On the return trip, they approached the Rocky Mountains in July 1806. On July 6, Clark recorded:The Indian woman informed me that she had been in this plain frequently and knew it well.… She said we would discover a gap in the mountains in our direction [i.e., present-day Gibbons Pass].A week later, on July 13, Sacagawea advised Clark to cross into the Yellowstone River basin at what is now known as Bozeman Pass. Later, this was chosen as the optimal route for the Northern Pacific Railway to cross the continental divide.