Nut Beds are fossils of tropical and subtropical nuts, fruits, roots, branches, and seeds. The Clarno Formation also contains bones, palm leaves longer than , 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 further 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