Abstract

The Farewell area of the western Alaska Range and the highest massifs of the neighboring Kuskokwim Mountains were repeatedly glaciated during late Cenozoic time.

Drift representing at least six episodes of glaciation has been identified in the Farewell area. The oldest drift, of Big Salmon Fork age, has been deformed tectonically and resembles glacial sequences of inferred late Tertiary age elsewhere in the Alaska Range. Till patches and isolated erratic boulders of the succeeding Lone Mountain glaciation have been deformed locally by faulting. Drift of the Selatna glaciation is more continuous, but has been highly eroded in some valleys. Drift sheets of the Farewell 1 and Farewell 2 glaciations retain recognizable depositional landforms; they are assigned early and late Wisconsin ages, respectively, on the basis of morphology, regional correlations, and a radiocarbon date. Moraines of probable late Holocene age commonly are ice cored and unstable.

Local climatic and physiographic variations in the Farewell area resulted in major differences in size and extent of glacial lobes in neighboring valleys. Moraines of similar ages also differ in degrees of weathering, erosion, and burial, making it difficult to apply relative-dating techniques for correlating between valleys. Apparent vertical displacements on active strands of the Farewell fault offset surficial deposits that range in age from at least early Pleistocene to the present.

Twelve isolated massifs in the Kuskokwim Mountains generated glaciers during Quaternary time. Four glaciations are recognized in the Beaver Mountains, and probably correlative drifts occur in the other rugged massifs. The Beaver Creek glaciation is marked by erratic boulders and drift patches as much as 10 kilometers beyond younger glacial deposits; breached divides and planated summit levels suggest that ice caps were present. All twelve massifs contain evidence of the succeeding Bifurcation Creek glaciation, characterized by highly modified morainal landforms and polymodal cirque orientations. Eight of the 12 massifs contain deposits of the Tolstoi Lake glaciation of inferred late Wisconsin age. Glaciers of Tolstoi Lake age, which developed mainly in north-trending valley systems formed sets of three and rarely four moraines in each valley. Drift of the Crater Mountain glaciation, confined to protected north-facing cirques, is assigned a Holocene age. Glacial processes have played an important role in the formation of placer deposits in the Kuskokwim Mountains.

The Farewell area and the Kuskokwim Mountains have similar glacial sequences. The Beaver Creek glaciation of the Kuskokwim Mountains probably represents several advances that are analogous to the multiple glacial episodes of pre-Farewell age in the western Alaska Range. The Bifurcation Creek and Tolstoi Lake glaciations of the Kuskokwim Mountains are believed correlative with the Farewell 1 and 2 succession farther east because of similar relative extents, moraine successions, and postglacial modifications. The Crater Mountain glaciation may be correlative with youthful advances in the Alaska Range.

Cirque floors rise northeastward across the study area, indicating that moisture sources in the Bristol Bay region were as important in the late Pleistocene as they are today.