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 Seattle lies within the Puget Sound Lowland, an elongate structural and topographic basin bordered by the Cascade and Olympic Mountains. The geology of the Seattle area is dominated by a complex, alternating, and incomplete sequence of glacial and interglacial deposits that rest upon an irregular bedrock surface. The depth to bedrock varies from zero to several kilometers below the ground surface. Bedrock outcrops in an east-west band across the lowland at the latitude of south Seattle and also around the perimeter of the lowland. Numerous faults and folds have deformed both the bedrock and overlying Quaternary sediments across the lowland, most notably the Seattle fault. During an earthquake on the Seattle fault ca. 980 A.D., 8 m of vertical offset occurred. The Seattle area has been glaciated at least seven times during the Quaternary Period by glaciers coalescing from British Columbia. In an area where each glacial and interglacial depositional sequence looks like its predecessor, accurate stratigraphic identification requires laboratory analyses and age determinations. The modern landscape is largely a result of repeated cycles of glacial scouring and deposition, and recent processes such as landsliding and river action. The north-south ridges of the lowland are the result of glacial scouring and subglacial stream erosion. The last glacier reached the central Puget Sound region ca. 15,000 years ago and retreated past this area by 13,650 14 C yr B.P. Post-glacial sediments are poorly consolidated, as much as 300 m thick in deep alluvial valleys, and susceptible to ground failure during earthquakes.

Abstract The Cordilleran ice sheet covered the northwest part of the North American continent during the last glaciation (Fig. 1). It developed from an initial core of coalescing mountain glaciers on Vancouver Island and the British Columbia mainland, spreading outward over a period of 10,000 to 15,000 yr. South of the ice-sheet limit, isolated alpine glaciers fluctuated in size, leaving a similar but not identical record of glacial advance and retreat. From the behavior of these glaciers, three questions have been posed and, in part, addressed by previous studies (including Crandell, 1965; Porter, 1976; Hicock and others, 1982; Clague, 1981; Waitt and Thorson, 1983). (1) Why did the Cordilleran ice sheet attain its maximum 5,000 yr later than its smaller counterparts to the south? (2) What was the extent of the alpine glaciers during the ice-sheet maximum, and why were most apparently well back from their maximum position? (3) What physical factors determined the rate and character of final ice-sheet retreat? This chapter approaches these questions by applying current knowledge of glacial mechanics, both theoretical and empirical, to various aspects of the inferred or reconstructed Cordilleran glaciers. The record of advance and retreat should reflect changes in the external environment (regional climate, sea-level changes), the glaciers’ physical responses to those changes, and changes that in turn result from ice growth and decay (isostasy, local climate). Existing data on the late-glacial advance-retreat chronology and climate constrain this analysis and provide an independent check on the conclusions suggested by this approach. Although the primary focus here is on the mechanics of deglaciation, this chapter considers the record of Cordilleran ice advance as well for several reasons. Retreat of most of the other North American ice sheets (Prest, 1969) coincided with the Cordilleran ice’s achievement of maximal advance, about 15 ka (Clague and others, 1980). Retreat of Cordilleran glaciers was neither monotonic nor entirely synchronous, in that some glaciers had retreated from their maximum positions 5,000 yr before others had finished advancing. Finally, the full advance-retreat record characterizes the physical behavior of the Cordilleran glaciers more completely, because the same physical principles determine glacier behavior regardless of the direction of motion of the ice terminus.