"The nativs," Clark wrote in his journal for July 25, 1806, "have ingraved on the face of this rock the figures of animals &c. near which I marked my name and the day of the month & year." He had done the same thing in the same sort of setting at least once before. Back on July 12, 1804, near the mouth of the Big Nemaha River, in Nebraska, he wrote,
No trace of the earlier inscription has ever been found, so the one on Pompey's Tower is the sole remaining material evidence of the Corps' presence in the Northwest.
We don't know what sort of instrument Clark used to engrave his signature, but we may suppose that he used something like a cold chisel. In any case, Private John Shields, the Corps' blacksmith, was with Clark's party, and he would have had a wide selection of suitable tools.
Over the years a number of other passersby have seized the opportunity to borrow a little status by scratching their tags over and around Clark's, but none more blatantly than a certain cavalryman in April of 1876.