Timing and Extent of Allerød and Younger Dryas Age (ca. 12,500–10,000 14C yr B.P.) Oscillations of the Cordilleran Ice Sheet in the Fraser Lowland, Western North America

The use of shaded digital topographic models to visualizes suites of topographic features, stratigraphy, and field mapping reveals newly recognized multiple moraines associated with oscillations of the remnants of the Cordilleran Ice Sheet in the Fraser Lowland along the western Washington–British Columbia border. Morphologic features show the extent of ice represented by Sumas Drift (ca. 11,600–10,000 14C yr B.P.), following ca. 3000 yr of retreat from the glacial maximum (Fraser Glaciation) positions 80 km south of Seattle and in the Strait of Juan de Fuca. The paleogeography of the ice margin and timing of ice retreat during the Sumas Stade is reconstructed and bracketed by 70 radiocarbon dates (24 on marine shells, 46 on wood and peat), which are secured by morphologic and stratigraphic evidence. Four topographically distinct phases of the Sumas deglaciation model are suggested.Phase SI: 11,600–<11,400 14C yr B.P., outermost drift, scattered remnants of ice-contact depositsPhase SII: 11,600–11,400 14C yr B.P., glacier readvance, building prominent moraine followed by glacier retreatPhase SIII: 10,980–10,250 14C yr B.P., glacier readvance building of multiple moraines followed by glacier retreatPhase SIV: >10,250–10,000? 14C yr B.P., glacier readvance, building of inner moraine followed by glacier retreat