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In addition, geophysical techniques
allow researchers to collect archeological
data without destroying the site; material
form and position are not impacted as with
traditional excavation. While these techniques
are not as useful in all archeological situations,
the sheer size of the Hopeton Earthwork,
coupled with the distinct nature of its
walls and other features makes it an excellent
candidate for the use of geophysics.
The geophysical survey work conducted by
the Midwest Archeological Center included
magnetic surveys using a Geoscan FM-36 fluxgate
gradiometer and a Geo-metrics G858 cesium
gradiometer and soil resistance surveys
with an RM-15 resistance meter. During the
course of this research, the Midwest Archeological
Center and Hopewell Culture National Historical
Park also sponsored a workshop on geophysical
survey techniques. The contemporaneous scheduling
of these two events allowed Dr. Bruce Bevan
to conduct independent geophysical prospection
studies at the Hopeton Earthworks site (Bevan
2001). Multiple survey results are useful
in that researchers can compare results,
interpreting individual anomalies in the
lab.
The 2001 geophysical survey
at Hopeton Earthworks was conducted on the
south side of the east-west farm road that
passes through the square. The 2002 survey
covered most of the rest of the square.
In total, geophysical survey included 104,000
square meters. The data from these surveys
were useful for two investigative goals:
to learn more about the earthwork walls
and to help investigate other aspects of
site structure.
Wall structure
One notable accomplishment of the geophysical
survey was the discovery that the western
and southern walls of the square are very
distinctly visible in the magnetic survey
data. The sharp boundaries on the interiors
and exteriors of the walls are in marked
contrast to existing topography, which is
very gradual due to years of agricultural
activities. The sharp magnetic contrast
between the core of the wall and the surrounding
soils suggested that the interior of the
wall must have been constructed from a material
that differed markedly from the soils that
occur naturally on the terrace.
Magnetic survey data were used to select
areas for test excavation that might best
expose some of the better preserved sections
of the wall. The strong magnetic lines marking
the earthwork wall lines are separated by
10 to 15 m. Breaks in the wall line anomalies
correspond to the gateways mapped by Squier
and Davis (1848) and Thomas (1894). The
east-west streaking that occurs at various
places is probably the result of deep plow
scars.
Non-architectural site structure
During the course of the magnetic survey
of the wall, data were also collected from
areas immediately inside and outside of
the square wall. It was hoped that this
survey data would identify anomalies that
might represent features resulting from
Hopewellian activities associated with the
earthwork. Examination of the survey data
resulted in the identification of 11 anomalies
that might represent prehistoric features.
These became candidates for strategic archeological
test excavation.
Conclusion
Geophysical survey and analyses contribute
greatly to research at Hopeton Earthwork,
which also includes topographic and excavation
data. This nested approach to this immense
site allows substantial coverage, allowing
archeologists to test hypotheses about overall
structure and formulate new ones about specific
aspects of this amazing feature NEXT
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Archeologists using the cesium gradiometer
