Case Study
Prehistoric Houses at CA-ORA-116
Newport Bay, Orange County, California
CA-ORA-116 is an Intermediate period (ca. 300 BC -
AD 700) midden and habitation site at Newport Bay, Orange County,
California. Lewis Somers of Archaeo-Physics conducted a geophysical
survey of the site as part of a multi-disciplinary study of the
site by Statistical research, Inc.
A number of archaeological features were identified
on the basis of the geophysical data. This allowed archaeologists
to pinpoint Features for excavation and to interpret excavation
results within the greater context of the site.
Excavation at CA-ORA-116 was guided in part by the
results of the resistance survey. The top image shows the Highpass
filtered resistance data in color. The bottom image shows excavation
units and identified house features overlaid on the resistance data.
In the image below, excavation units are overlaid in
blue; the high resistance features circled in yellow were identified
as house pits. These results show a clear correlation between resistance
anomalies and prehistoric features.
Two interpretations of the magnetic field gradient
data emphasize anomalies of different magnitudes. Prehistoric cultural
features are likely to produce only very weak magnetic anomalies,
typically less than 10 nanoTeslas (positive or negative) from the
local mean. Anomalies of this magnitude can be difficult to detect
in the presence of ferrous iron. Even small pieces of modern iron
or steel debris can create anomalies of several hundred nT, obscuring
any weak anomalies in the vicinity.
With appropriate processing it is possible to distinguish
weak anomalies even in the presence of strong anomalies caused by
ferrous iron. The top image shows the magnetic data within the expected
range of prehistoric features. For example, house features identified
by excavation (see resistivity map) are associated
with weak magnetic highs. Some anomalies within this range are obviously
historic in origin, such as the two parallel linear magnetic lows
caused by modern vehicle tracks. While anomalies of modern origin
may not directly inform us of prehistoric site use, they can reveal
modern disturbance which may impact the site. The bottom image shows
high amplitude magnetic anomalies which are caused by historic iron
debris.
Reference: House Pits and Middens, a Methodological
Study of Site Structure and Formation Processes at CA-ORA-116,
Newport Bay, Orange County, California. Donn R. Grenda, Christopher
J. Doolittle, and Jeffrey H. Altschul, eds. Technical Series 69,
Statistical Research, Inc. Tuscon, AZ; Redlands, CA 1998.
