Abstract The advance of the Cordilleran Ice Sheet (CIS) during the Vashon Stade is limited by 14 C dates from sediments beneath Vashon till, which indicate that ice advanced southward across the Canadian border sometime after ca. 18 ka 14 C yr B.P. and reached the Seattle area soon after 14.5 ka 14 C yr B.P. The Puget lobe underwent sudden, large-scale terminus recession and downwasting not long after 14.5 ka 14 C yr B.P., and backwasted northward from its southern terminus past the Seattle area by ca. 14 ka 14 C yr B.P. Rapid thinning of Vashon ice after the terminus had receded north of Seattle allowed marine water from the Strait of Juan de Fuca to flood the lowland, floating the remaining ice and disintegrating the remaining CIS northward all the way to Canada, except for a narrow band along the eastern margin of the lowland. Everson glaciomarine drift (gmd), consisting mostly of poorly sorted stony clay deposited from floating ice, was deposited essentially contemporaneously over the central and northern Puget Lowland. Unbroken, articulated, marine shells, some in growth positions, indicate that the gmd represents in situ deposition. More than 150 14 C dates from Washington and British Columbia fix the age of the Everson gmd at 11,500 to ca. 12,500 14 C yr B.P., making it a valuable stratigraphic marker over the central and northern Puget Lowland. Ice-contact marine deltas and shorelines were produced on Whidbey Island as the CIS thinned and disintegrated in the central Puget Lowland, allowing marine water from the Strait of Juan de Fuca to penetrate beneath the ice. During this time, the CIS had disintegrated in the deeper water of the inland waterways, but grounded ice remained along the eastern side of the mainland, changing the ice flow direction from N-S to NE-SW, from the grounded ice on the mainland toward the open deep water to the west at the Strait of Juan de Fuca. A well-defined, marine ice-margin existed along the south and west sides of Penn Cove and isostatically raised shorelines and marine deltas were formed at elevations up to ~33 m on southern Whidbey Island and up to ~88 m on northern Whidbey. The shorelines are best preserved along the sides of marine embayments on the island. Following the deposition of Everson gmd and the emergence of the northern Puget Lowland, the CIS readvanced several times, defining four phases of the Sumas Stade: Sumas I represents grounding of the CIS and deposition of till in the western Fraser Lowland. Sumas II consists of a well-defined moraine and meltwater channels deeply incised into Everson gmd. A series of Sumas III moraines that occur in British Columbia shed meltwater that built a broad outwash plain behind the Sumas II moraine. A Sumas IV moraine occurs across a Sumas III meltwater channel at the eastern margin of the Fraser Lowland.

ABSTRACT The northern Puget Lowland of Washington State, USA, provides an exceptional opportunity not only to examine grounding line processes associated with marine-based ice sheets, but also to relate subaerial outcrop to marine geological observations of grounding line landforms and sedimentary processes in Antarctica and the deglaciated Northern Hemisphere. During this trip, we visit outcrops that record the interaction of the Cordilleran Ice Sheet and its bed, starting with locations where the ice sheet slowly flowed across crystalline bedrock. We also visit locations where the ice flowed across unconsolidated deposits, allowing discussions of subglacial bed deformation and grounding zone wedge development. Evidence shows that grounding line retreat across Whidbey Island was punctuated by periods of grounding line position stability and local ice advance during the growth of multiple grounding zone wedges. We will discuss the criteria for identifying grounding zone wedges, including diamicton units with foreset bedding that downlap onto a regional glacial unconformity at the base, and are truncated at the top by localized unconformities indicative of ice advance across the foreset beds. Grounding zone wedge foreset beds are composed of debris flows sourced from a deformation till and from sediment transported to the grounding line by subglacial meltwater. The overlying surface unconformity is associated with a laterally discontinuous till and pervasive glacial lineations. Other field stops focus on iceberg scouring and evidence of subglacial meltwater drainage, as well as the transition from marine to subaerial conditions during retreat of the Cordilleran Ice Sheet from the northern Puget Lowland.

Abstract As the late Pleistocene Cordilleran Ice Sheet (CIS) retreated from the southern Puget Lowland and thinned rapidly, marine waters invaded the central and northern lowland, floating the residual ice and causing wholesale collapse of the CIS from southern Whidbey Island to southern British Columbia. Massive, poorly sorted Everson glaciomarine drift was deposited contemporaneously over the entire central and northern lowland. More than 160 14 C dates show that the Everson interval began 12,500 14 C yr B.P. and ended 11,700 14 C yr B.P. Numerous marine strandlines record the drop in relative sea level in the Fraser Lowland from ~180 m (600 ft) at the end of the Everson interval to near present sea level. Following emergence of the Fraser Lowland, a lobe of the CIS advanced from the Fraser Canyon near Sumas to Bellingham during the Sumas Stade. As the ice retreated, at least eight end moraines were built successively across the lowland, each marking a position of ice advance or stillstand that records late Pleistocene climatic fluctuations. About 40 new 14 C dates indicate that the ages of these moraines span the Inter-Allerød–Younger Dryas intervals between 11,700 and 10,000 14 C yr B.P. The 14 C chronology allows correlation of the Sumas moraines with moraines in the Cascade Range, Rocky Mountains, Canada, Scandinavia, the European Alps, New Zealand, South America, and elsewhere. Late in the retreat of the ice, large outburst floods from an ice-dammed lake in British Columbia swept across the Sumas outwash plain, resulting in fluted topography and giant ripples on dune forms.

Abstract As the Vashon glacier retreated from its terminal position in the southern Puget-Lowland and thinned rapidly, marine waters invaded the central and northern lowland, floating the ice and depositing Everson glaciomarine drift over a wide area from southern Whidbey Island to southern British Columbia. The Everson deposits are characterized by vast areas of massive, poorly sorted stony silt and clay commonly containing marine shells. At Bellingham Bay and elsewhere in the Fraser Lowland, Deming sand is overlain by massive, poorly sorted, Bellingham glaciomarine drift to elevations of 180–210 m above present sea level and is underlain by Kulshan glaciomarine drift. Following deposition of the Everson glaciomarine drift, ice readvanced into northern Washington and deposited Sumas Drift and meltwater channels were incised into the glaciomarine deposits. Four moraine-building phases are recognized in the Sumas, the last two in the Younger Dryas. Rapid deglaciation between 14,500 and 12,500 14 C yr B.P. resulted in lowering of the surface the Cordilleran Ice Sheet below ridge crests in the Nooksack drainage and glacial activity thereafter became topographically controlled. Local valley glaciers in the upper Nooksack Valley were fed by alpine glaciers on Mount Baker, Mount Shuksan, and the Twin Sisters Range that were no longer connected to the Cordilleran Ice Sheet. Remnants of the Cordilleran Ice Sheet persisted in the Fraser Lowland at that time but were separated from the Nooksack Valley glaciers by several ridges 1200 m higher than the surface of the ice sheet. Alpine glaciers deposited drift in the Middle and North forks of the Nooksack drainage 25–45 km down-valley from their sources. Large mega-landslides in the Nooksack drainage are associated with an area of unusually high seismic activity, whereas nearby areas having the same geology, topography, climate, and vegetation have no such mega-landslides, suggesting that the landslides are seismically induced. Five Holocene tephras have been recognized in the region around Mount Baker–Schreibers Meadow scoria, Mazama ash, Rocky Creek ash, Cathedral Crag ash, and the 1843 tephra.