.
"It's much too large to be a stock enclosure; it's got to be a ceremonial enclosure," said study co-author Alex Bayliss, a statistical archaeologist with Historic England. "It's completely unlike anything we've ever found in the British prehistory." [In Photos: A Walk Through Stonehenge]
The area around Stonehenge is dotted with ancient historical sites, revealing shrouded glimpses of Britain's prehistoric past. Bones found at a site near Stonehenge suggest that the site was a sacred wild-auroch hunting ground long before the monument was built. Nearby, the largest prehistoric human-made chalk mound, called Silbury Hill, looms over the landscape, though its original purpose remains obscure. Avebury is also home to Avebury henge, a historic stone monument similar to Stonehenge. And the remains of a Neolithic settlement called Durrington Walls shows signs of ancient barbecues and may have been where Stonehenge's builders lived while they were erecting the epic monument.
The wooden circles in Avebury, located about 23 miles (37 kilometers) from the mysterious stone circle at Stonehenge, were discovered when a pipeline was being laid in the 1960s and 1970s. In the late 1980s, Cardiff University archaeologist Alasdair Whittle conducted a small excavation at the site. He and his colleagues found the charred remains of a massive monument that sprawled across the landscape. Based on the charring, the team deduced that two massive, side-by-side circles dominated the landscape, together spanning about 2.5 miles (4 km). One of the large circles was about 820 feet (250 m) in diameter.
"It's like a pair of glasses: There's two circles with a small gap in between them," Bayliss told Live Science.
Workers in ancient times likely constructed the site first by digging large ditches and then placing oak posts into sockets in the earth, Whittle told Live Science. Then, they used the dug-out earth to backfill and cover the base of the posts and create a huge palisade. The posts were very closely set, so it's likely that hundreds and hundreds of trees would have been cut down to build the monument, Whittle added.
"It looks like a serious, big enterprise," Whittle told Live Science.
During that excavation, scientists dated a shard of pottery found in one of the post holes, using the ratio of carbon isotopes, or versions of carbon with different numbers of neutrons. Based on that analysis, the team determined that the site was used around 2500 B.C., the same time as the first stones were raised at Stonehenge.
However, in recent years, carbon-dating techniques have improved dramatically, so the team revisited the analysis. This time, they carbon-dated the charred remains in the post holes, along with animal bones at the site and fragments of pottery, with improved techniques.
It turned out that the site was 800 years older than the prior research had suggested. The monument was erected during a dark period in this region of Britain's history, for which relatively little archaeological evidence exists.
"There's shed loads of stuff happening in the 700 years before and shed loads of stuff after, but there's almost nothing in the middle," Bayliss said.
The team suspects that the two great enclosures were used as a gathering place — though not for long, as there were few other remains of human settlement or occupation dating to the period, Bayliss said. It's possible that one of the enclosures was for women and the other for men. People would have gathered, and then burnt the massive timber circles to the ground in what would have been an "amazing spectacle," Bayliss said.
Right by the wooden circles, archaeologists also unearthed artifacts such as animal bones, remnants of ancient structures and pottery shards suggestive of late Neolithic housing. Those remains also date to about 2500 B.C.
"That's significant in the local context because it shows people coming back to an old site" — a trend that seems to crop up across southern England in this rough period of prehistory, Whittle said.
The settlement also neatly overlaps with the raising of Silbury Hill, just two or three fields away, suggesting that some people from the settlement may have been involved in its construction, Whittle said.
"They could have been the people who were doing this," Whittle speculated.
The new findings will be published Friday (June 9) in the journal British Archaeology.
Originally published on Live Science.
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Tia is the managing editor and was previously a senior writer for Live Science. Her work has appeared in Scientific American, Wired.com and other outlets. She holds a master's degree in bioengineering from the University of Washington, a graduate certificate in science writing from UC Santa Cruz and a bachelor's degree in mechanical engineering from the University of Texas at Austin. Tia was part of a team at the Milwaukee Journal Sentinel that published the Empty Cradles series on preterm births, which won multiple awards, including the 2012 Casey Medal for Meritorious Journalism.