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 This field guide focuses on glacial history, dynamics and processes, and postglacial landscape adjustments in the southern Fraser Plateau region. Located between the Coast and Columbia Mountains in south-central British Columbia, Canada, the southern Fraser Plateau was near the geographic center of the last (marine oxygen isotope stage [MIS] 2) Cordilleran Ice Sheet (CIS). The transition from cold to warm-based ice during MIS 2 is recorded in till sedimentology and structural geology. The perceived absence of large deglacial recessional moraines has been used as evidence that ice regionally stagnated because of a rapid rise in equilibrium line altitude. However, glacioisostatic rebound orientations, ice-marginal channel and grounding-line and push moraine distributions, and reconstructions of late-glacial ice-marginal lake evolution suggest a systematic northwestward pattern of active ice-margin retreat toward the Coast Mountains, accompanied by regional thinning. Eskers and erosional corridors record drainage of supraglacial lakes or ice-marginal water sources in or over thin ice. Many ice-dammed lakes drained catastrophically. Following lake drainage, streams incised valley fills, leaving behind terraces capped by paraglacial fans and eolian sediment. In sum, we examine (1) valley-fill sediments that record Quaternary history dating back to the early or mid-Pleistocene; (2) till, moraines, erosional corridors, and eskers that provide evidence for MIS 2 CIS dynamics and hydrology; (3) late-glacial ice-marginal lake sediments and landforms that allow reconstruction of lake evolution and drainage, and changing ice-margin positions; and (4) the character and ages of river terraces, paraglacial fans, and eolian sediments that record the timing and nature of postglacial landscape adjustments.

ABSTRACT Volcanoes that interact with the cryosphere preserve indicators of their eruption environments. These glaciovolcanoes and their deposits have powerful potential as proxies of local and global paleoclimates. The Garibaldi volcanic belt is the northern (Canadian) segment of the Cascade volcanic arc. In this study, we compiled a comprehensive database of Quaternary volcanic landforms and deposits in the Garibaldi volcanic belt. We found that the region exhibits a high degree of volcanic diversity, and a significant component of this diversity is due to the abundance of glaciovolcanoes. These include: tuyas, tindars, subglacial tephra cones, ice-impounded lavas, subglacial domes and breccias, subglacial lava flows, and lava-dominated tuyas. As a group, they inform the presence, thickness, and transient properties of ancient, continental-scale ice sheets (i.e., the Cordilleran ice sheet) that have waxed and waned in thickness and extent across the region. We ascribe much of the character of glaciovolcanism in the Garibaldi volcanic belt to a wide range of magma compositions (alkaline basalt to rhyolite) and to the extreme relief of the landscape. We used forensic volcanologic evidence, in conjunction with our database, to define a terrestrial-based reconstruction of ice-sheet thickness and extent that spans the latter half of the Quaternary (i.e., past ~1 m.y.). We then compared our reconstruction to the marine isotope stage (MIS) record and found a number of positive correlations and discordances. We show glaciovolcanoes to be an excellent, and underutilized, proxy for Earth’s paleoclimate, and a powerful tool for reconstructing ice sheets predating the last glaciation.

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.