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Large glaciers descended western valleys of the Olympic Mountains six times during the last (Wisconsin) glaciation, terminating in the Pacific coastal lowlands. The glaciers constructed extensive landforms and thick stratigraphic sequences, which commonly contain wood and other organic detritus. The organic material, coupled with stratigraphic data, provides a detailed radiocarbon chronology of late Pleistocene ice-margin fluctuations. The early Wisconsin Lyman Rapids advance, which terminated prior to ca. 54,000 14C yr B.P., represented the most extensive ice cover. Subsequent glacier expansions included the Hoh Oxbow 1 advance, which commenced between ca. 42,000 and 35,000 14C yr B.P.; the Hoh Oxbow 2 advance, ca. 30,800 to 26,300 14C yr B.P.; the Hoh Oxbow 3 advance, ca. 22,000–19,300 14C yr B.P.; the Twin Creeks 1 advance, 19,100–18,300 14C yr B.P.; and the subsequent, undated Twin Creeks 2 advance. The Hoh Oxbow 2 advance represents the greatest ice extent of the last 50,000 yr, with the glacier extending 22 km further downvalley than during the Twin Creeks 1 advance, which is correlative with the global last glacial maximum. Local pollen data indicate intensified summer cooling during successive stadial events. Because ice extent was diminished during colder stadial events, precipitation—not summer temperature—influenced the magnitude of glaciation most strongly. Regional aridity, independently documented by extensive pollen evidence, limited ice extent during the last glacial maximum. The timing of glacier advances suggests causal links with North Atlantic Bond cycles and Heinrich events.

The timing of Late Pleistocene mountain glacier fluctuations varied widely across the western United States. Glaciers in the maritime Olympic Mountains reached local maxima 125-60 ka and 34-31 ka, and were far less extensive during the global Last Glacial Maximum (LGM). Glaciers in the Cascade Mountains showed a similar general pattern but with contrasts in detailed timing. Most mountain glacier systems further inland exhibited maximum ice extent during Marine Isotope Stage (MIS) 2. In the Wallowa, Wind River, and possibly the Sawtooth Mountains, maximum ice extent occurred ca. 20-23 ka. Mountain glacier systems of the Cordilleran Ice Sheet (Puget Lobe), greater Yellowstone region and Uinta Mountains reached maximum extent ca. 19-16 ka, and those in the Wallowa, Sawtooth and Wind River Ranges were at near-maximum positions ca. 17 ka.The variable timing of maximum and near-maximum ice advances appears to be related to climatic setting and glacier mass balance characteristics. Mountain glacier systems under the strongest maritime influences reached maximum extent at times of strong westerly atmospheric flow. Olympic and Cascades glaciers appear to have been negatively influenced by weakened westerly flow at the time of the global LGM, perhaps associated with ice-sheet-induced anticyclonic flow. Glacier systems with maximum advances correlative with the global LGM appear to have responded more strongly to depressed temperature. The ca. 17 ka ice advances of several glacier systems are broadly correlative with Heinrich Event 1 in the North Atlantic region, and imply a cooling event and reinvigoration of moisture transport into the region. Post-LGM advances of Uinta Mountain glaciers appear to have arisen from moisture influences of pluvial Lake Bonneville.

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