Geologic units in Yakima county, Washington

Additional scientific data in this geographic area

Miocene volcanic rocks (Middle Miocene) at surface, covers 35 % 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.

Pleistocene-Recent volcanic rocks (Pleistocene to Holocene) at surface, covers 16 % 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.

Glaciolacustrine deposits (Pleistocene) at surface, covers 12 % of this area

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

Quaternary nonmarine deposits (Pleistocene) at surface, covers 10 % of this area

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

Miocene-Pliocene nonmarine rocks (Miocene-Pliocene) at surface, covers 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.

Eocene-Oligocene volcanic rocks (Eocene-Oligocene) at surface, covers 6 % 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-Miocene volcanic rocks (Miocene) at surface, covers 4 % 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.

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.

Pliocene-Pleistocene volcanic rocks (Mostly Pleistocene) at surface, covers 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.

Tertiary granitic rocks (Probably mostly Eocene; ranges from Miocene to Paleocene) at surface, covers 1 % of this area

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

Quaternary nonmarine deposits (Pleistocene) at surface, covers 1 % 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.

Miocene-Pliocene volcanic rocks (Miocene-Pliocene) at surface, covers 1 % 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.

Pre-upper Eocene rocks (Eocene (Olympic Peninsula); Cretaceous(?) (Yakima County)) at surface, covers 1.0 % 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.

Landslide and mudflow deposits (Holocene) at surface, covers 0.5 % 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.

Eocene nonmarine rocks (Eocene) at surface, covers 0.2 % 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.