North American Database of Archaeological Geophysics
North American Database of Archaeological Geophysics
Abstract/Summary:
Project Name: Bunker Hill National Monument, MA;
Reference: Kvamme, K.L. (2001) Current Practices in Archaeogeophysics: Magnetics, Resistivity, Conductivity, and Ground-Penetrating Radar. In Earth Sciences and Archaeology, P. Goldberg, V. Holliday, and R. Ferring, eds., Kluwer/Plenum Publishers, New York, pp. 353-384.
The Battle of Bunker Hill, of the American Revolutionary War, was actually fought on Breed's Hill, now part of the Bunker Hill National Monument. During the night and early morning of June 17, 1775, American patriots created a redoubt and breastworks on the rest of the hill. It was promptly and successfully attacked by the British who were occupying nearby Boston, but only after repeated assaults with great loss. The size and form of the redoubt are variously reported, and many discrepancies exist between two maps made after the battle by British engineers. It is reported as generally square, but with projecting angles, including a redan (a triangular projection) facing west and ramparts that were about 2 m in height with a small ditch at their base. Unfortunately, it was probably destroyed by the British later that year. While the surrounding battlefield has succumbed to urbanization, a core four-acre area at the locus of the redoubt is maintained within the park. Yet, this area has been greatly disturbed by monument construction since the 1820s with a large obelisk, a lodge (now a museum), extensive pavements, stairways, and landscaping. The last three have been modified several times during this area's long history as a monument; more recent additions include modern plumbing, electricity, wheel chair access, audio markers, and signage. The policy of the National Park Service is to preserve sites rather than to excavate them for research purposes. Nevertheless, limited archaeological excavations were allowed in 1996 prior to the construction of a new irrigation system. A ditch similar to ones found at other Revolutionary War forts was located on the west side of the monument that contained late 18th century artifacts, including two musket balls. It was uncertain, however, whether it represented the famous redoubt or remains from a British garrison and fortification constructed after the battle.
Limited geophysical surveys were conducted soon after these investigations that included GPR, resistivity, and magnetometry. The GPR profiles immediately gave definition to the ditch feature, allowing its extent to be mapped for over 20 meters. It also revealed a second, less robust ditch-like feature aligned at a slightly different angle inside the first. A larger resitivity survey of much of the available unpaved park area was later conducted that sheds further light on these buried features, but raises many more questions. While a hodge-podge of anomalies is evident, the extent of the mapping gives continuity and context to many of them. The sharp, narrow, high resistivity anomaly to the east is undoubtedly some sort of pipe slit trench. Broader, diagonal linear anomalies can also be seen that point to the northeast, southeast, and southwest corners of the park; they represent former walks from the 19th century that extended from the park corners to the obelisk unlike the present ones that traverse the site from the sides. The remaining anomalies probably represent a mixture of American and British constructions from the Revolutionary War period, subsequent alterations to the area by landscaping practices, and particularly construction activities associated with the monumental obelisk. Which anomalies represent the American redoubt? It cannot be said with certainty, but the broad low resistivity features corresponding to the large ditch revealed by GPR and excavations to the west, and similar anomalies to the east and southeast are tantalizing. They also enclose a region within which the monument to the patriots is placed. The complexity of this site simply means that the geophysical results cannot provide unambiguous answers. Nevertheless, the evidence given here provides the best picture to date of what might lie beneath the ground at this important site.