National Park Service archeologist William Griswold uses a magnetometer to search for anomalies in the soil that may be remnants of earlier human settlement at Adams National Historic Park in Quincy.
National Park Service archeologist William Griswold uses a magnetometer to search for anomalies in the soil that may be remnants of earlier human settlement at Adams National Historic Park in Quincy.
courtesy of William Griswold

Archeology and physics: the two fields seem about as different as can be. Archeologists are concerned with human history, their interests extending as far back as maybe a few million years, when stone tools began to appear. Physicists think on a grander scale, concerned with understanding laws of nature and phenomena that would be the same, whether or not there were human beings around to figure them out.

On Wednesday, William Griswold, an archeologist with the National Park Service, will explain how the two fields intersect, at Boston College’s Weston Observatory.

Griswold uses geophysical instruments, including ground-penetrating radar, an electrical conductivity meter, and a magnetometer to explore archeological sites. Those tools enable archeologists to do remote sensing—getting an idea of what’s underfoot without disturbing the soil.

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“What we try and do is identify anomalies below the ground that could be archeological features,” Griswold said in an interview. “Archeology, kind of by its very nature, is a very destructive science, in that whatever we dig up we can never put back. So we have to be meticulous in our examination.”

A few years ago, Griswold worked at a site in Minute Man National Park called Bull Tavern. It’s thought to be where an 18th century tavern existed. The building is long gone, but Griswold thinks his instruments have helped reveal where the foundation was.

“I feel pretty confident we did locate the remains of a building,” Griswold said. “But here again, there’s a difference: geophysics identifies anomalies, archeology identifies features.”

Griswold said that it’s not quite as simple as just using geophysics tools to see clear details from centuries ago embedded in the ground, the way it might be depicted on a television show. The machines give scientists anomalies—unexpected differences in the conductivity of soil, or hints of what might be a building foundation or a trench—that need to be confirmed by old-fashioned digging.

“For instance,” Griswold said, “the ground-penetrating radar will show us pit-related features. You can see where the ground has been dug into, and you can extrapolate from that the dimensions of the hole that was dug.”

But how was it dug? It could have been a person from the 1700s. It could have been created by a large tree falling over and taking all the soil with it in a big knot of roots. It might even be a rodent tunnel.

“All these types of non-human events can pose as anomalies,” Griswold said.

Griswold works up and down the East Coast—he was recently at Valley Forge National Park in Pennsylvania, helping on a project in which some huts are being relocated. The park service needed to know what it might be disturbing. In another indication of the complication of getting clear answers from such work, Griswold found himself probing not Revolutionary War-era features, but litter left behind from three Boy Scout jamborees held in the park in the 1950s.

Griswold’s talk will be held Wednesday, May 22, at 7 p.m. at the Weston Observatory at 381 Concord Road, Weston. The colloquium is free, but reservations are required. Call (617) 552-8300.