Читать книгу Prairie - Candace Savage - Страница 6

CHAPTER 2

DIGGING INTO THE PAST

A prairie requests the favor of your closer attention. It does not divulge itself to mere passersby.

SUZANNE WINCKLER, PRAIRIE: A NORTH AMERICAN GUIDE, 2004

THERE IS AN unseen dimension to the prairies, and that dimension is time. At first glance, this landscape may seem to have avoided the ravages of the past. The level plains and soft, rolling hills appear to have settled here quietly, their surface unmarred by signs of geological strife. But appearances can be deceiving. The great grasslands of central North America have been shaped over the past three or four billion years by the same forces that raised the Rockies and excavated the Grand Canyon. Their surface has been seared by the sun, scoured by ice, blasted by blowing sand, and buried in deep drifts of gravel. As a result of immense energies beneath the surface of the Earth, the plains have been raised up, forced down, drowned by oceans, and blanketed in ash. They have experienced every shudder and wrench as continents have collided and torn away from each other, only to collide and tear away again.

The traces left on the surface of the prairies by this planetary bump and grind are surprisingly minimal. Yet if you know what to look for and where to look for it, the subtleties of the prairie landscape become eloquent. An oil well bears witness to ancient tropical seas. A vast level plain provides an unexpected reminder of the protracted violence of mountain building. A hummocky wheat field speaks of the lumbering passage of glaciers. To an observer with a little basic geological knowledge, even the most unspectacular prairie landscape suggests a long and spectacularly interesting history.

UNDER THE WAVES

TO GO BACK into the prairie’s history means to go down. The record and residue of times past lie beneath our feet, so wherever we go on the prairies, we are traveling across vanished worlds. Straight beneath you, for example, at a depth of between 1,800 and 4,000 miles (2,900 and 6,400 kilometers), lies the Earth’s core—the yolk of the planetary egg. This partly solid, partly fluid center is encased in an equally ancient layer of rock called the mantle. Surrounding the mantle, in turn, is a covering of waxlike malleable material known as the asthenosphere, which is kept at a lethargic boil by the heat of its own radioactive decay. As the source of the molten magma that periodically shoots up through volcanic fissures and rifts in the ocean floor, the asthenosphere is the main powerhouse of geological turmoil.

The asthenosphere is overlain by a relatively thin and fragile shell of rock known as the lithosphere. The outermost membrane of this rocky shell is the Earth’s crust, a layer that is thinner, proportionately speaking, than the skin of an apple. On the prairies, the crust extends to an average depth of 25 to 30 miles (roughly 40 to 50 kilometers). Yet this comparatively short vertical distance takes us back in time some 3.8 billion years, to an era when the flying debris of creation had begun to subside and the Earth’s crust was finally able to stabilize. In this remote and inhospitable age, we find the first traces of life—microscopic stains, a few microns long, made by filaments of cyanobacteria, or blue-green algae.

Rocks from this primordial era lie right out in the open on the Precambrian Shield, but they seldom break through to the surface of the Great Plains. Instead, these ancient granites generally lie several miles beneath our feet, providing the foundation, or “basement rock,” on which the prairies have been built. Our region lies on what geologists call the North American craton, the stable core of the continent. This is a large fragment of the Earth’s crust that sheared away from an unnamed supercontinent toward the end of the Precambrian Era. By the time this happened—some 600 million or 700 million years ago—the Earth (and the prairie region along with it) had already endured more than three billion years of mountain building, erosion, glaciation, deglaciation, and general geological Sturm und Drang. But things must have been starting to settle down, because when the supercontinent tore itself apart, it produced a North American continent-in-the-making that has persisted until the present.

GEOLOGICAL TIMESCALE

AGE	ERA	PERIOD	EPOCH	YEARS AGO
	Cenozoic	Quaternary	Holocene	10,000
		Tertiary	Pleistocene	1.6 million
			Pliocene	5 million
Age of Mammals			Miocene Oligocene	24 million 34 million
			Eocene	55 million
			Paleocene	65 million
	Mesozoic	Cretaceous		145 million
Age of Reptiles		Jurassic		200 million
		Triassic		250 million
	Paleozoic	Permian		300 million
Age of Amphibians		Carboniferous		355 million
Age of Fishes		Devonian		420 million
		Silurian		440 million
		Ordovician		490 million
		Cambrian		545 million
	Precambrian			4.5 billion

This infant continent was not exactly the landmass that we know today. The entire Western Cordillera was missing, with the result that the west coast of the craton ran south through present-day British Columbia and the Pacific states (much closer to the prairies than it is today). At first, the cratonic landmass lay exposed—a low, eroding plain, as barren as the face of Mars. But, as the geological strife continued, sea levels began to rise and the land was gradually overrun by the ocean. In time, the entire continent (with the periodic exception of a chain of tropical islands that ran diagonally across the plains, from Lake Superior toward Arizona) had disappeared beneath the waves.

For roughly the next 55 million years (from about 545 million to 490 million years ago), much of the North American craton lay under a shallow sea. Wherever the land remained exposed, it was eroded by water and wind, which ground the gritty Precambrian rocks into rounded grains of quartz sand. This sand was then swept to the coasts and out into the sea, where it settled to the bottom in beds tens to thousands of yards (or meters) thick. Eventually, these lustrous sediments were overlain by layers of fine-grained mud. But whether sandy or silty, this ocean floor was literally crawling with life, particularly three-lobed, many-legged, bottom-feeding arthropods known as trilobites. After an agonizingly slow start with the cyanobacteria, evolution was finally hitting its stride, producing a menagerie of weird and wonderful undersea life. As generation upon generation of these animals lived and died, their remains settled onto the ocean floor, where they were buried under thick layers of sediments. Today these fossil-rich deposits—now compressed into solid sandstone and shale—are buried some 3 miles (5 kilometers) beneath the wheat fields of the northern plains and at lesser depths in other parts of the prairies. But in a few places—like the Judith and Little Rocky mountains and the northern Black Hills—they have been pushed up to the surface, exposing their maritime history to plain view.

When the Cambrian sea finally withdrew and dry land emerged again, the forces of erosion immediately began to tear away at the newly formed rocks. But soon, geologically speaking—after a break of little more than 20 million years—the water rose and slowly spread over the land. This time, even the transcontinental island chain was bathed in the warm, clear seas. Now primitive snails munched on algae and were themselves preyed upon by giant squidlike nautiloids, with shells up to a couple of yards in length. Hundreds of new species of shelled animals evolved, including crinoids, or “sea lilies” (distantly related to modern sea urchins), and exotic reef-forming corals. There was so much life in these oceans that when they finally withdrew some 440 million years ago, they left behind thick deposits of shell fragments and calcium-rich debris, which eventually solidified into fossil-rich limestones. These Late Ordovician deposits include the elegant Tyndall stone that is quarried in Manitoba and graces so many buildings in the Prairie provinces.

And so things continued for about the next 100 million years, as shallow oceans advanced across the North American craton, only to withdraw and then flood back in. If the run and roll of the grasslands sometimes make us think of the sea, surely this is a reminder of the landscape’s long marine history. With every advance and retreat of the ocean, the land was burdened with fresh deposits of sand, silt, and crushed shells, which built up, year by year, in nearly horizontal, banded layers. Although some of these contrasting sea floors have since been exposed by erosion (where rivers have cut deeply down through the sediments), for the most part they lie thousands of yards (or meters) beneath the grasslands.

The oceans that left these deposits behind were hospitable to life—shallow, warm, well lit, and typically tropical. During the Silurian and Devonian periods in particular (between about 440 million and 355 million years ago), these waters provided ideal conditions for reef-forming sponges and corals. In what would one day become the Canadian Prairie provinces, the reef builders of the Devonian had a heyday, constructing barrier reefs and ringlike walls that rose to heights of 300 feet (90 meters). Wherever the sea was constricted by these limestone palisades, the water gradually became super-salty. If the circulation of the sea was inhibited, water lost to evaporation could not readily be replaced, and the concentration of salts steadily increased. In time, the salts precipitated out of the sea water in these areas, leaving thick beds of potash and other salts, notably under present-day Saskatchewan. The potash deposits in Texas were formed by a similar process but some millions of years later, during the Permian Period.

The last truly continentwide inundation withdrew from the North American craton about 300 million years ago. The next time the sea attempted to overrun the land—as it would continue to do for millions of years to come—it found itself lapping around the shores of a rocky upland that had started to rise in the eastern half of North America. Apparently, the asthenosphere had heated up and begun to force masses of molten rock up through rifts in the ocean floor. This event had sent the continental plates on a slow and perilous collision course. First, Europe smashed against North America from the northeast. Then a massive supercontinent called Gondwana (the combined landmasses of South America, Africa, India, Antarctica, and Australia) crunched into North America from the south, causing the land to buckle and forcing the Appalachian Mountains to lift along the east coast. The forces involved in these mighty adjustments were even felt in the middle of the craton, where a range of mountains 3,000 feet (900 meters) high rose out of the plains of present-day Oklahoma and Colorado. Known as the Ancient Rockies, these mountains have since been eroded to their roots by the action of water and wind.

Apart from the appearance of these new highlands, the west coast of the craton was comparatively untouched by these titanic collisions. Through all the commotion, the sea continued to wash up over the land, even splashing around the base of the Ancient Rockies and turning them into a cluster of islands. With every advance and retreat, the sea again left behind layers of sediment and the fossilized remains of a strange coterie of underwater life. In addition to the crinoids, corals, and other unusual beasts that had occupied Devonian waters, there were now small filter-feeders, called archimedes, that had perfect corkscrew skeletons, and others, called productids, that held themselves up off the sea bottom by perching on stiltlike spines. (A wonderful jumble of 250-million-year-old sea life has been preserved in the Guadalupe Mountains of western New Mexico and Texas, which were once a complex of reefs in the western ocean.) Bony fishes swam through these waters, sometimes hotly pursued by large, saw-toothed sharks. The game of evolution was being played with feverish exuberance.

Nautiloids

Productid

Meanwhile, back on dry land, the surface of the continent was continuing to buckle and twist. As the Appalachians were thrust upward, land in the center of the craton was forced to rise along with them. A broad plain formed along the edge of the eastern highlands, sloping gently toward the western sea. When the waters receded, this coastal plain extended all the way west to present-day Alberta and Texas. And even when the sea rose up and flowed across the land, the eastern margin of the plain (roughly from present-day Manitoba south to Kansas and Missouri) was now high enough to escape all but the most severe flooding.

A new frontier for life was emerging not only in North America but on the other continents as well. Land plants, which had put in their first appearance some millions of years before, had never made much of a showing. But as stable new habitats became available, the evolutionary tree began to bud and sprout with explosive energy, producing more and larger species of land plants than ever before. In time, the soggy, boggy landscapes left by the retreating oceans were filled with riotous jungles of giant sphenopsids, or scouring rushes, tree-sized ferns and leafy conifers. These tremendous swamps, which flourished between about 355 million and 300 million years ago, disappeared soon afterward, probably as a result of a cooling and drying trend in the climate. Buried where they fell—in modern-day Iowa, Missouri, and Kansas, among other places—the swamp plants eventually turned into coal, the characteristic rock of the Pennsylvanian, or Upper Carboniferous, Subperiod.

Through much of the next 50 million years (the Permian Period), the land shriveled in the sun. Swamps decayed, seas shrank, and the exposed plains along the west coast blew with sand and salt. But life was not to be stopped. Insects, which had dominated the wetlands of the Carboniferous, now gave rise to new dry-land forms such as beetles and the distant ancestors of crickets and grasshoppers. Amphibians, too, crawled out of the swamps and began to invent the technology they needed for life on the land—notably a soft-shelled, amniotic egg that could develop out of the water. In time, new life-forms developed that could live their whole lives on land, including massive, lizardlike creatures known as stem reptiles. Basking alongside these primitive organisms on the arid coastal plains were their near relatives, the synapsids—the direct ancestors of modern mammals. At the root of our family tree is Dimetrodon, a burly, fin-backed synapsid with two stabbing canine teeth, which it used to snap up slow and unwary amphibians. We know these creatures once roamed the savannas of the western plains because wonderfully preserved skeletons of Dimetrodon and many of its equally bizarre contemporaries have been dug out of Permian “red bed” deposits in New Mexico, Texas, and Oklahoma.

Dimetrodon

And so it is that we find ourselves near the end of the Permian, watching a lumpish, beaked synapsid called Kannemeyeria breaking off the tough stem of a broad-leafed conifer somewhere along the west coast of Texas. Under our feet lie the accumulated sediments of 3.5 billion years, or more than 90 percent of the geological timeline. Yet except for the wide spread of the horizon, there is little in this scene to put us in mind of the modern prairies. No grass, no gophers, no pronghorns, no playas or sloughs. Something radical will have to happen to create the landscape that we see around us today. Something revolutionary.

TERRIBLE LIZARDS

THE PERMIAN PERIOD ended in a biological catastrophe—the most severe mass extinction in all of geological history. During a period of several million years, over 95 percent of all the species living in the oceans were eliminated, together with 75 percent of terrestrial vertebrates. Why did this disaster occur? No one knows for sure, but the continuing gyrations of the continental plates may have been partly to blame. By the Permian Period, the continents had become temporarily fused into one gigantic landmass, called Pangaea. At the same time, the floor of the oceans apparently warped downward, drawing the sea away from the land and exposing a vast and inhospitable heartland of hot, dry silt and sand. These deserts had little to offer to life-forms that, in ages past, had flourished in a watery world of lagoons and swamps. Perhaps this change in conditions is enough to explain the huge loss of life. But whatever the probable causes (and many have been invoked), the impact was severe, and, despite the nonstop creativity of evolution, it would take millions of years for the Earth to repopulate itself with a full range of plants and animals.

At the same time that this biological revolution was occurring, a major geological upheaval was also underway. The continents, after docking together in Pangaea for some millions of years, began to tear away from one another. As Europe sheared off to the east and the Atlantic Ocean opened up, the North American craton was shoved slowly westward. Eventually, about 165 million years ago, the drifting continent ran into a small fragment of the Earth’s crust (perhaps an island chain), known to geologists as a terrane. As the continent plowed onward, it contacted other, similar obstacles in its path. One by one, these terranes were crushed against the west coast of the craton and added to its mass. The impact of these collisions—which would continue sporadically for about the next 100 million years—caused the western margin of the craton to fold, twist, crack, and rise up mightily, until ranges of ragged peaks ran along the length of the continent. The present-day plains (which for so long had lain along the west coast, exposed to the run of the sea) were now guarded by the serried ranks of the Rockies.

THE EARTH IN UNSTEADY MOTION

Sometime in the first half of the nineteenth century, a man named Little Hill, “a Winnebago chief from the upper Mississippi, west,” shared this story with a U.S. government official and amateur ethnographer named Henry Schoolcraft. Little Hill’s narrative reflects the beliefs of the Winnebago Buffalo Society about the creation of the world.

Little Hill on Creation

The Great Spirit awoke as from a dream, and found that he was alone. He created the four winds by taking a piece of flesh from near his heart and mixing it with the substance upon which he sat. For these brothers he created a woman, our Grandmother the earth. She was sent down below, but she was unstable, and rocked about violently. To steady the world below, the Great Spirit sent down four giant snakes and four giant animals of another kind, and they were able to hold down the corners of the earth. However, when the winds blew across this creation, it fell back into unsteady motion again; so he created a gigantic buffalo, who is the land, and placed it in the center of the earth to make it steady.

Early in this process, before the wall of mountains was complete, the ocean still sometimes slipped through gaps in the palisade and washed across the plains. This happened several times during the Triassic and Jurassic periods (between 250 million and 145 million years ago), culminating in a huge incursion, known as the Sundance Sea, which swept east into present-day Saskatchewan, Nebraska, and Texas. But these waters were soon expelled from much of their floodplain by a deluge of a completely different sort—an influx of mud and sand that washed down off the slopes of the newly formed mountains. No sooner had the mountains raised their heads than erosion began to level them. Mixed with generous quantities of volcanic ash from the tumult of mountain building, these sediments were strewn across the plains as far east as the Dakotas. Today they form brightly banded sandstones and shales—the Success, Kootenay, and Morrison formations by name—that bear witness to a titanic struggle among rivers, mountains, and seas. They also contain evidence of an awe-inspiring bestiary of ancient life.

The cataclysmic extinctions at the end of the Permian had left a biological void, but by the Middle Triassic (about 225 million years ago), this vacuum had been filled to bursting with reptiles. Creeping, crawling, swimming, flying, stomping across the land, reptiles had become the dominant animal group on Earth. Chief among them were the dinosaurs, including the 80- to 100-ton Brachiosaurus, which raised its ultralong neck to browse in the treetops, and the plated Stegosaurus, which had seventeen trapezoidal shields of bone embedded along its spine. Unfortunately for them (but fortunately for succeeding generations of dino-enthusiasts), hundreds of these large-bodied, small-brained animals apparently tramped into the rushing rivers, got stuck in the mud, and died. Their bones were then swept away by the currents and dropped on snags and in backwaters, where they lay in thick beds. These Morrison deposits provide the focus for the Dinosaur National Monument in Colorado and Utah. Although now in the mountains, the deposits formed on level terrain, and the same or similar species must have lounged under ginkgo trees and trudged through the spiky underbrush of what is now the Great Plains.

The sea, which in the Late Jurassic had been driven off the continent by sediment from the mountains to the west, managed to creep in one last time during the Cretaceous Period (between 145 million and 65 million years ago). By then, the Rocky Mountains formed an unbroken dyke along the west coast, but the rising waters rushed around it at both ends, flowing south from the western Arctic and north from the Gulf of Mexico. By the time the waters met in Colorado about 100 million years ago, almost the entire prairie region was inundated. It would take about 40 million years for the sea to make its final retreat, but when the water was finally gone, a vast plain lay exposed, stretching farther than the eye could see across the interior of the continent.

The first eyes to gaze across those broad, unfettered vistas were no doubt reptilian. By the Late Cretaceous, herds of heavyset Triceratops, with their wide, frilled collars and clustered horns, were roaming across the countryside and foraging in lush stands of horsetails, ferns, gingkoes, and palms, keeping an eye peeled for their most dangerous enemy, Tyrannosaurus rex. In case we were in any doubt about what T. rex preyed upon, a paleontologist in Saskatchewan once found a large sample of fossilized tyrannosaur dung. The two-quart (two-liter) lump contains what appears to be the crushed head frill of a juvenile Triceratops. Much of what we know about Cretaceous dinosaurs, both trivial and profound, comes from sites on the Great Plains, including the Red Deer River Valley in Alberta and the Frenchman Valley in Saskatchewan.

Tyrannosaurus rex

Succored by a mild and equable climate, much improved since Permian times, life in the Cretaceous was full. The air thrummed with insects, including moths and bees. The massive flying reptile Quetzalcoatlus rode the updrafts over the southern plains on a 40-foot (12-meter) span of wings—wider than those of a single-engine plane—searching for the bodies of the dead and dying. Frogs and salamanders hid in the underbrush, a habitat they shared with cowering, timorous mammals, few of them bigger than mice. Yet by the end of the Cretaceous Period, about half of this rich assembly of species—including all of the flying reptiles and the dinosaurs—had vanished. In fact, you could say they literally disappeared in a flash. One day, about 66 million years ago, an enormous meteorite (6 miles, or 10 kilometers, in diameter) crashed into the ocean off the present-day Yucatan coast with the force of a 100-million-megaton bomb, creating a worldwide holocaust of toxic vapor and soot. Beads of sizzling-hot glass were ejected from the blast, setting the rain forests and swamps alight as far north as Saskatchewan and Alberta. A newly discovered fossil site in southwestern North Dakota captured the catastrophe as it occurred: Charred trees. Shocked minerals. Marine and freshwater creatures all thrown in together. Fish with fused glass in their gills. By the time things eventually settled, the dinosaurs were extinct, and the plains were open for a new group of species to fill the vacancy.

HIGH AND DRY

NOT LONG AFTER the last dinosaur drew its last breath, something strange began to happen along the western margin of the Great Plains, in the heart of present-day Montana and Wyoming. About 50 million years ago, for reasons that no one can explain (more crashing and grinding off the west coast?), the level plains of the Cretaceous seabed began to heave upward, bend, and in places, crack open. Molten rock from the asthenosphere bubbled up through the fissures, sometimes crystallizing before it reached the surface, sometimes pouring out across the land to form dykes, domes, and ridges of lava. When the smoke cleared, mountains stood right out in the middle of the level plains. Subsequently honed by erosion, these unexpected rocky peaks still punctuate the western landscape from the Sweet Grass Hills east through the Bears Paw and the Little Rocky mountains, and south to the Crazy Mountains and the Black Hills.

At the time of their formation, the isolated “prairie mountains” did not have the presence that they do today. Even the main ranges of the Rockies were little more than bumps that protruded above a muddy, gravel-strewn landscape. The higher the mountains had thrust themselves up, the faster erosion had worn them away, until they lay buried, neck-deep, in their own shed silt, sand, rock, and clay. (The thick coal deposits in the Powder River Basin of northeastern Wyoming were formed when tons of this muck overran a peat bog some 50 million years ago and buried the vegetation under 10,000 feet, or 3,000 meters, of sediment.) Year after year, rivers carried a massive tonnage of this debris eastward to the central plains, depositing it as a broad, sloping alluvial fan. As the braided streams of the floodplain washed over the sediments, they gradually licked the surface smooth, creating a landscape that in places is so level that it almost seems supernatural. This stunning flatland once extended from the knob-peaked Rockies across southern Alberta and Saskatchewan, south through the eastern Dakotas, east to the Flint Hills of Kansas, and down to central Texas. Today, though much diminished by erosion, this landscape persists as the High Plains of Nebraska, Kansas, Oklahoma, and Texas. Legend has it that when the Spanish first crossed the plains of Texas in the 1500s, they used stakes to mark their route because the land was so spectacularly featureless. Hence the name Llano Estacado, or the Staked Plains, of northern Texas.

Some 45 million years ago, when the High Plains landscape was still being shaped, it would have taken more than stakes to help travelers find their way, for it was covered by a dripping, tangled forest. Globally, the climate had never been more amenable to life—there were dawn redwoods near the North Pole—and the plains basked in warm, wet, subtropical weather. A lush woodland spread across the midcontinent, alive with an impressive variety of birds and mammals. Ancestral squirrels and monkeys leapt through the overstory, while down below, titanotheres—beasts the size of rhinos, with knobby horns and sharp tusks—shuffled across the forest floor feeding on shrubs. Among the other browsing animals of the time was an early ancestor of the horse, Orohippus by name, which had four toes on its front feet and three on the back and grew to be about the size of a large Shetland sheepdog.

Life was easy. But then a sequence of unrelated events halfway around the world sent the climate into a nosedive. Beginning about 37 million years ago, the average global temperature dropped by 14°F (8°C) over the span of a million years. Thereafter, despite brief periods of recovery, the climate continued to cool. As the weather became cooler and drier, the tropical forests of the North American plains began to wither and die away.

But conditions that were death for palm trees were ideal for another group of plants. Relative newcomers on the evolutionary scene, grasses had first appeared shortly after the extinction of the dinosaurs but had met with limited success. They were drought specialists, and while humid conditions prevailed, they had been confined to small patches of ground that had somehow been deprived of abundant rainfall. Now, not only were the tropical rains failing because of a global drying trend, but the North American plains were under a special disadvantage. With the Rockies in place, storms that rolled in from the Pacific tended to drop their precipitation as they swept up the western slopes. By the time they reached the plains, they were pretty much wrung out. But grasses don’t require much moisture, and this characteristic gave them a competitive edge. Over the next several million years (between about 24 million and 3 million years ago), grasses gradually became the dominant plants across the Great Plains.

If we could slip through a crack in time and go back to the plains of Nebraska some 20 million years ago, we would find ourselves in a landscape that is at once familiar and wonderfully strange. This is big sky country, an open landscape of shoulder-high grasses dotted with walnuts and other broad-leafed trees, vaguely reminiscent of the savannas of East Africa today. A broad river courses across the plain, its margins fringed by willows and its current murky with sediment from the constantly eroding Rockies. Whenever this river floods, it coats the land with yet another layer of silt and sand.

The river is the main source of water in this increasingly arid land, and wildlife flocks to its banks. Herds of miniature rhinos (about the size of domestic pigs but with two horns sprouting from the ends of their snouts) plunge into the shallows to find refuge from biting flies. Ancestral horses called Parahippus, somewhat bigger than Orohippus but still the size of dogs, come down to the river to drink at dawn and dusk. The rest of the time, 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.

Dwarf rhinoceros

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 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.

PERMANENT WINTER

NOBODY KNOWS FOR sure why the cold settled in as it did. Whatever the cause, by between about three million and two 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 massed into mounds, and mounds into mountains, until the snow compacted into ice under its own tremendous weight. 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 two 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 over the Canadian Prairie provinces and south through northeastern Montana, the Dakotas, and 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.

After that ice sheet 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 Sandhills 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 side of the Missouri River and provide the matrix for rich, rolling farmlands in Nebraska, Kansas, Iowa, Missouri, and elsewhere.

The ice began its final, halting retreat about eighteen thousand years ago. Over the succeeding millennia, a block of ice larger than present-day Antarctica gradually melted away, and it didn’t go quietly. Torrents gushed from the eroding ice sheets, gouging out meandering coulees and wide, flat-bottomed river valleys as they coursed eastward over the plains. Today, dry coulees writhe incongruously across the northern prairies, from nowhere to nowhere, and glacial spillways seem ludicrously oversized for the quiet streams, like the Milk River, 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 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.

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. Yet if the new habitats were insufficient to sustain mammoths, they appeared perfectly suited to the bison, which soon emerged as the dominant grazing animal on the open range.

As the great herds drummed across the prairie, they of course had no conception of how their homeland had been shaped. The evidence of that drama lay unnoticed beneath their hooves—long-buried evidence of onrushing seas, rising mountains, silt-burdened rivers, and towering cliffs of ice. But the bison were untroubled by the traumas of the past, as they flowed across the horizon toward the present.