against this assault can the remnants of the older landscape be seen. The tops of the buttes and mesas that tower over the eroded plains were once a part of this continuous high plain.
Saber-toothed cat
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 landscapes. 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. Across these rolling hills 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 10 feet, or almost 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.
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.
Columbian mammoth
Skulls of Bison latifrons and Bison antiquus
Plains bison, Bison bison
Of all the species that arrived on the North America 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 creation 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 recently, most archaeologists asserted that humans (members of a genus that was born in East Africa some 2 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 believed to have happened within the last 11,000 to 12,000 years. Recently, however, archaeologists have uncovered evidence that people were living in Chile at least 1,000 years earlier than this, a discovery that has made the old timelines suspect. But whether the first people arrived 60,000 or 30,000 or 15,000 years ago, over the Bering land bridge or (as some suggest) by boat, as a massed arrival or in staggered groups, we know that by about 11,000 years ago, they were established across the north-central and southern plains. Equipped with elegantly chipped fragments of stone and bone, these hunters 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 success of these recent immigrants.
Three-flowered avens, seen in bud at left, is also known as prairie smoke or old man’s beard in tribute to its feathery seedheads, seen here.
But inevitably, on a planet where change is the only constant, their success was fleeting. By 13,000 years ago, the fabulous array of large mammals that the newcomers had encountered 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 archaeologists suggest? Or was climate change the culprit? The evidence suggests that, between about 13,000 and 10,000 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 (relatively speaking), a carpet of grasses spread out across the plains, blued by sage and beardtongue and enlivened by patches of golden beans, blazing stars, and prairie smoke, or three-flowered avens. The prairies of historic times had finally been created.
Until quite recently (geologically speaking), North America was home to many different kinds of pronghorns— some diminutive, some with multiple horns. The modern species is the only one to survive the extinction crisis at the end of the Ice Age.
Although the species is old as the hills, researchers are only now beginning to map their preferred calving grounds and seasonal migrations.
This new grassland was big and bold, but it was also much less varied than the mixed landscape of tundra, grass, and forest that it had displaced. And perhaps this in itself is enough to explain the disappearance of the Ice Age megamammals, which required a rich and varied supply of foods that grasslands alone could not provide.
