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Geologic units in Grays Harbor county, Washington

[Additional scientific data in this geographic area]

Tertiary dikes, sills, and small intrusive bodies (Middle to Late Tertiary) at surface, covers 0.1 % of this area
Dikes are commonly diabase; plugs and sills are generally andesite porphyry and dacite.
Lithology: quartz monzonite; granite; basalt; quartz diorite; rhyodacite; tonalite; gabbro; andesite; dacite; rhyolite
Terrace deposits (Pleistocene) at surface, covers 31 % 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
Younger glacial drift (Pleistocene) at surface, covers 0.5 % 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
Glacial drift, undivided (Pleistocene) at surface, covers 0.2 % of this area
Glacial and glaciofluvial sand, gravel, and till; includes alpine glacier outwash and till as well as some Recent alluvium.
Lithology: glacial drift; till; outwash; alluvium
Eolian deposits (Holocene) at surface, covers 0.8 % of this area
Active dune sand; includes beach sand along southwestern Washington coast.
Lithology: beach sand; dune sand; mud flat
Alluvium (Holocene) at surface, covers 5 % 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.
Lithology: alluvium; alluvial terrace; peat; glacial drift
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.
Lithology: siltstone; sandstone; conglomerate
Pre-upper Eocene rocks (Eocene (Olympic Peninsula); Cretaceous(?) (Yakima County)) at surface, covers 0.6 % 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.
Lithology: sandstone; argillite; siltstone; conglomerate; mudstone; limestone; chert; greenstone; basalt
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.
Lithology: sandstone; siltstone; shale; olistostrome; tuff; conglomerate
Oligocene marine rocks (Late Eocene to Late Oligocene) at surface, covers 4 % 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.
Lithology: siltstone; sandstone; conglomerate; tuff; mudstone
Mesozoic-Tertiary volcanic rocks, undivided (Oligocene to Eocene) at surface, covers 0.5 % 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.
Lithology: basalt; sandstone; greenstone; diabase; gabbro; argillite
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.
Lithology: graywacke; siltstone; slate; phyllite; argillite; arkose; siltstone; conglomerate; coal; basalt; gabbro
Miocene volcanic rocks (Middle Miocene) at surface, covers 0.3 % 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.
Lithology: tholeiite; andesite; dacite; rhyolite; sandstone; siltstone; conglomerate
Miocene-Pliocene marine rocks (Miocene-Pliocene) at surface, covers 4 % 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.
Lithology: sandstone; siltstone; conglomerate; mudstone
Miocene marine rocks (Miocene) at surface, covers 8 % of this area
Massive to thin-bedded, friable, basaltic to feldspathic sandstone, with shale, siltstone, and local pebble conglomerate interbeds.
Lithology: sandstone; siltstone; shale; conglomerate
Middle and lower Eocene volcanic rocks (Eocene) at surface, covers 12 % 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.
Lithology: tholeiite; siltstone; sandstone
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.
Lithology: sandstone; siltstone; conglomerate; lava flow; volcanic breccia (agglomerate); tuff; claystone; shale; coal; sedimentary breccia; olistostrome
Upper upper Eocene nonmarine and marine rocks (Middle Eocene) at surface, covers 0.4 % 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.
Lithology: sandstone; siltstone; shale; coal; conglomerate; olistostrome; mafic volcanic rock

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