YellowstoneUsing spreadsheet modeling techniques, I and undergraduate students in my 1994 junior-level Geomorphology course reconstructed the Late Pleistocene ("Pinedale") ice cover of the greater Yellowstone area (Locke, 1995, in Rocky Mountain Cell Friends of the Pleistocene Field Conference Guidebook, G. A. Meyer, ed.). The area is topographically asymmetrical, with highest elevations (above 3000 m shaded) along the southeastern margin of Yellowstone in the Absaroka Range, and to the northeast on the Beartooth plateaus. Major rivers draining the area include (clockwise from north) the Boulder and Stillwater (to the N), the Clark Fork of the Yellowstone (NE), the forks of the Shoshone (E), the Snake (S), the Madison (W), and of course, the Yellowstone River itself (to the NW and N). Each of these rivers contained a distributary lobe of Yellowstone ice. The morphology of the former ice cap reflects the topography of the catchment area (both high ground and low), the topography of the ice cap itself, and the climate which fed it. |
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| The Yellowstone Ice Cap as modeled consisted of three major domes (250 m
contour interval, shaded over 3250 m): one on the Beartooth Plateau, one over the Lamar
River valley north of Yellowstone Lake, and the last nestled in the high central Absaroka
Range. The Beartooth Plateau icecap was topographically defined and constrained, draining
away from Yellowstone down the Stillwater and Clark Fork systems. The dome over the Lamar
valley was largely unconstrained, the result of a long flowline to the dominant outlet of
the Yellowstone River (see K. L. Pierce, 1979, USGS PP 729-F). Secondary outlets were to
the NE out the Stillwater and Clark Fork valleys and across the divide to the SE into the
Shoshone River valley. The largest dome, in the Absaroka Range, was maintained by a
combination of topographic forcing and ice rheology. The ice cap asymmetry, with perhaps 99% of the ice mass to the east of the topographic divide, was maintained by Pacific moisture, as indicated by lowest equilibrium line altitudes below 2500 m in the NW Teton Range but above 3250 m in the SE Absaroka Range. This model used effective basal shear stresses of 100 kPa (1 bar) except in the Jackson Lake basin (25 kPa) and matches mapped nunatak areas well, with one exception. K. L. Pierce (personal communication) described significantly higher ice margins in the upper Clark Fork drainage than predicted by the model, suggesting compressive flow in the canyon itself driving effective basal shear stresses well in excess of 100 kPa. A similar circumstance, with effective basal shear stresses locally exceeding 600 kPa required to match ice surfaces to moraines, is found in and above the canyons of the eastern Wind River Range. |
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