Ringing Rocks

A genuine geological oddity you can hear for yourself

In the woods near Upper Black Eddy, in Bucks County, a field of dark boulders covering about seven acres lies bare among the trees. Some of the boulders ring like struck metal when hit with a hammer. The rest give the dull clunk anyone would expect from a rock.

The land came to the Penn family from the Lenape through the 1737 Walking Purchase, and the earliest published description of the Bridgeton boulder field appeared in 1876. In 1895 Abel B. Haring, president of the Union National Bank in Frenchtown, New Jersey, bought the field and turned down a maker of Belgian paving blocks who wanted to quarry it. On 22 August 1918 he gave it to the Bucks County Historical Society. It later became a county park, and land purchases have grown it to 129 acres.

Rocks like these once gave a concert. In June 1890 John J. Ott, a physician from Pleasant Valley, played a set of boulders of about 200 pounds apiece, gathered from the Stony Garden field on Haycock Mountain, at a meeting of the Buckwampun Historical Society. He tuned them by chipping, struck them with a steel hammer, and played with a brass band; the bell-like tones carried over the horns. The selections included "Home Sweet Home".

The boulders are diabase, from sills formed when magma pushed into the Newark Basin some 200 million years ago, early in the Jurassic. Crystals of olivine and pyroxene settled to the bottom of the sills and hardened into a separate layer 10 to 15 feet thick, harder and denser than the diabase above it. During the Pleistocene the ice sheets stopped short of these outcrops, but the hard freezing near their edge shattered the rock into boulder fields of the kind called felsenmeer, German for sea of rock. On the gentle slope the fine debris washed away before soil could form.

The ringing is only partly explained. In 1965 Richard Faas of Lafayette College tested rocks in his lab and found that they produce tones below the range of human hearing, and the audible sound comes from those tones interacting. In the 1960s a Rutgers professor sawed live and dead boulders into thin slices; only the ringing ones expanded within a day, a sign of internal elastic stress. Iron, a popular explanation, sits within the normal range for basalt. Brett McLaurin, a geologist at Commonwealth University, says only certain boulders vibrate freely enough to produce sound, and that the mystery is still not fully solved.