ONGOING INVESTIGATION OF THE LATE PLEISTOCENE YELLOWSTONE AND GRAND TETON ICE CAP HISTORIES, MONTANA AND WYOMING, USA

In prior research, we established a high-resolution cosmogenic chronology for the well-preserved moraine sequence deposited by the northern outlet glacier of the Yellowstone ice cap. Cosmogenic ³He and ¹⁰Be ages indicate that the outlet glacier reached its terminal position at 16.5 ± 0.4 ³He ka and 16.2 ± 0.3 ¹⁰Be ka, respectively, and ages from recessional moraines upvalley from the terminal moraine suggest deglaciation of the Yellowstone plateau by ~14 ¹⁰Be ka. These cosmogenic chronologies identify a late Pinedale glacial maximum in northern Yellowstone that is younger than that in the nearby Wind River Mountains by 3-4 kyr. This age difference may be explained by the time interval required for the progressive buildup of the Yellowstone ice cap, or alternatively by contrasting regional glacier responses that reflect a climatic difference related to the thermal and orographic influences of the proximal Laurentide Ice Sheet. In order to evaluate these hypotheses, our continuing investigations involve the application of surface exposure dating techniques to glacial deposits in additional drainages within the Yellowstone and Grand Teton regions. Sampling has focused on glacial deposits marking the southern margin of the former Yellowstone ice cap, as well as moraine sequences deposited by former outlet glaciers along the eastern front of the Teton Range. Pending results from these key sites are expected to reveal complexities of the glacial histories of these ranges that will enable a more detailed assessment of spatial and temporal patterns of glacier responses in this region of the Rocky Mountains.