Читать книгу Prairie - Candace Savage - Страница 27

The mysterious boulders, or concretions, at Red Rock Coulee near Medicine Hat, Alberta, were formed on the floor of a shallow, inland sea about 75 million years ago, during the Cretaceous Period.

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 leaped 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.

Eocene herbivores

At Toadstool Geologic Park, near Chadron, Nebraska, sediments that originally eroded off the young Rocky Mountains were subsequently thrust up by faulting and then sculpted by erosion. The park also features a trackway of fossil footprints left millions of years ago by giant pigs, rhinos, camels, and other prehistoric beasts.

Life was easy. But then a sequence of unrelated events halfway around the world sent the climate into a nosedive. (According to one theory, the separation of Antarctica from Australia caused a major rerouting of oceanic currents, with the result that water from the poles no longer mingled with water from the equator. The South Pole thus became an isolated refrigeration cell that eventually spread a chill around the entire planet.) Beginning about 37 million years ago, the average global temperature dropped by 14˚F (8˚C) over the 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.

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.