A Magnetic Gradiometer Survey of the Waterline Corridor at Fort Union Trading Post National Historic Site

In August 1999, archeologists from the Midwest Archeological Center conducted a magnetic survey of the existing waterline alignment at Fort Union Trading Post National Historic Site. This was done as the first phase of a three-phase project whose overall goal is to assist the park in achieving Section 106 compliance in conjunction with proposed Fiscal Year 2000 installation of a new waterline. The routes of the current waterline and its replacement transect two known significant sites - the Garden Coulee site (32WIl8) and Fort Union Trading Post (32WIl7). Significant numbers of buried features are known to occur in both sites. The route is also very close to the suspected location of Fort William, a large palisaded subsidiary complex associated with Fort Union. The geophysical survey was designed to locate a route which would have the least impact on the park's cultural resources. The survey corridor was composed of twenty-eight 20-x-20-m geophysical survey grids with a total survey coverage of 11,200 m2• Identification of buried archeological features was assisted by the observation of several large pits during the 1977 monitoring of the original waterline. One of these was shown to produce a substantial magnetic anomaly. Other work in the region indicated that many archeological features, both those associated with the fort and those of Native American construction, would be detectable with a magnetometer. The 1999 magnetic survey documented at least 53 anomalies that are either certain or highly likely to result from archeological features. These features are distributed along the length of the line but are most concentrated at each end of the survey corridor.