CHAPTER 2

       DIGGING INTO THE PAST

       A prairie requests the favor of your closer attention. It does not divulge itself to mere passersby.

      SUZANNE WINCKLER, PRAIRIE: A NORTH AMERICAN GUIDE, 2004

      THERE IS AN unseen dimension to the prairies, and that dimension is time. At first glance, this landscape may seem to have avoided the ravages of the past. The level plains and soft, rolling hills appear to have settled here quietly, their surface unmarred by signs of geological strife. But appearances can be deceiving. The great grasslands of central North America have been shaped over the past three or four billion years by the same forces that raised the Rockies and excavated the Grand Canyon. Their surface has been seared by the sun, scoured by ice, blasted by blowing sand, and buried in deep drifts of gravel. As a result of immense energies beneath the surface of the Earth, the plains have been raised up, forced down, drowned by oceans, and blanketed in ash. They have experienced every shudder and wrench as continents have collided and torn away from each other, only to collide and tear away again.

      The traces left on the surface of the prairies by this planetary bump and grind are surprisingly minimal. Yet if you know what to look for and where to look for it, the subtleties of the prairie landscape become eloquent. An oil well bears witness to ancient tropical seas. A vast level plain provides an unexpected reminder of the protracted violence of mountain building. A hummocky wheat field speaks of the lumbering passage of glaciers. To an observer with a little basic geological knowledge, even the most unspectacular prairie landscape suggests a long and spectacularly interesting history.

       UNDER THE WAVES

      TO GO BACK into the prairie’s history means to go down. The record and residue of times past lie beneath our feet, so wherever we go on the prairies, we are traveling across vanished worlds. Straight beneath you, for example, at a depth of between 1,800 and 4,000 miles (2,900 and 6,400 kilometers), lies the Earth’s core—the yolk of the planetary egg. This partly solid, partly fluid center is encased in an equally ancient layer of rock called the mantle. Surrounding the mantle, in turn, is a covering of waxlike malleable material known as the asthenosphere, which is kept at a lethargic boil by the heat of its own radioactive decay. As the source of the molten magma that periodically shoots up through volcanic fissures and rifts in the ocean floor, the asthenosphere is the main powerhouse of geological turmoil.

      The asthenosphere is overlain by a relatively thin and fragile shell of rock known as the lithosphere. The outermost membrane of this rocky shell is the Earth’s crust, a layer that is thinner, proportionately speaking, than the skin of an apple. On the prairies, the crust extends to an average depth of 25 to 30 miles (roughly 40 to 50 kilometers). Yet this comparatively short vertical distance takes us back in time some 3.8 billion years, to an era when the flying debris of creation had begun to subside and the Earth’s crust was finally able to stabilize. In this remote and inhospitable age, we find the first traces of life—microscopic stains, a few microns long, made by filaments of cyanobacteria, or blue-green algae.

      Rocks from this primordial era lie right out in the open on the Precambrian Shield, but they seldom break through to the surface of the Great Plains. Instead, these ancient granites generally lie several miles beneath our feet, providing the foundation, or “basement rock,” on which the prairies have been built. Our region lies on what geologists call the North American craton, the stable core of the continent. This is a large fragment of the Earth’s crust that sheared away from an unnamed supercontinent toward the end of the Precambrian Era. By the time this happened—some 600 million or 700 million years ago—the Earth (and the prairie region along with it) had already endured more than three billion years of mountain building, erosion, glaciation, deglaciation, and general geological Sturm und Drang. But things must have been starting to settle down, because when the supercontinent tore itself apart, it produced a North American continent-in-the-making that has persisted until the present.

       GEOLOGICAL TIMESCALE