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 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,
