An ancient stone circle built 50 years before Stonehenge has been uncovered on the site of its quarry after lying buried in peat on a Scottish island for thousands of years.
The structure, Na Dromannan, is 100 feet (30 metres) wide and overlooks the four other Standing Stones of Callanish, a pattern resembling a Celtic cross on the Isle of Lewis, in the Outer Hebrides. Built in 3,000BC, it is considered second in importance to Stonehenge and archaeologists believe it will offer new insights into the social function of the stones.
Colin Richards, a senior lecturer at Manchester University's school of art history and archaeology and leader of a team that began excavating the site three weeks ago, said Na Dromannan was unique.
"It is the only stone circle we have found built on a quarry, and the circle could commemorate the quarry as sacred. It has led us to reassess the function of these circles. Where the stones come from and the act of moving them from one place to another seems to be more important than the finished circle.
"We found little activity went on around these circles after they were built. The meaning may lie in the gathering of hundreds of people to move these huge stones, the biggest to be moved at the time."
Although archaeologists have traced the geological site of Stonehenge to a mountain in south Wales from which the stones came, no evidence of its quarry has been found.
The Lewis stones, many measuring 13ft (four metres) long, are on a rocky outcrop rather than the soft ground in which henges are normally found. They were propped up by stones encircling their bases, though many have fallen.
"There are not many stone circles in this condition and I have never seen this type of construction before," Mr Richards said. "It was long thought there might be a further stone circle on the site but until now, it has lain buried in the peat. It looks spectacular, on a ridge overlooking a loch and the Callanish stones. It should be good for the tourist trade."
The discovery of the quarry also sheds light on how rocks were transported on wooden sledges to their final destination. The structure is made of Lewissian gneiss, a brilliant white metamorphic rock with black bands running through it, the rock containing feldspar, quartz and mica, which gives it its sheen. One of the stone faces is quartz.
About half of the stones in the circle have been unearthed and the full structure should be fully uncovered next year. The circle is among the oldest known structures in the country.
Two stone structures in Orkney, the Stones of Staenness and the Ring of Brodgar, and the Callanish stones on Lewis are also 5,000 years old. Mr Richards, who is two years into a five-year research project in north-west Scotland, found Britain's first ancient quarry in Orkney this year.
The great stone circles, including Stonehenge, were built during the new Stone Age, which began in 3,700BC. The original uses of ancient structures have been a source of speculation and some theories consider them to be altars and burial sites. Others believe they are astronomical guides.