There’s a bed of ringing rocks in Bucks County that make music with the help of a hammer

"That's about a quarter-tone flat," Curtis Nolley said. "Try it again."

He paused. Listened.

"That was pretty good," he said with the self-assured satisfaction of a music teacher.

Nolley, a visiting professor of choral music at Virginia's Bridgewater College, would usually conduct in a classroom. Not on this day.

Sitting with his wife and friends in a craggy sea of rocks that shoots off a dirt trail veined with bumpy tree roots, Nolley was in Bucks County's Ringing Rocks Park, hammer in hand as he tapped on these curious stones.

 >>READ MORE: Down on the farm with the robots and drones

Your average rock would emit a most uninspiring thud. But this is no regular rock. It chimes, with a delicate tink.

To Nolley, 64, of Harrisonburg, Va., that sound could be the beginning of a song. He just had to find the remaining notes.

Hearing a rock ring — or sing, or chime like a bell, others say — doesn't simply happen anywhere. It's a phenomenon found at only a few places in the country, including Upper Black Eddy's Ringing Rocks Park, home to thousands of boulders made up of iron-rich diabase, a type of igneous rock. The 128-acre park, with a waterfall, is aptly named after its standout spectacle, the naturally forming rocks, which peal noisily when struck.

Fields of ringing rocks are "quite rare" in the United States, said Helen Delano, senior geologic scientist at the Pennsylvania Geological Survey, adding that this park is one of the largest, if not the largest, diabase boulder fields in the East.

The boulders ended up at the site after the ground thawed after the Ice Age, she said, becoming "kind of a soupy mess." The finer sediment, sand, and mud would have floated out.

 >>READ MORE: Why a Rutgers scientist heated up moon rocks

"So we've got this pile of boulders sitting there, and there's no soil or very little soil between them, so they can't support plant growth," she said.

The rocks that ring usually sit atop other rocks and have weathered over the years, she said. The ones that don't sing — at least not at a range detectable by human ears — are frequently buried, wedged between the blunt corners and angled points of other grayish-brown stones.

Delano cited a 1912 journal article that said there were five "ringing" fields of rocks in Southeastern Pennsylvania. Two of the fields are in Bucks County, two in Montgomery County, and one in Chester County, wrote botanist and mineralogist Edgar T. Wherry in the article, published in Proceedings of the Academy of Natural Sciences of Philadelphia.

Most people who visit the park on Ringing Rocks Road, just past Lonely Cottage Road, haven't read that century-old article. They go because sites such as TripAdvisor and Yelp boil it down to the basic: Ringing rocks are cool.

Last week, somewhere on this rocky plain, a child shouted, "I found one!"  One second later came the excited rhythm of tink, tink, tink, tink. Every minute or so, there was a clunk as someone used a hammer to hit a rock that didn't ring.

The technique to find a ringing rock? Look for the ones sitting on top that aren't too tightly crammed between other stones. Another visual giveaway: rock surfaces that have been rubbed white by hammer marks.

Nolley, perched on a rock, had tapped on several ringing rocks. Some produced musical notes, and he strung those notes together.

"We found some chords," he said. "Some major thirds, some major sixths, some intervals that are next to each other." He quietly hummed as he tried to decide what simple tune he could hammer out. Around him sat his wife, Andrea, a school librarian who plays the flute, and their friends Sandy Fyke and David Foster of Pattenburg, N.J.

Nolley decided on the wedding march. Was anyone getting married at this boulder field? No. (It wouldn't be a good venue, anyhow — it's too treacherous to walk in any way but slow, measured steps.) But the beginning notes — G, C, C, C — could be played, albeit a bit flat. He enlisted his wife and Fyke to contribute. Could they, he asked, find rocks near them that could be played for the next couple notes — G, D, B, C?

"It's not going to be perfect pitch," said Nolley, who at one point pulled out his mobile phone to access a tuner app. "You're going to have to fudge a little here."

Still, music is music.

And if music was what people wanted to make with ringing rocks, the park will allow it, acting as a de facto recording studio set against a clear blue sky and acres of emerald-green woods. Nolley and his crew persistently tapped on various stones, searching for the closest notes to play a portion of the wedding march. Sitting in a circle, legs splayed before them, the Nolleys and Fyke rehearsed.

 >>READ MORE: Philly entrepreneurs recycle glass into rock — and envision gold

Days before, two giggly young adults had clambered over the boulders. One thwacked the rocks with a hammer, thrashing his head up and down in imitation of a rock star on stage.

And in the late 19th century, a Pennsylvania native, J.J. Ott, used ringing rocks of different pitches to play music with a group called the Pleasant Valley Band. He performed for the Buckwampum Historical Society of Bucks County.

"I think it's very hard to envision what it can really mean to have rocks that sound like they're hollow and ring like a bell," said David Hanauer, who grew up around Doylestown and highlighted the ringing rocks in a website he created in the late 1990s.

"It's really hidden away," Hanauer, now a doctor and clinical associate professor at the University of Michigan Medical Center, said of Ringing Rocks Park. "Very few people know it exists."

He said he hasn't updated the website in more than a decade. But the internet doesn't leave many stones unturned.

Sites like Atlas Obscura, which point out unusual travel destinations, have appealed to the senses of people who seek a little adventure and don't mind sitting for a winding car ride along the edges of Bucks County.

That includes people like Raymond Helmich, an 11-year-old from Hamilton, N.J.

Armed with a hammer in each hand, Raymond raised his arms and slammed the tools facedown on a hunk of rock.

"The way I kind of see it, the rocks that aren't touching as many things — like that one right over there," he said, standing and pointing to a large, flat rock perched at the top "– it's higher up and not touching as much. It makes a cooler sound than one that's over there." He pointed to a rock tightly wedged between dozens of other rocks.

He's right. It's what scientists like Delano, of the Pennsylvania Geological Survey, and Wherry, author of the 1912 article about the ringing rocks, have pointed out.

"It's just science in action. It's very cool," Delano said. "I'm just like everybody else. I might have a rock hammer instead of a carpenter's hammer, but it's cool to walk around and tap on things."