LATE FRASER ALPINE GLACIER ACTIVITY IN THE NORTH CASCADES, WASHINGTON

Recently discovered alpine glacier moraines in the northern North Cascades add important new detail to our understanding of climate in the Pacific Northwest at the end of the Fraser Glaciation (ca. 14,000-10,000 14C ybp). Terminal moraines in five valleys span a range of climatic zones, from maritime (west) to continental (east), and define a regional advance and stillstand of 2.5-11-km-long valley glaciers. In all five valleys, the late Fraser glaciers terminated beyond Neoglacial limits, and are likely tens of kilometers upvalley of positions attained by alpine glaciers at the Fraser Glaciation maximum. Equilibrium line altitudes (ELA), reconstructed for these glaciers using the AAR method, rise from west to east along a gradient of approximately 11m/km, paralleling the ELA trend line of modern glaciers. ELA depression associated with formation of the moraines is ~400-600m, which is in agreement with published data from 25 other valleys in the region. The moraines postdate the late Wisconsinan maximum advance of the Cordilleran Ice Sheet at 14,500 14C ybp, because the ice sheet obliterated all older alpine moraines in this region. Their age is also constrained by Mazama tephra layer “O” (6700 14C ybp), which lies on the moraines. Logs buried in till beneath late Fraser Glaciation moraines in the Middle Fork of the Nooksack River provide a closely limiting age for this regional advance of approximately 10,600 14C ybp. Further age constraints will be obtained using radiocarbon, tephrochronology, and cosmogenic dating methods. The moraines are tentatively correlated with the late Fraser Hyak and Rat Creek moraines in the southern North Cascades.