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.
Glacial erratics—boulders of granite from the Canadian Shield that were carried south by advancing ice—still stand where they were dropped at the end of the last glaciation.
After that ice sheet (the Kansan) 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 Sand Hills 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 18,000 years ago, a transition that marked the beginning of the present interglacial period. (Unless the buildup of greenhouse gases warms the climate enough to break the Ice Age cycle of retreat and advance, a new glaciation can be expected to begin within the next several thousand years: some experts expect it within decades.) Over the succeeding 10,000 years, 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 Lowland 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 wide flat 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 Rock-ies, 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 down about 1,600 to 2,000 feet (500 to 600 meters) below the level of the High Plains, right down to—and into—the seabeds of the Cretaceous. Only where erosion-resistant
