THE LAST GLACIAL MAXIMUM IN THE NEVADA MOUNTAINS, MONTANA

Late Pleistocene glacial systems in the Nevada Mountains consisted of small cirque and valley glaciers. Field mapping supplemented by analyses of topographic maps, digital elevation models, and aerial photos provided the basis for reconstruction of four glaciers from which equilibrium-line altitudes (ELAs) were obtained. ELAs were determined by the accumulation-area ratio method using a ratio of 0.65 ± 0.05. Preliminary results suggest ELAs ranged from approximately 1750 to 2000 m during the local last glacial maximum (LGM). Circumstances have thus far precluded direct dating of LGM moraines in the Nevada Mountains, and therefore ¹⁰Be exposure ages from moraine boulders in the Boulder Mountains, 30 km away, are assumed to loosely constrain the local LGM to be between 27.0 and 16.3 ka. A temperature-index approach was used to infer late Pleistocene climate by determining the changes in temperature and/or precipitation required to maintain LGM ELAs and steady-state mass balances of the paleoglaciers. Assuming LGM precipitation was comparable to that of today, a cooling of between 11-13°C is suggested. General circulation models (GCMs) indicate, however, that drier conditions prevailed during the LGM in the northern Rocky Mountains. If correct, temperature depression could have been slightly greater in magnitude. In either case, it appears therefore that climate here during the LGM was colder than those to the south, consistent with results from GCMs and available paleoclimate proxies.