A GEOPHYSICAL INVESTIGATION OF THE FORT LA PRESENTATION ARCHAEOLOGICAL SITE, OGDENSBURG, NY

Fort La Presentation is located in the St. Lawrence Valley at Lighthouse Point at Ogdensburg, NY. It was the first settlement of Ogdensburg and the site of the last battle of the French and Indian War. The fort was not just a French military site but also a religious community, a trading post and a major Native American outpost for the Iroquois. The purpose of this project was to locate the remnants of the fort by using geophysical methods of prospecting.

Seismic, electrical resistivity and magnetic traverses were conducted to locate the remnants of the fort. These remnants include the wall, foundation and cornerstones. The seismic refraction survey indicated about 30' (10 meters) of glacial till overlying the Ogdensburg dolostone bedrock. Vertical electrical sounding (VES) at the site indicates 6' of high resistivity sand overlying a low resistivity material, probably clay.

The magnetic profiles indicated several magnetic anomalies with magnitudes of 2000 nanotesla and steep magnetic gradient. Normally magnetic anomalies of this magnitude are due to Precambrian basement rock however, the steep gradients indicate a depth of about 2 meters placing their source in the overlying glacial till.

Four resistivity profiles were conducted across the site. Resistivity measurements were made at 2 meter intervals and an electrode spacing of 4 meters. The resistivity profiles indicate several resistivity anomalies as high as 200 ohm-meters in the upper 2 meters. In summary, even though the source of the magnetic and resistivity anomalies is unknown, it's possible they are related to some of the remnants at the site.