Geologic units in Clark county, Washington

Upper Eocene volcanic rocks (Late Eocene to Oligocene) at surface, covers 43 % of this area

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

Quaternary nonmarine deposits (Pleistocene) at surface, covers 25 % 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.

Pliocene nonmarine rocks (Pliocene) at surface, covers 17 % of this area

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

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.

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.

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.

Eocene-Oligocene volcanic rocks (Eocene-Oligocene) at surface, covers 0.9 % 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.

Tertiary granitic rocks (Probably mostly Eocene; ranges from Miocene to Paleocene) at surface, covers 0.8 % of this area

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

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.

Oligocene-Miocene volcanic rocks (Miocene) at surface, covers < 0.1 % 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.

Pleistocene-Recent volcanic rocks (Pleistocene to Holocene) at surface, covers < 0.1 % 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.

Miocene volcanic rocks (Middle Miocene) at surface, covers < 0.1 % 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.