Magnetic Gradient Survey of Seminoe's Fort on the Tom Sun Ranch Along the Oregon and California National Historic Trails, Natrona County, Wyoming

Seminoe’s Fort was a trading post along the California, Mormon Pioneer, and Oregon National Historic Trails near Devils Gate in southwestern Natrona County, Wyoming, which was used in various ways from 1852 to 1857. Over the decades since the abandonment and destruction of the fort, the exact location of the small trading post, designated 48NA288, was lost in the memory of the American people.
The National Park Service’s Long Distance Trail Office in Salt Lake City requested the Midwest Archeological Center and the Wyoming State Archaeologist’s Office to identify the location of the trading post using geophysical investigation techniques. Midwest Archeological Center staff conducted geophysical investigations at the Tom Sun Ranch between May 22 and May 26, 2001. Two techniques were initially identified for the investigations, including a magnetic gradient survey and a resistance or ground conductivity survey; however, due to the flooding of the project area by irrigation waters, the archeological team could only conduct the magnetic gradient survey of the project area.
An area of 14,800 m2 was initially laid out in a hayfield on the right side of Pete Creek near the Mormon Handcart Visitor Center. The geophysical investigations covered a slightly smaller area due to the presence of Pete Creek and an irrigation ditch. The data from the magnetic gradient survey indicated the location of the trading post, a rectangular area to the southwest of the post, trash dumps associated with the trading post and with more recent ranching activities, as well as several isolated dipole anomalies. The investigations provided substantial information about the trading post’s location, layout, and integrity for archeological excavations, which followed in the summer of 2001.