Reinterpretation of the depositional environment of the Yellowstone “fossil forests”

Volcaniclastic sedimentary rocks preserving the “fossil forests” of the Eocene Lamar River Formation in Yellowstone National Park were deposited by a complex alluvial system. Mud flows and braided streams originating on surrounding volcanic peaks transported plant parts, including some logs and stumps, from high-elevation, cool-temperate sites and mixed them with tropical lowland forms. Because there is evidence of mixing from several habitats, these “fossil forests” should not be used as a type area for arguing that Eocene plants had climatic tolerances different from their modern counterparts.