Geologic units in Cowlitz county, Washington

Additional scientific data in this geographic area

Upper Eocene volcanic rocks (Late Eocene to Oligocene) at surface, covers 55 % of this area

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

Miocene volcanic rocks (Middle Miocene) at surface, covers 13 % 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.

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.

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

Eocene-Oligocene volcanic rocks (Eocene-Oligocene) at surface, covers 7 % 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.

Miocene nonmarine rocks (Late Miocene) at surface, covers 3 % 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.

Terrace deposits (Pleistocene) at surface, covers 2 % 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 1 % 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.

Lower upper Eocene marine and nonmarine rocks (Eocene) at surface, covers 0.9 % 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 nonmarine rocks (Pliocene) at surface, covers 0.7 % of this area

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

Middle and lower Eocene volcanic rocks (Eocene) at surface, covers 0.7 % 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.

Oligocene nonmarine rocks (Oligocene) at surface, covers 0.6 % 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.

Quaternary nonmarine deposits (Pleistocene) at surface, covers 0.5 % 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.

Landslide and mudflow deposits (Holocene) at surface, covers 0.4 % 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.

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 < 0.1 % of this area

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

Quaternary nonmarine deposits (Pleistocene) at surface, covers < 0.1 % 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.