Maps depicting thicknesses of sedimentary units (isopach maps) are standard tools for interpretation of processes and patterns of sedimentation within basins (Potter and Pettijohn, 1977, p. 283-287). Isopach maps showing the thickness of unconsolidated sedimentary deposits overlying bedrock may help locate sources of aggregate and potential ground-water aquifers. These maps also aid in the prediction of ground response resulting from earthquakes (Hays, 1980). In the Puget Lowland this latter use is of particular importance. Strong ground motions were highly variable throughout the Puget Sound area during the 1965 Seattle earthquake (M_s = 6.5); complex topography of the bedrock surface plus thick sections of young unconsolidated and semiconsolidated sediment were responsible for this variability (Langston, 1981; Shakel and Toksoz, 1980). Hall and Othberg (1974) outlined the major patterns of unconsolidated sediment accumulation in Puget Sound, including the unusually thick Sedimentary deposits beneath the city of Seattle.

Since Hall and Othberg's 1971, study, new geotechnical drilling and marine seismic reflection profiling information have become available and were used to prepare this map, which shows the depth to bedrock in the Seattle 30' by 60' quadrangle (scale 1:100,000). The map shows bedrock depth, in meters, beneath the land surface or the sediment-sea water interface.

Bedrock throughout the Seattle quadrangle is presumed to be volcanic rock, conglomerate, Sandstone, or Shale and is Tertiary in age. With the exception of a few reports of age or lithology collected from oil wells (Livingston, 1958), the subsurface information used for this map sheds little light on the nature and distribution of the various Tertiary rocks in the subsurface. It is assumed, on the basis of pronounced lithologic differences in drill holes and widespread unconformable relationships with underlying bedrock units seen in marine seismic reflection profiles, that the deposits overlying bedrock are Quaternary in age, but no direct dating of materials has been done to confirm this assumption.