The gist of the article is that Native Americans have visited and lived in the Yellowstone Park area for centuries. The article cites obsidian from Yellowstone dating back 3,000 years.
This is the relevant sentence in the article: "Each obsidian flow has its own chemical signature, which can be identified by X-ray fluorescence, a technique developed in the 1960s. Artifacts made of Yellowstone obsidian from Obsidian Cliff have been found all over the Rockies and the Great Plains, in Alberta, and as far east as Wisconsin, Michigan and Ontario."
- Bookwood Historical Collection, Star Lake
- UW Madison/Wis Hist Soc