# Crescent Formation Canonical page: https://lithomap.app/resources/geologic-formations/united-states/crescent-formation Last reviewed: September 17, 2026 Page purpose: Comprehensive United States geologic formation reference for human readers, search systems, and language models. Major region: Pacific Coast Regional setting: Olympic Peninsula and southern Vancouver Island region Age: Early to middle Eocene Age detail: Mostly early to middle Eocene, with age varying by mapped subdivision Depositional setting: Submarine volcanic plateau and seamount environments Rock types: Basalt, Pillow lava, Volcanic breccia, Tuff, Tuffaceous sediment States in recognized usage: Washington ## Summary The Crescent Formation is an immense Eocene basaltic foundation of the Olympic Peninsula, assembled largely by submarine eruptions before the volcanic crust was attached to western North America. ## Distribution Crescent rocks form much of the eastern and northern Olympic Peninsula and extend beneath younger rocks. Excellent exposures occur around parts of the Strait of Juan de Fuca, Hood Canal, and the Olympic Mountains, although vegetation and deformation obscure many contacts. Regional maps divide the thick volcanic package in different ways, and closely related Siletzia rocks continue southward into Oregon. The formation name should not be assumed for every basalt in western Washington. ## Geological history Basalt erupted onto and beneath the ocean, creating pillow lava, massive flows, breccia, and volcanic sediment. Continued eruption built unusually thick oceanic crust that was later accreted to the continent and uplifted with the Olympic Peninsula. Some upper flows erupted in shallower water or on volcanic islands, while marine sediment accumulated between eruptive episodes. Folding, faulting, metamorphism, and hydrothermal alteration subsequently modified the original volcanic minerals. ## Fossil record Most basalt flows contain no fossils, but interbedded marine sediment and limestone locally preserve foraminifera and other microfossils that constrain the formation’s Eocene age. Fossil occurrence depends on finding sediment deposited between eruptions. A volcanic outcrop alone is unlikely to show body fossils even where fossil-bearing beds occur elsewhere in the formation. ## Minerals and geological resources Plagioclase and pyroxene are major primary basalt minerals, with magnetite and volcanic glass also common. Alteration can produce chlorite, clay minerals, calcite, quartz, and zeolites in fractures and cavities. Secondary mineral fillings record seawater circulation and later burial, but collectible material is localized and may be protected on public land or inaccessible on steep coastal outcrops. ## Field context - Wet basalt, moss, and steep coastal slopes create serious fall hazards. - Rock collecting is prohibited in Olympic National Park and may be restricted elsewhere. - Distinguish Crescent basalt from younger Cascade and local intrusive rocks with a current map. ## Connected field guides - Basalt (rock; Named rock type and mineral context) — https://lithomap.app/resources/rock-identification/basalt - Breccia (rock; Named rock type) — https://lithomap.app/resources/rock-identification/breccia - Calcite (mineral; Mineral discussed in this formation) — https://lithomap.app/resources/mineral-identification/calcite - Chlorite (mineral; Mineral discussed in this formation) — https://lithomap.app/resources/mineral-identification/chlorite - Magnetite (mineral; Mineral discussed in this formation) — https://lithomap.app/resources/mineral-identification/magnetite - Quartz (mineral; Mineral discussed in this formation) — https://lithomap.app/resources/mineral-identification/quartz Mapped location and formation context support an identification but cannot establish one. Compare several physical properties and use laboratory analysis when the material remains ambiguous. ## Connected state geology guides - No directly overlapping state guide is currently published. ## Authoritative sources - Geolex Crescent references — U.S. Geological Survey: https://ngmdb.usgs.gov/Geolex/UnitRefs/CrescentRefs_5110.html - Geologic history of Siletzia — U.S. Geological Survey: https://www.usgs.gov/publications/geologic-history-siletzia-a-large-igneous-province-oregon-and-washington-coast-range ## Formation photograph - Image: https://lithomap.app/_delivery/formations/crescent-formation.webp - Description: Basalt flows and flow breccias of the Crescent Formation on Blue Mountain in Olympic National Park - Creator: Walter Siegmund - License: CC BY-SA 3.0 — https://creativecommons.org/licenses/by-sa/3.0/ - Source record: https://commons.wikimedia.org/wiki/File:Blue_Mountain_4874.JPG Formation-level information does not prove that a fossil or mineral occurs at a specific coordinate. It also does not establish ownership, access, collecting permission, present conditions, or specimen identity.