Pompeys Pillar

This postmark advertises 'Clark Days' held annually in July by the Pompey's Pillar Historical Association, commemorating Clark's stop here in 1806. More than just a ledger for Clark's name, of the Lewis and Sacagewa fame. Pompeys Pillar, the prominent sandstone outcrop in the Montana landscape has a ton of rock art on it, with some dating over 11,ooo years old. 
Situated on the Yellowstone river, it is a well known transit marker in oral histories called  Iischibii 'Where the Mountain Lion Lies'. Campsites with bitched bison and mussel shell remains are found in the vicinity of the pillar. 
Over 147 petroglyphs are recorded - though owing to the rate of erosion and 11,000 years of episodic camping it is not hard to imagine this landmark as a palimpsest of communications.