Vol. 17, No. 25	March 26, 1998

Non-invasive radar reveals area's ancient harvests

William J. Chadwick has been combining the disciplines of geology and archaeology to learn more about such mounds of shells using high-tech Ground Penetrating Radar (GPR). His studies represent one of the first uses of GPR in areas surrounded by a salt-water marsh environment.

His paper, "Ground Penetrating Radar: Its use in understanding the geologic context of a prehistoric archaeological site, Cape Henlopen, Delaware,'' explores the geology of the cape, deposition of the seashells and evidence of sea level changes in the area since the beginning of Native American occupation, about 10,000 years ago.

"One of the good things about this technique is that it's non-invasive," Chadwick said. "You can get a view of what the site looks like before excavating. And what's unique is applying (GPR) to an archaeological site surrounded by a marsh. I've done a pretty extensive search in libraries and the Internet, and people haven't used it in a coastal zone upland surrounded by a marsh."

The site, originally discovered by archaeologists with the Delaware Department of Natural Resources and Environmental Control, is listed on the National Register of Historic Places, and at some point may again be the target of an archaeological investigation, according to Chadwick. He said much of the pile, known to anthropologists as a "shell midden'' lies at a depth below the marsh and dunes. But it was discovered because the tip of the midden was exposed at the ground's surface.

Investigators were able to determine its origin from artifacts that accompanied the shells: "A shell midden is a trash heap. The Native Americans were harvesting oysters, clams, conchs, in the bay. They'd wade out and dig them up. Most were broken up, crushed, and the meat taken out and cooked. The Native Americans would heat rocks in a fire and drop the meat and rocks into a pot of water'' to cook the food, said Chadwick. Shards of pottery, pieces of stone tools and fire-cracked rock were among the diagnostic items found on the surface amid the shells, he said.

The Native Americans "were doing this in a pretty wide time range, from maybe 1,000 AD until 1600 AD,'' before the time of contact with Europeans, Chadwick said. Many piles of shells were found along the shoreline when settlers arrived, but the numbers have dwindled since. "Geologists in the 1800s suggested farmers harvest the middens to lime their fields. Farmers came down in wagons and picked them up. But now these sites are protected,'' said Chadwick.

To measure the midden site, Chadwick and other members of the UD Geology Department carried GPR equipment out into the marsh and moved across the midden. "In the system, you have a transmitter, a receiver and a power source. You carry it out in backpacks. We used two or three people. You send an electromagnetic pulse into the ground and the signal reflects back to the receiver. We can identify different layers by changes in their electrical properties, which cause the signals to be reflected, and these reflections can be graphed on a portable laptop computer," said Chadwick.

GPR has been used by archaeologists to look at hidden underground building foundations in places like Greece, and geologists use the equipment to identify subterranean features such as aquifer sands, Chadwick said. But the signals aren't effective in penetrating salt water, so in the past, GPR wasn't used in coastal marsh areas because nobody thought it would work. Now, Chadwick said, that supposition has been proven incorrect.

In his studies, Chadwick said, '' I've been getting down to about 25 feet,'' learning that sand underlies the shell midden, which is about 6 feet deep and about 90 feet long and about 60 feet wide. "A loss of signal indicates there's a saltwater marsh or lagoon at the base. The ideal would be to 'ground-truth'' the GPR data by drilling a core sample and see if it conforms to what I see from the surface,'' Chadwick said.

The midden study is ideal for a Ph.D. candidate with Chadwick's background. The 31-year-old researcher grew up in the western mountains of Maine and earned a bachelor's degree in geology and geography from the University of Maine at Farmington and a master's in geology from Temple University in Philadelphia. He minored in anthropology as an undergraduate and worked as an archaeological researcher for four years before heading for Delaware for doctoral studies.

"My ultimate purpose is to come up with a history of the evolution of Cape Henlopen from a geological and archaeological point of view. We know the sites as you go west from the Atlantic to the Delaware Bay get younger...so we may be able to define the local sea level curve for the area,'' said Chadwick.

The study of both the geology of the area and the lives of the native peoples are enriched by such projects. As Chadwick notes in his paper, "Definition of the paleogeography and paleoenvironments that surround a coastal archaeological site increases our knowledge of prehistoric peoples' occupancy of past terrains.''

Advising Chadwick as he pursues his doctorate are faculty members John C. Kraft, James Pizzuto, John M. Madsen and Billy Glass, geology, and Jay Custer, anthropology. Chadwick is working on the leading edge of research with GPR, said Madsen: "It's all a relatively new technology. We've used it before to get very high resolution pictures, down to tenths of centimeters, of what the subsurface looks like. And we used (GPR) at Dover Air Force Base to look for evidence of buried drums,'' he said. But Chadwick's work "is exciting. It's really a new way to look at an archaeological site like this,'' he said. And Madsen said that presenting a paper to such a prestigious professional society "is a good thing for a graduate student to do.''

-Phil Milford
Photo by Robert Cohen