ARCHEOLOGISTS have started to reveal the secrets held by an ancient stone circle discovered high on Dartmoor.

The structure, at least 4,300 years old, has been found on a boggy plateau near Sittaford Tor, west of Fernworthy Reservoir.

At an altitude of approximately 1,700 feet it is the highest yet known in the South West and takes the total on Dartmoor to 16.

With a diameter of 104ft it is one of the largest and made up of 30 granite markers, now fallen — the stones probably having been taken from the nearby outcrop.

Less than a mile away is Grey Wethers, a unique double circle which shares the same arc across north east Dartmoor as Sittaford, White Horse Hill and a number of other monumental sites.

Experts believe they may have at one time been part of a giant circular network with other circles still waiting to be found.

The existence of a Sittaford circle was suggested in 2007 by Alan Endacott, a museum curator from Throwleigh who now lives near Launceston and who hopes to complete a PhD on the moor's ceremonial landscapes.

Volunteers from the Dartmoor Preservation Association set about clearing the site and found the proof.

Now a £2,000 geophysical survey, funded by the Moor Than Meets The Eye project, has allowed the investigation to begin into who built it, when and why.

Already revealed is evidence of a lost trench that ran close to the stones on a line pointing to the winter solstice sunrise.

An upright stone nearby could be an 'outlier' — an additional boulder that features to the south east of other circles from the Bronze Age or Late Stone Age (Neolithic).

Jane Marchand, senior archeologist with Dartmoor National Park Authority, has examined burned heather from underneath the main stones which, having been carbon dated, suggests they were already lying flat by 2,300BC.

'It's really exciting to have a new important circle like this,' she said.

'These are really nice big stones, carefully chosen. Stone circles are an absolute enigma, however. The ones on Dartmoor never have burial mounds within them so they were probably used for some ceremonial purposes.

'Other circles excavated in the past revealed large amounts of charcoal.

'There's one theory I really like that suggests they were used at night with fires lit within the stones for chosen people only, producing a magical quality with the flames all going and those there not being able to see out.'

Mrs Marchand is keen to see what else the examination can reveal. With the exception of the circle at Tottiford reservoir, dug in 2010 by Channel Four's Time Team, Sittaford is the first discovery of its kind since the 19th century.

'This one's really exciting as it's the first time we've been able to carry out an analysis using up to date methods,' she said.

'What we hope to do eventually is some trial excavation as we'd like more dating material.

'We'd also be looking for other features as circles were often the start of larger ceremonial monuments bringing in burial chambers and other standing stones.

'It's a fantastic site here with 360 degree views and the other important thing is White Horse Hill, where the extraordinary Bronze Age hoard was found, is just on the skyline.

'The idea of all these monuments is you can stand on one and see another.

'In this area we've got what we call the sacred circle of stone circles. Why we have them, however, we don't know.'

Mr Endacott's belief that a circle existed at Sittaford was given a boost in 2007 when fire swept the area.

Among the blackened gorse lay the pillow-like stone he'd been hoping to find.

'I was quite euphoric but trying to contain my excitement as I knew it would not be easy to prove,' he said.

'I mentioned it to Jane but within 12 months the turf and grass had grown over it again and I thought they'd never believe me.

'But a couple of years later she took a look and agreed with my interpretation.'

Mr Endacott also predicted the presence of a 'linear feature' such as a ditch which has now been confirmed by the geophysical survey.

He believes the site's roots could date back deep into prehistory, possibly as far as the Middle Stone Age (Mesolithic) of up to 11,600 years ago.

'The history of these monuments goes back a lot further than we'd assumed hitherto,' he said.

'There's a lot of Mesolithic finds associated with stone circles and my theory is the sites were already being used and seen as sacred long before being monumentalised, maybe in timber initially and then stone.

'At Tottiford material from 10,000 years ago was discovered so it's very exciting to think what we may have here.'