Late Quaternary history of the northern Uinta Mountains, northeastern Utah

Nineteen valley glaciers covering approximately 940 km2 were present in the northern Uintas during the peak of the Smiths Fork Glaciation. Deglaciation from the Smiths Fork maximum was complete by 15–14 ka B.P. Low ablation gradients for three of the reconstructed glaciers (1.4 to 2.8 mm/m) indicate that glaciers in the northern Uintas were sustained more by cool summer temperatures than extreme winter snow accumulation. This conclusion is supported by study of modern climate at the locations of the reconstructed equilibrium lines, which suggests that mean summer temperatures were 5.5°C lower than modern (3.2 to 5.7°C) with mean winter precipitation from 943 to 3038 mm (modern 354 to 590 mm).; The modern equilibrium line altitude (ELA) in the northern Uintas averages 3970 m asl, approximately 100 m below the highest peaks in the center of the range. Glaciers are absent in the range because no cirque floors intersect the ELA. Previous estimates of 1000 to 1475 m of equilibrium line depression during the last glacial maximum are too large; last glacial maximum ELA depression in the northern Uintas was closer to 900 m.; Cirque floor moraines, which are present in 12 of 19 north slope valleys, were deposited shortly before 10 ka B.P. These moraines may represent the final pulse of a latest Pleistocene glacial episode that began during the Younger Dryas and persisted until shortly before 10 ka B.P. Pollen spectra suggest that an open Picea parkland was established at timberline by 9.5 ka B.P., accompanied by mean growing season temperatures ∼1.0°C greater than modern. Increasing warmth into the middle Holocene allowed Pinus to grow up into the parkland by 7.5 ka B.P. A period of maximum warmth was reached shortly after 5.5 ka B.P., and conditions were near modern by 3.8 ka B.P.; High-altitude moraines postdate the cirque floor moraines. Absolute age estimates obtained through application of lichen growth-rate curves indicate that these moraines predate the Little Ice Age and were formed during a localized pulse of glaciation ∼1650 to 1300 B.P.