Soil Evidence for Postglacial Forest-Grassland Fluctuation in the Absaroka Mountains of Northwestern Wyoming, U.S.A.

Abstract

Soils on the west side of Dead Indian Pass (2414 m) in the Absaroka Mountains of northwestern Wyoming give indication of climatic and vegetational shift. This change is marked by a buried, truncated calcareous soil with abundant grass phytoliths (perhaps a Calcic Cryoboroll) which is unconformably overlain by a forest soil without abundant grass phytoliths (Typic Cryochrept). Using archaeological, geological, and soil data at the Dead Indian Creek archaeological site (48PA551), this shift appears to have happened sometime between about 5400 and 4400 BP, that is, near the Altithermal-Neoglacial boundary when climates shifted from dry to more humid, respectively. During the Altithermal, the west side of Dead Indian Pass, and much of the general area to elevations at least as high as 2400 m, were probably covered by grassland, including many north-facing slopes. In the Neoglacial, however, greater moisture probably encouraged forestation of north-facing slopes and others, as well as some stream terraces and bottom lands.