Tower Rock marked the point where the Corps of Discovery left the plains and entered the Rocky Mountains, leaving the familiar prairies for a new terrain. On July 16, 1805, Lewis recorded that at this place there was a large rock 400 feet high standing immediately in the gap the Missouri makes on a passage from the mountains. He wrote that it was insulated from the neighboring mountains by a handsome little plain surrounding its base on three sides, while the Missouri washed its base on the other, and that it could be ascended with some difficulty nearly to its summit, from which there was a pleasing view of the country they were about to leave.