# Ohanapecosh Formation Canonical page: https://lithomap.app/resources/geologic-formations/united-states/ohanapecosh-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: Southern and central Cascade Range Age: Late Eocene to Oligocene Age detail: Age assignments vary; much of the mapped unit is late Eocene to Oligocene Depositional setting: Submarine and island volcanoes with water-laid volcanic sediment Rock types: Volcanic breccia, Tuff, Volcanic sandstone, Lava, Conglomerate States in recognized usage: Washington ## Summary The Ohanapecosh Formation is a thick, altered volcanic succession beneath parts of the Washington Cascades, built by ancient eruptions, underwater debris flows, ash, and lava long before modern Mount Rainier formed. ## Distribution Ohanapecosh rocks are exposed in and around Mount Rainier National Park and elsewhere in the southern and central Cascades. Folding, faulting, alteration, and younger volcanic cover make contacts difficult to trace in some areas. The formation can exceed thousands of feet in thickness and includes interlayered lava-flow complexes, coarse volcanic debris, water-laid sandstone and silt, ash-flow deposits, and rhyolitic rock. ## Geological history Eruptions supplied andesite-dominated lava and fragmental material to a landscape with shallow marine water and volcanic islands. Ash clouds, lahars, and pyroclastic flows entered water and became thick breccia and volcaniclastic sediment. Burial and low-grade metamorphism altered volcanic glass and primary minerals to zeolite-facies assemblages. The formation was then folded, uplifted, deeply eroded, and weathered before younger Cascade volcanic units covered it. ## Fossil record Fossils are sparse across much of the coarse volcanic succession. Plant remains have helped constrain age in selected finer-grained beds, but many classic sections yielded no identifiable fossils. The uneven record reflects depositional conditions: hot lava, debris flows, and coarse breccia rarely preserve organisms, whereas quieter water-laid sediment offers a better chance of retaining plant material. ## Minerals and geological resources Original volcanic rocks contain plagioclase, pyroxene, and other minerals typical of andesite, with less common basaltic, dacitic, and rhyolitic compositions. Alteration produced zeolites, clay minerals, chlorite, calcite, and related secondary phases. The altered mineral assemblage records burial temperature and water-rock reaction. It is not uniform enough to infer a collectible pocket from the formation name or a broad map polygon. ## Field context - Steep Cascade exposures are commonly fractured, vegetated, wet, and prone to rockfall. - Natural materials are protected within Mount Rainier National Park. - Do not confuse ancient Ohanapecosh volcanic rocks with much younger Mount Rainier lava and lahar deposits. ## Connected field guides - Breccia (rock; Named rock type) — https://lithomap.app/resources/rock-identification/breccia - Conglomerate (rock; Named rock type) — https://lithomap.app/resources/rock-identification/conglomerate - Sandstone (rock; Named rock type) — https://lithomap.app/resources/rock-identification/sandstone - Andesite (rock; Mineral discussed in this formation) — https://lithomap.app/resources/rock-identification/andesite - 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 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 - Geology of Mount Rainier National Park — U.S. Geological Survey: https://www.usgs.gov/publications/geology-mount-rainier-national-park-washington - Geolex Ohanapecosh references — U.S. Geological Survey: https://ngmdb.usgs.gov/Geolex/UnitRefs/OhanapecoshRefs_11474.html - Geologic formations at Mount Rainier — National Park Service: https://www.nps.gov/mora/learn/nature/geologicformations.htm ## Formation photograph - Image: https://lithomap.app/_delivery/formations/ohanapecosh-formation.webp - Description: Roadcut exposing volcanic rocks of the Ohanapecosh Formation in Washington - Creator: Chris Light - License: CC BY-SA 4.0 — https://creativecommons.org/licenses/by-sa/4.0/ - Source record: https://commons.wikimedia.org/wiki/File:Ohanapecosh_formation_7452.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.