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Detailed geologic mapping has been done in east and central Jefferson by Carson (1976a, b) and in north Mason County by Birdseye (1976). These maps include areas bordering western and north-northeastern Hood Canal. However, no detailed geologic maps have been completed that include areas along the eastern side of Hood Canal.
The western side of Hood Canal is predominantly comprised of upper-to-lower Eocene basalt, interbedded conglomerate, and minor sedimentary deposits of the Crescent Formation (Figure 14). Outcrops of dark brown, columnar to massive Crescent basalt with flow tops dipping east at angles of 20° to > 65° are common along the mid-to-northern shoreline (Carson, 1976a, b). For example, basalt flows in the Dosewallips River valley (Figure 4), are oriented "…near vertical with tops facing east" (Babcock et al., 1992). Overlying the upper Crescent Formation basalt are Oligocene(?) Twin River Formation volcanoclastics, pre-Fraser deposits of Olympic Mountain and Puget lobe glacial tills and outwash, and non-glacial sediments (Carson 1976a, b). Further up-section are variably thick layers of late Pleistocene Fraser glaciation sand and gravel tills, proglacial lacustrine silt, sand, clays, glacial drift, and glacial outwash deposits. Overlying these late Pleistocene sediments are recent alluvial fan deposits of sand and gravel deposited by rivers draining the western Olympic Mountains. Both older deglaciation and younger (< 50 yr old) landslide deposits are mapped along the southwestern shoreline of Hood Canal (Carson, 1976a, b). East of Dabob Bay, the Toandos Peninsula (Figure 1) is topped by thick Fraser advance outwash and lodgment tills with no apparent Crescent Formation outcrops (Birdseye, 1976).
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Figure 13: Location and 14C age of Puget Sound coseismic geologic features. The apparent coeval relationship of deformation structures, possibly related to displacement on the Seattle fault 1100 ca, is remarkable. The Saddle Mountain East fault parallels southern Hood Canal’s western shoreline and is shown about 5 km to the W-NW of Hoodsport (modified from Bucknam et al., 1992).
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Figure 14: Upper Crescent Formation outcrops. Pictures taken looking W-NW (see Figure 3 for locations). Shown is east dipping (20-30° ) columnar basalt along Hood Canal’s western shoreline. The outcrop in picture ‘A’ may be landslide-emplaced, although its dip angle is consistent with Crescent Formation mapped by Carson (1976b). Picture B was taken along Seal Rock beach (Figure 1) 0.5 km west of trackline 45 (08:09 GMT) and shows basalt overlain by a dense layer of basal till.
