The so-called Archaic period starts around 9000 years ago when people in the Northern Plains started to eat more diversified foods in addition to bison, particularly plants. Bison hunting techniques also changed, with the appearance of driving systems such as jumps and corrals at the latest 4000 years ago (but probably earlier).
A major event of the Archaic was the ash fall from the Mazama eruption (today Crater Lake, in Oregon) 7600 years ago. It covered the Blackfoot territory with a thick layer of ash. This event, combined with the effects of an unusually hot and arid climate, probably explains the relative rarity of archaeological sites around that time.
Population probably increased again around 5000 years ago. The earliest materials that archaeologists recognize as specifically Blackfoot, including medicine wheels and iniskim, also date to that time. So do large communal burials (cemeteries) in the vicinity of the Great Sand Hills.