Prairie. Candace Savage

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