ABSTRACT Seattle’s geologic record begins with Eocene deposition of fluvial arkosic sandstone and associated volcanic rocks of the Puget Group, perhaps during a time of regional strike-slip faulting, followed by late Eocene and Oligocene marine deposition of the Blakeley Formation in the Cascadia forearc. Older Quaternary deposits are locally exposed. Most of the city is underlain by up to 100 m of glacial drift deposited during the Vashon stade of Fraser glaciation, 18–15 cal k.y. B.P. Vashon Drift includes lacustrine clay and silt of the Lawton Clay, lacustrine and fluvial sand of the Esperance Sand, and concrete-like Vashon till. Mappable till is absent over much of the area of the Vashon Drift. Peak local ice thickness was 900 m. Isostatic response to this brief ice loading was significant. Upon deglaciation, global ice-equivalent sea level was about −100 m and local RSL (relative sea level) was 15–20 m, suggesting a total isostatic depression of ~115–120 m at Seattle. Subsequent rapid rebound outstripped global sea-level rise to result in a newly recognized marine low-stand shoreline at −50 m. The Seattle fault is a north-verging thrust or reverse fault with ~7.5 km of throw. Conglomeratic Miocene strata may record initiation of shortening. Field relations indicate that fault geometry has evolved through three phases. At present, the north-verging master fault is blind, whereas several surface-rupturing faults above the master fault are south verging. The 900–930 A.D. Restoration Point earthquake raised a 5 km × 35 km (or larger) area as much as 7 m. The marine low-stand shoreline is offset by a similar amount, thus there has been only one such earthquake in the last ~11 k.y. Geomorphology is largely glacial: an outwash plain decorated with ice-molded flutes and large, anastomosing tunnel valleys carved by water flowing beneath the ice sheet. Euro-Americans initially settled here because of landscape features formed by uplift in the Restoration Point earthquake. But steep slopes and tide flats were not conducive to commerce: starting in the 1890s and ending in the 1920s, extensive regrading removed hills, decreased slopes, and filled low areas. In steep slopes the glacial stratigraphy is prone to landslides when saturated by unusually wet winters. Seismic hazards comprise moderately large (M 7) earthquakes in the Benioff zone 50 km and more beneath the city, demi-millennial M 9 events on the subduction zone to the west, and infrequent local crustal earthquakes (M 7) that are likely to be devastating because of their proximity. Seismic shaking and consequent liquefaction are of particular concern in Pioneer Square, SoDo, and lower Duwamish neighborhoods, which are largely built on unengineered fill that was placed over estuarine mud. Debris from past Mount Rainier lahars has reached the lower Duwamish valley and a future large lahar could pose a sedimentation hazard.

Abstract The primary objective of this two-day field trip is to examine sediments from the Evans Creek stade of the early Fraser Glaciation at four key sections along the Skagit River near Concrete and the shoreline of Ross Lake. These sediments provide important new information on the timing and extent of alpine glacier advances during the Evans Creek stade (early Fraser Glaciation). In lower Skagit valley at Cedar Grove, glacial drift overlies an organic bed that yielded a radiocarbon age of 25,040 14 C yr B.P.; this age is a maximum limiting date for the Evans Creek stade. Three radiocar-bon ages within 400 years of 24,000 14 C yr B.P. record damming of upper Skagit valley by the Big Beaver alpine glacier. The ice dam created glacial Lake Skymo, which persisted until at least 18,020 14 C yr B.P., suggesting that Cascade glaciers remained at advanced positions throughout most of the Evans Creek stade. However, growth of a forest on early Evans Creek drift at Cedar Grove 20,730 14 C yr B.P. requires at least some recession of the Baker valley glacier. An increase in the number of lowland and montane macrofossils in glacial Lake Skymo sediments after 20,770 14 C yr B.P. is consistent with a mid–Evans Creek stade warm interval. Sometime after 20,730 14 C yr B.P., the Baker valley glacier overrode the forest bed and deposited till at Cedar Grove. The advance dammed Skagit River and created glacial Lake Concrete, which persisted until about 16,400 14 C yr B.P.