Geologic units in San Juan county, Washington

Additional scientific data in this geographic area

Glacial drift, undivided (Pleistocene) at surface, covers 38 % of this area

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

Pre-Tertiary sedimentary and metasedimentary rocks, undivided (Mostly Early Cretaceous to Middle Jurassic, possibly includes minor Eocene rocks) at surface, covers 15 % of this area

Graywacke, argillite, phyllite, chert, talc, and graphite schist; some faulted-in blocks of serpentinite and greenstone. Includes minor limestone on San Juan Island.

Triassic sedimentary rocks, undivided (Triassic with Permian where impossible to differentiate) at surface, covers 12 % of this area

Predominantly limestone, marble, and dolomite near Riverside in Okanogan County. Conglomerate, shale, graywacke, gritstone, and limestone on San Juan Island. Siltstone with greenstone locally on Orcas Island. Graywacke conglomerate, cherty greenstone, and limestone in northern Ferry County.

Pre-Carboniferous intrusive rocks (Paleozoic) at surface, covers 11 % of this area

Meta-quartz diorite, hypersthene diorite, and gneissose and directionless quartz diorite of eastern Skagit County. Quartz diorite and diorite in the San Juan Islands. Includes amphibolite and gneiss locally.

Permian rocks (Permian-Triassic) at surface, covers 7 % of this area

Conglomerate, graywacke, siltstone, argillite and interbedded fossiliferous limestone, greenstone, and minor angular conglomerate in northwestern Stevens and Ferry Counties. Impure quartzite, sandstone, graywacke, greenstone, ribbon chert, chert breccia, and limestone in Snohomish County and on San Juan Island. Lower Permian limestone on Black Mountain in northwestern Whatcom County. Middle Permian rocks in northeastern Washington.

Upper Jurassic-Lower Cretaceous sedimentary and volcanic rocks (Late Cretaceous-Jurassic) at surface, covers 5 % of this area

Sedimentary and volcanic rocks, undivided. Graywacke, argillite, siltstone, slate, volcanic rocks, phyllite, greenschist, and greenstone.

Upper Jurassic-Lower Cretaceous volcanic rocks (Jurassic-Cretaceous on San Juan Islands; possibly Eocene in King County) at surface, covers 4 % of this area

Predominantly volcanic rocks, mostly metamorphosed to greenstone and greenschist; includes some sedimentary rocks

Cretaceous sedimentary rocks, mostly marine (Late Cretaceous; possibly Eocene) at surface, covers 3 % of this area

Cretaceous conglomerate, sandstone, shale, breccia, limestone, and gritstone in San Juan Islands.

Carboniferous rocks (Late Devonian to Mississippian) at surface, covers 3 % of this area

Thin-bedded graywacke, shale, argillite, slate, schist, volcanic breccia, gritstone, conglomerate, and limestone on northeast shore of Orcas Island. Limestone or dolomitic limestone, apparently interbedded with limy argillite and graywacke, forms belt of small separate outcrops between Springdale and Valley in southeastern Stevens County. Late Devonian to Early Pennsylvanian in age.

Devonian rocks (Devonian to Permian) at surface, covers 2 % of this area

Thin alternating beds of cherty quartzite and argillite, occasional limestone lenses, volcanic rocks, sandstone, and graywacke on northwest shore of Orcas Island. Small outcrops of Devonian limestone included under Pzu in southeastern Stevens County and under PMPms in northwestern Whatcom County.

Pre-Tertiary metamorphic rocks, undivided (Probably Permian) at surface, covers 0.2 % of this area

Schist, gneiss, marble, quartzite, amphibolite, greenstone, metaconglomerate, graywacke; includes metasedimentary, volcanic, and intrusive rocks. Some areas, as on San Juan Islands, show little if any metamorphism.