that now occupy their broad channels.
And it wasn’t only moving water that left its mark on the land. In many places, meltwater was prevented from flowing away by ice dams, and the silt-laden water pooled to form shallow, milky lakes, such as Glacial Lake Regina in south-central Saskatchewan and Lake Dakota in east-central South Dakota. The largest of these “proglacial” lakes, Glacial Lake Agassiz, flooded some 135,000 square miles (350,000 square kilometers) at its maximum extent (three times the size of Lake Superior, the largest modern freshwater lake), including extensive tracts in Saskatchewan and Manitoba and the Red River Lowlands in eastern North Dakota and northern Minnesota. When the ice and then the water finally retreated from the land, these lake bottoms stood exposed as broad plains bounded by terraced beaches, all covered with a dressing of mineral-rich silt.
The land that emerged directly from under the ice sheets, by contrast, was a rough-and-tumble mess, strewn with the rubble that the glaciers had dropped as they retreated. Sinuous ridges of gravel and silt, called eskers, marked the courses of streams that had once flowed under or through the ice; strange conical hills, called kames, stood where streams pouring out of the glaciers had deposited gravel and sand. One of the most prominent glacial features on the northern plains was a long, broken ridge of hill country, called the Missouri Coteau, that meandered (and still meanders) across central Saskatchewan and south through the Dakotas. Geologists refer to the Coteau as “dead ice moraine,” because it formed when hunks of ice became buried in gravel and lay there for centuries, ever so gradually rotting away. As each block of ice melted, the gravel that had been lying on top of it sagged to form a depression, or prairie pothole.
Meanwhile, south of the reach of the glaciers, on the foreshore of the Rockies, the landscape had also been undergoing renovations. Sometime before the Ice Age set in, the entire western prairies had inexplicably begun to rise. As a result, the rivers, which previously had been building up the plains with loads of gravel and silt, now began to cut through the very layers they had previously deposited. This erosion was most dramatic along the slopes of the Rockies, where the rivers were powerful enough to wear through 70 million years of sediments. Along the Front Range of the mountains in Colorado, for example, the South Platte and Arkansas rivers have dug 1,600 to 2,000 feet (500 to 600 meters) below the level of the High Plains. Only where erosion-resistant layers of rock have stood against this assault can the remnants of the older landscape be seen. The tops of the buttes and mesas that tower over the eroded landscape were once a part of this continuous high plain.
By the end of the glaciation, the Great Plains of North America had been transformed from the seabed of ancient times into a mosaic of distinctive landforms. To the north extended a rumpled terrain of glacial debris. Beyond the limit of the glaciers, to the south and east, lay a softer landscape of ancient ocean floors, much of it now blanketed in wind-shaped drifts of glacial sand and silt. To the west, the flatlands of the High Plains stepped up steadily toward the front ranks of the Rockies. And everywhere, rivers were cutting down into the land, etching deep valleys, canyons and, where the land was suitably dry and bare, badlands.
But if the varied landforms of the plains were beginning to look more like those of the present, many of the life-forms still did not. Disadvantaged by the cool, wet weather of the Ice Age, the grasses that had previously dominated the plains had lost ground to other plants. Now a band of tundra skirted the retreating ice, while to the south, dark coniferous forests spanned much of the continent. Pure grasslands were restricted to scattered meadows and, perhaps, to a relict prairie crammed into the southernmost plains. Together, these diverse habitats were occupied by a stunning array of life, including white-tailed and mule deer, caribou, several species of pronghorns, black bears, cougars, bobcats, lions, cheetahs, saber-toothed cats, horses, llamas, one-humped camels—even Ice Age elephants. Woolly mammoths (shaggy beasts that stood almost 10 feet, or 3 meters, tall) browsed on the tundra, while Columbian mammoths (just as unkempt and much larger) appear to have favored the remnant patches of grassland. Meanwhile, in the forests, their somewhat daintier relatives, the mastodons (the size of Indian elephants) fed on a diet of black-spruce boughs and other woody tidbits.
Columbian mammoth
The mammoths and mastodons were relatively recent arrivals on the plains, Ice Age immigrants that migrated across the Bering land bridge from Eurasia during intermittent cold spells. Whenever the climate worsened and the glaciers advanced, water became locked up in the ice and sea levels dropped, exposing a bridge of land across the Bering Strait between Siberia and Alaska. When the glaciers receded again, the land bridge was drowned, but a passageway simultaneously opened to the south through the Canadian plains, which allowed the newcomers to wander into the heart of the continent. Some mammals, including ancestral camels and horses, made this journey in reverse, moving north when the plains corridor was open and then migrating across to Asia when the land bridge appeared.
Of all the species that arrived on the North American plains during the Ice Age—a menagerie that included not only elephants but also grizzlies, elk, and moose—two demand special notice. The steppe bison, Bison priscus, was a magnificent, thick-maned animal with flamboyant curving horns (attributes that are dramatically depicted in the cave art at Lascaux, France). The first bison herds likely poured across the Bering land bridge a few hundred thousand years ago and eventually made their way south to the Great Plains. Over the millennia that followed, successive waves of steppe bison made the same long trek, eventually meeting and mingling with the descendants of the pioneer herds. Meanwhile, that pioneer stock had been changing, shaped by life on the steppes and forests of a new continent. The result of this complex process of immigration, adaptation, and interbreeding was the emergence of several distinctively North American types, notably the giant, long-horned Bison latifrons and the somewhat smaller Bison antiquus. In time these species were displaced by an even more compact version, Bison bison, the shaggy beast that, in historic times, provided food and shelter to the first people of the plains.
Exactly when the first of those hunting people arrived on the scene is a mystery. Until quite recently, most archeologists insisted that humans (members of a genus that was born in East Africa some two million years ago) entered North America from Asia, by crossing the Bering land bridge and traveling down an ice-free corridor into the plains. This migration was said to have happened about 13,000 years ago. Then, in the 1970s, researchers working in Chile uncovered evidence that people had been living there for two thousand years before this supposed first-arrival date. Subsequent discoveries in Bell County, Texas, and elsewhere have pushed the timelines back even further, to at least 15,500 years before the present. This accords with the spirit of Indigenous memory, which affirms that their forebears were here at the beginning. Equipped with elegantly chipped fragments of stone and bone, these ancestral hunting people killed and butchered not only bison but also camels, horses, mastodons, and—their specialty—mammoths. At sites from Alberta to Texas, the proof of their presence—blackened hearths, discarded tools, and cracked marrow bones—lies buried where they left it so long ago. In some places, the skeletons of several large mammals lie strewn about the camps, testimony to the prowess of these big-game hunters.
But inevitably, on a planet where change is the only constant, this regime was fleeting. By thirteen thousand years ago, the fabulous array of large mammals on the plains was already disappearing. As many as fifty species—including giant beavers, ground sloths, lions, cheetahs, dire wolves, saber-toothed cats, horses, camels, mammoths, and mastodons—all became extinct within a few thousand years. Did an exploding population of well-fed humans hunt the animals into oblivion, as some archeologists believe? Or was climate change the culprit? The evidence suggests that, between about thirteen thousand and ten thousand years ago, average global temperatures first dropped abruptly and then rebounded. On the North American plains, these climatic changes ultimately translated into a dramatic shift in vegetation patterns. Pushed by warmer, drier conditions, the spruce forests gave way to pines, then in places to open, mixed woodlands, and ultimately to grass. In the blink of an eye (geologically speaking), a carpet of grasses spread out
