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 Geological and human forces have created some spectacular treasures at the boundary between the Central Lowlands and the Great Plains, and three of them are explored in this guide. In northern Nebraska, the Ashfall Fossil Beds site, a world-class Lagerstätte of articulated mammal, reptile, and bird skeletons, reveals the mass death of a Miocene biotic community. Chapter 1 provides a detailed overview of the geology, paleontology, and reconstructed paleocommunity at Ashfall. The bluffs of the Missouri River in eastern Iowa contain some classic type sections of Pleistocene stratigraphic units. Chapter 2 explores the historical development of Pleistocene stratigraphy in this area and presents new data to refine understanding of the area’s complex geological history. Finally, Chapter 3 presents a unique tour of the Nebraska State Capitol in Lincoln, which is clad with Indiana limestone and adorned with igneous, metamorphic, and sedimentary rocks from European and U.S. quarries. The field guide describes the historical, architectural, and geological aspects of these stones.

Geomorphic data from rivers in the Tanana foreland basin and northern foothills of the Alaska Range indicate that this is an actively deforming landscape. The Tanana basin is an alluvial and swampy lowland of ∼22,000 km 2 located in south-central Alaska between the northern flank of the Alaska Range and the Yukon-Tanana uplands. The major axial drainage of the basin is the Tanana river, which is fed by large transverse braided rivers flowing northward out of the Alaska Range. To better define active structures and the neotectonic configuration of the basin, we have constructed a series of longitudinal stream profiles along the major rivers of the Tanana basin. Stream profiles along with changes in channel morphologies delineate four main areas of active deformation. (1) In the western part of the basin, major rivers in the Kantishna Hills area have stream profiles and changes in channel morphologies that indicate that the northeast-trending Kantishna Hills anticlinorium is an active structure. All longitudinal stream profiles in this area exhibit convexity, suggesting tectonic perturbation, as they cross the trend of this 85-km-long structure. In addition, the channel of the McKinley River clearly becomes entrenched as it flows around the southwestern nose of the Kantishna Hills anticlinorium suggesting that the structure may be propagating southwestward. Our geomorphic data from this area are consistent with well-documented seismicity along the southwestern part of the Kantishna Hills. (2) In the central part of the basin, the Nenana River area, changes in channel morphology, stream profile perturbations, and uplifted Pliocene-Pleistocene erosional surfaces coincide with a series of east-trending anticlines. We interpret these folds as part of an active Neogene thrust belt that forms the foothills of the north-central Alaska Range. This active thrust belt is propagating northward and deforming the proximal part of the Tanana foreland basin. North of the topographic front of the foothills, stream profiles indicate active subsidence of the basin. (3) In the eastern part of the Tanana basin, the Delta River area, stream profiles and channel morphologies delineate active deformation along the strike-slip Denali fault and the Granite Mountain/Donnelly Dome thrust fault system. (4) In the northern part of the Tanana basin, the Fairbanks area, stream profiles and channel morphologies delineate northeast-trending active structures that coincide with known seismic zones. These structures are most likely related to block rotation between the Denali and Tintina fault systems along northeast-trending sinistral strike-slip faults. An interesting result of our analysis of the Fairbanks area is the hypothesis that the Tanana River has been forced to abandon its previous channels due to progressive uplift along an active northeast-trending structure. This forced migration has resulted in a series of watergaps, with the modern Tanana River having been deflected around the southwestern culmination of this structure. Interactions between fluvial systems and active structures of the Tanana basin provide a surface record of regional transpressional deformation. This deformation is accommodated by strain partitioning between strike-slip faults like the Denali fault, an active thrust belt along the northern flank of the Alaska Range, and rotation of crustal blocks between the Denali and Tintina fault systems.

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.