Deglaciation of the Mountainous Region of Northwestern Montana, U.S.A., as Indicated by Late Pleistocene Ashes

Abstract

During the late Wisconsin glaciation, the mountainous regions of northwestern Montana were covered by glacial ice. Marias Pass, on the Continental Divide immediately south of Glacier National Park, was covered by a local ice field. This ice and that from other glaciers to the north and south flowed eastward onto the plains of Montana to form the Two Medicine Glacier, a large piedmont glacier that extended 55 km beyond the mountain front. The presence of the Glacier Peak G ash and the underlying St. Helens Jy ash in laminated lake sediments near Marias Pass indicates that in this region the Continental Divide was ice free before about 11,400 BP. Macrofossils, pollen, and spores in these same sediments indicate establishment of shrubs, herbs, and scattered conifers by that time. At Sun River Canyon, about 90 km south of Marias Pass, glaciers also flowed beyond the mountain front onto the plains to form the Sun River Glacier, another large piedmont glacier that extended beyond the mountain front for 25 km. The presence of the Glacier Peak G ash in a postglacial alluvial fan indicates that glacial ice had receded upvalley from the canyon mouth and that the Sun River Glacier no longer existed by 11,200 BP.