FLORISSANT FORMATION

division of middle and upper Oligocene rocks in the United States (the Oligocene Epoch lasted from 36.6 to 23.7 million years ago). The Florissant Formation occurs in central Colorado, where it overlies the White River Group; it was named for exposures studied near Florissant, Colo. The Florissant consists of shales that contain a rich and varied fossil assemblage. Many kinds of Oligocene plants are represented in the Florissant, including the leaves and twigs of redwoods, hackberries, and sycamores. These remains represent a temperate upland forest that existed in the region during the Oligocene. Well-preserved fish skeletons have been found in the fine-grained Florissant sediments, as have freshwater mollusks and an excellently documented record of Oligocene insects and spiders. The Florissant contains the earliest record of the tsetse fly and some mammalian fossils.