# Clarno Formation Canonical page: https://lithomap.app/resources/geologic-formations/united-states/clarno-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: Central and north-central Oregon Age: Eocene Age detail: Approximately 54–39 million years ago Depositional setting: Volcanic highlands, forests, rivers, and debris-flow aprons Rock types: Lava, Tuff, Lahar deposits, Conglomerate, Mudstone States in recognized usage: Oregon ## Summary The Clarno Formation records Eocene volcanoes and subtropical forests in Oregon, where lava, ash, and lahars buried plants and animals and created unusually rich fossil sites. ## Distribution Clarno rocks crop out widely in central Oregon, including the Clarno Unit of John Day Fossil Beds National Monument. Thick volcanic and volcaniclastic successions preserve deposits formed near active Eocene volcanic centers. The formation includes many rock types and local volcanic packages. Fossil localities such as the Clarno Nut Beds and Hancock Mammal Quarry represent small, exceptional windows within a much larger volcanic landscape. ## Geological history Andesitic volcanoes produced lava flows, ash, breccia, and fast-moving lahars. Rivers and basins between volcanic centers reworked debris into conglomerate, sandstone, mudstone, and ancient soils. Some lahars rapidly buried forests, while silica-rich fluids and burial cemented wood and sediment. Long pauses between eruptions allowed soils and ecosystems to develop before the next volcanic episode. ## Fossil record Leaves, wood, fruits, nuts, seeds, pollen, and vertebrates document a warm Eocene forest. The Nut Beds are especially important for plant diversity, while the Hancock Mammal Quarry preserves land mammals and other vertebrates. Fossil concentration depends on the deposit: lahars may entomb woody debris, quiet sediment can preserve leaves, and channel or quarry beds may concentrate bone. Most lava flows contain no fossils. ## Minerals and geological resources Plagioclase feldspar and pyroxene are common in volcanic rock, with volcanic glass and ash altered to clay minerals. Silica cement and replacement helped preserve wood, nuts, and other plant material. Iron oxides color ancient soils and weathered volcanic debris. Secondary minerals may line cavities or fractures, but their occurrence is local and protected within the monument. ## Field context - Collecting is prohibited within John Day Fossil Beds National Monument. - Volcanic breccia and lahar cliffs contain poorly supported blocks that can fall without warning. - Clarno and John Day rocks record different volcanic episodes and should not be treated as interchangeable. ## Connected field guides - Conglomerate (rock; Named rock type) — https://lithomap.app/resources/rock-identification/conglomerate - Mudstone (rock; Named rock type) — https://lithomap.app/resources/rock-identification/mudstone - Feldspar (mineral; Mineral discussed in this formation) — https://lithomap.app/resources/mineral-identification/feldspar 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 - Oregon geology — https://lithomap.app/resources/geology/united-states/oregon ## Authoritative sources - Clarno Formation — National Park Service: https://www.nps.gov/joda/learn/nature/clarno-formation.htm - NPS Geodiversity Atlas—John Day Fossil Beds — National Park Service: https://www.nps.gov/articles/nps-geodiversity-atlas-john-day-fossil-beds-national-monument-oregon.htm ## Formation photograph - Image: https://lithomap.app/_delivery/formations/clarno-formation.webp - Description: Eroded Clarno-area rock formations above the John Day River in Oregon - Creator: Jsayre64 - License: CC BY-SA 4.0 — https://creativecommons.org/licenses/by-sa/4.0/ - Source record: https://commons.wikimedia.org/wiki/File:John_Day_River_canyon_near_Clarno%2C_OR.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.