Prairie. Candace Savage

they range across the savanna, plucking leaves off the trees and grazing on grasses that tower over their heads. Because grass is very abrasive, Parahippus have acquired specially ridged teeth that are able to withstand the daily grinding. Llamalike camels (members of a family that evolved in North America and only later migrated to South America and Eurasia) lounge in the willows but keep an eye out for any suspicious shadows moving through the bushes. In this world, danger takes the forms of saber-toothed cats and long-jawed dogs, some of them as large as coyotes and wolves. Smaller dogs, the size of foxes, prey on the Paleolagus, or “ancient rabbits,” that burrow into the roots of shade trees, and on Paleocastor, or “ancient beavers,” that, amazing as it seems, occupy deep, corkscrew burrows in the middle of the dry prairie.

      One of the distinctive species of the northern and central Great Plains, the white-tailed jackrabbit traces its ancestry back to the Miocene Period, some 37 million years ago.

      Dwarf rhinoceros

      Orohippus and Parahippus against the silhouette of the modern horse, Equus

      Paleocastor

      Traces of these animals, and others like them, have been preserved at the Agate Fossil Beds National Monument on the Niobrara River in northwestern Nebraska. Here, the buried beds of bone testify not only to remarkable lives but also to miserable deaths. It seems that the drying trend, which had driven back the rain forest and allowed the lush parklands to spread, occasionally became so severe that it stressed even the savannas, causing rivers to dry up and trees to blacken. Animals gathered alongside the dying rivers and died along with them. Later, when floods flashed down out of the mountains, the currents gathered up the bones, massing them into backwaters and oxbows.

      As the centuries ticked by, the climate became progressively more arid. Soon, in place of the lush savannas, a tawny, almost-treeless grassland sprawled across the plains. And although many mammalian species survived—including rhinos, horses, camels, rodents, cats, and dogs—all were challenged by their changed and unforgiving environment. An unremitting diet of grass pushed grazing animals to develop high-crowned teeth, which grew in to replace themselves as they were worn away. The absence of hiding places put a premium on speed, forcing both predator and prey to adopt the runner’s long-legged physique. Hunter and hunted also came to rely on their quick wits, as the brain power of both players was augmented.

      As it turned out, these hard-won adaptations would offer little protection against the trauma that was about to unfold—the Ice Age.

       > PRAIRIE MOUNTAINS

      The Black Hills, which straddle the border between Wyoming and South Dakota, are the most easterly outliers of the Rocky Mountains. (On some geological maps, they are actually identified as the Central Rocky Mountains.) These hills rose up out of the plains between 62 million and 48 million years ago. The core of the hills, extending roughly from Deadwood to Wind Cave, is a massive, elliptical dome of granite, some of it as much 2.5 billion years old, that was thrust up from deep underground and exposed by erosion. Now carved into spires and peaks, the dome lies shoulder deep in a broad, encircling apron of younger rocks, formerly ocean floors, that is known as the Limestone Plateau.

      At one time, about 37 million years ago, this plateau was completely buried in sediments that had washed down off the dome, but that overburden— and much more besides—has since been washed away. Erosion has also created the remarkable Racetrack, or Red Valley, that runs around the foot of the plateau. But nothing has been able to wear away the hogback of resistant Cretaceous sandstone that encloses both valley and hills like a fortress wall. Chosen by Americans as a site to honor past presidents (four of whom are represented on Mt. Rushmore), the Black Hills also stand as a natural monument to the colossal energies that shaped the continent.

       Permanent Winter

      Nobody knows for sure why the cold settled in as it did. Perhaps (as one theory suggests) the chill from the Antarctic refrigeration cell crept gradually north until the Arctic Ocean froze and exerted its own cooling force. Or maybe the expansion of the continental land mass over several hundred million years had caused the global temperature to trend downward. (Since land holds less heat than water, a larger land mass might logically translate into lower temperatures.) The rise of the Rockies and other mountain ranges around the world may also have contributed to the decline by disrupting the jet stream and causing cold Arctic air to spill south across the land. Or perhaps all these Earthbound events were irrelevant in the grandeur of space, where a wobble in the Earth’s orbit and the inconstancy of the sun may have triggered subtle changes in the climatic system.

      Be that as it may, between about 3 million and 2 million years ago, the Earth had cooled so much that permanent winter had settled over the northern reaches of the continent. The tepid summers no longer melted away the preceding winters’ snows. Beginning at high latitudes and progressing southward, drifts built up into mounds, and mounds into mountains, until the snow compacted into ice under its own tremendous mass. Eventually, after several thousand years, these glaciers began to advance, flowing almost imperceptibly but relentlessly south over the Central Lowlands. In time, the northern third of North America was buried under some 2 miles (3 kilometers) of ice; that’s about the height, from base to peak, of Mount Everest. In its heartland on the Precambrian Shield, the ice reached a maximum depth of about 16,000 feet, or 5,000 meters.

      Geologists used to believe that the glaciers advanced and retreated four times over a span of about 2 million years. These successive incursions were known in North America as the Nebraskan, Kansan, Illinoian, and Wisconsin glaciations, in honor of their southernmost extent. But more recent research suggests that the glaciers probably made many more than four sweeps down the continent, each time grinding away the traces left by previous glaciations. Since much of the record has been wiped clear, a detailed chronology of the Ice Age on the prairies cannot be reconstructed. But we do know that by about 1.2 million years ago, a vast slab of ice had bulldozed its way almost to the present-day confluence of the Missouri and Mississippi rivers. At its maximum, the ice sheet probably extended beyond the Canadian provinces, sweeping across northeastern Montana and south through the Dakotas to northeastern Kansas. From there it cut across the plains of northern Missouri and then eastward, across the continent, to the ice-stricken valley of the St. Lawrence.

      Glacial erratics—boulders of granite from the Canadian Shield that were carried south by advancing ice—still stand where they were dropped at the end of the last glaciation.

      After that ice sheet (the Kansan) retreated, the glaciers never again penetrated quite so deeply into the plains. The final glaciation, for example, which began some 100,000 to 75,000 years ago, didn’t progress much farther south than central Iowa. But the devastation that the glaciers inflicted was not limited to their actual footprint. Whenever the glaciers melted back, they left behind outwash plains of sand and silt. Ferocious winds that developed over the ice fields picked up this grit and hurled it around the interior of the continent. In a number of places (notably, the Great Sand Hills of Saskatchewan and the Sand Hills of western Nebraska) the wind laid down its burden in vast fields of dunes. Elsewhere, the storms whipped up clouds of dust—rock that had been ground into flour by the glaciers—and broadcast it over the land. Today, these silt, or loess, deposits, often several yards thick, form the bluffs along the Iowa