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90 GEOLOGY AND GLACIATIONS Five million years ago, in the center of the North American continent, an immense grass-covered and treeless plain sloped eastward from the mountains. Across this plain flowed large, broad, shallow and braided rivers carrying the sands and gravels that had eroded from the mountains. These streams were carrying so much sand and gravel that these often choked the river's own channel causing it to spill over its banks and move sideways into a new channel. These rivers created what we now call the Great Plains, extending from Canada to Mexico. Over these Great Plains roamed herds of grazers like horses, camels, mastodons and bison which were stalked by predators like saber-toothed cats. Then this plain gradually lifted while its western (next to the mountains) and its eastern edges were eroded away. A large section of the center of this plain is still present as the High Plains stretching from the Nebraska/South Dakota border south to Texas. Some 2.5 million years ago, the Pleistocene epoch began. During the Pleistocene, the planet had cooled to the point that the polar ice caps had formed and wobbling of the planet's axis caused the climate to alternately warm and cool. During the cool periods, massive, continent-wide ice sheets formed and ground their way south. But there wasn't just one Pleistocene glaciation; there may have been as many as 20 in North America over that 2.5 million year period. [That may sound like a long time (well, actually, it is) but, if we compress the age of the earth into a 60 minute basketball game, the Pleistocene began in the last 2 seconds of the game]. Traditionally, these glaciations were lumped into four main periods. The oldest was the Nebraskan (2.5 to 0.5 million years ago), followed by the Kansan, the Illinoian and, finally, the Wisconsin, ending some 10,000 years ago. Currently the geology literature doesn't recognize most of these as separate, well- defined periods. But, I don't really care if there were two or 22. The point is that there were major, early glacial periods that had a hand in forming Nebraska's watersheds. Well before the Pleistocene began, the ancestral North Platte River, heading in the Laramie Range of Wyoming, flowed northeast to the Red River of the North. The ancestral South Platte was a tributary of the North Platte and, of course, it too, flowed northeast. The ancestral Republican River was about where it is now, flowing southeast. Only one million years later, the North/South Platte Rivers were now flowing southeast into Kansas. 218 This may have been the major drainage flowing southeasterly from southern Nebraska/northern Kansas and through central Missouri (called the Grand or Old Grand-Missouri). 30, 39 During the Pleistocene glaciations, the whole region became wetter and colder, with a treeless tundra nearest the glacial ice giving way to spruce forests growing as far south as Kansas. With the coming of the ice, the rivers that had been flowing to the northeast and to the east were now blocked by the ice sheet. That must have been something to see. The rivers flowing northeast and mixing with meltwater off the ice with nowhere to go. The river valleys would have filled creating huge lakes. Then the lakes would have overtopped the divides between watersheds cutting new channels to the south and southeast. Only the first two (Nebraskan and Kansan) ice sheets reached into Nebraska and the location of the edges