The survey officially began on June 8, 1871 when it departed Ogden, Utah , although many survey members were already making observations and collecting specimens while the team was assembling in Salt Lake City and then Ogden. The party traveled north, reaching Taylor's Bridge (now Idaho Falls) on the Snake River on June 25, 1871. On June 30, 1871, the survey party had reached into Montana, camping just over the Continental Divide near Monida Pass . Hayden and his survey party reached Virginia City, Montana on July 4, 1871 and Fort Ellis near Bozeman, Montana, on July 10, 1871. By this time, Thomas Moran, the guest artist had joined the survey. At Fort Ellis, both George Allen, the botanist and Cyrus Thomas, the agricultural statistician and entomologist, then left the party for health reasons, while José, the guide, joined the team. After resupplying and coordinating with the U.S. Army at Fort Ellis, the survey departed south along the Yellowstone River on July 15, 1871. For the next 45 days, the Hayden Survey would coordinate efforts with the Barlow-Heap expedition under the command of Colonel John W. Barlow, Chief Engineer for General Philip Sheridan that the U.S. Army was sending into Yellowstone at the same time.