Quaternary (present to 1.6 Ma)
Holocene (present to 10,000 years ago)--Beach deposits characterize coastal areas. Stream alluvial and deltaic deposits. Dune deposits along the southwest coast, Puget Lowland, and in the Columbia Basin. Erosion of Cascades and Okanogan Highlands. Mountain glaciation. Landslides, talus, and colluvium. In the Cascade mountains, stratovolcanoes of dacite, andesite, and basalt and associated pyroclastic deposits, lahars, volcaniclastic sediments, and tephra.
Pleistocene (.01 to 1.6 Ma)--Repeated continental and alpine glaciation. Widespread loess deposition. Breaching of huge glacial lakes causing immense floods that scoured the Columbia Basin and cut major rivers such as the Columbia. Basins east and west of the Cascades, including the Puget Lowland, filled with clay, silt, sand, and gravel. Along the coast, thick fluvial terrace deposits and building of river deltas. Continued mountain building by deposition of volcanic rocks. Birth of present-day Cascade stratovolcanoes. Lahars on the flanks of the Cascade mountains.
Tertiary (Neogene) (1.6 to 23.7 Ma)
Pliocene (1.6 to 5.3 Ma)--Marine clastic deposition along the coast. Outwash gravels along flanks of the Cascades. Infilling of basins by fluvial and lacustrine sediments. Cascade mountains continue to build through addition of silicic and basic volcanic rocks as well as growth of dacite domes. Angular unconformities at the base of the Pliocene.
Miocene (5.3 to 23.7 Ma)--Marine sediments in coastal basins. Maximum number of batholiths of silicic to intermediate composition emplaced in the Cascades during this period. Columbia Basin subject to repeated enormous flood basalt events. Some basalts flow to Pacific Ocean following the ancestral Columbia River valley. Fine sediments, diatomite, sands, and locally gravels deposited during periods of volcanic quiescence. Compression of crust causing formation of large regional folds in the Columbia Basin. Uplift of the Blue Mountains. Melange formation on the west flank of the Olympic Mountains.
Tertiary (Paleogene) (23.7 to 66.4 Ma)
Oligocene (23.7 to 36.6 Ma)--Marine sedimentary rocks in the Olympics and Willapa Hills. Extrusive rocks and associated sediments deposited in Cascades. Volcanism continues into the Miocene. Emplacement of stocks and batholiths in the northern part of the Cascade mountains. Clockwise rotation of Cascade arc and Willapa Hills.
Eocene (36.6 to 57.8 Ma)--Maximum areal extent of Tertiary seas west of the Cascades. Formation of extensive coal deposits in coastal swamps. Sedimentary rocks with coal east of the Cascades in strike-slip basins. Lahars, tuffs, and volcaniclastic rocks with dacite and andesite fill grabens in Okanogan Highlands. Marine basalts with interbedded sediments formed offshore and subsequently thrust on to the continent and uplifted. Widespread intermediate to silicic plutonism in Okanogan Highlands. Formation of gneiss domes in Okanogan Highlands. Maximum crustal deformation with faulting and folding. Strike-slip faults translate large portions of the Cascades, jumbling geologic units. Cascade magmatic arc initiated. North half of the state uplifted.
Paleocene (57.8 to 66.4 Ma)--Major hiatus with erosion. Geologic record consists of one small area of continental sediments. Minor acidic intrusives.
Cretaceous (66.4 to 144 Ma)
Marine sedimentary rocks in northern Puget Lowland. Chert-rich marine sandstones, mudstones, shales, and greenstone in central Cascades. Grabens with terrestrial clastic sedimentary rocks. Intermediate to silicic intrusive rocks. Collision of North America with Wrangellia. Metamorphic rocks of quartzite schist, amphibolite, and orthogneiss. Thrusting in northwest Washington. Transition rocks into Jurassic period as above with marble, high grade schist, ultrabasic intrusives and igneous-metamorphic complexes.
Jurassic (144 to 208 Ma)
Marine sediments. Alkalic, basic, intermediate, and acidic intrusives. Metasediments, phyllite, metavolcanics, schist, and orthogneiss. Formation of melange. Multiple thrusting and folding in eastern Okanogan Highlands. Intrusives and metamorphic rocks transition into the Triassic.
Triassic (208 to 245 Ma)
Isolated blocks of carbonate rocks, marine metasediments, and metavolcanics. Break-up of the continent Pangea.
Permian (245 to 286 Ma)
Sedimentary and volcaniclastic rocks in northern Cascades. Marine metasedimentary, carbonate rocks, marble, and metavolcanics in Okanogan Highlands.
Pennsylvanian (286 to 320 Ma)
Carbonate rocks, volcanics, basalt flows, metasedimentary rocks, and metavolcanic rocks in Okanogan Highlands as well as the northern Cascades.
Mississippian (320 to 360 Ma)
One small area in Okanogan Highlands contains carbonate rocks.
Devonian (360 to 408 Ma)
Marine sediments and volcanics in northern Cascades. Marine metasedimentary rocks, carbonate rocks, phyllite, and metagreywacke in Okanogan Highlands.
Silurian (408 to 438 Ma)
Sporadic outcrops of conglomerate, siltstone, slate, and limestone in northeastern Okanogan Highlands.
Ordovician (438 to 505 Ma)
Thick sequences of marine metagreywacke, quartzite, slate, and carbonate rocks in northeast corner of the state. Intermediate intrusives and metavolcanic rocks in Okanogan Highlands.
Cambrian (505 to 570 Ma)
Thick sequences of limestone, phyllite, and quartzite in northeast Washington. Limestone transitional into Ordovician.
Proterozoic (570 to 2500 Ma)
Marine and continental argillite, siltstone, and quartzite in the eastern portion of the Okanogan Highlands. Metavolcanic rocks and basic intrusive sills. Metamorphic rocks with paragneiss in central Okanogan Highlands. Rifting of sedimentary basins.
The above text is modified from the following article: Lasmanis, Raymond, 1991, The geology of Washington: Rocks and Minerals, v. 66, no. 4, p. 262-277. © Copyright Heldref Publications (Helen Dwight Reid Educational Foundation). Used with permission.