There are four hypergraphic links in the photo below.
Detail from a photo (below) of Pompeys Pillar by Brent Phelps.
On the relatively sheltered face of Pompy's Tower where Clark engraved his tag in 1806 are three expressions of official, collective graffiti, dignified in bronze. In 1928, the Billings chapter of Daughters of the American Revolution commemorated the patriotic significance of the Expedition with a plaque. In 1938, members of the Masonic Lodge in Billings acknowledged their own brotherly links with Meriwether Lewis and William Clark, both of whom were Freemasons. The third tablet is a memorial to Don Foote, a local real estate developer, farmer, and amateur historian, who purchased the land around it in 1965, to ensure its preservation.
Pompeys Pillar--editor Nicholas Biddle's guess as to what Clark meant; the name-minding federal Board of Geographic Names deletes apostrophes--is now the center of a state park. Surveillance cameras help rangers apprehend a number of would-be graffitists each year, in the act of embellishing the famous rock with memorials to themselves.
A comprehensive study of the markings on Pompeys Pillar has recently been undertaken by Minot State University, so we know there is a total of some 2,500 tags on all surfaces of the rock, not counting those that were on chunks of sandstone that have fallen, face down, on the ground around it.