...mark your route in all places where there will be a portage to pass from one river to another or from one water-fall to another by cutting or notching some trees or by some piles of stones engraved and cut; and take care to place in large letters Charles IV King of Spain and below Company of Missouri, the day, the month, and the year when you do this in order to serve as unquestionable proof of the journey that you are going to make.The procedure was inherently powerful. Mackay warned Evans that if he were to come upon the Russian settlement that was rumored to exist somewhere north of California, "you will cease to make any sign of taking possession, for fear of having spring up with these foreigners any jealousy which would be prejudicial to the success of your journey."1
Three years earlier, Alexander Mackenzie, in search of a navigable water route across North America, had left his mark, his tag, at the end of his long trek from Montreal to the Pacific Ocean.
I now mixed up some vermilion in melted grease, and inscribed, in large characters, on the South East face of the rock on which we had slept last night, this brief memorial – "Alexander Mackenzie, from Canada, by land, the twenty-second of July, one thousand seven hundred and ninety-three."2
Similarly, at their camp on Chinook Point ("Point Distress") on November 23, 1805, William Clark marked his name, "the Day of the month & year on a Beech trees & (By Land). Capt. Lewis Branded his and the men all marked their nams on trees about the Camp." A modern graffiti writer might call that a "throwup."
At times the men of the Corps added "U.S." to their tags, as well as the latitude of the place if Lewis had taken the necessary observations.
They marked their passage almost from the start. On May 23, 1804, two days after leaving St. Charles, Missouri, they stopped to inspect Tavern Cave where, Clark said, "many names are wrote up on the rock. Mine among others."