ABSTRACT The northern Rocky Mountain Trench of eastern British Columbia is a broad valley mantled by glaciolacustrine terraces supporting a complex mix of mesic-temperate (“interior wet belt”) forests that are strongly affected by terrain and substrate. Neither the geomorphic history during early Holocene deglaciation nor the vegetation history of the origin of the Tsuga heterophylla (western hemlock) and Thuja plicata (western redcedar) populations in the interior wet-belt forest is well understood. Sediment cores were obtained from two lakes, 10 km apart and occupying different terraces (83 m elevational difference), and these were compared to existing fire-history and paleoclimate reconstructions. Radiocarbon dates and a mapped terrain classification indicate the upper terrace formed as a lacustrine and glaciofluvial kame terrace hundreds of years prior to the lower terrace, which was formed by glaciolacustrine sediments of a proglacial lake. The minimum limiting ages of these terraces correlate with dated jökulhlaup deposits of the Fraser River. The upper site’s first detectable pollen at >11.0 ka was dominated by light-seeded pioneer taxa (Poaceae [grasses], Artemisia [sagebrush], and Populus [aspen]) followed by a peak in Pinus (pine) and finally dominance by Betula (birch) at 10.2 ka. Pollen data suggest an earlier invasion of T. heterophylla (western hemlock) (by ca. 8 ka) than previously understood. Wetlands on extensive, poorly drained, glaciolacustrine soils promoted the persistence of boreal taxa and open forests (e.g., Picea mariana [black spruce]), while the better-drained upper kame terrace promoted development of closed-canopy shade-tolerant taxa. Invasion and expansion of mesic cedar-hemlock taxa progressed since at least the middle Holocene but was highly constrained by edaphic controls.

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.