Prairie. Candace Savage

      This infant continent was not exactly the landmass that we know today. The entire Western Cordillera was missing, with the result that the west coast of the craton ran south through present-day British Columbia and the Pacific states (much closer to the prairies than it is today). At first, the cratonic landmass lay exposed—a low, eroding plain, as barren as the face of Mars. But, as the geological strife continued, sea levels began to rise and the land was gradually overrun by the ocean. In time, the entire continent (with the periodic exception of a chain of tropical islands that ran diagonally across the plains, from Lake Superior toward Arizona) had disappeared beneath the waves.

      For roughly the next 55 million years (from about 545 million to 490 million years ago), much of the North American craton lay under a shallow sea. Wherever the land remained exposed, it was eroded by water and wind, which ground the gritty Precambrian rocks into rounded grains of quartz sand. This sand was then swept to the coasts and out into the sea, where it settled to the bottom in beds tens to thousands of yards (or meters) thick. Eventually, these lustrous sediments were overlain by layers of fine-grained mud. But whether sandy or silty, this ocean floor was literally crawling with life, particularly three-lobed, many-legged, bottom-feeding arthropods known as trilobites. After an agonizingly slow start with the cyanobacteria, evolution was finally hitting its stride, producing a menagerie of weird and wonderful undersea life. As generation upon generation of these animals lived and died, their remains settled onto the ocean floor, where they were buried under thick layers of sediments. Today these fossil-rich deposits—now compressed into solid sandstone and shale—are buried some 3 miles (5 kilometers) beneath the wheat fields of the northern plains and at lesser depths in other parts of the prairies. But in a few places—like the Judith and Little Rocky mountains and the northern Black Hills—they have been pushed up to the surface, exposing their maritime history to plain view.

      When the Cambrian sea finally withdrew and dry land emerged again, the forces of erosion immediately began to tear away at the newly formed rocks. But soon, geologically speaking—after a break of little more than 20 million years—the water rose and slowly spread over the land. This time, even the transcontinental island chain was bathed in the warm, clear seas. Now primitive snails munched on algae and were themselves preyed upon by giant squidlike nautiloids, with shells up to a couple of yards in length. Hundreds of new species of shelled animals evolved, including crinoids, or “sea lilies” (distantly related to modern sea urchins), and exotic reef-forming corals. There was so much life in these oceans that when they finally withdrew some 440 million years ago, they left behind thick deposits of shell fragments and calcium-rich debris, which eventually solidified into fossil-rich limestones. These Late Ordovician deposits include the elegant Tyndall stone that is quarried in Manitoba and graces so many buildings in the Prairie provinces.

      And so things continued for about the next 100 million years, as shallow oceans advanced across the North American craton, only to withdraw and then flood back in. If the run and roll of the grasslands sometimes make us think of the sea, surely this is a reminder of the landscape’s long marine history. With every advance and retreat of the ocean, the land was burdened with fresh deposits of sand, silt, and crushed shells, which built up, year by year, in nearly horizontal, banded layers. Although some of these contrasting sea floors have since been exposed by erosion (where rivers have cut deeply down through the sediments), for the most part they lie thousands of yards (or meters) beneath the grasslands.

      The oceans that left these deposits behind were hospitable to life—shallow, warm, well lit, and typically tropical. During the Silurian and Devonian periods in particular (between about 440 million and 355 million years ago), these waters provided ideal conditions for reef-forming sponges and corals. In what would one day become the Canadian Prairie provinces, the reef builders of the Devonian had a heyday, constructing barrier reefs and ringlike walls that rose to heights of 300 feet (90 meters). Wherever the sea was constricted by these limestone palisades, the water gradually became super-salty. If the circulation of the sea was inhibited, water lost to evaporation could not readily be replaced, and the concentration of salts steadily increased. In time, the salts precipitated out of the sea water in these areas, leaving thick beds of potash and other salts, notably under present-day Saskatchewan. The potash deposits in Texas were formed by a similar process but some millions of years later, during the Permian Period.

      The last truly continentwide inundation withdrew from the North American craton about 300 million years ago. The next time the sea attempted to overrun the land—as it would continue to do for millions of years to come—it found itself lapping around the shores of a rocky upland that had

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