CONTRASTING MODES OF PLEISTOCENE GLACIATION ALONG THE CONTINENTAL DIVIDE, WESTERN MONTANA: A NATURAL EXPERIMENT IN LANDSCAPE RESPONSE TO CLIMATE CHANGE

A western Montana transect along the continental divide between Butte and Lincoln reveals a range of Pleistocene glacial styles: a 500 km² ice cap system occupied the northern Boulder Mountains (BMC); small cirque glaciers developed in the Nevada mountain area (NMA); and terminal deposits of the 15,000 km² northern Rocky Mountain ice cap (RMC) are found in the Lincoln area. These changes in glacial style represent a transition from localized alpine to continental glaciation, and present an opportunity to examine the interaction between cordilleran landscapes and Pleistocene climate change. We present the results of new field mapping, GIS analysis, and spreadsheet modeling aimed at reconstructing the extent and thickness of Pleistocene ice in the BMC and NMA systems. These systems contrast markedly in size and expression. Our efforts in the BMC system have focused on new 1:24,000-scale field mapping coupled with digitization and field-checking of pioneering maps by Ruppel in order to constrain ice margins and the altitudes of nunataks and breached divides. From the NMA, we present new 1:24,000-scale mapping which significantly revises the known extent of glaciation in the area. The small, relatively accessible cirques of the NMA system provide an opportunity to determine Pleistocene Equilibrium-Line Altitudes (ELA) for comparison with the larger, less readily constrained BMC system, as well as results from an earlier study which calculated ELAs for the RMC. ELA calculations, coupled with in-progress cosmogenic nuclide exposure dating of terminal deposits of the BMC, will allow us to examine regional influences on Pleistocene climate, and place these systems within the context of existing literature documenting Pleistocene environments in the northern Rocky Mountains. Regional influences on climate and ELAs may have included proximity to the Cordilleran and Laurentide ice sheets, and glacial lakes Missoula and Great Falls. Proterozoic quartzites and felsic intrusive rocks are common along this transect, providing potential targets for additional surface-exposure dating work. Sediment records from ice-block lakes developed in hummocky end moraines of the RMC may provide additional opportunities to examine the chronology of and landscape response to deglaciation.