GLACIOLOGY OF THE LAST GLACIAL MAXIMUM, UINTA MOUNTAINS, NORTHEASTERN UTAH

The Uinta Mountains are an east-west trending range in the U.S. Rocky Mountains, displaying abundant evidence in north- and south-flowing drainages of the extensive (~2400 km²) system of mountain glaciers it hosted during the Pleistocene. A recent synthesis of glacial geologic mapping of the Uinta Mountains supports detailed reconstructions of former ice extent, which permit calculations of ice dynamics during the Last Glacial Maximum. Reconstructed glaciers in forty-four valleys with the best-preserved glacial geomorphic records indicate that the surface area of discrete valley glaciers ranged from 3.5 to 257 km². Five main distributary glaciers with a combined surface area of 685 km² drained the largest ice mass in the range, the Western Uintas Ice Field. Glacier lengths ranged from 4 to 43 km, mean thicknesses from 40 to 286 m, and mean surface slopes (as represented by continuous lateral moraine crests) from 1.4 to 5.2 degrees. Basal shear stresses reconstructed from these dimensions and adjusted with appropriate valley cross-section shape factors ranged from 14 to 91 kPa. Average basal shear stresses were greater on the south slope (69 kPa) compared to the north slope (41 kPa). Given the overall similarity in mean ice surface slopes, the difference in reconstructed basal shear stress is primarily due to greater mean thickness of glaciers on the south slope (170 m) compared to those on the north slope (95 m). Greater ice thicknesses on the south slope of the Uinta Mountains are attributed to two factors. First, due to asymmetry of the Uinta anticline, glacier accumulation areas on the south slope were located on flat to gently-dipping bedrock strata. This positioning facilitated extensive lateral erosion at the heads of glacial valleys, which, through repeated glaciations, generated increasingly broad areas for snow accumulation. As a result, the reconstructed accumulation areas for glaciers on the south slope averaged 85 km² compared to just 31 km² for the north slope, and the greater ice flux draining from these accumulation zones required larger valley cross sections. Second, the base level for streams on the south slope is ca. 300 m lower than that for streams on the north slope. Over time, this situation may have facilitated the incision of deeper canyons capable of accommodating thicker glaciers.