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Geophysics For Archaeology Assessment: Fort
William Discovered? Fort Laramie National Historical Site, Wyoming.
by
Clark Davenport, Don Heimmer, John Lindeman and John Gilmore
ABSTRACT
Fort William, antecedent of present day Fort Laramie, was built at the
confluence of the Laramie and North Platte Rivers in 1834. The
historical significance of the fort is that it provided the
infrastructure for supporting early western fur trading activities,
offering an operational base for Western U.S. expansion. Inadequate
historical recording of the fort's location during its construction and
subsequent relocation in 1841 to the site of Fort Laramie has been the
cause of much speculation. No observable surface expression of the
older fort exists.
A reconnaissance program of geophysical surveying was conducted at Fort
Laramie National Historic Site in an area designated to be the most
likely site of Fort William. To facilitate the survey, aerial
photography and on-site surfacial geology were reviewed. Several
geophysical methods including magnetometer, ground conductivity and
ground penetrating radar were employed. The ground penetrating radar
provided significant data indicating the presence of a remnant structure
eighteen inches below ground surface. The anomalous area delineated on
the radar profiles are believed to be expressions of cultural
influences. The location and dimensions are consistent with the
historically reported proportions and location of Fort William.
INTRODUCTION
A reconnaissance program of geophysical surveying was conducted at the Fort
Laramie National Historic Site, Fort Laramie, Wyoming during the month
of September, 1988. The program was devised to evaluate the application
of several geophysical techniques for characterization of subsurface
disturbances associated with historical/cultural activities.
A full day of geophysical equipment evaluation, consisting of ground
penetrating radar, magnetometer and ground conductivity surveying were
conducted in known areas of cultural activities on the site occupied by
present day Fort Laramie. Remnant structual features consisting of
calvary stables, storage houses, corrals, fortification trenches and
graves were examined. Knowledge and experience gained from this
investigation were applied to an adjacent property historically reported
to be the site of Fort William, antecedent of Fort Laramie.
HISTORICAL BACKGROUND
With the development of fur trade along the North Platte River in the early
1800's, came the need to provision the fur trappers and the opportunity
to trade with the Indians. Often fur trappers, or mountain men", banded
together to form trading or fur companies. These companies established
outposts throughout the fur trapping regions. Most of these outposts
were supplied with goods transported from St. Louis. These trading
posts served trappers, Indians and the growing traffic resulting from
exploration activities.
One of the early mountain men, William Sublette, established a fort in 1834
near the confluence of the North Platte and Laramie Rivers, in
southeastern Wyoming, near the present town of Fort Laramie. The actual
site of the fort, Fort William, has never been found, although local
legend places the site either within the grounds of the present day Fort
Laramie National Historic Site, or near an old steel bridge,
approximately two miles east of Fort Laramie. Fort William would have
been established close to drinking water, out of reach of flooding and
near existing trails.
Little is known about the design and construction of Fort William. It is
believed that the Fort would have been laid out in a manner similar to
other trading posts, i.e. rectangular in shape, surrounded by a wooden
stockade and having elevated blockhouses at opposite corners. Most
likely the stockade was constructed of square-hewn cottonwood logs, set
upright and side by side in a trench approximately three feet deep.
Travellers who visited the Fort are not in agreement about its size,
reporting sizes ranging from 150 feet square to a 100 by 80 foot
rectangular shape.
Due to the strong competition among the fur trading companies, and the
deterioration of the wooden stockade, Fort William was abandoned in 1841
in favor of an adobe constructed trading post, Fort John, located within
the confines of Fort Laramie National Historic site.
STUDY AREA AND FIELD METHODS
Fort William, built in 1834 and occupied through 1841, is reported, in
historical records, to be located in the southwest quarter of section
22, T26N, R64W, Goshen County, Wyoming. Analysis of available aerial
photography, vintages 1948 and 1976, and surface reconnaissance of the
site indicated no observable surface or subsurface expression of a
remnant structure. The absence of surface expression may be attributed
to site cultivation activity prior to it's acquisition by the National
Park Service.
Knowledge of previous site activities dictated reliance on historical
accounts of the fort's location for layout of the geophysical survey
locations. The site map identifying the reconnaissance grid and later
detail gridding of January, 1989 is provided in Figure 1.
A total of five ground penetrating radar profiles were collected over the
area in September, 1988. Additionally, Lines 1, 2 and 4 were examined
with a magnetometer and an EM-31 ground conductivity meter for
corroborative purposes. Radar data was collected with an 80 MHz
transducer/antenna with range settings calibrated at 150-200 nano-seconds
and 25.6 scans per second. A number of oil pipelines transacting the
site, buried at a depth of five feet, provided a crude, but convenient
time/depth conversion calculation. The sand and gravel materials
deposited on site by fluvial processes, offered an excellent medium for
radar surveying. All radar data was recorded on digital tape for
processing purposes.
FIGURE 1
DISCUSSION
Ground penetrating radar profile Lines 2, 5 and 3, Figures 2, 3 and 4
respectively, are examples of field data acquired in the area. Profile
2, indicates a disturbed area located on the eastern end of the line.
Depth of disturbance begins below a calculated plow depth of 1.5 feet
and extends to 4.5 feet. A deeper disturbance occurs at a depth of 6.0
feet on the far eastern end of the line, it's origin may be associated
with deeper foundation (blockhouse?) material or other possible
excavations. Total maximum length of the disturbed area is
approximately 175 feet. This distance is calculated between points of
noticeable subsurface terminations.
Profile 5, offers a depiction of a north-south transect across the
anomalous area observed on Profile 2. The perpendicular intersections of
the lines are identified in the figures. Depth of disturbance, as on
Line 2, coincidently begins at a plow depth of 1.5 feet and continues to
4.5 feet. Total length of the feature is approximately 200 feet. This
distance is also calculated between points of observed subsurface
terminations.
Figure 4 is offered as an example of an undisturbed (except for near
surface plowing) natural soil horizon located west of the indicated
anomaly. The radar signature is representative of signatures observed
on the remaining profiles.
All profiles were processed employing a number of high and low pass
filtering programs. Signature terminations observed within the
anomalous area were enhanced by these techniques.
Due to time constraints, the cursory survey conducted with the magnetometer
and ground conductivity meter provided data of limited use.
CONCLUSIONS
The anomaly delineated in Figures 2 and 3 is believed to represent an area
of abnormal subsurface conditions, the expression of which are
inconsistent with known land use activity on the site. The possibility
that historical/cultural activities are related to this shallow
disturbance is likely, based on historical reports. The location and
dimensions of the observed radar anomaly is consistent with the reported
proportions and location of Fort William.
The analysis of additional radar data collected in January, 1989, to detail
the anomalous area, is currently in progress. The ultimate test of all
geophysical data interpretation, "ground truthing" awaits.
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