Geologic units in Pierce county, Washington

Additional scientific data in this geographic area

Oligocene-Miocene volcanic rocks (Miocene) at surface, covers 17 % 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.

Younger glacial drift (Pleistocene) at surface, covers 15 % of this area

Advance and recessional outwash, stratified drift, and associated deposits. Primarily silt, sand, and gravel with some clay. Includes alluvium locally and scabland deposits of eastern Washington.

Younger glacial drift (Pleistocene) at surface, covers 14 % 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.

Eocene-Oligocene volcanic rocks (Eocene-Oligocene) at surface, covers 10 % 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.

Younger glacial drift (Pleistocene) at surface, covers 9 % of this area

Younger glacial drift, undivided. Till, outwash, and associated deposits; sorted and unsorted sand, gravel, silt, and clay. Includes some alluvium.

Pleistocene-Recent volcanic rocks (Pleistocene to Holocene) at surface, covers 7 % 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.

Upper Eocene volcanic rocks (Late Eocene) at surface, covers 5 % of this area

Predominantly andesite flows and breccia; includes some basalt flows. Contains basaltic conglomerate, pyroclastic rocks, tuff beds, and sandstone in Chehalis-Centralia coal district, Lewis County.

Alluvium (Holocene) at surface, covers 5 % 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.

Glacial drift, undivided (Pleistocene) at surface, covers 4 % of this area

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

Tertiary granitic rocks (Probably mostly Eocene; ranges from Miocene to Paleocene) at surface, covers 4 % of this area

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

Older glacial drift (Pleistocene) at surface, covers 3 % 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 upper Eocene marine and nonmarine rocks (Eocene) at surface, covers 2 % 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.

Upper upper Eocene nonmarine and marine rocks (Middle Eocene) at surface, covers 2 % 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.

Landslide and mudflow deposits (Holocene) at surface, covers 1 % 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 0.8 % of this area

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

Upper Eocene volcanic rocks (Late Eocene) at surface, covers 0.7 % of this area

Andesite and basalt flows and associated breccia in central Lewis County. Pyroclastic rocks, mudflows, flow breccia, and volcanic-rich sedimentary rocks in King and Pierce Counties.

Miocene nonmarine rocks (Late Miocene) at surface, covers < 0.1 % 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.