Once again the sea covered the State and left the rust-colored Fox Hills sandstone, which is particularly conspicuous along the Cannonball River where action of underground water has formed it into the rusty-looking spheres which give the river its name. Similarly formed cylinders of this sandstone in concretionary form are also found along the stream, and large cylinders protrude from the top of Cannon Butte in the Badlands like the barrels of cannon from the turret of a huge battleship.
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.
The recession of the seas left a broad and gently rolling plain cut by sluggish rivers whose wooded valleys were inhabited by the descendants of the great swamp beasts. When the waters again invaded the plain, the bones of these monsters were embedded in the deposits which became the White River formation, youngest bedrock underlying the State. So numerous are the fossil remains in the lower White River beds that these strata are called the titanothere beds. Fossils are found throughout the formation, however, ranging from mammal bones to the remains of fish and turtles. Erosion has worn away much of the White River formation in North Dakota, but it is conspicuously revealed on the summits of White or Chalky Butte, Sentinel Butte, Black Butte, and the Killdeer Mountains, and also in a few other small isolated areas in the Missouri Slope.
Gradually, during the time these formations were being laid down, the winters of this region were becoming more and more severe. Masses of ice moved slowly southward from the Arctic Region, covering much of the land and transforming the nearby forests, meadows, and swamps into a treeless plain of black mucky soil with a permanently frozen subsoil overgrown with moss, lichens, and dwarf shrubs. Fierce wintry storms took their toll of the mammoths, rhinoceroses, and reindeer living upon the tundras.
As the glaciers moved south, the animals were forced to flee to warmer lands. Soon the ice mass had covered all of North Dakota except a very small region in the southwest corner beyond the Killdeer Mountains. When at length it receded, it left in its wake boulders, gravel, and till—a drift soil composed of clay, sand, gravel, and boulders. Much of this now has been worn away; on the west side of the Missouri only a few scattered areas remain, and on the east side the till, though more continuous, is often merely a veneer a few feet in thickness.