PALEOINDIAN SITES AND THE PLEISTOCENE-HOLOCENE TRANSITION; WYOMING AND SOUTHWESTERN MONTANA

The last major Pleistocene glacial episode in the region occurred between 30,000 – 22,000 C¹⁴ years B.P. The major mountain ranges contained extensive glaciers e.g., a 1,000 m thick, 110 km long ice sheet covered the Yellowstone Plateau. The plains areas were characterized by a periglacial environment and the presence of artic steppe grasslands that supported an extensive mega-fauna, many species of which later became extinct. Warming and deglaciation commenced approximately 18,000 C¹⁴ B.P. By 14,000 C¹⁴ years B.P., much of the Yellowstone Plateau was ice-free and a tundra steppe prevailed. By 11,500 C¹⁴ years B.P., most mountain ranges were essentially free of ice and a coniferous forest prevailed. Between 11,500 – 10,000 C¹⁴ years B.P., the regional climate was generally cooler and more moist than present. Aquoll soils are superimposed on Clovis, Folsom and Hell Gap cultural horizons at a number of sites. A change to a semi-arid to arid climate took place in Late Paleoindian times. Depending on location, this change occurred between 9,300 – 7,000 C¹⁴ years B.P. At the Indian Creek Site, located in the Elkhorn Mountains of Montana, an abrupt change from coniferous forest to sagebrush steppe occurred approximately 8,200 C¹⁴ years B.P. Evidence of climatic change is also present in the stratigraphic record. The melting of glaciers with consequential high water run-off resulted in the regional deposition of gravel, which ceased just prior to 11,100 C¹⁴ years B.P. The transition to a more xeric environment during late Paleoindian time is also recognizable regionally by abrupt changes in depositional patterns, e.g. initiation of debris flow deposition, cessation of alluvial fan deposition and change in alluvial sedimentation patterns from steady state to erratic.