ABSTRACT High-resolution light detection and ranging (lidar) data and new stratigraphic, lake sediment, and radiocarbon constraints help to resolve a long-standing dispute regarding the timing and nature of the Everson interstade and the Sumas stade, the last major events of the Cordilleran ice sheet in the Fraser Lowland. The new data indicate that: (1) an early, maximum Sumas advance occurred roughly 14,500 cal yr B.P. (calibrated 14 C years before 1950), extending into the Salish Sea near Bellingham, Washington; (2) ice retreated north of the International Boundary long enough for forests to establish in deglaciated lowland sites; (3) a rapid, short-lived rise in local relative sea level (RSL) of ~20–30 m, possibly related to meltwater pulse 1A or the collapse of a glacio-isostatic forebulge, inundated the U.S. portion of the lowlands up to ~130 m above modern sea level; and (4) directly following this transgression at ca. 14,000 cal yr B.P., ice readvanced across the border to nearly the same limit as reached during the early Sumas period. Distinct crosscutting marine strandlines (erosional and depositional remains of emerged marine shorelines), subaerial moraines, and till plains imaged in lidar data indicate that following the maximum extent of the second Sumas advance, local RSL progressively lowered as the glacier fluctuated and gradually thinned. By ca. 13,000 cal yr B.P., ice had retreated north of the border, and local RSL had fallen to within ~4 m of modern. A layer of possible loess in sediments in Squalicum Lake suggests a possible third and final Sumas readvance between 13,000 and 11,150 cal yr B.P., at which time a moraine was constructed ~8 km south of the border near the town of Sumas, Washington. Together, our results suggest that the concept of a distinct Everson interstade and Sumas stade should be abandoned in favor of a more nuanced “Sumas episode” that encompasses the sequence of events recorded in the Fraser Lowland.

ABSTRACT Recently obtained radiocarbon ages from the southern Puget Lowland and reevaluation of limiting ages from the Olympic Peninsula in the light of new light detection and ranging (LiDAR) data suggest that the Juan de Fuca and Puget lobes of the Cordilleran ice sheet reached their maximum extents after 16,000 calibrated yr B.P. Source areas for both lobes fed through a common conduit, likely requiring that downstream responses to changes in either source area were similar. Dates for ice-sheet retreat are sparse and contradictory, but they suggest that retreat was rapid. Depositional and geomorphic evidence shows that retreat of the Juan de Fuca lobe predated retreat of the Puget lobe. No recessional end moraines have been identified in the Puget Lowland, in contrast to numerous recessional end moraines constructed by the Okanogan lobe east of the Cascade Range, and in contrast to later ice-sheet retreat in western Whatcom County north of the Puget Lowland. These observations lead to the hypothesis that collapse of the Juan de Fuca lobe, hastened by the instability of a marine-based ice sheet, steepened the ice-sheet surface over the eastern Strait of Juan de Fuca and diverted ice flow upstream of the Puget lobe to the west. Starved of ice, the Puget lobe retreated quickly.

ABSTRACT In late Wisconsin time, the Purcell Trench lobe of the Cordilleran ice sheet dammed the Clark Fork of the Columbia River in western Montana, creating glacial Lake Missoula. During part of this epoch, the Okanogan lobe also dammed the Columbia River downstream, creating glacial Lake Columbia in northeast Washington. Repeated failure of the Purcell Trench ice dam released glacial Lake Missoula, causing dozens of catastrophic floods in eastern Washington that can be distinguished by the geologic record they left behind. These floods removed tens of meters of pale loess from dark basalt substrate, forming scars along flowpaths visible from space. Different positions of the Okanogan lobe are required for modeled Missoula floods to inundate the diverse channels that show field evidence for flooding, as shown by accurate dam-break flood modeling using a roughly 185 m digital terrain model of existing topography (with control points dynamically varied using automatic mesh refinement). The maximum extent of the Okanogan lobe, which blocked inundation of the upper Grand Coulee and the Columbia River valley, is required to flood all channels in the Telford scablands and to produce highest flood stages in Pasco Basin. Alternatively, the Columbia River valley must have been open and the upper Grand Coulee blocked to nearly match evidence for high water on Pangborn bar near Wenatchee, Washington, and to flood Quincy Basin from the west. Finally, if the Columbia River valley and upper Grand Coulee were both open, Quincy Basin would have flooded from the northeast. In all these scenarios, the discrepancy between modeled flood stages and field evidence for maximum flood stages increases in all channels downstream, from Spokane to Umatilla Basin. The pattern of discrepancies indicates that bulking of floods by loess increased flow volume across the scablands, but this alone does not explain low ­modeled flow stages along the Columbia River valley near Wenatchee. This latter discrepancy between modeled flood stages and field data requires either additional bulking of flow by sediment along the Columbia reach downstream of glacial Lake Columbia, or coincident dam failures of glacial Lake Columbia and glacial Lake Missoula.

ABSTRACT The Pleistocene Okanogan lobe of Cordilleran ice in north-central Washington State dammed Columbia River to pond glacial Lake Columbia and divert the river south across one or another low spot along a 230-km-long drainage divide. When enormous Missoula floods from the east briefly engulfed the lake, water poured across a few such divide saddles. The grandest such spillway into the Channeled Scabland became upper Grand Coulee. By cutting headward to Columbia valley, upper Grand Coulee’s flood cataract opened a valve that then kept glacial Lake Columbia low and limited later floods into nearby Moses Coulee. Indeed few of the scores of last-glacial Missoula floods managed to reach it. Headward cutting of an inferred smaller cataract (Foster Coulee) had earlier lowered glacial Lake Columbia’s outlet. Such Scabland piracies explain a variety of field evidence assembled here: apparently successive outlets of glacial Lake Columbia, and certain megaflood features downcurrent to Wenatchee and Quincy basin. Ice-rafted erratics and the Pangborn bar of foreset gravel near Wenatchee record late Wisconsin flood(s) down Columbia valley as deep as 320 m. Fancher bar, 45 m higher than Pangborn bar, also has tall foreset beds—but its gravel is partly rotted and capped by thick calcrete, thus pre-Wisconsin age, perhaps greatly so. In western Quincy basin foreset beds of basaltic gravel dip east from Columbia valley into the basin—gravel also partly rotted and capped by thick calcrete, also pre-Wisconsin. Yet evidence of late Wisconsin eastward flow to Quincy basin is sparse. This sequence suggests that upper Grand Coulee had largely opened before down-Columbia megaflood(s) early in late Wisconsin time. A drift-obscured area of the Waterville Plateau near Badger Wells is the inconspicuous divide saddle between Columbia tributary Foster Creek drainage and Moses Coulee drainage. Before flood cataracts had opened upper Grand Coulee or Foster Coulee, and while Okanogan ice blocked the Columbia but not Foster Creek, glacial Lake Columbia (diverted Columbia River) drained over this saddle at about 654 m and down Moses Coulee. When glacial Lake Columbia stood at this high level so far west, Missoula floods swelling the lake could easily and deeply flood Moses Coulee. Once eastern Foster Coulee cataract had been cut through, and especially once upper Grand Coulee’s great cataract receded to Columbia valley, glacial Lake Columbia stood lower, and Moses Coulee became harder to flood. During the late Wisconsin (marine isotope stage [MIS] 2), only when Okanogan-lobe ice blocked the Columbia near Brewster to form a high lake could Missoula floodwater from glacial Lake Missoula rise enough to overflow into Moses Coulee—and then only in a few very largest Missoula floods. Moses Coulee’s main excavation must lie with pre-Wisconsin outburst floods (MIS 6 or much earlier)—before upper Grand Coulee’s cataract had receded to Columbia valley.

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 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 The Channeled Scabland of east-central Washington comprises a complex of anastomosing fluvial channels that were eroded by Pleistocene megaflooding into the basalt bedrock and overlying sediments of the Columbia Plateau and Columbia Basin regions of eastern Washington State, U.S.A. The cataclysmic flooding produced huge coulees (dry river courses), cataracts, streamlined loess hills, rock basins, butte-and-basin scabland, potholes, inner channels, broad gravel deposits, and immense gravel bars. Giant current ripples (fluvial dunes) developed in the coarse gravel bedload. In the 1920s, J Harlen Bretz established the cataclysmic flooding origin for the Channeled Scabland, and Joseph Thomas Pardee subsequently demonstrated that the megaflooding derived from the margins of the Cordilleran Ice Sheet, notably from ice-dammed glacial Lake Missoula, which had formed in western Montana and northern Idaho. More recent research, to be discussed on this field trip, has revealed the complexity of megaflooding and the details of its history. To understand the scabland one has to throw away textbook treatments of river work. —J. Hoover Mackin, as quoted in Bretz et al. (1956, p. 960)

Abstract The natural landscape of the North Shore of Vancouver is a mountainous one extending from sea level to ~1400 m. Land below ~400 m has been undergoing increasing urbanization since the 1950s. Development has encroached on areas subject to natural hazards such as floods, debris flows, slope failures, and coastal inundation. We will visit examples of these urban hazards, discuss problems of hazard identification in a forested landscape, and review urban planning and engineering responses to hazard management.

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 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.