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.
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
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.
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
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
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.
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.