NUMERICAL MODELING OF PLEISTOCENE VALLEY GLACIERS IN THE NORTHERNMOST U.S. ROCKY MOUNTAINS, LEWIS RANGE, MONTANA

During the last Pleistocene glaciation, the Lewis Range of northwestern Montana featured the northernmost discrete valley glaciers in the U.S. Rocky Mountains. The shapes of these glaciers are delimited by well-preserved glacial deposits and landforms in the east-flowing valleys of Lake and Cut Bank Creeks. Here, ice advanced beyond the mountain front forming piedmont lobes that deposited a broad, hummocky moraine complex in each valley. Cosmogenic ¹⁰Be ages of boulders atop the ice distal and ice proximal sectors of the terminal moraine in Cut Bank Creek indicate that moraine deposition spanned at least the interval 18.5 ± 0.5 ka to 17.7 ± 0.5 ka. A numerical, two-dimensional glacier mass balance and ice flow model was applied to these valleys to simulate the known maximum ice extent and to constrain the likely temperature and precipitation changes during the construction of the terminal moraine complex. After model parameters were validated by simulating the modern glaciers in the headwaters of Lake and Cut Bank Creek valleys, simulations of the maximum Pleistocene ice extents yielded a range of possible temperature and precipitation combinations. Results of model experiments indicate that near modern precipitation rates would have been accompanied by a temperature depression of ~8°C. This result is in contrast to outputs of regional and global-scale paleoclimate models, which indicate a temperature depression of more than 10°C and a strong reduction in precipitation during the Last Glacial Maximum. However, the late onset of deglaciation observed here and elsewhere in the Northern Rocky Mountains suggests that enhanced precipitation may have sustained glaciers at their maximum extent until 17.7 ka.