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Geologic units in Grays Harbor county, Washington

Additional scientific data in this geographic area

Terrace deposits (Pleistocene) at surface, covers 45 % 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.

Lithology: glacial drift; terrace

Middle and lower Eocene volcanic rocks (Eocene) at surface, covers 17 % 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 11 % of this area

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

Alluvium (Holocene) at surface, covers 7 % 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.

Oligocene marine rocks (Late Eocene to Late Oligocene) at surface, covers 6 % 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.

Miocene-Pliocene marine rocks (Miocene-Pliocene) at surface, covers 5 % of this area

Brown-gray, coarse-grained, moderately consolidated, commonly cross-bedded sandstone. Grades locally into gritstone and conglomerate lenses; includes minor shale beds; sandstone often shaly and in some areas grades into claystone.

Oligocene-Miocene marine rocks (Oligocene) at surface, covers 3 % of this area

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

Eolian deposits (Holocene) at surface, covers 1 % of this area

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

Pre-upper Eocene rocks (Eocene (Olympic Peninsula); Cretaceous(?) (Yakima County)) at surface, covers 0.9 % of this area

Argillite and graywacke between inner and outer volcanic belts in Olympic Peninsula. Sheared carbonaceous argillite, argillite, graywacke, and minor conglomerate lenses and altered lava flows in western Yakima County.

Younger glacial drift (Pleistocene) at surface, covers 0.8 % of this area

Younger glacial drift, undivided. Till, outwash, and associated deposits; sorted and unsorted sand, gravel, silt, and clay. Includes some alluvium.

Lithology: till; outwash; alluvium; peat

Mesozoic-Tertiary volcanic rocks, undivided (Oligocene to Eocene) at surface, covers 0.8 % of this area

Altered basalt, pillow lavas, and flow breccia of inner volcanic belt of Olympic Peninsula; includes minor interbedded red limy argillite and associated manganese ore.

Upper upper Eocene nonmarine and marine rocks (Middle Eocene) at surface, covers 0.6 % 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.

Miocene volcanic rocks (Middle Miocene) at surface, covers 0.4 % 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.

Glacial drift, undivided (Pleistocene) at surface, covers 0.4 % of this area

Glacial and glaciofluvial sand, gravel, and till; includes alpine glacier outwash and till as well as some Recent alluvium.

Tertiary dikes, sills, and small intrusive bodies (Middle to Late Tertiary) at surface, covers 0.2 % of this area

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

Lower upper Eocene marine and nonmarine rocks (Eocene) at surface, covers 0.1 % 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.

Pliocene marine rocks (Pliocene) at surface, covers < 0.1 % of this area

Coarse conglomerate, shale, and minor sandstone; along ocean beaches in Grays Harbor County.

Mesozoic-Tertiary marine rocks, undivided (Miocene to Eocene) at surface, covers < 0.1 % of this area

Dark-gray, massive to poorly bedded gray-wacke of the interior Olympic Peninsula; commonly with interbedded slate, argillite, volcanic rocks, and minor arkosic sandstone. Includes rocks both older and younger than Ev2, some of which may be Paleozoic.