A HAZY PICTURE OF A LONG UNTOLD STORY
Although we know that there were people in what is now Yellowstone National Park, (on & off,) for more than 11,000 years – that’s about all we know. 
In 1959 an isolated Clovis Point was found at the site of the old Gardiner post office; it was made of material from Obsidian Cliff. Other Yellowstone obsidian can be dated to about 13,000 years ago at the Bostrom Site in Illinois.
Most of the evidence is indirect but substantial. It comes from finds of Yellowstone Obsidian throughout North America. Geological studies of Yellowstone Lake have also aided in the research to understand the human prehistory of Yellowstone.The studies by Weeks & Blakeslee have been instruemental in shedding light on the earliest prehistory of Yellowstone National Park.
A Shoreline Survey of Yellowstone Lake
Yellowstone Lake is a large high-altitude freshwater lake with a shoreline of approximately 110 miles. Old beach lines exist at elevations of up to 150 meters above the present beach and at ca. 7 meters below present level. The main high and low water stands appear to reflect variations in climate, glacial blockage of the Yellowstone River valley, and the presence of erosion-resistant strata in the valley. The present lake level is controlled by the elevation of the XX rhyolite at the Le Hardy rapids, just below the outlet of the lake. Minor variations in the level of old shorelines probably reflect minor tectonic shifts.
Human use of the Yellowstone plateau goes back at least to Clovis times. No sites of that age have yet been documented in the park, but Clovis points of Obsidian cliff obsidian have been found outside the park boundaries. The oldest artifact that has been reported professionally is a Goshen point (ca. 11,000 BP) from the surface of the Osprey Beach site. It may derive from a paleo beachline that is obscured by a later, Cody-aged (ca. 9,500 BP) beach of similar elevation. Cody complex campsites are buried in that beachline at both the Osprey Beach site and at 48YE1, at the northern end of the lake.
Later Holocene use of the park area appears to have been variable. In theory, the warmer climate of the Altithermal should have encouraged more use of the high country, in contrast to the dearth of sites of this age on the adjacent Great Plains. Still, no sites of this time period have yet been identified in the park. Middle Archaic sites (4700-3000 B.P.) are common, but very Late Prehistoric (ca. 300 BP) materials are scarce. The latter may reflect the effects of the Little Ice Age, perhaps because glacier growth inhibited travel though mountain passes.
Wave action maintains cut banks along much of the present shoreline, and site visibility is high. Except for areas of high tourist traffic, where souvenir collecting has affected the remains, prehistoric lithic debris is common. A complete shoreline survey should provide a good sample of the variations in human use of the park area, so long as old shorelines are surveyed.
The sketchy picture of the time between 11,000 years ago and the arrival of Euro-Americans is still to be fleshed out. Recent studies of prehistory center on the occupation of Yellowstone by the Bannock, Shoshone & Sheepeater Shoshone.
There is a brief summary at the Yellowstone Park Historical Highlights page.
