A publication of the Archaeological Institute of America
Geologists at the University of Delaware have successfully used ground penetrating radar (GPR) to measure a midden site on an upland surrounded by a saltwater marsh at Cape Henlopen in Delaware. Previously, the technique had not been used in coastal marsh areas because GPR signals do not effectively penetrate salt water. Because of its proximity to the marsh, it was thought that the ground water at the site was salty.
GPR allows archaeologists to identify underground features non-invasively. The process involves sending an electromagnetic pulse from an antenna into the ground which reflects back to a receiver. Different layers in the ground are identified by the changes in the electrical properties of the signal, and the time it takes for the signal to return is used to determine depth. A profile is generated as the antenna is moved along the surface.
The Cape Henlopen site has been profiled down to depths of roughly 25 feet. The shell midden is six feet deep, 90 feet long, and 60 feet wide, and it is underlain by Holocene sand. The GPR reading will be confirmed by archeological excavations at a future date.
The site, which dates to between A.D. 1000 and 1600, was discovered by the Delaware Department of Natural Resources and Environmental Control in 1976 and is listed on the National Register of Historic Places. Items found on the surface of the shell mound include pieces of pottery, stone tools, and fire-cracked rock. It is thought that Native Americans waded into the water and collected seafood, then heated rocks to drop into pots, cooking the meat. There were many such coastal shell middens when Europeans settlers first arrived, but nineteenth-century farmers, after a suggestion by geologists, used them to fertilize their fields.
William J. Chadwick, a graduate student at the University of Delaware, is using the GPR information in his study of the deposition of shell middens and the evidence of sea-level changes during the evolution of Cape Henlopen over the past 2000 years. Data from work done on other similar sites will be presented at the Geological Society of America Annual Meeting during the last week of October in Toronto, Canada.