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ICE ON THE ROCKS
The Mississippi River was old long before the first giant sloth faced down a dire wolf or the last short-faced bear stood up to her full thirteen feet and bared her teeth to an eight-foot-long beaver. It was old before the first woman to see it got her feet muddy. The river is older than the entire fabulous menagerie of strange and outsized mammals that roamed the watershed during the two-and-a-half-million-year Pleistocene epoch, which ended about twelve thousand years ago with the most recent retreat of the glaciers. It was that most recent ice, however, that sculpted the northern features of the Mississippi River watershed into their current forms.
All across the top of the continent, the ice dammed up the northward progress of prehistoric rivers and sent them south, into the Mississippi watershed. The melting ice sheets didn’t drain into the Gulf of Mexico in any kind of measured or consistent pattern but rather in fits and starts, and floods of diluvian scope. For several thousand years, when the ice had retreated into Canada, but not far enough to allow the northern rivers to flow into Hudson Bay and the Arctic Ocean, a gigantic lake covered northern Minnesota, western South Dakota, and most of central Canada. This prehistoric Lake Agassiz, named for the nineteenth-century Swiss-born geographer who pioneered the radical idea of prehistoric ice ages, was larger than all of the Great Lakes combined, larger than the Caspian Sea. When at last it broke through the moraine of glacial rubble that was its southern boundary and drained for a time through Minnesota and Wisconsin, it carved the outsized gorge between those states through which the upper Mississippi River now flows. The Minnesota, Illinois, Ohio, and Wisconsin Rivers all flow through valleys that are far broader than the present water levels could have carved.
None of this is to say that the last ice age created either the Mississippi River or the land across which it meanders. The northern boundary of the watershed was shaped by glacial ice, which measures its workday in tens of thousands of years. The eastern and western walls, however, were caused by the drift of continents, which operates over hundreds of millions of years. Beneath all the lists of clay, most of the basin rests on top of some of the oldest rocks on earth. This shield of granite and gneiss known as the North American Craton, or Laurentia, has been drifting around smashing into, and breaking away from, other ancient pieces of the earth’s crust for more than two billion years. During that span Laurentia has been a component of more than a half-dozen supercontinents, including the all encompassing Pangea, which formed out of a series of titanic collisions that began five hundred million years ago.
When continents collide, oceans slowly disappear and mountains creep upward; the formation of Pangea eventually threw up a mountain range along one side of Laurentia that was higher than the Himalayas are today. Remnants of that Central Pangean Range still exist, in the Anti-Atlas Mountains of Morocco and the Scottish Highlands. Much of what is left of the old Pangean Range, however, is now the Appalachian Mountains of North America—in other words, the eastern boundary of the Mississippi River basin.
Some rocks, when they are formed, align their internal magnetism with the earth’s poles, allowing paleomagnetologists to say with a surprising degree of confidence that those vertiginous peaks of the Central Pangean Range half a billion years ago ran roughly east-west, rather than north-south as the Appalachians do today. Rain nonetheless fell on those same slopes that would become Kentucky and Tennessee. Rain fell in showers and torrents and began the long work of tearing down the range and carrying it to the sea. Mountain brooks seem so ephemeral when approached on foot in a dry summer, or when buried under the ice and snow of winter, at least when compared with grand continental currents such as the Mississippi, the Missouri, the Ohio, and the Arkansas. But they are not.
Bounded as they are by metamorphic walls high above the rise and fall of the seas—what geologists call basement rocks—those tiny seasonal alpine rills are more permanent in their ways than the mightiest lowland rivers. The latest sediment of the day can be found on the soles of your shoes by the banks of Old Man River, but geologists know to look for the truly ancient up in the hills. The oldest river in America, and possibly the oldest in the world, is the ironically named New River. It flows off the western slopes of the Appalachians in North Carolina through a corner of Virginia and West Virginia, where it merges with the Gauley to form the Kanawha, which joins the Ohio at Point Pleasant, West Virginia, and then the Mississippi at Cairo. The New River is older than the Atlantic Ocean, older than the dinosaurs.
Roughly two hundred million years ago the supercontinent of Pangea began to stretch and rift apart, tearing its central mountain range into pieces. The first break occurred between New Jersey and Morocco, slowly opening what became the Atlantic Ocean between Trenton and Marrakech; between, if you will, the Appalachian Mountains of the explorer Daniel Boone and the Anti-Atlas Mountains of the explorer Abu Abdullah Muhammad Ibn Abdullah Al Lawati Al Tanji Ibn Battuta; between “Dueling Banjos” and The Sheltering Sky. Laurentia drifted away from the rest of Pangea at a rate of several centimeters a year, which suggests that for many thousands of years what would become the Atlantic Ocean was strictly a tidal creek. Meanwhile, rain that fell on those proto-Appalachian ranges gathered into rivulets and brooks, and into streams and rivers, from which thirsty triceratops quenched their dry throats.
The water flowing off the Central Pangean Range did not, however, gather itself into a single proto-Mississippi. With no Rocky Mountains to corral them at the other side of the continent, various streams and rivers wound their way independently across a vast, flat, and periodically marshy land populated by the familiar cast of hulking dinosaurs and skulking mammals of the late Triassic and early Cretaceous eras. Along the way, Europe and Asia broke off from New England and drifted toward their current positions, and an upwelling of magma known as the Bermuda hotspot split the Ouachita Mountains off from the main branch of the Appalachians and sent them toward their current location in Arkansas. The new ocean widened, as the Atlantic is still widening today. What would become the Pacific shrank, as it still is shrinking today.
Finally, some hundred million years ago the ocean floor to the west of Laurentia began to slide under the crust of the neighboring plate, pushing up ranges along the leading edge of the drifting continent including, most importantly, the Rocky Mountains. The familiar pieces of North America were coming together: an older, decaying range of mountains running up its eastern coast and a younger, sharper, taller spine of ranges rising in the west.
Instead of a river between them, there was a sea. The lowlands between the new Rocky Mountains and the old Appalachians initially buckled downward, the way a piece of cardboard might pop down in the middle if pressure is exerted along two sides. Into these lowlands salt water flowed from both the Arctic Ocean and the Gulf of Mexico, creating a vast, shallow sea that, at its largest, stretched the entire distance between the two great ranges. For forty million years this “Western Interior Seaway” was a warm and fertile place, full of whale-size toothy mosasaurs, along with sharks and rays, finny fish, horseshoe crabs, and clams. Rockhounds today find shark teeth a thousand miles from the nearest salt water in the watershed.
Ultimately, however, the colossal uplift that forged Pikes Peak and the Grand Tetons, and lesser ranges as far east as the Black Hills of South Dakota, raised the land between the Rockies and the Appalachians. The sea retreated, until by sixty-five million years ago the mouth of what can now rightly be called the Mississippi was around Memphis. North America was just beginning to look recognizably like itself, with mountains to the left and mountains to the right and a big winding river coming down the middle when a massive meteor struck the Yucatán Peninsula, just across the Gulf of Mexico. This set in motion the great extinction of the dinosaurs and 75 percent of the other species around the globe, and gave the skulking mammals their opportunity to evolve: into mammoths, sloths, and, eventually, archaeologists and anthropologists.
Copyright © 2013 by Paul Schneider