ALPINE ENVIRONMENTS OF HUMANS AT THE PLEISTOCENE-HOLOCENE BOUNDARY IN IDAHO AND ADJACENT AREAS

In the 1960's Earl Swanson and associates excavated archaeological sites in Birch Creek Valley, east-central Idaho, mainly rock shelters on the flood plain margin and in lower reaches of tributary canyons in the flanking Lemhi and Beaverhead Mountains. These sites had considerable depth, yielding cultural material at least as old as 10,500 - 11,000 14C yr BP. Concurrently and continuing, Dort and students studied regional environments that affected these early people.

The Lemhi crest averages about 11,000 feet elevation. Most major, east-facing valleyheads were occupied by glaciers in late Pinedale time (ca. 12,000 14C yr BP). Summits and upper interstream divides must have had perennial snowfields and even ice carapaces. High alpine areas were thus available to Paleoindians, and trans-mountain travel was effectively blocked. The floor of Birch Creek Valley was nearly as arid as it is now; trees were generally absent, although mountain slopes were forested. Glaciers retreated mainly be sublimation; meltwater streams were minor.

This local situation was part of a regional pattern. The 80-mile-long Lemhi Range had at least 250 definable glaciers, mainly on its eastern, sheltered side. Glacierization of other high ranges in Idaho was closely similar. So also in adjacent states, e.g., the Wind River Mountains were nearly buried by ice. Interacting chronologies of maximum expansion of Pinedale Glaciers, not everywhere synchronous, with local arrival times of humans determine specific affects. In any event, remnant glaciers were still present well after the first people appeared.