 Mullan was spearheading the construction of a military road from Walla Walla to Fort Benton, and Sohon surveyed routes and monitored the construction progress.In 1860 Sohon guided the first wagon party to cross the Rocky and Bitterroot Mountains to the Columbia Plateau by a route other than the more accustomed Overland Trail.When Mullan's Road was complete, Sohon accompanied him to Washington, DC, to assist in the preparation of topographical data, maps, and illustrations for a report on the road's construction.