Groton — At about 5 p.m. Wednesday, a saber of sunlight penetrated through the small window at the rear of the drystone chamber in the woodlands known as Gungywamp, reaching across its length to the opening of a small side room near the door.

Particulates from a small fire lit by archaeologist Vance Tiede hung in the air inside the chamber, silhouetting the shaft of light that enters at that precise angle only twice a year: the fall equinox, which occurred Wednesday, and mirrored six months later on the spring equinox.

At the winter and summer solstice, the light shaft enters the same chamber through the doorway.

“Can you all see it now?” Tiede asked the small group of hikers gathered at site to see the phenomenon.

“It’s cool,” said Christie Flemming of Gales Ferry, who had come with her father, David. “I’ve been coming here since I was five. I’ve always heard it’s a calendar cave, but one time we were here and someone was trying to convince us that aliens built it.”

Tiede, the owner of a Guilford-based company that specializes in a branch of archaeology focused on the relationship of ancient structures to astrological alignments, scoffs at the notion that the chamber and other stone ruins at Gungywamp are the work of aliens.

But neither does he believe the prevailing explanation that all the structures are the work of English colonists. Instead, he propounds another intriguing possibility.

Based on his preliminary research, Teide — like others who have put forward a similar theory on the formation's origins — believes there is evidence to suggest that 6th-century Irish monks built the structures, and is working to enlist support for a full-scale archaeological investigation.

As part of that effort, he invited State Archaeologist Brian Jones along for the equinox display, and showed him other curiosities at Gungywamp.

These included carvings on several stones he said match script used at Irish monastic sites that he said contain the phrase “Christ Lord and Master” and “Jesus Christ.”

Jones recalled that one of his predecessors as state archaeologist published an academic paper in the 1980s that attributed all the Gungywamp structures to colonists.

But the paper, he noted, didn’t mention the markings, which Tiede referred to as “petroglyphs.”

“The earlier guys had never seen this?” he asked Tiede.

“No,” Tiede replied.

The style of architecture of the chambers, Tiede said, is unique to monastic sites in western Ireland, western Scotland, Iceland and other North Atlantic islands designed to track the equinox and solstice so monks would know the timing of Easter and Christmas.

“The risk to reward ratio tells me (further investigation) is worth doing,” Tiede told Jones, as Jones ran his hand across one of the gouges in a granite boulder. That work would include some forensic archaeology — searching soil samples for microscopic ancient remains of sheep’s wool and plants such as rye and barley that are not native to North America but that would have been brought over and grown by monks.

Jones was obviously intrigued by the ruins and attentive to Tiede’s encyclopedic explanations of how this site could fit with what’s known about the Irish monks who followed Saint Brendan’s quest across the islands of the North Atlantic.

While maintaining his skepticism, he agreed with Tiede that the site warrants a closer look and offered to send him laser maps of the area.

“I’m honestly troubled that these could be natural intrusions,” Jones said, examining one of the marked stones. “But the thing to do would be to get an archaeological preserve established here.”

The 270-acre site, formerly owned by the now-defunct YMCA of Southeastern Connecticut, has reverted to state ownership and is slated to become a state park because of a provision in the original bequest.

A court proceeding to finalize state ownership could take place by the end of the year, according to officials at the state Department of Energy and Environmental Protection.

Tiede has recently been working with Tyler Beebe, a Groton resident who’s been hiking at Gungywamp since he was a boy.

Working as Tiede’s assistant, Beebe, who hopes to become a professional archaeologist, since this spring has been combing the area with a metal detector and doing excavations, uncovering hand-hewn nails, an old knife blade, slag from iron mining, pottery shards and other artifacts.

“There’s quite a bit of this area that has not been explored” by past archaeological investigations, Beebe said.

He and Tiede had been at the site all day, doing GPS mapping of the ruins.

“We’re trying to see where things are in relation to each other, and then we’ll look at the land records,” Beebe said.

Tiede said they have found several other sites of archaeological interest in the area surrounding the 270-acre state-owned parcel that are privately owned.

Some of the owners would be willing to sell those properties to the state, he said, and others would not.

Both Jones and Tiede conceded that the challenge of an archaeological investigation at Gungywamp would be sorting out what is original and what has been disturbed or altered.

The now-defunct Gungywamp Society, a group of amateur archaeologists who oversaw the ruins for the YMCA, did some restoration work in the main chamber, for example.

“Everything on the back wall is original, and the side chamber is all original,” Tiede told Jones as the two stood inside the partially submerged structure.

Later, walking a deer trail between the sites, Jones posed a question to Tiede.

“In your mind, why would they have come to this location?”

“You have to put this in the context of what’s going on in Christianity in 6th-century Ireland,” Tiede said, beginning an explanation of the monastic communities that spread across the Celtic world at the time.

The Gungywamp site, he said, would have been “more defensible” against Native Americans than an exposed coastal site, yet had water access.

The 14-person currachs the monks would have sailed across the Atlantic could have come from Long Island Sound up the Thames River into a stream that is near the ruins.

The stream is now dammed off into a pond.

If they were followers of Saint Brendan, he said, they would have stayed two or three years, then considered their pilgrimage complete and headed back.

"This stuff is all maybes," he said. "These (chambers) could be root cellars, but root cellars are not known to have astrological alignments. This doesn't prove anything, but English Puritans did not do this."

j.benson@theday.com

Twitter: @BensonJudy