the time the last of the great reptiles disappeared. The evidence suggests that some 65 million years ago, a hunk of rock 6 miles (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, possibly setting the rain forests and swamps alight as far north as Saskatchewan and Alberta. Swirling clouds of ash and dust blotted out the sun, as the world settled into the gloom of “impact winter.”
Yet despite the magnitude of this disaster, time has removed almost every trace. Today—apart from a 100-mile-wide (180-kilometer-wide) crater off the Mexican coast—little remains except for a narrow band of whitish clay that is visible in several dozen places around the world. Not only does this layer contain residues of the rain of molten glass, it is also distinguished by the presence of “shocked quartz” (grains of sand that have been distorted by an impact) and by a high concentration of iridium, an element that is rare on Earth but common in meteorites and other space objects.
Once derided as sci-fi fantasy, the idea that a giant meteorite struck the Earth is now generally accepted by experts. But is that really how the large dinosaurs met their end? New findings suggest that the answer may be “yes.” Analysis of fossilized pollen from in and around the impact zone appear to reflect a sudden die-off of photosynthetic plants. Perhaps, in the dark days after the collision, food webs collapsed and great beasts like T-Rex and Triceratops starved to death. Key sites for answering these intriguing questions include the Cretaceous/Tertiary, or K/T, Boundary strata in the Red Deer Valley at Drumheller, Alberta; the Frenchman Valley in southwestern Saskatchewan; Dogie Creek in eastern Montana; and Badlands National Park in South Dakota.
High and Dry
Not long after the last dinosaur drew its final 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 Bear Paws 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 onto a broad, eastward-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.
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