North American Database of Archaeological Geophysics
North American Database of Archaeological Geophysics
Abstract/Summary:
Project Name: Richland Archaeological Project
Reference: Hargrave,
M., S. Alt, D. Maki, and T. Pauketat (2002). Using Magnetic Gradiometry to Investigate
an Early Mississippian Settlement. Poster presented at the Southeastern Archaeological
Conference, Biloxi, MS.
Grossmann Site is an early
Mississippian period settlement located in the Illinois uplands about 18 km
east of Cahokia. In 1998, ITARP archaeologists excavated 17 structures located
in the right-of-way of a new road. In 1999, ITARP conducted a controlled surface
collection of artifacts across the remainder of the site. Dr. Michael Hargrave
(ERDC/CERL) conducted a magnetic survey of the site in 2001. Dr. Tim Pauketat
(UIUC) and Susan Alt (UIUC) directed large-scale excavations at Grossmann, under
the aegis of the University of Illinois Field School in Archaeology and the
Richland Archaeological Project. An area of 3,188 m2 was mechanically stripped,
exposing 42 Mississippian period structure complexes (55 building episodes)
and 58 other features (pits, post pits, truss trenches, hearths, etc.). Grossmann
is an unusual site in many ways. Numerous structures are closely packed into
a relatively small area. By regional standards, the structures tend to be quite
large. One feature included a cache of 70 ground stone celts.
The magnetic gradient survey at Grossmann was conducted using a Geoscan FM36.
Data were collected along transects spaced at .5-m intervals, resulting in 16
data values per m2. Overall, the Grossmann magnetic survey was quite successful.
Using a statistical threshold of 4 standard deviations, about 60% of the structures
and 40% of the other features were detected. Less than 20% of the anomalies
were false positives (i.e., anomalies that were not associated with prehistoric
features).
Magnetic lab tests of soil and artifact samples conducted by David Maki (Archaeo-Physics)
helped explain factors that influenced feature detection and false positives.
Features with larger volumes and evidence for burning, and structures with basins
were more likely to be detected. Some of the false positives may relate to the
remains of features preserved in the plow zone. Based on the ratio of anomalies
to detected and undetected structures, we estimated that an additional 49 structures
are present outside the excavated area.