On August 13, 1805, William Clark took three compass readings from atop an exposed limestone bluff, selecting geographic landmarks as points on which to take bearings while looking out across what is now Montana. His maps became an important resource for future travelers. The Corps of Discovery had begun their journey with no printed materials such as maps or reports to guide them, and besides the constant challenge of choosing which water route to take, they also had to decide which of the countless land formations to plot and describe as they attempted to fulfill their mission. President Thomas Jefferson instructed Lewis to describe all features that could be positively identified by future travelers. The landmarks on which Clark took bearings that day remain as he saw them, including Point of the Beavers head hill hears N24 E. 12 Ms., the course of the Wisdom River N25 W., and the gap at the place the river passes thro' a mountain in advance is 18 w 10 Ms.