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.