A team of archaeologists is braving horse flies, spiky vegetation and murky ditches to hunt for mysterious standing stones lost beneath a West Country salt marsh.

The Yelland stone row at Isley Marsh disappeared beneath a thick blanket of silt after the closure of a power station changed the flow of sediment in the Taw and Torridge estuary in north Devon in the 1980s.

This week experts are probing and digging for the prehistoric stone row to make sure it is stable and to try to understand why ancient people built the monument on a spot that may have only been accessible at low tide.

Charlotte Russell, of Historic England, which is funding the work, said the stones were surveyed in the 1930s, at which point they were clearly visible. “They sat there in view happily for decades but when the power station was shut down the stones disappeared from sight and haven’t been seen since.”

Facebook Twitter Pinterest The standing stones in 1953, with the two parallel lines still slightly visible. Photograph: Historic England Archive

Consisting of at least 18 small stones arranged in a pair of parallel rows, the structure is a nationally important site with scheduled monument status.

The idea now is to find a couple of the stones – there is some uncertainty about their exact spot – and check they are intact.

Another aspect is to dig to the level just below the stones to try to get clues about what the landscape was when the stones were set up. “We’ll be reaching a landscape that hasn’t been touched by humans for 4,000 years,” she said.

The team hopes they will find preserved remnants of pollen, seeds or signs of insects that will tell them more about what the landscape was like when the stones were set up.

Russell said it was possible the area was wooded or farmed – or was a wetland as it is is today.

There are no plans to try to completely uncover the stones or keep any they find exposed because it would be too difficult and expensive to keep the water at bay. The area is also delicate because it is an important nature reserve beloved of wading and over-wintering birds.

“As long as we know they are there and safe and haven’t been disturbed, then they are really quite snug and safe in their blanket of silt,” said Russell.

As with Stonehenge, the meaning of the stone row will probably remain a mystery though, intriguingly, similar rows have been found on Exmoor. “They may have been visible from Exmoor on a good day,” she said. “So there may be some relationship between the people at Yelland and those on Exmoor.”

When the work is done, Historic England and the conservation charity the RSPB, which manages the site, plan to set up an information board telling the story of the stones, and will then leave them in peace, unseen and as mysterious as ever.

The team on site this week dodging the flies and trying not to fall into the ditches was led by Martin Bates, a geoarchaeologist from the University of Wales Trinity Saint David, Lampeter.

He said: “We’ve established where they should be. But they are only small stones, not like the bluestones of Stonehenge.” The team thinks the stones lie under about 30cm of silt. After their initial digging they believed they had reached the surface the stones had been placed on – but as yet none of the stones.

The team has had to move in on a neap tide, when there is no danger of incoming water catching them out. But having surveyed the landscape, Bates believes the row was built on land between low and high tide, which suggests whoever built them had a strong connection with the estuary and the ocean. “It’s certainly a strange place to put a stone row,” he said.