Geologic Units
The study area is located within the urban area of southern Seattle. Much of the topography in the study area has been shaped by glacial erosion or deposition. Within this region the surficial deposits are predominantly of Quaternary age and consist of relatively flat-lying interbedded glacial and non-glacial materials (Yount et al., 1990).
The glacial sequences can be very compact and generally consist of clayey to sandy till containing abundant cobbles and boulders (Yount et al., 1990). Interbedded non-glacial deposits are made up of moderately consolidated fluvial sand, silt, clay and minor gravel (Yount et al., 1990).
Most of the primary surficial geologic units in southern Seattle are sampled in this study. About half of the sites in this study are located on Vashon till sediments; two sites in West Seattle are located on an older glacial outwash sand; four sites in the Duwanish River industrial area are located on Holocene alluvium and artificial fill; one site at Seward Park (SQ0) is located on poorly cemented Tertiary tuffaceous sandstone and conglomerate (Waldron et al., 1962). The sites within the Duwamish industrial area are near the mouth of the Duwamish River and are underlain by as much as 75 m of alluvial loose to dense sand and silt (Waldron et al., 1962).
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