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.

Abstract The city of Seattle, Washington State, lies within the Puget Sound Lowland, an elongate structural and topographic basin between the Cascade Range and Olympic Mountains. The area has been impacted by repeated glaciation in the past 2.4 m.y. and crustal deformation related to the Cascadia subduction zone. The present landscape largely results from those repeated cycles of glacial scouring and deposition and tectonic activity, subsequently modified by landsliding, stream erosion and deposition, and human activity. The last glacier to override the area, the Vashon-age glacier of the Fraser glaciation, reached the Seattle area ca. 14,500 14 C yr B.P. (17,400 cal yr B.P.) and had retreated from the area by ca. 13,650 14 C yr B.P. (16,400 cal yr B.P.). The Seattle area sits atop a complex and incomplete succession of glacial and nonglacial deposits that extends below sea level and overlies an irregular bedrock surface. These subsurface materials show spatial lithologic variability, are truncated by many unconformities, and are deformed by gentle folds and faults. Sediments that predate the last glacial–interglacial cycle are exposed where erosion has sliced into the upland, notably along the shorelines of Puget Sound and Lake Washington, along the Duwamish River valley, and along Holocene streams. The city of Seattle straddles the Seattle uplift, the Seattle fault zone, and the Seattle basin, three major bedrock structures that reflect north-south crustal shortening in the Puget Lowland. Tertiary bedrock is exposed in isolated locations in south Seattle on the Seattle uplift, and then it drops to 550 m below ground under the north half of the city in the Seattle basin. The 6-km-wide Seattle fault zone runs west to east across the south part of the city. A young strand of the Seattle fault last moved ~1100 yr ago. Seattle has also been shaken by subduction-zone earthquakes on the Cascadia subduction zone and deep earthquakes within the subducting plate. Certain postglacial deposits in Seattle are prone to liquefaction from earthquakes of sufficient size and duration. The landforms and near-surface deposits that cover much of the Seattle area record a brief period in the geologic history of the region. Upland till plains in many areas are cut by recessional meltwater channels and modern river channels. Till plains display north-south drumlins with long axes oriented in the ice-flow direction. Glacially overridden deposits underlie the drumlins and most of the uplands, whereas loosely consolidated postglacial deposits fill deep valleys and recessional meltwater channels. Ice-contact deposits are found in isolated locations across the uplands and along the margins of the uplands, and outwash deposits line upland recessional channels. Soft organic-rich deposits fill former lakes and bogs. A preliminary geologic map of Seattle was published in 1962 that is only now being replaced by a detailed geologic map. The new map utilizes a data set of 35,000 geotechnical boreholes, geomorphic analyses of light detection and ranging (LIDAR), new field mapping, excavation observations, geochronology, and integration with other geologic and geophysical information. Findings of the new mapping and recent research include recognition of Possession- and Whidbey-age deposits in Seattle, recognition that ~50% of the large drumlins are cored with pre-Vashon deposits and 50% with Vashon deposits, and that numerous unconformities are present in the subsurface. Paleotopographic surfaces display 500 m (1600 feet) of relief. The surficial deposits of Seattle can be grouped into the following categories to exemplify the distribution of geologic materials across the city: postglacial deposits 16%, late glacial deposits 12%, Vashon glacial deposits 60%, pre-Vashon deposits 9%, and bedrock 3%. of these, 49% are considered fine-grained deposits, 19% are considered intermediate or interbedded deposits, and 32% are considered coarse-grained deposits. These percentages include only the primary geologic units and not the overlying fill and colluvial deposits.

Abstract Our detailed mapping in the south Puget Sound basin has identified two tephras that are tentatively correlated to tephras from Mount St. Helens and Mount Rainier dated ca. 100-200 ka and 200 ka, respectively. This, plus the observation that fluvial and lacustrine sediments immediately underlying the Vashon Drift of latest Wisconsin age are nearly everywhere radiocarbon infinite, suggests that glacial and nonglacial sediments of more than the past five oxygen-isotope stages are exposed above sea level. Distal lacustrine advance outwash equivalent to the Lawton Clay in the Seattle area is conspicuously absent. Instead, a thick (>120 ft) glaciolacustrine silt below the Vashon sediments contains dropstones and is radiocarbon infinite. Elsewhere, coarsegrained advance Vashon outwash rests unconformably on radiocarbon-infinite non-glacial sediments. These relationships may imply that late Pleistocene tectonic activity has modified the paleotopography and stratigraphy of the south Puget Sound area.