Miocene, in geology, the intermediate of the three epochs of the tertiary or mammalian age, having the eocene below and the pliocene above. The term is derived from Gr. less, and recent, from less than half its species being of living forms. Some geologists make a fourth division, called oligocene, by separating an upper portion of the eocene and uniting it with the lower section of the miocene. The beds of the miocene epoch are of either marine or fresh-water formation. The marine beds cover a large part of the Atlantic border of the United States, belonging to what is known in American geology as the Yorktown period. They are full of fossils, and occur at Gay Head on Martha's Vineyard, in Cumberland co., N. J., on both sides of the Chesapeake in Maryland, and in Virginia at Yorktown, Suffolk, Smithfield, and other places. Fresh-water beds of miocene occur in the upper Missouri region, along the White river, called onauvaiscs terres or "bad lands." They constitute the "White river" group of Hay den, and have a thickness of 1,000 ft. and upward. In these beds are found the remains of the titanotherium, which also occurs in the eocene. There are also in the Wind river valley and on the west side of the Wind River mountains other fresh-water deposits from 1,500 to 2,000 ft. thick, called the Wind river group.
In California and Oregon the miocene formation consists of sandstone and shale, in some places attaining a thickness of 4,000 or 5,000 ft. They occur near Astoria on the Columbia river, and also in the coast ranges both north and south of San Francisco, in the Santa Inez mountains, and at various other places.