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 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 Formation, 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 contains 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 laboratory 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