Near the close of the Mesozoic era, the climate of North Dakota became warmer, almost like that of the South Atlantic States. Through the swamps roamed horned carnivorous dinosaurs, especially triceratops, which had "the largest head with the smallest brain of the reptile race." Again and again the sea invaded this swampland, depositing the Lance formation, comprising layers of massive sandstone and shale in which the luxuriant plant life of the area created thin beds of lignite coal. The Lance formation underlies most of the Missouri Plateau, and comes to the surface in two places--in the vicinity of Bismarck, and near Marmarth. Reptilian fossils are found in both the lignite and the intervening layers of rock. At the dawn of the Cenozoic age, as mammalian life began to develop on the globe, another invasion of the sea left behind it a great plateau interspersed with swamps, marshlands, rivers, and lakes. On the plain grew giant sequoia, cypress, juniper, and other semitropical trees. Over the thick mat of mosses, lichens, and liverwort in the swamps crept turtles, alligators, lizards, and other reptiles, monstrous in size. King of this jungle was the titanothere, with its great body, short stocky neck, and columnar legs. Long-jawed shaggy mastodons and gigantic rhinoceroses challenged its supremacy. As these titans of the forest lumbered through the underbrush, herds of _Merycoidodon culbertsoni_ or ruminating hogs, _Leptomeryx evansi_, dainty deerlike creatures no larger than jack rabbits, and little three-toed horses scampered out of their way. The Fort Union formation, created through successive fresh water deposits of sediment in the swamps, contained vast quantities of rank swamp vegetation. In the intervening millions of years this has been turned into lignite, a very soft coal which has become North Dakota's most valuable mineral resource. The lignite veins in the formation indicate that the sea covered this area at least eleven times during the period when the formation was being deposited. In addition to lignite, the Fort Union clay shales and sandstones contain pure plastic clay beds and some bentonite, a claylike mineral of commercial value.