It’s all about the fossils at John Day Fossil Beds National Monument in Oregon. This park is one of six National Park System units established specifically to preserve and protect Cenozoic Era (66 mya – present) fossils. The fossils found within the boundaries of the national monument span almost 44 million years of life, narrating a tale of climate change, evolution, and adaptation.
Geology
Map of the physiographic provinces of Oregon showing the location of the three units comprising John Day Fossil Beds National Monument / NPS
The geology of this national monument spans a considerably longer timeframe (>90 mya), and the integration of both fossils and geology helps scientists understand the habitats in which this flora and fauna thrived, and how both life and environments changed over geologic time.
The National Park Service divides the geology of John Day Fossil Beds into two types: physical and historical. Physical geology describes the structure, composition (the rocks and minerals), and geologic processes (weathering, erosion). Historical geology studies earth’s reconstruction over time using geologic principals and processes (plate tectonics, stratigraphy, structural geology, sedimentology, and paleontology) as well as climate studies.
John Day Fossil Beds National Monument is situated within two physiographic provinces in eastern Oregon: the Blue Mountains (Sheep Rock Unit) and the Deschutes-Columbia Plateau (Clarno and Painted Hills units). A physiographic province is a geographic area with characteristic physical features and geology (including specific rock types and structural elements).
The rugged Blue Mountains in central and northeastern Oregon range between 6,000 – 9,000 feet (1,829 – 2,743 meters) in elevation and sit within the rain shadow of the Cascade Range. The resulting environment created by the Blue Mountains’ arid setting differs from that of mountain ranges further west. A series of volcanic island arcs and marine basins, collectively called “terranes,” are the framework composing the Blue Mountains Province, and dates to around 145 million years old. These terranes were subsequently overlain by basalt flows from the Columbia River Basalt Group beginning some 17 million years ago. Uplift, folding and faulting, and erosion completed this mountain province.
The Deschutes-Columbia Plateau Province extends from the Blue Mountains to the Cascade Range and north to the Columbia River. As the name suggests, copious basalt flows (more than 300) of the Columbia River Basalt Group erupted through fissures to create a relatively level, broad plateau covering 56,000 cubic miles (230,000 cubic km) of landscape.
The fossils at John Day Fossil Beds are preserved in sedimentary rocks. With all this talk of metamorphic and volcanic rocks, you are probably wondering where the sedimentary rocks come in to play.
Sedimentary rocks are rocks composed of bits and pieces of other rocks and minerals, cemented together with silica, calcite, or hematite (the three most common cements). The bits and pieces composing the sedimentary rocks in which this national monument’s fossils are found originate from volcanic eruptions.
According to park staff:
Once deposited, the ash and tuff from these eruptions weather into soils, and solidify with burial and pressure into claystones and siltstones. The original sources for these volcanic materials can be found throughout eastern and central Oregon. The earliest of these is the Clarno Formation, whose primary source is an extinct range of volcanoes that now make up part of the Ochoco Mountains. Remnants of this volcanic chain can still be seen in the cone-shaped hills near Mitchell.
The younger rocks of the John Day Formation were likely formed due to the activities of a large volcanic field. Calderas are all that remain of these volcanoes, which would have produced alternating basalt and ash eruptions throughout the late Eocene and early Oligocene. The three known calderas of this volcanic field include the Crooked River caldera, near present-day Prineville, the Wildcat Mountain caldera between Prineville and the Painted Hills, and the Tower Mountain caldera northeast of Sheep Rock.
The Picture Gorge Basalts were part of an extensive period of volcanism in the Pacific Northwest. The most likely source for these particular basalts was a series of fissures just east of the Sheep Rock area.Picture Gorge Basalts, Sheep Rock Unit, John Day Fossil Beds National Monument / Rebecca Latson
The volcanic source for the Mascall Formation has not yet been pinpointed, but possibilities include the modern Cascades and the McDermitt volcanic field near the Oregon-Nevada border.
A view of the Mascall Formation at the Sheep Rock Unit of John Day Fossil Beds National Monument / Rebecca Latson
The ash in the most recent fossil formation, the Rattlesnake, originates from a source near the town of Burns, Oregon.
Climate Change And The Painted Hills Unit
The alternating colors of these sediments at the Painted Hills Unit striking displays of past climate changes over millions of years at John Day Fossil Beds National Monument / Rebecca Latson
Perhaps one of the best places to view the past sequence of climate change occurring over millions of years is the Painted Hills Unit, approximately 10 miles (16 km) northwest of Mitchell, Oregon. The red, yellow, black, and tan sedimentary rocks of which these hills are composed are claystones created by ash, pumice, and tuff from volcanic eruptions beginning about 35 million years ago.
The red and yellow-tan sediment layers represent cycling climate conditions, with alternating warm and wet versus cool and dry periods. This lasted over the space of about 6 million years, as the Eocene epoch (55.8 to 33.9 million years) gave way to the Oligocene epoch (33.9 to 23 million years ago).
The red soils are floodplain deposits and the color comes from iron oxides formed during the more tropical (warmer and wetter) periods.
A close-up look at the crumbly red soil indicating a warm, wet, tropical environment where the Painted Hills Unit now sit, John Day Fossil Beds National Monument / Rebecca Latson
The yellow-tan colors are the result of iron and magnesium oxides formed during those drier, cooler times.
A close-up look at the crumbly yellow-tan soil indicating a much cooler, drier environment where the Painted Hills Unit now sit, John Day Fossil Beds National Monument / Rebecca Latson
You will also see some black layers of soil as well as black patches on the red and yellow hills. The layers of black are lignite (brown coal) originating from vegetation growing along that tropical floodplain, and the black stains and patches seen here and there upon the hills get their color from manganese oxide.
Leaf impression fossils found in the Painted Hills indicate the presence of redwood, cinnamon, and alder trees, and vertebrate fossils of camels, early horses, and rhinoceroses have also been discovered.
Fossils
Frontier minister Thomas Condon was the first to recognize the importance of eastern Oregon’s paleontology back in the 1860s. A self-trained scientist, Condon ministered to his Dalles, Oregon congregation by day and presented geology lectures in the evening. He learned about the fossils found from the soldiers attending his lectures. Condon’s own fossil discoveries spurred other scientists to venture out and explore the fossil-bearing landscape that would later become a national monument in 1975.
Fossil oreodont skulls, John Day Fossil Beds National Monument / Rebecca Latson
A superbly-preserved fossil flower with visible veining in the petals, John Day Fossil Beds National Monument / Rebecca Latson
Fossil insects and vegetation found at John Day Fossil Beds National Monument / Rebecca Latson
Today, the museum collections at John Day Fossil Beds National Monument include more than 60,000 well-preserved specimens of plant, insect, and animal fossils, ranging from oreodonts (extinct sheep-sized hoofed mammals) to flowers, to leaves, to bean seeds, to cicadas and water beetles. One area of the national monument – the Clarno Nut Beds – has yielded 173 species of fruits and seeds.
There are seven fossil assemblages within the national monument. Each assemblage is analogous to chapters of a story describing the environments and life forms existing during those time periods.
According to park staff:
Clarno Nut Beds (44 million years ago)
Oregon was a hot, wet, semitropical place filled with a wide diversity of plants including palm trees and avocados.
Hancock Mammal Quarry (40 million years ago)
Temperatures and rain decrease compared to the Clarno Nut Beds but remained mostly above freezing.
Bridge Creek (33 million years ago)
A wide variety of plant material has been preserved in fine grain lake sediment including the Metasequoia, Oregon's state fossil.
Turtle Cove Assemblage (30 – 25 million years ago)
Turtle Cove is the thickest and most productive fossil-bearing layer within the John Day Fossil Beds, yet few leaf fossils were preserved.
Upper John Day Assemblage (24 – 20 million years ago)
The ecosystem became an open habitat with the appearance of burrowing and running animals.
Mascall Assemblage (15 million years ago)
Long-legged hoofed animals including horses, camels, and giraffe-deer swiftly crossed open meadows to escape beardogs and cats.
Rattlesnake Assemblage (7 million years ago)
The ecosystem preserved here is more familiar to modern eyes- except for the occasional elephant and giant sloth.
For more detailed information about each assemblage, click on the assemblage’s name.
For a more comprehensive look at the geology of John Day Fossil Beds National Monument, you can open and read the 2014 PDF Geologic Resources Inventory Report by the National Park Service.
You can read more in-depth information about the geology, fossils, history, and general information about John Day Fossil Beds in this NPS history publication.
