PLEISTOCENE MOUNTAIN GLACIATION IN MONTANA, USA

A variety of Pleistocene glacier types occurred in Montana, including continental ice sheets (Laurentide and Cordilleran), large mountain ice caps (Northern Rockies and Yellowstone-Beartooth), small mountain ice caps (e.g. Boulder Highlands and West Pioneers), and transection glaciers (e.g. East Pioneers and Flint Creek Range). Numerous valley, cirque and niche glaciers also existed in more than sixty distinct mountain ranges. Glacier distribution and equilibrium-line altitudes are consistent with a climatic pattern where moist air masses entered the NW corner of the state. Prevailing winter westerly winds were directed south of the Cordilleran ice sheet at the glacial maximum.

The extents of Pleistocene glaciers of western Montana were mapped from topographic map and aerial photo interpretation, with limited field verification. The results generally conform to previous regional work, however detailed field studies of glacial extents are scant. In the few places where deposits of older glaciations were mapped, last-glacial maxima—“Pinedale” (Wisconsinan, Weichselian, Würm)—extents closely mimicked older glacial extents except on the west flank of the Yellowstone Ice Cap and in localities where glaciers extended onto the Great Plains. However, most glacial extents have not been field-mapped and meaningful patterns of difference between last-glacial and earlier extents may yet emerge.

Nowhere in Montana are the ages of glacial episodes well-constrained. Most last-glacial moraines are correlated with the Pinedale glaciation (~25,000 to ~16,000 14C yr BP) elsewhere in the Rocky Mountains, rather than dated directly. Radiocarbon dates in the Yellowstone region and south of Glacier National Park, and occurrences of Glacier Peak tephra in the Flathead region show massive retreat or disintegration of the major ice caps prior to the Younger Dryas Chron. It is assumed that many mountain glaciers also retreated at this time. Younger Dryas glaciation was restricted to cirques.

Glacial drainage modifications in western Montana resulted from sedimentation and erosion associated with Glacial Lake Missoula, and local rerouting of pre-glacial streams. Ice-marginal lakes existed in some areas, however their ages and extents are not well known.