job, he became involved with a group of French bankers interested in the lands of the recently negotiated Gadsden Purchase. In late 1853, with the bankers' backing, Poston joined with mining engineer Herman Ehrenberg to organize an expedition into the territory Mexico was expected to sell to the United States. Taking a ship from San Francisco, the two became shipwrecked near the Mexican port of Guaymas. They were then detained temporarily by Mexican authorities as suspected filibusters before heading north to the Gadsden territory. The expedition visited San Xavier del Bac and Ajo, collecting mineral samples along the way, before traveling down the Gila River. At Fort Yuma, a U.S. Army post near the confluence of the Gila and Colorado River, Poston first met the fort's commander Major Samuel P. Heintzelman. While at Fort Yuma Poston surveyed a townsite on the south side of the river a mile below the fort, at Jaeger's Ferry. Poston sold the townsite called Colorado City for $20,000 when he returned to San Francisco.