Did an Ice Cap Cover the Flint Creek Range?

Abstract

During the last glacial maximum (LGM), glaciers occupied many ranges in the mountain west. In the vicinity of Racetrack Creek and Fred Burr Creek in the Flint Creek Range, west-central Montana, glaciers left clear moraines marking maximum extent into lower valleys on either side of the range. ‘Trimlines’ marking former ice surface suggest that an ice cap might have joined the glaciers at the top of the range. Surficial geologic mapping of the lower Racetrack Creek valley allowed us to reconstruct ice thickness as it exited the range. We then used a mathematical model (based on physical properties of ice, slope, and basal shear stress) to reconstruct former ice surface to the divide. Our model matched closely with the constraining physical data, indicating that a small ice cap fed the two outlet glaciers during the LGM. The ice cap did not completely cover the top of the range, leaving protruding peaks or ‘nunataks’. The glacial equilibrium line altitude (ELA) was 2,130 meters in the Fred Burr Creek valley and 70 meters lower in the Racetrack Creek valley, indicating that ice extended lower on the east of the range. These results provide an important contribution to the understanding of glacial history in the Flint Creek Range and growing body of data on glacial and climate history of the mountain west. The ELA we determined are over 250 meters lower than those previously estimated for this area, suggesting that the local climate was colder and/or wetter than previously interpreted.