UD Geology News: Non-Invasive Radar Reveals "Ancient Harvests," Dotting Delaware's Shoreline

March 20, 1998

PORTLAND, MAINE--Dotting the shoreline near Delaware's Cape Henlopen, seashells evoke Native Americans boiling oysters, clams and conchs 1,000 years ago, says a University of Delaware geologist whose work should help archaeologists "see through" salt marshes--without digging them up.

Piles of seashells near Cape Henlopen, a spit of land jutting into Delaware Bay where it meets the Atlantic Ocean near the town of Lewes, aren't just debris washed ashore by historic storms, UD doctoral candidate William J. Chadwick reported March 20, 1998, during a Geological Society of America meeting. "These piles, or 'shell middens,' are ancient harvests," Chadwick says. "The Native Americans would wade into the water and dig up seafood. Then, they would heat rocks in a fire and drop the rocks and meat into a pot."

Combining geology and archaeology, Chadwick's research marks one of the first uses of high-tech Ground Penetrating Radar (GPR) in areas surrounded by a saltwater marsh. "This technique lets you see what the site looks like before you excavate," he explains. Archaeologists have previously used GPR to examine hidden underground building foundations, Chadwick says, and geologists use the same equipment to identify subterranean features such as aquifer sands.

Because GPR signals aren't effective in penetrating salt water, however, researchers haven't used the technique in coastal marsh areas because "nobody thought it would work," Chadwick says. Now, that supposition has been disproved, thanks to UD research. Chadwick's presentation at the Geological Society of America meeting explored the geology of ancient spits at Cape Henlopen, the deposition of seashell middens and evidence of sea-level changes in the area since the beginning of Native American occupation, about 10,000 years ago.

Seaside Meals

Much of the Cape Henlopen shell middens lies beneath marsh and dunes but was discovered by archaeologists with the Delaware Department of Natural Resources and Environmental Control because their tips were sticking up at the ground's surface, Chadwick says. Shards of pottery, pieces of stone tools and fire-cracked rock were among the diagnostic items found on the surface of the site, which is listed on the National Register of Historic Places and may someday be the target of an archaeological investigation, he says.

Native Americans cooked seafood on the site "in a pretty wide time-range, from maybe 1000 A.D. until 1600 A.D.," before the time of their first contact with Europeans, Chadwick says. Many piles of shells were found along the shoreline when settlers arrived, but the numbers have since dwindled. "Geologists in the 1800s suggested farmers harvest the middens to lime their fields," he says. "Farmers came down in wagons and picked them up. Now, these sites are protected."

To measure the midden site, Chadwick and his colleagues from the UD Department of Geology carried GPR equipment into the marsh. The GPR system includes a transmitter, a receiver and a power source, which are carried into the field in backpacks. "You send an electromagnetic pulse into the ground and the signal reflects back to the receiver," he explains. "We can identify the 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," he says.

Thus far, Chadwick says, he has measured the site down to depths of roughly 25 feet. Sand underlies the shell midden, which is about 6 feet deep, 90 feet long and 60 feed wide, according to UD data. "A loss of signal indicates a saltwater marsh at the base," he says, adding that his goal is to eventually "ground-truth" or confirm GPR findings by drilling a core sample to make sure it conforms with data retrieved non-invasively, from the surface. Advising Chadwick are John C. Kraft, H. Fletcher Brown Professor of Geology; Billy P. Glass, a professor of geology; James Pizzuto and John M. Madsen, associate professors of geology; and Jay F. Custer, professor of anthropology. Madsen says Chadwick is working on the leading edge of GPR research: "It's all a relatively new technology," he notes.

"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. But, he adds, Chadwick's work is exciting because "it's a new way to look at an archaeological site."


PRESENTATION INFORMATION: Chadwick's paper was presented during Session 24 of the Geological Society of America meeting Friday, March 20, at 2:40 p.m. EST, in Portland, Maine.

University of Delaware

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