Timing and extent of Allerod 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 C-14 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 C-14 yr B.P., outermost drift, scattered remnants of ice-contact deposits Phase SII: 11,600-11,400 C-14 yr B.P., glacier readvance, building prominent moraine followed by glacier retreat Phase SIII: 10,980-10,250 C-14 yr B.P., glacier readvance building of multiple moraines followed by glacier retreat Phase SIV: >10,250-10,000? C-14 yr B.P., glacier readvance, building of inner moraine followed by glacier retreat. (C) 2002 University of Washington.