Non-invasive technique reveals "ancient harvests," dotting Delaware's shore.*

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" shell middens--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 during a Geological Society of America meeting. "These piles, or `shell middens,' are ancient trash heaps from harvests," Chadwick explains. "The Native Americans would wade into the water and dig up seafood. Then, they would heat rocks in a fire and drop the meat and rocks in a pot full of water to cook"

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

Because GPR signals aren't effective in penetrating salt water, however, researchers haven't used the technique on archaeological sites 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 Cape Henlopen, the deposition of seashell middens and evidence of sea-level changes in the area for the past 1000 years.

Native American Harvests

Many of the Cape Henlopen shell midden sites lie beneath the present marsh and relict dunes, but this site was identified by the Delaware Department of Natural Resources and Environmental Control because it lies exposed at the 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 1,000 AD until 1,600 AD," 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 use the shell in the middens to lime their fields," he says. "Farmers came down in wagons and picked them up. Now, these sites are protected."

To image 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, all of which had to be carried into the field in backpacks. "You send an electromagnetic pulse into the ground and the signal reflects off an interface between electrically contrasting materials. The signal is received by the receiver and recorded on a laptop computer," he explains. "The signals identify the difference in density of the sediments by changes in the electrical properties". This data is then plotted 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 or lagoon at the base," he says, adding that his goal is to eventually "ground-truth" or confirm GPR findings by drilling a core sample or excavating the site in conjunction with DNREC to make sure it conforms with data retrieved non-invasively, from the surface.

Advising Chadwick are Geology Professor John C. Kraft, with James E. Pizzuto and John M. Madsen, both associate professors of geology. 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: This paper was presented at the Geological Society Meeting of America in Portland, Maine, on March 20, 1998.

*This web page was modified from a Press Release from the University of Delaware Public Relations Office.

Last updated: March 25, 1998

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