The previously unknown Neolithic structure was found within Castilly Henge in Bodmin.
A team of volunteers cleared Castilly Henge near Bodmin of bracken and scrub so archaeologists could map the stone circle using modern surveying techniques.
This mapping found at least seven regularly spaced pits in the interior, forming a shape resembling a crooked horseshoe.
A “henge” refers to a circular or oval-shaped bank with a ditch around the inner edge constructed during the Neolithic period between 3,000 en 2,500 BC.
Not all henges contain a stone circle, and there is only one other in Cornwall – Stripple Stones on the slopes of Hawk’s Tor on Bodmin Moor.
The researchers believe the pits at Castilly may once have formed a complete ring, but ground conditions at the time of the survey left them unable to gather clear data on the northern part of the henge interior.
They found some stones had been removed and taken elsewhere, while some of the others have probably been pushed face down into the pits in which they once stood upright.
Archaeologists believe henge sites would have been used for gatherings and rituals.
Castilly Henge was used for more earthly purposes in more recent times, archaeologists think.
There is evidence to suggest it was used as a theatre in the Middle Ages, and a gun emplacement during the English Civil War.
The research at Castilly Henge began in 2021 when it was included in a monument management scheme by Historic England.
The scheme aims to conserve and repair monuments on Historic England’s at-risk register.
Volunteers led by the Cornwall Archaeology Unit (CAU) cleared the site of vegetation that was threatening features of the site hidden below ground.
Peter Dudley, senior archaeologist at Cornwall Archaeological Unit, gesê: “Over the winter, 13 people gave 111 hours of their time and now the monument is looking so much better.
“The project has also re-fenced the field and the farmer is happy to start grazing again, improving the long-term management of this amazing archaeological site.”
Ann Preston-Jones, a project officer for at-risk heritage sites with Historic Engeland, gesê: “The research at Castilly Henge has given us a deeper understanding of the complexity of this site and its importance to Cornish history over thousands of years.
“It will help us make decisions about the way the monument is managed and presented, so that it can be enjoyed by generations to come.”