Aerial view of the henge. © Highways Agency

Archaeologists working on the site of a new bypass in Cornwall have discovered a ceremonial henge - the first of its kind to be found in the region.

The circle-henge or pit circle was found during excavations at Deep Tye Farm between Bodmin and Indian Queens, where improvements to the A30 are being carried out to ease traffic and bring environmental gains to Tregoss and Goss Moors.

The ceremonial monument was built in the late Neolithic period (3000-2000BC) and consists of pits in two arc formations, one inside the other. The outer arc forms a ditch 10metres in diameter while the inner arc has 10 postholes that may have contained freestanding wooden posts or a more elaborate structure. There is a wide gap to the south.

“The Deep Tye Farm site is a modest example of this type of monument,” said Stuart Foreman of Oxford Archaeology, who carried out the excavation work on behalf of the Highways Agency, “which can reach quite lavish proportions. Stonehenge is the best known example.”

Archaeologists standing in the henge formation. © Highways Agency

The Deep Tye henge is a particularly rare type of structure, largely because it can only be revealed by open excavation. Only about 50 hengi-forms have been identified nationally, mostly in the south of England.

“Such monuments were built by the early farming communities of Britain during the Neolithic period,” explained Mr Foreman. “The purpose of these monuments is not certain, although it is generally accepted that they acted as arenas for ceremonial gatherings and ritual.”

“They were places of ritual offerings, sometimes involving sacrifice of animals and more rarely people,” he continued. “Some sites also served as cemeteries for human cremation burials.”

Henges sometimes incorporate astronomical alignments, but this does not appear to be the case at Deep Tye Farm.

Work on the roundhouse excavation. © Highways Agency

Other large-scale henges are known in the south-west, like the Stripple Stones on Bodmin Moor and the earthwork Castilly Henge. The new A30 route passes through an area rich in archaeological remains – careful planning has avoided damage to any of the known monuments, including Castle-an-Dina, Bronze Age barrows and tin mining features from the 18th to 20th centuries.

The search continues for artefacts at the Deep Tye Farm site, but at nearby Lower Trenoweth, the remains of a large Romano-British roundhouse were found, along with a large section of a pottery jar dating back about 2,000 years.

It is speculated that the shelter may have been a seasonal dwelling used by tin miners, given the scarcity of artefacts found. However, despite the rich sources of tin on Goss Moor, no evidence for tin working has been found at the site so far.

Finds will go to the Royal Cornwall Museum in Truro following analysis.