A GEOPHYSICAL SURVEY OF AN AMERICAN REVOLUTIONARY WAR FORT

To determine the full extent and actual shape of the defensive ditch of Gridley's star fort, an important American Revolutionary War site, a geophysical survey using ground penetrating radar (GPR) and electromagnetic induction was conducted. Archaeological excavations in the mid 1990's confirmed the existence of sections of the lowest levels of the fort – the defensive ditch, and magazine and gate foundations – but the size of the ditch was at odds with published plans of the fort. The GPR, with a 400mHz antennae, and the electromagnetic induction instrument collected three frequencies (13kHz, 9kHz and 6kHz). The star shape of the fort meant that geophysical transects could not be oriented normal to the feature. The results of the survey were fairly conclusive showing that the location of the ditch did not agree with the published plans and had a consistent width of three meters on four bastion faces and a width of 7 meters on one face adjacent to the gate complex. The width of the ditch on the second face adjacent to the gate is unknown due to the angle at which the radar lines intersected the ditch on this face. An 8 meter broad glacis with fraise trench was discovered adjacent to the north and west facing ditches. It was not observed on other faces but the absence could either be the result of the angle of the profile lines or lack of construction. A cunette is visible on many of the radar lines. The survey was successful and uncovered many previously unknown features, most notably the existence of the glacis. The 13 and 9kHz frequencies proved to be the most useful of the EM frequencies owing to the variable depth of the ditch. However, the GPR provided the most consistent and reliable survey data.