Geologic units in Pacific county, Washington

Middle and lower Eocene volcanic rocks (Eocene) at surface, covers 25 % of this area

Dark-gray, course- to fine-grained, strongly chloritized basalt flows and breccia; includes pillow lava, deeply altered palagonite beds, amygdoidal and vesicular flows, and, locally, sedimentary rocks. Comprises outer volcanic belt in Olympic Peninsula, where manganese ore is associated with some submarine lavas.

Miocene marine rocks (Miocene) at surface, covers 23 % of this area

Massive to thin-bedded, friable, basaltic to feldspathic sandstone, with shale, siltstone, and local pebble conglomerate interbeds.

Oligocene marine rocks (Late Eocene to Late Oligocene) at surface, covers 10 % of this area

Massive, tuffaceous and nontuffaceous sandstone and siltstone; locally concretionary; includes conglomerate along the north coast of Olympic Peninsula and basaltic sandstone east of Chehalis.

Terrace deposits (Pleistocene) at surface, covers 10 % of this area

Unconsolidated to partly consolidated fluvial and glaciofluvial sand and gravel with minor amounts of silt and clay. Includes marine terrace along west coast of the Olympic Peninsula.

Upper upper Eocene nonmarine and marine rocks (Middle Eocene) at surface, covers 9 % of this area

Massive to thin-bedded, feldspathic to arkosic sandstone, siltstone, shale, and carbonaceous shale; becomes mostly marine in the western foothills of Cascade Mountains where coal beds are abundant. Basaltic sandstone and siltstone in northern Olympic Peninsula.

Alluvium (Holocene) at surface, covers 6 % of this area

Mostly unconsolidated silt, sand, and gravel valley fill with some clay; includes low-level terrace, marsh, peat, artificial fill, and glacial deposits locally.

Eolian deposits (Holocene) at surface, covers 4 % of this area

Active dune sand; includes beach sand along southwestern Washington coast.

Miocene volcanic rocks (Middle Miocene) at surface, covers 3 % of this area

Dark-gray to black, dense aphanitic basalt flows; commonly columnar jointed, less commonly irregularly and platy jointed; some flows vesicular, grading to scoriaceous; includes minor pillow lava, palagonite beds, and interbedded soil profiles and sedimentary beds; contains diatomite beds locally. Maximum thickness in south-central Washington may be in excess of 10,000 feet; much thinner in western Washington, where flows are mostly associated with marine sedimentary rocks. Includes acidic and intermediate volcanic rocks in northern Cascade Mountains.

Lower upper Eocene marine and nonmarine rocks (Eocene) at surface, covers 3 % of this area

Predominantly massive to well-bedded tuffaceous marine siltstone with interbedded arkosic and basaltic sandstone. Includes conglomerate in King County and along north side of Olympic Peninsula. Minor lava flows and breccia in western Lewis County and eastern Grays Harbor County. Coal seams in central Lewis County and north-central Pierce County.

Upper Eocene volcanic rocks (Late Eocene to Oligocene) at surface, covers 3 % of this area

Predominantly basalt flows and flow breccia; includes some pyroclastic and andesite rocks. Chiefly in western Washington.

Oligocene-Miocene marine rocks (Oligocene) at surface, covers 2 % of this area

Massive to thin-bedded, coarse-grained sandstone, conglomerate, conglomeratic sandstone, shale, and sandy shale.

Tertiary dikes, sills, and small intrusive bodies (Middle to Late Tertiary) at surface, covers 1 % of this area

Dikes are commonly diabase; plugs and sills are generally andesite porphyry and dacite.

Upper Eocene volcanic rocks (Late Eocene) at surface, covers 0.2 % of this area

Andesite and basalt flows and associated breccia in central Lewis County. Pyroclastic rocks, mudflows, flow breccia, and volcanic-rich sedimentary rocks in King and Pierce Counties.