Up to several feet of volcanic ash (or tephra) covered much of the Northwestern Plains following the eruption of Mount Mazama (now Crater Lake, in Oregon) about 7600 years ago. Since much of the vegetation must have died, animals and people were probably forced to move out of the region for some time before they could return.
Mazama ash is often found at archaeological sites, where it provides a useful chronological marker. Everything found below it is older than 7600 years.