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. The early glacier was followed by the Wisconsin ice sheet, the Dakota lobe of which covered a large part of this State, pushing back the Missouri River, which had previously flowed north into Hudson Bay, into its present channel. Eventually this glacier, too, melted and receded, leaving a great lake about 650 feet deep, nearly 700 miles long, and 200 miles wide, with an area of not less than 110,000 square miles, including the region now known as the Red River Valley. This lake has been named Lake Agassiz, in honor of Louis Agassiz, first prominent advocate of the theory that drift was formed by land ice. Lake Agassiz existed some 10,000 years ago, lasted for probably 1,000 years, and covered an area greater than the present Great Lakes. Its sole remnants today are Lakes Winnipeg, Winnipegosis, and Manitoba, and Lake of the Woods.