Geologic units in Stevens county, Washington

Additional scientific data in this geographic area

Glacial drift, undivided (Pleistocene) at surface, covers 34 % of this area

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

Mesozoic granitic rocks, undivided (Mostly Cretaceous-Jurassic) at surface, covers 18 % 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.

Precambrian rocks, undivided (Proterozoic) at surface, covers 8 % 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.

Upper Paleozoic rocks, undivided (Ordovician) at surface, covers 5 % 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.

Ordovician rocks (Middle Ordovician) at surface, covers 5 % 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.

Cambrian phyllite (Cambrian) at surface, covers 4 % 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).

Cambrian limestone and dolomite (Early Cambrian-Middle Ordovician) at surface, covers 4 % 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.

Cambrian quartzite (Cambrian) at surface, covers 4 % 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.

Glaciolacustrine deposits (Pleistocene) at surface, covers 3 % of this area

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

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.

Mesozoic volcanic rocks, undivided (Jurassic) at surface, covers 2 % of this area

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

Permian rocks (Permian-Triassic) at surface, covers 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.

Alluvium (Holocene) at surface, covers 2 % 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.

Precambrian volcanic rocks (Late-Proterozoic) at surface, covers 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.

Tertiary volcanic rocks, undivided (Mostly Oligocene-Eocene) at surface, covers 0.9 % of this area

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

Tertiary nonmarine rocks, undivided (Tertiary; mostly Eocene) at surface, covers 0.8 % of this area

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

Oligocene volcanic rocks (Oligocene) at surface, covers 0.7 % of this area

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

Older glacial drift (Pleistocene) at surface, covers 0.4 % 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.

Lower Paleozoic rocks, undivided (Paleozoic; likely Ordovician) at surface, covers 0.4 % 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.

Carboniferous and Permian volcanic rocks (Devonian to Permian; Triassic in Asotin County) at surface, covers 0.4 % 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.

Precambrian (?) phyllite (Cambrian-Precambrian boundary) at surface, covers 0.3 % 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.

Tertiary dikes, sills, and small intrusive bodies (Middle to Late Tertiary) at surface, covers 0.3 % of this area

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

Precambrian rocks, undivided (Middle to Late Proterozoic) at surface, covers 0.3 % of this area

Magnesite-bearing Stensgar dolomite; southwestern Stevens County.

Precambrian conglomerate (Late-Proterozoic) at surface, covers 0.2 % 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.

Pre-Tertiary basic intrusive rocks (Triassic-Permian) at surface, covers 0.2 % 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.

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-Tertiary metamorphic rocks, undivided (Probably Permian) at surface, covers < 0.1 % 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.

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.

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.

Younger glacial drift (Pleistocene) at surface, covers < 0.1 % 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.

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.