EVIDENCE FOR COALESCENCE OF LAURENTIDE AND ROCKY MOUNTAIN GLACIERS EAST OF GLACIER NATIONAL PARK, MONTANA

Abstract

Evidence indicates that Laurentide and Rocky Mountain piedmont glaciers coalesced in the Willow Creek drainage east of Glacier National Park. Wisconsin-aged moraines of these glaciers converge and overlap at 1384 to 1432 m elevation about 1 km east of Hall Coulée and 1 to 2 km north of Galbreath Ridge. This zone of glacier contact extended about 7 km northward to the International Boundary where moraines of the two glaciers again converge at 1341 to 1432 m elevation. Piedmont moraines also mantle the floors of Emigrant Gap and the North Fork of the Milk River glacial spillways. For alpine ice to have been diverted into these spillways, the glaciers must have coalesced near the (northern) heads of the spillways. Based on relative soil development, alpine tills in the North Fork of the Milk River could be two ages (late Wisconsin and earlier Wisconsin or late Illinoian [?]), suggesting two distinct coalescence events. Up to 1-m diameter Shield erratics occur on Galbreath Ridge south of Willow Creek drainage at elevations between 1425 and 1478 m. These boulders probably were ice-rafted across a proglacial lake (held in on the south by Galbreath Ridge, on the north and east by the Laurentide glacier, and on the west by the corresponding piedmont glacier and/or topography), which formed prior to the cutting of the larger spillways, probably during the Illinoian stage. The glaciers may have also coalesced here at that time. [Key words: Laurentide glacier, piedmont glacier, coalescence, Glacier National Park.]