Oligocene epoch (ŏlˈəgōsēnˌ) [key], third epoch of the Tertiary period in the Cenozoic era of geologic time, lasting from 38 to 24 million years ago. More of North America was dry land during the Oligocene than in the preceding Eocene epoch. The Gulf Coast was flooded, but the Atlantic coast N of South Carolina became emergent; the principal formation of the Gulf district was the Vicksburg limestone. The Pacific coast, like the more northern Atlantic coast, was largely elevated; erosion led to the deposition of the Oligocene portion of three sediments (the Sespe conglomerate, sandstone, and shale), which contains red beds like those of the Permian period. The great erosion of the Rockies was responsible for the deposition of the fossil-rich White River clays and sands over large areas of W Nebraska and NE Colorado and parts of Wyoming and the Dakotas. Late in the Oligocene, the John Day deposits of volcanic ash, notable for their included fossils, were formed in Oregon. In S Europe, the formations are somewhat similar to those of the Eocene; a sandstone and shale formation, the Flysch, was laid down in regions adjacent to mountain systems. The Alpine mountain building episode reached peak intensity as Africa further impinged against the Eurasian plate (see plate tectonics). In the middle Oligocene a sea extended over N Europe as far east as the Urals and was connected with the greater Mediterranean through the present Rhine valley. There are extensive deposits of Oligocene lignite in Germany, indicating swamp conditions either before or during the flood, and the Alsatian potash, salt, and gypsum are Oligocene. During the Oligocene there was considerable volcanic activity in central Europe, Scotland, Ireland, and Iceland, as well as in the San Juan Mts. of Colorado and the Absaroka Mts. of Wyoming where remnants of this volcanism persist in Yellowstone National Park. The life of the Oligocene was marked in Europe and North America by the virtual disappearance of the archaic mammals of the Paleocene. Carnivorous mammals—ancestral dogs and cats—made their appearance, along with beavers, mice, rabbits, and squirrels. A more highly developed type of horse, giant hogs, and camels were other new arrivals. The titanotheres—mammals remotely related to the horse and the rhinoceros—evolved to types of great size, then died out. The brontotherium, which appeared in North America, was the largest mammal to ever live on that continent. Running and aquatic rhinoceroses developed. The earliest elephant and a primitive anthropoid ape appeared in Africa. The climate of the Oligocene was mild and temperate in North America.
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