Geology and Paleontology
The John Day Fossil Beds National Monument lies within the Blue Mountains physiographic province, which originated during the late Jurassic and early Cretaceous, about 118 to 93 million years ago. Northeastern Oregon was assembled in large blocks (exotic terranes) of Permian, Triassic, and Jurassic rock shifted by tectonic forces and accreted to what was then the western edge of the North American continent, near the Idaho border. By the beginning of the Cenozoic era, 65 million years ago, the Blue Mountains province was uplifting (that is, was being pushed higher by tectonic forces), and the Pacific Ocean shoreline, formerly near Idaho, had shifted to the west.
Volcanic eruptions about 44 million years ago during the Eocene deposited lavas accompanied by debris flows (lahars) atop the older rocks in the western part of the province. Containing fragments of shale, siltstone, conglomerates, and breccias, the debris flows entombed plants and animals caught in their paths; the remnants of these ancient flows comprise the rock formations exposed in the Clarno Unit. Preserved in the Clarno Nut Beds are fossils of tropical and subtropical nuts, fruits, roots, branches, and seeds. The Clarno Formation also contains bones, palm leaves longer than 24 inches (61 cm), avocado trees, and other subtropical plants from 50 million years ago, when the climate was warmer and wetter than it is in the 21st century. Large mammals that inhabited this region between 50 and 35 million years ago included browsers such as brontotheres and amynodonts, scavengers like the hyaenodonts, as well as Patriofelis and other predators. Eroded remnants of the Clarno stratovolcanoes, once the size of Mount Hood, are still visible near the monument, for example Black Butte, White Butte, and other buttes near Mitchell.
After the Clarno volcanoes had subsided, they were replaced about 36 million years ago by eruptions from volcanoes to the west, in the general vicinity of what would become the Cascade Range. The John Day volcanoes, as they are called, emitted large volumes of ash and dust, much of which settled in the John Day basin. As with the earlier Clarno debris flows, the rapid deposition of ash preserved the remains of plants and animals living in the region. Because ash and other debris fell during varied climatic and volcanic conditions and accumulated from many eruptions extending into the early Miocene (about 20 million years ago), the sediment layers in the fossil beds vary in their chemical composition and color. Laid down on top of the Clarno Strata, the younger John Day Strata consist of several distinct groups of layers. The lowermost contains red ash such as that exposed in the Painted Hills Unit. The layer above it is mainly pea-green clay. On top of the pea-green layer are buff-colored layers. Fossils found in the John Day Strata include a wide variety of plants and more than 100 species of mammals, including dogs, cats, oreodonts, saber-toothed tigers, horses, camels, and rodents. The Blue Basin and the Sheep Rock unit contain many of these same fossils, as well as turtles, opossums, and large pigs. More than 60 plant species are fossilized in these strata, such as hydrangea, peas, hawthorn, and mulberry, as well as pines and many deciduous trees. One of the notable plant fossils is the Metasequoia (dawn redwood), a genus thought to have gone extinct worldwide until it was discovered alive in China in the early 20th century.
After another period of erosion, a series of lava eruptions from fissures across northeastern Oregon, southeastern Washington, and western Idaho inundated much of the Blue Mountain province with liquid basalt. Extruded in the middle Miocene between 17 and 12 million years ago, more than 40 separate flows contributing to the Columbia River Basalt Group have been identified, the largest of which involved up to 400 cubic miles (1,700 km3) of lava. The most prominent of these formations within the monument is the Picture Gorge Basalt, which rests above the John Day Strata.
Subsequent ashfall from eruptions in the Cascade Range in the late Miocene contributed to the Mascall Strata, layers of stream-deposited volcanic tuffs laid atop the Picture Gorge Basalt. Preserved in the Mascall are fossils of animals such as horses, camels, rhinoceroses, bears, pronghorn, deer, weasels, raccoons, cats, dogs, and sloths. These fauna thrived in the monument's open woodland and savannah between 15 and 12 million years ago. The fossils of oak, sycamore, maple, ginkgo, and elm trees reflect the area's cool climate during this time period.
The last major eruption occurred in the late Miocene, about 7 million years ago. The resulting stratum, the Rattlesnake Formation, lies on top of the Mascall and is capped by an ignimbrite. The Rattlesnake stratum has fossils of mastodons, camels, rhinoceroses, the ancestors of dogs, lions, bears, and horses, and others that grazed on the grasslands of the time. Two fossilized teeth found recently in the Rattlesnake stratum near Dayville are the earliest record of beaver, Castor californicus, in North America. The beaver teeth, which are about 7 million years old, have been scheduled for display at the Condon Center.
The monument contains extensive deposits of well-preserved fossils from various periods spanning more than 40 million years. Taken as a whole, the fossils present an unusually detailed view of plants and animals since the late Eocene. In addition, analysis of the John Day fossils has contributed to paleoclimatology (the study of Earth's past climates) and the study of evolution.
Paleontologists at the monument find, describe the location of, and collect fossil-bearing rocks from more than 700 sites. They take them to the paleontology lab at the visitor center, where the fossils are stabilized, separated from their rock matrix, and cleaned. The fossil specimens are then catalogued, indexed, stored in climate-controlled cabinets, and made available for research. In addition to preparing fossils, the paleontologists coordinate the monument's basic research in paleobotany and other scientific areas and manage the fossil museum in the visitor center.
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