# John Day Formation Canonical page: https://lithomap.app/resources/geologic-formations/united-states/john-day-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: John Day Basin and central Oregon Age: Late Eocene to early Miocene Age detail: Approximately 39–18 million years ago Depositional setting: Volcanic ash falls, floodplains, lakes, rivers, and ancient soils Rock types: Tuff, Claystone, Siltstone, Sandstone, Conglomerate States in recognized usage: Oregon ## Summary The John Day Formation is a long, ash-rich record of changing Oregon landscapes, preserving plants and mammals through a transition from warm forests to cooler, drier open environments. ## Distribution The formation is widespread in north-central Oregon and is spectacularly exposed in the Sheep Rock and Painted Hills areas. It includes several members and fossil-bearing intervals deposited over roughly twenty million years. Blue-green claystone, painted paleosols, pale tuff, and channel sandstone are all expressions of the same broad formation. No single color or landform represents its full history. ## Geological history Explosive eruptions from regional volcanic centers repeatedly spread ash across rivers, lakes, floodplains, and soils. Ash was reworked by water, altered to clay, welded into tuff, or mixed with ordinary sediment. Members record environmental change from humid lake and forest settings to progressively cooler, drier floodplains. Dated ash layers provide time markers that let geologists compare biological and climatic change. ## Fossil record The formation preserves fossil leaves, seeds, fish, amphibians, reptiles, and a rich mammal record including early horses, oreodonts, rhinoceros relatives, predators, and rodents. Different members contain distinct communities. Fine lake beds preserve delicate plants and small animals, while floodplain paleosols and channel deposits more often contain mammal bones and teeth. Stratigraphic position is essential to interpreting any specimen. ## Minerals and geological resources Volcanic glass altered into clay minerals, zeolites, and other secondary phases. Feldspar and quartz occur in ash and sediment, while the blue-green Turtle Cove beds contain celadonite formed during water-rock alteration. Iron minerals color red and yellow paleosols, and silica helps cement some ash beds. The formation’s colors reflect alteration and soil development rather than a single collectible mineral seam. ## Field context - All fossils, rocks, and natural materials are protected within John Day Fossil Beds National Monument. - Ash-rich clay slopes become slick when wet and erode easily under foot traffic. - A fossil removed without exact member and bed information loses much of its scientific meaning. ## Connected field guides - Conglomerate (rock; Named rock type) — https://lithomap.app/resources/rock-identification/conglomerate - Sandstone (rock; Named rock type) — https://lithomap.app/resources/rock-identification/sandstone - Siltstone (rock; Named rock type) — https://lithomap.app/resources/rock-identification/siltstone - Feldspar (mineral; Mineral discussed in this formation) — https://lithomap.app/resources/mineral-identification/feldspar - 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 - Oregon geology — https://lithomap.app/resources/geology/united-states/oregon ## Authoritative sources - John Day Formation — National Park Service: https://www.nps.gov/joda/learn/nature/john-day-frm.htm - Geologic formations at John Day Fossil Beds — National Park Service: https://www.nps.gov/joda/learn/nature/geologicformations.htm - John Day Formation reconnaissance — U.S. Geological Survey: https://pubs.usgs.gov/publication/b1161D ## Formation photograph - Image: https://lithomap.app/_delivery/formations/john-day-formation.webp - Description: Blue-green John Day Formation badlands along the Lower Blue Basin Trail in Oregon - Creator: Finetooth - License: CC BY-SA 3.0 — https://creativecommons.org/licenses/by-sa/3.0/ - Source record: https://commons.wikimedia.org/wiki/File:Blue_basin_lower_trail.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.