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Geologic units in Skamania county, Washington
[Additional scientific data in this geographic area]
- Tertiary dikes, sills, and small intrusive bodies (Middle to Late Tertiary) at surface, covers 0.7 % of this area
Dikes are commonly diabase; plugs and sills are generally andesite porphyry and dacite.
- 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.
- Alluvium (Holocene) at surface, covers 0.3 % 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.
- 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 3 % of this area
Predominantly basalt flows and flow breccia; includes some pyroclastic and andesite rocks. Chiefly in western Washington.
- Eocene-Oligocene volcanic rocks (Eocene-Oligocene) at surface, covers 43 % 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.
- Oligocene nonmarine rocks (Oligocene) at surface, covers 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.
- Eocene nonmarine rocks (Eocene) at surface, covers < 0.1 % 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.
- Pleistocene-Recent volcanic rocks (Pleistocene to Holocene) at surface, covers 28 % 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.
- Open Water (Holocene) at surface, covers < 0.1 % of this area
- Landslide and mudflow deposits (Holocene) at surface, covers 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.
- Oligocene-Miocene volcanic rocks (Miocene) at surface, covers 13 % 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.
- Pliocene-Pleistocene volcanic rocks (Mostly Pleistocene) at surface, covers 3 % 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.
- 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.