Geologic units in Washington (state in United States)

Additional scientific data in this geographic area

Miocene volcanic rocks (Middle Miocene) at surface, covers 14 % 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.

Quaternary nonmarine deposits (Pleistocene) at surface, covers 13 % of this area

Periglacial eolian deposits. Buff to light-brown, massive, homogenous, unconsolidated loessial silt; some water-laid material locally. Probably early Pleistocene.

Mesozoic granitic rocks, undivided (Mostly Cretaceous-Jurassic) at surface, covers 9 % of this area

Granite, quartz monzonite, quartz diorite, granodiorite, and trondhjemite. Includes diorite in southeastern Washington; diorite and gabbro near Concunully in Okanogan County; gneiss, schist, and migmatites in areas of Chelan, Colville, and Okanogan batholiths. Includes high-grade metamorphic rocks of Precambrian age in Spokane area.

Glacial drift, undivided (Pleistocene) at surface, covers 5 % of this area

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

Younger glacial drift (Pleistocene) at surface, covers 4 % of this area

Advance and recessional outwash, stratified drift, and associated deposits. Primarily silt, sand, and gravel with some clay. Includes alluvium locally and scabland deposits of eastern Washington.

Alluvium (Holocene) at surface, covers 4 % 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.

Younger glacial drift (Pleistocene) at surface, covers 3 % of this area

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

Eocene-Oligocene volcanic rocks (Eocene-Oligocene) at surface, covers 3 % of this area

Predominantly light-green, bedded andesite breccia with interbedded andesite and basalt flows, mudflows, and tuff beds; becomes more tuffaceous near top of unit. Includes tuffaceous and arkosic sandstone, shale, and carbonaceous shale beds in central and southern Cascade Mountians. Rhyodacite and quartz latite flows in northwestern Ferry County.

Terrace deposits (Pleistocene) at surface, covers 3 % 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.

Pleistocene-Recent volcanic rocks (Pleistocene to Holocene) at surface, covers 3 % of this area

Predominantly dark-gray to black vesicular basalt; olivine-rich in part. Includes andesite flows and pyroclastic rocks of Mount Baker, Glacier Peak, Mount Rainier, Mount Adams, Mount St. Helens, and all cinder cones in southern part of the State. Also includes some Recent flows southeast of Mount St. Helens.

Middle and lower Eocene volcanic rocks (Eocene) at surface, covers 2 % 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.

Pre-Upper Jurassic gneiss (Mostly Cretaceous) at surface, covers 2 % of this area

Biotite, quartz diorite, trondhjemite, and hornblende gneisses, many of which are migmatitic; includes small granitic bodies locally. Small areas of mica schist, marble, amphibolite, and lime-silicate rocks in Entiat Mountains area of Chelan County.

Younger glacial drift (Pleistocene) at surface, covers 2 % of this area

Till. Hard, blue-gray to gray concrete-like mixture of clay, silt, sand, and gravel deposited as end or recessional moraine. Principally Wisconsin in age.

Mesozoic-Tertiary marine rocks, undivided (Miocene to Eocene) at surface, covers 2 % 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.

Glaciolacustrine deposits (Pleistocene) at surface, covers 2 % of this area

Fine-grained sand and silt, well-stratified, with some gravel, clay, and diatomaceous earth. Contains clastic dikes in Walla Walla area.

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

Andesite flow breccia, andesite flows, and minor tuff beds; includes some basalt flows and flow breccia. Commonly more massive and less altered than similar-appearing Eocene-Oligocene volcanic rocks. Clastic flows and flows of black glass, and course to fine-grained clastic and pyroclastic rocks in the Republic and Curlew areas of Ferry County.

Tertiary granitic rocks (Probably mostly Eocene; ranges from Miocene to Paleocene) at surface, covers 2 % of this area

Granite, quartz monzonite, quartz diorite, granodiorite, and trondhjemite. Includes dacite porphyry and granite breccia near Bumping Lake, Yakima County.

Upper Eocene volcanic rocks (Late Eocene to Oligocene) at surface, covers 2 % of this area

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

Paleocene-Cretaceous nonmarine rocks (Eocene (see age coding)) at surface, covers 2 % of this area

Brown-gray to light-gray, medium- to course-grained massive cross-bedded arkose with interbedded conglomerate and siltstone. Contains several coal seams in Whatcom County. Iron-rich laterite at base near Cle Elum and Blewett Pass in Kittitas and Chelan Counties.

Pre-Upper Jurassic metamorphic rocks of the medium and high-grade zone (Early Jurassic-Triassic) at surface, covers 1 % of this area

Schist, amphibolite, and minor lime-silicate rocks, marble, quartzite, and metaconglomerate.

Precambrian rocks, undivided (Proterozoic) at surface, covers 1 % of this area

Predominantly phyllite with some schist, limestone, dolomite, quartzite, and volcanic rocks; northeastern Pend Oreille County. Mainly quartzite sandstone in upper part, dark-gray argillite with sandstone and limestone in middle part, and sandstone with argillite in lower part; southeastern Pend Oreille County. Banded slate with quartzite and dolomite; southwestern Stevens County. Quartzite, siliceous argillite, and argilliceous quartzite grading into argillite and quartz-mica schists form south ot north; southeastern Stevens County. Quartzite, argillite, quartz-feldspar gneiss, and other metamorphic rocks in northeastern Whitman and southeastern Spokane Counties are partly if not all extenstions of the Belt strata.

Pre-Upper Jurassic metamorphic rocks of the low-grade zone (Jurassic) at surface, covers 0.9 % of this area

Greenschist, phyllite, and slate; includes some limestone, quartzose phyllite, schistose metaconglomerate, breccia, and basic igneous rocks. Includes schist locally.

Quaternary nonmarine deposits (Pleistocene) at surface, covers 0.8 % of this area

Periglacial lacustrine deposits. Light-brown, well-sorted and bedded clayey sandstone and sandy clay with interbeds of volcanic ash and calcareous cemented gravels.

Upper Paleozoic rocks, undivided (Ordovician) at surface, covers 0.8 % of this area

Mostly graywacke, interbedded quartzite and phyllite, greenstone and serpentine, and black shale with minor limestone. Some quartz-mica schist in Bald Knob area of Ferry County. Schist, gneiss, and amphibolite in other parts of Ferry County. Some rocks of lower Paleozoic age, possibly Precambrian, and Mesozoic may be included.

Cretaceous sedimentary rocks, mostly marine (Cretaceous) at surface, covers 0.8 % of this area

Lower Cretaceous greenish-gray massive arkosic sandstone and light-gray sandy shale in western Okanogan County. Includes phyllite in eastern Whatcom County.

Oligocene marine rocks (Late Eocene to Late Oligocene) at surface, covers 0.7 % 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.

Upper upper Eocene nonmarine and marine rocks (Middle Eocene) at surface, covers 0.7 % 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.

Upper Eocene volcanic rocks (Late Eocene) at surface, covers 0.7 % of this area

Predominantly andesite flows and breccia; includes some basalt flows. Contains basaltic conglomerate, pyroclastic rocks, tuff beds, and sandstone in Chehalis-Centralia coal district, Lewis County.

Miocene marine rocks (Miocene) at surface, covers 0.6 % of this area

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

Miocene-Pliocene nonmarine rocks (Miocene-Pliocene) at surface, covers 0.6 % of this area

Tuffaceous and pumiceous andesitic sandstone and siltstone with interbedded conglomerate and claystone. Conglomerate beds chiefly andesitic, but also quartzitic, granitic, and basaltic; includes basalt flows locally.

Lower upper Eocene marine and nonmarine rocks (Eocene) at surface, covers 0.5 % 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.

Pre-Tertiary metamorphic rocks, undivided (Probably Permian) at surface, covers 0.5 % of this area

Schist, gneiss, marble, quartzite, amphibolite, greenstone, metaconglomerate, graywacke; includes metasedimentary, volcanic, and intrusive rocks. Some areas, as on San Juan Islands, show little if any metamorphism.

Older glacial drift (Pleistocene) at surface, covers 0.5 % of this area

Till, outwash, and associated interglacial deposits; sorted and unsorted gravel, sand, silt, and clay. Includes peat beds and non-glacial sediments locally. Mostly pre-Wisconsin in age.

Upper Jurassic-Lower Cretaceous sedimentary and volcanic rocks (Late Jurassic-Early Cretaceous) at surface, covers 0.4 % of this area

Predominantly sedimentary rocks. Graywacke, argillite, and siltstone with some slate and phyllite; includes graywacke breccia and ribbon chert with minor local limestone lenses and basalt flows.

Pre-upper Eocene rocks (Eocene (Olympic Peninsula); Cretaceous(?) (Yakima County)) at surface, covers 0.4 % 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.

Tertiary-Cretaceous granitic intrusive rocks (Early Tertiary-Late Cretaceous) at surface, covers 0.4 % of this area

Granite, granodiorite, trondhjemite, and quartz diorite. Late Cretaceous and/or early Tertiary.

Quaternary nonmarine deposits (Pleistocene) at surface, covers 0.4 % of this area

Predominantly a well-cemented, heterogeneous mixture of volcanic gravel, sand, silt, and clay. Contains some till and in western Washington is commonly deeply weathered.

Tertiary volcanic rocks, undivided (Mostly Oligocene-Eocene) at surface, covers 0.4 % of this area

Includes andesite, basalt, and rhyolite flows, and associated pyroclastic rocks. In isolated areas across the northern part of the State.

Upper Jurassic-Lower Cretaceous sedimentary and volcanic rocks (Late Cretaceous-Jurassic) at surface, covers 0.4 % of this area

Sedimentary and volcanic rocks, undivided. Graywacke, argillite, siltstone, slate, volcanic rocks, phyllite, greenschist, and greenstone.

Eocene nonmarine rocks (Eocene) at surface, covers 0.4 % of this area

Predominantly sandstone and shale. Includes some conglomerate in the Cle Elum area in Kittilas County. Contains extensive coal seams near Roslyn and carbonaceous shale and coal beds in White Pass area. Contains tuff beds in northwestern Ferry County.

Cretaceous sedimentary rocks, mostly marine (Cretaceous) at surface, covers 0.4 % of this area

Upper Cretaceous black siltstone, graywacke, and silica pebble conglomerate in western Okanogan County.

Lower Paleozoic rocks, undivided (Paleozoic; likely Ordovician) at surface, covers 0.3 % of this area

Predominantly gneiss and schist in northern Ferry County. Quartzite, partly interbedded with dolomitic and calcitic marble, lime-silicate gneiss, and amphibolite; including in places forsterite marblean diopside marble, quartz-biotite-sillimanite schist and sillimanite-orthoclase gneiss in northeastern Ferry County. Quartzite with overlying limestone in southeastern Stevens County. Some upper Paleozoic and Precambrian rocks may be included.

Oligocene-Miocene marine rocks (Oligocene) at surface, covers 0.3 % of this area

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

Pre-Tertiary sedimentary and metasedimentary rocks, undivided (Mostly Early Cretaceous to Middle Jurassic, possibly includes minor Eocene rocks) at surface, covers 0.3 % of this area

Graywacke, argillite, phyllite, chert, talc, and graphite schist; some faulted-in blocks of serpentinite and greenstone. Includes minor limestone on San Juan Island.

Cambrian phyllite (Cambrian) at surface, covers 0.3 % of this area

Predominantly gray-green, banded phyllite, some sericite schist, abundant thin beds of quartzite, especially in lower part of unit, and limestone in upper part; northern Pend Oreille County. Much the same lithology but more conspicuous subunits of quartzite, limestone, and schist in northeastern Stevens County. Gray phyllite, greenish argillite, andalusite schist, minor inerbedded quartzite and siliceous dolomite, especially in lower part, and much gray limestone in upper part; north-central Stevens County. Limestone bed in lower part of unit contains Lower Cambrian fossils (Archaeocyathus).

Permian rocks (Permian-Triassic) at surface, covers 0.2 % of this area

Conglomerate, graywacke, siltstone, argillite and interbedded fossiliferous limestone, greenstone, and minor angular conglomerate in northwestern Stevens and Ferry Counties. Impure quartzite, sandstone, graywacke, greenstone, ribbon chert, chert breccia, and limestone in Snohomish County and on San Juan Island. Lower Permian limestone on Black Mountain in northwestern Whatcom County. Middle Permian rocks in northeastern Washington.

Tertiary marine and nonmarine rocks (Eocene) at surface, covers 0.2 % of this area

Predominantly light-gray to olive-gray, fine to course-grained, thick-bedded sandstone with laminae to thick beds of medium dark gray siltstone, in northern Olympic Peninsula. Probably late Eocene to Miocene.

Pliocene-Pleistocene volcanic rocks (Mostly Pleistocene) at surface, covers 0.2 % of this area

Light-gray andesite, andesite porphyry, and open-textured basalt flows with minor associated mudflows and breccia. Includes restricted areas of valley flow basalt in Snake River Canyon in southeastern Washington and in Spokane area.

Carboniferous-Permian sedimentary and volcanic rocks (Devonian to Permian; some Jurassic) at surface, covers 0.2 % of this area

Sedimentary and volcanic rocks, undivided. Cherty and slaty argillite, siltstone, graywacke, chert, greenstone, tuff, andesite, and spilitic volcanics.

Landslide and mudflow deposits (Holocene) at surface, covers 0.2 % of this area

Predominantly landslide debris. Includes till-like mudflow deposits of andesitic rock fragments in clayey sand matrix in Buckley-Enumclaw area of Kind and Pierce Counties.

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.

Cambrian quartzite (Cambrian) at surface, covers 0.2 % of this area

Gritstone with conglomerate in lower part and gray, white, and buff platy quartzite in upper part grading upward into phyllite; northern Pend Oreille County. Light-gray, platy, medium- to thick-bedded quartzite in upper part and darker impure quartzite with some interbedded argillite or phyllite in lower part; north-central Stevens County. White, gray, and reddish, medium- to thick-bedded quartzite in lower part and thin interbeds of quartzite and argillite in upper part, with very low Lower Cambrian fossils (Nevadia, Hyolthellus, and three genera of brachipods) near base of upper part; Addy district of central Stevens County. Thick argillite and thin quartzite units form an uppermost zone in southwestern Stevens County. Conspicuous phyllite units near base in north-central Lincoln County.

Pre-Upper Jurassic metamorphic rocks of the low-grade zone (Early Cretaceous metamorphism (Jurassic protolith)) at surface, covers 0.2 % of this area

Greenschist of central Whatcom and Skagit Counties.

Miocene-Pliocene volcanic rocks (Miocene-Pliocene) at surface, covers 0.2 % of this area

Dark-gray, fine-grained, dense, porphyritic in part, basalt flows in central and south-central part of State; commonly interbedded with conglomerate, sandstone, and siltstone. Includes small areas of rhyolite north of Cle Elum in Kittitas County, and andesite north of Leavenworth in Chelan County.

Miocene nonmarine rocks (Late Miocene) at surface, covers 0.2 % of this area

Poorly to moderately consolidated tuffaceous sandstone, conglomerate, siltstone, and claystone in western Washington. Chiefly clay and shale with minor sand, gravel, and diatomaceous earth near Spokane. Includes diatomite beds near Yakima and Quincy, and some marine beds in Western Washington.

Cambrian limestone and dolomite (Early Cambrian-Middle Ordovician) at surface, covers 0.2 % of this area

Mostly massive dolomite, with a basal unit of gray to dark-gray limestone interbedded with limy shale, and an upper unit of fine-grained massive limestone with some marble; Pend Oreille and northern and central Stevens Counties. Three-fold division less evident in Colville area. Dolomite, with minor basal unit of interbedded limestone and phyllite in the Addy-Dunn Mountain area of Stevens County. Marble, dolomite, limestone, and limy slate in Hunters' district. Dolomitic marble in southern Stevens and northern Lincoln Counties. Middle Cambrian fossils near base in Metaline district, and Bathyuriscus-Elrathina fauna in lower unit in Leadpoint district. Phosphatic brachiopods in upper unit in Leadpoint district tentatively assigned to Middle and Upper Cambrian.

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

Predominantly andesite flows and breccia; includes interbedded sedimentary rocks south of Startup in Snohomish and King Counties.

Ordovician rocks (Middle Ordovician) at surface, covers 0.2 % of this area

Mainly black to gray slate or slaty argillite, argillite, black to dark-gray siltstone in north-central Stevens County and grayish olive-green silty argillite in west-central Stevens County. Many occurrences of Early and Middle Ordovician graptolites; also rare conodonts.

Carboniferous and Permian volcanic rocks (Devonian to Permian; Triassic in Asotin County) at surface, covers 0.2 % of this area

Predominantly altered andesite, basalt, and diabase with interbedded chert and argillite; includes some tuff, greenstone, and spilitic volcanic rocks; northern Cascade Mountains. Mostly schistose greenstone, some agglomerate, and rarely lapilli; includes minor beds of limestone with associated argillite and graywacke; northwestern Stevens County.

Carboniferous-Permian sedimentary and volcanic rocks (Devonian to Permian, minor Mesozoic) at surface, covers 0.2 % of this area

Predominantly sedimentary rocks. Graywacke, argillite, and slate; includes minor marble, siltstone, arkose, conglomerate, ribbon cherts, and volcanic rocks. Some Devonian rocks may be included in northwestern Washington.

Pre-Tertiary ultrabasic intrusive rocks (Late-Jurassic) at surface, covers 0.2 % of this area

Peridotite and pyroxenite; generally altered partly or completely to serpentine. Includes serpentinite and saxonite on Sumas Mountain, Whatcom County.

Pliocene nonmarine rocks (Pliocene) at surface, covers 0.2 % of this area

Conglomerate, sandstone, shale, and mudstone. Tuffaceous in part; contains alluvial fan type material locally.

Miocene-Pliocene marine rocks (Miocene-Pliocene) at surface, covers 0.2 % 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.

Mesozoic volcanic rocks, undivided (Jurassic) at surface, covers 0.1 % of this area

Includes latite, andesite and basalt flows, tuff, and agglomerate. Interbedded sedimentary rocks in Orient area of Stevens County.

Eolian deposits (Holocene) at surface, covers 0.1 % of this area

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

Upper Eocene volcanic rocks (Late Eocene) at surface, covers 0.1 % 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.

Upper Cretaceous-lower Tertiary volcanic rocks (Late Cretaceous) at surface, covers 0.1 % of this area

Dark-gray to green andesite tuff, breccia, and flows in Methow River area of Okanogan County.

Triassic sedimentary rocks, undivided (Triassic with Permian where impossible to differentiate) at surface, covers 0.1 % of this area

Predominantly limestone, marble, and dolomite near Riverside in Okanogan County. Conglomerate, shale, graywacke, gritstone, and limestone on San Juan Island. Siltstone with greenstone locally on Orcas Island. Graywacke conglomerate, cherty greenstone, and limestone in northern Ferry County.

Lower Tertiary volcanic rocks, undivided (Eocene) at surface, covers < 0.1 % of this area

Predominantly andesite flows and flow breccia; includes basalt flows, minor rhyolitic rocks, and some sedimentary rocks.

Pre-Carboniferous intrusive rocks (Paleozoic) at surface, covers < 0.1 % of this area

Meta-quartz diorite, hypersthene diorite, and gneissose and directionless quartz diorite of eastern Skagit County. Quartz diorite and diorite in the San Juan Islands. Includes amphibolite and gneiss locally.

Pre-Middle Jurassic sedimentary and volcanic rocks (Jurassic) at surface, covers < 0.1 % of this area

Predominantly sedimentary rocks. Graywacke, argillite, and slate; includes minor marble, siltstone, arkose, conglomerate, ribbon cherts, and volcanic rocks.

Tertiary nonmarine rocks, undivided (Tertiary; mostly Eocene) at surface, covers < 0.1 % of this area

Sandstone, shale, conglomerate, agglomerate, and tuff; includes some lava flows. Massive conglomerate, sandstone, siltstone, and ferruginous shale in northwestern Whatcom County.

Precambrian volcanic rocks (Late-Proterozoic) at surface, covers < 0.1 % of this area

Mostly homogenous schistose greenstone; in places massive, mottled, and containing conspicuous calcite and epidote. Tuffaceous chlorite schist in upper part in northern Pend Oreille County. Amphibolite and plagioclase amphibolite in Little Pend Oreille Lakes district. Massive to sheared or schistose greenstone with dark-green ovoid spots; agglomeratic and amygodaloidal in places; sheared pillows near Blue Creek, central Stevens County; minor intrusive phase and probable center of eruption west of Finch magnesite quarry; central to southwestern Stevens County.

Mesozoic-Tertiary volcanic rocks, undivided (Oligocene to Eocene) at surface, covers < 0.1 % 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.

Pre-Tertiary volcanic rocks, undivided (Probably mostly Jurassic) at surface, covers < 0.1 % of this area

Andesite and basalt flows, and greenstone; includes minor interbedded limestone, arkose, quartzite, and chert beds.

Tertiary dunite intrusive rocks (Late Cretaceous) at surface, covers < 0.1 % of this area

An intimate mixture of saxonite and dunite, partly serpentinized; Twin Sisters mountain area of Whatcom and Skagit Counties. Predominantly dunite and associated serpentinite in Jumbo Mountain area of central Snohomish County.

Oligocene nonmarine rocks (Oligocene) at surface, covers < 0.1 % of this area

Andesite conglomerate, tuff beds, and mudflow material. Includes some interbedded andesite flows in Columbia River Gorge. Lake sediments with Oligocene flora in Republic area in Ferry County. Massive tuffaceous sandstone and siltstone with beds of coal and high-alumina clay in Castle Rock-Toledo coal district in Cowlitz and Lewis Counties; include local interbedded basalt flows and some marine and late Eocene rocks.

Precambrian conglomerate (Late-Proterozoic) at surface, covers < 0.1 % of this area

Gray-brown, coarse, poorly sorted pebbles and cobbles of limestone, dolomite, reddish-brown quartzite, black slate or phyllite, and rarely granitic rocks in a gray sandy phyllite matrix; northeastern Pend Oreille County and southwestern Stevens County. Rocks become finer grained and more schistose and the unit becomes thicker toward the southwest, where there is included an isolated subunit which may be a tillite, consisting of cobbles, boulders, and blocks of argillite and carbonate rocks in a fine silty matrix.

Upper Eocene volcanic rocks (Late-Middle Eocene) at surface, covers < 0.1 % of this area

Rhyolite flows and some interbedded tuff beds in Cle Elum area, Kittitas County.

Precambrian (?) phyllite (Cambrian-Precambrian boundary) at surface, covers < 0.1 % of this area

Mostly phyllite with interbedded carbonate rocks, quartzite, and gritstone; some tufflike beds and conglomerate at the base. Rocks confined to northeastern Pend Oreille County and central Stevens County.

Basic intrusive rocks, undivided (Jurassic in central Cascades; Oligocene to Eocene in northern Cascades) at surface, covers < 0.1 % of this area

Predominantly gabbro; includes some serpentine. Age undetermined.

Pre-Tertiary basic intrusive rocks (Triassic-Permian) at surface, covers < 0.1 % of this area

Predominantly gabbro and metagabbro; includes hornblendite, peridotite, and pyroxenite. In Nighthawk district and near 49th Parallel in Okanogan County and in Orient district of Stevens County.

Oligocene volcanic rocks (Oligocene) at surface, covers < 0.1 % of this area

Andesite and rhyodacite flows, tuff, and tuffaceous sandstones of northern Lincoln County and southwestern Stevens County.

Cretaceous nonmarine rocks (Late Cretaceous) at surface, covers < 0.1 % of this area

Redbeds of thick-bedded sandstone, shale, and pebble conglomerate in upper Methow River area of Okanogan County.

Pre-Carboniferous crystalline complex (Devonian) at surface, covers < 0.1 % of this area

Metahornblendite, amphibolite, gneiss, metadiorite, meta-quartz diorite, and trondhjemite. As klippes along western slope of northern Cascade Mountains.

Pre-Middle Jurassic sedimentary and volcanic rocks (Late Paleozoic deposition? with Cretaceous metamorphism?) at surface, covers < 0.1 % of this area

Sedimentary and volcanic rocks, undivided. Graywacke, argillite, slate, greenstone, and spilitic volcanic rocks.

Tertiary-Cretaceous basic intrusive rocks (Cretaceous-Jurassic) at surface, covers < 0.1 % of this area

Diorite and gabbro in western Snohomish County.

Upper Jurassic-Lower Cretaceous volcanic rocks (Jurassic-Cretaceous on San Juan Islands; possibly Eocene in King County) at surface, covers < 0.1 % of this area

Predominantly volcanic rocks, mostly metamorphosed to greenstone and greenschist; includes some sedimentary rocks

Middle Jurassic volcanic rocks (Middle Jurassic) at surface, covers < 0.1 % of this area

Andesite, dacite, and minor basalt with slate and graywacke interbeds in the Nooksack River region of Whatcom County.

Mesozoic alkali intrusive rocks (Jurassic-Cretaceous) at surface, covers < 0.1 % of this area

Syenite in Nighthawk and Buckhorn Mountain areas of Okanogan County; malignite on Kruger Mountain near Nighthawk.

Cretaceous sedimentary rocks, mostly marine (Late Cretaceous; possibly Eocene) at surface, covers < 0.1 % of this area

Cretaceous conglomerate, sandstone, shale, breccia, limestone, and gritstone in San Juan Islands.

Upper Tertiary volcanic rocks, undivided (Miocene-Oligocene) at surface, covers < 0.1 % of this area

Mostly massive andesite flows, flow breccia, and pyroclastic material; includes some basalt flows and sedimentary rocks.

Carboniferous rocks (Late Devonian to Mississippian) at surface, covers < 0.1 % of this area

Thin-bedded graywacke, shale, argillite, slate, schist, volcanic breccia, gritstone, conglomerate, and limestone on northeast shore of Orcas Island. Limestone or dolomitic limestone, apparently interbedded with limy argillite and graywacke, forms belt of small separate outcrops between Springdale and Valley in southeastern Stevens County. Late Devonian to Early Pennsylvanian in age.

Pre-Middle Jurassic volcanic rocks, undivided (Early Cretaceous-Late Jurassic) at surface, covers < 0.1 % of this area

Predominantly greenstone and spilitic volcanic rocks; includes some slate, argillite, and graywacke.

Precambrian rocks, undivided (Middle to Late Proterozoic) at surface, covers < 0.1 % of this area

Magnesite-bearing Stensgar dolomite; southwestern Stevens County.

Upper Triassic and/or Lower Jurassic Marine Rocks (Cretaceous-Triassic) at surface, covers < 0.1 % of this area

Conglomerate, gritstone, graywacke, and carbonaceous argillite of northwestern Whatcom County.

Devonian rocks (Devonian to Permian) at surface, covers < 0.1 % of this area

Thin alternating beds of cherty quartzite and argillite, occasional limestone lenses, volcanic rocks, sandstone, and graywacke on northwest shore of Orcas Island. Small outcrops of Devonian limestone included under Pzu in southeastern Stevens County and under PMPms in northwestern Whatcom County.

Lower and Middle Jurassic marine rocks (Devonian-Permian) at surface, covers < 0.1 % of this area

Dark-gray, massive to thick-bedded, siliceous argillite, siltstone, and shale; includes minor graywacke and limestone. Confined to northwestern Whatcom County.

Silurian-Devonian rocks (Silurian-Devonian) at surface, covers < 0.1 % of this area

Mainly black argillite; some limestone, thin beds of conglomerate, sandstone, quartzite, and dolomite. Rocks confined to northwestern Pend Oreille County and adjacent Stevens County. Silurian age based on Monograptus and corals of several unusual genera; Devonian age based on coral fauna.

Tertiary alkali intrusive rocks (Probably Eocene) at surface, covers < 0.1 % of this area

Syenite sills in Phalen Lake area, northwestern Stevens County. Porphyritic syenite in Danville area of Northern Ferry County.

Mesozoic alkali intrusive rocks (Likely Eocene) at surface, covers < 0.1 % of this area

Albite granite as small plugs and sills near Boundary in Stevens 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.

Columbia River Basalt Group. (Miocene) at surface, covers < 0.1 % of this area

Large-volume lava flows of tholeiitic basalt, basaltic andesite, and subordinate andesite in western Idaho; consists of Imnaha Basalt (17.5-16.5 Ma), Grande Ronde Basalt (16.5-15.6 Ma), Wanapum Basalt (15.6-14.5 Ma), and Saddle Mountains Basalt (14.5-6 Ma). Includes porphyritic basalt and basaltic andesite in western Owyhee County. (Quaternary to Eocene Continental Volcanic and Intrusive Rocks).

Precambrian intrusive rocks (Middle Proterozoic(?)) at surface, covers < 0.1 % of this area

Dark-gray diorite sills from 3 to 1200 feet thick. Confined to Bead Lake district, southeastern Pend Oreille County. Tentatively assigned to the Precambrian.

Grande Ronde Basalt (Early to Middle Miocene) at surface, covers < 0.1 % of this area

Flows of dark-gray to black, aphyric tholeiitic basalt, including both high- and low-Mg chemical types (Swanson and others, 1979). Potassium-argon ages mostly in the range of 15 to 17 Ma (Lux, 1982; Watkins and Baksi, 1974; Fiebelkorn and others, 1983)

Pre-Tertiary dunite intrusive rocks (Middle Jurassic) at surface, covers < 0.1 % of this area

Dunite associated with serpentine on Cypress Island in San Juan Islands and on Mount Chopaka near Oroville in Okanogan County.

Windermere Supergroup. (Cambrian and Neoproterozoic) at surface, covers < 0.1 % of this area

Upper part consists of shallow marine and fluvial quartzose sandstone, and minor siltite, shale, and limestone of the Brigham Group of southeast Idaho, Wilbert Formation of east-central Idaho, and quartzites of unknown affinity west of Borah Peak. More highly metamorphosed equivalents are quartzite, metaconglomerate, metasiltite, calc-silicate marble, and schist of Gospel Peaks successions C and D in central Idaho; mature quartzite, biotite schist, and minor calc-silicate rocks of Syringa metamorphic sequence east of Moscow; and schist and quartzite in the Albion Range (Elba Quartzite, schist of Upper Narrows, quartzite of Yost, schist of Stevens Spring, quartzite of Clarks Basin, schist of Mahogany Peaks, and Harrison Summit Quartzite). Lower part consists of diamictite, immature sandstone, and bimodal volcanic rocks related to continental rifting. Includes Pocatello Formation (700-665 Ma), formation of Leaton Gulch near Challis, Shedroof Conglomerate in extreme northwest Idaho, and metamorphic equivalents (schist, marble, calc-silicate rocks, metaconglomerate, and ~686 Ma metavolcanic rocks) of Gospel Peaks successions A and B in central Idaho. (Paleocene to Neoproterozoic Sedimentary and Igneous Rocks of the Cordilleran System).

Lake Bonneville deposits. (Pleistocene) at surface, covers < 0.1 % of this area

Silt, clay, sand, and gravel deposited in and at margins of former Lake Bonneville (33-11 calibrated ka) and sand and gravel deposited in giant flood bars by outburst floods from the lake (17.4 calibrated ka). The 1,575-m (5,170 ft) Lake Bonneville shoreline was used to determine the maximum extent of the lake deposits. Flood deposits follow Bonneville flood path from near Downey and the Portneuf River westward along the Snake River to Lewiston. They include sand and silt deposited in slack-water areas to 740 m (2,430 ft) elevation in the Boise, Weiser, Payette, and Snake river drainages. (Quaternary Sediments).

Saddle Mountains Basalt (Middle to Late Miocene) at surface, covers < 0.1 % of this area

Petrographically diverse flows of basalt erupted between about 13.5 and 6 Ma (McKee and others, 1977; Swanson and others, 1979)

Seven Devils Group. (Triassic and Permian) at surface, covers < 0.1 % of this area

Basaltic to rhyolitic (largely mafic) arc-derived volcanic and volcaniclastic rocks of the Wallowa terrane; includes minor limestone. Composed of Windy Ridge, Hunsaker Creek, Wild Sheep Creek, and Doyle Creek formations. (Mesozoic-Paleozoic Rocks of the Blue Mountains Island-Arc Complex).

Glaciofluvial, lacustrine, and pediment sedimentary deposits (Pleistocene) at surface, covers < 0.1 % of this area

Unconsolidated, poorly sorted silt, sand, and gravel. Includes lacustrine deposits west of Columbia River Gorge (Trimble, 1963). Mostly in northern Morrow and Umatilla Counties where unit represents deposits of swollen late Pleistocene Columbia River (Hogenson, 1964)

Coon Hollow and Weatherby formations. (Cretaceous and Jurassic) at surface, covers < 0.1 % of this area

Marine mudstone and subordinate conglomerate and sandstone of the Coon Hollow Formation south of Lewiston and turbiditic sandstone, mudstone, volcanic conglomerate, and andesite and rhyolite tuff of the Weatherby Formation north of Weiser. (Mesozoic-Paleozoic Rocks of the Blue Mountains Island-Arc Complex).

Sedimentary and volcanic rocks. (Jurassic and Triassic) at surface, covers < 0.1 % of this area

Marine limestone and marble of the Martin Bridge Formation and calcareous mudstone and phyllite of the Hurwal Formation exposed west of Riggins and south of Lewiston; basaltic andesite, rhyolite tuff (~202 Ma), and conglomerate along Salmon River southwest of Grangeville; and rhyolite tuff at Pittsburg Landing (~198 Ma). (Mesozoic-Paleozoic Rocks of the Blue Mountains Island-Arc Complex).

Landslide deposits. (Quaternary) at surface, covers < 0.1 % of this area

Unsorted gravel, sand, and clay of landslide origin; includes rotational and translational blocks and earth flows. (Quaternary Sediments).

Quartz diorite. (Cretaceous and Jurassic) at surface, covers < 0.1 % of this area

Primarily quartz diorite, with subordinate diorite, gabbro, granite, and amphibolite; largely undeformed except near eastern part of Blue Mountains island-arc complex; wide age range (160-90 Ma). (Mesozoic-Paleozoic Rocks of the Blue Mountains Island-Arc Complex).

Granodiorite and two-mica granite. (Cretaceous) at surface, covers < 0.1 % of this area

Granodiorite and granite containing biotite, commonly with muscovite; includes bulk of Atlanta lobe (85-67 Ma) and isolated plutons in northern Idaho (107-67 Ma). (Paleocene and Cretaceous Idaho Batholith and Older Cretaceous and Jurassic Intrusive Rocks).

Columbia River Basalt Group and related flows (Miocene) at surface, covers < 0.1 % of this area

Subaerial basalt and minor andesite lava flows and flow breccia; submarine palagonitic tuff and pillow complexes of the Columbia River Basalt Group (Swanson and others, 1979); locally includes invasive basalt flows. Flows locally grade laterally into subaqueous pillow-palagonite complexes and bedded palagonitic tuff and breccia. In places includes tuffaceous sedimentary interbeds. Joints commonly coated with nontronite and other clayey alteration products. Occurs principally in the Willamette Valley from Salem north to the Columbia River, and in the northern Coast Range. Unit includes correlative Cape Foulweather and Depoe Bay Basalts in the Coast Range (Snavely and others, 1973, 1976a, 1976b; Swanson and others, 1979; Wells and others, 1983). In Eastern Oregon, occurs principally in Deschutes-Umatilla Plateau and in the Blue Mountains. K-Ar ages range from about 6 to about 16.5 Ma (McKee and others, 1977; Swanson and others, 1979; Sutter, 1978; Lux, 1982). Locally separated into Tcs, Tcw, Tcg, Tcp, and Tci

Alluvial-fan deposits. (Quaternary) at surface, covers < 0.1 % of this area

Gravel and subordinate sand and silt deposited at mouths of canyons; largest fans are in Basin and Range Province in east-central and southeastern Idaho. (Quaternary Sediments).

Silicic vent deposits (Pliocene to Pleistocene) at surface, covers < 0.1 % of this area

Complex domal masses of rhyolite and dacite that include near-vent flows, breccia, pumicite, perlite, obsidian, and ash-flow tuff

Wanapum Basalt (Middle Miocene) at surface, covers < 0.1 % of this area

Flows of gray to dark-gray, medium-grained, commonly plagioclase porphyritic basalt of Frenchman Springs petrochemical type (Wright and others, 1973). Generally exhibits blocky to platy jointing. Potassium-argon ages mostly about 15 Ma (Lux, 1982; Fiebelkorn and others, 1983)

Sedimentary rocks associated with flood basalts. (Miocene) at surface, covers < 0.1 % of this area

Fluvial and lacustrine deposits associated with Columbia River Basalt Group and equivalent basalts (17-8 Ma); consolidated to weakly consolidated sandstone, siltstone, arkose, conglomerate, claystone, and tuffaceous sediment; subordinate intercalated basalt and rhyolitic tuff. Includes Payette and Sucker Creek formations in southwestern Idaho, sediments associated with basalt of Weiser in western Idaho, and Latah Formation in northern Idaho. Includes sedimentary rocks of uncertain origin in southwest corner of Idaho. (Quaternary to Eocene Continental Sediments and Sedimentary Rocks).

Alluvial deposits. (Quaternary) at surface, covers < 0.1 % of this area

Deposits in valleys consisting of gravel, sand, and silt. Includes younger terrace deposits. May contain some glacial deposits and colluvium in uplands. (Quaternary Sediments).