MUNROE, Jeffrey S., Quaternary Research Group, Dept. of Geology & Geophysics, University of Wisconsin-Madison, 1215 W. Dayton St., Madison, Wisconsin, 53706, jmunroe@geology.wisc.edu

Investigations of upper subalpine geomorphology are being combined with palynology and radiocarbon geochronology to reconstruct the geomorphic and climatic history of the northern Uinta Mountains since the last deglaciation.  The timing of the Last Glacial Maximum in the Uintas is unknown.  However, a basal AMS radiocarbon date of 12,190 (plus or minus 120) radiocarbon yrs BP from a tarn in the eastern Uintas indicates that the deglaciation there was complete by approximately 14,500 cal yrs BP.
        Other dates on Salix fragments and organic-rich sediments deposited directly on till behind cirque-floor end moraines range from 10,500 to 9,500 cal yrs BP.  These closely limiting minimum dates suggest the possibility that ice persisted in the highest cirque basins until the latest Pleistocene.  Alternatively, glaciers may have temporarily reformed in the most suitable locations in response to a short-lived return to glacial conditions following the main deglaciation.
        Information about Holocene treeline dynamics in the northern Uintas is provided by peat layers exposed in river cutbanks at two upper subalpine locations.  Over 400 pollen grains (excluding Cyperaceae) were identified for each of nine discrete organic layers.  Based on a relationship between Picea/Pinus ratios and site elevation derived by Maher (1963) for the floristically similar San Juan Mountains of southern Colorado, apparent timberline was up to 100 m higher than modern in the northern Uintas during the period 9000 to 7000 cal yrs BP.  Timberline then fell up to 200 m below modern through 4000 cal yrs BP.
        Finally, three additional basal radiocarbon dates in the literature from the Uintas hint at the magnitude of the early Holocene warm interval.  The dates, all of which are on organic material retrieved from wet meadow environments, are all less than 7,500 cal yrs BP; no organic material dating to before that time has been discovered from outside of lake basins.  Therefore, it appears possible that the early Holocene climate favoring elevated timberline resulted in desiccation of wet meadows, leading to oxidation and elimination of the pre-middle Holocene organic record from the sites studied to date.