Grand Canyon: Solving Earth's Grandest Puzzle
In the year 1540, a squadron of men from Coronado’s expedition, seeking the fabled Seven Cities of Gold, came instead upon the rim of a great canyon. Though their Indian guides said the stream at the bottom was half a league (about a mile) across, the Conquistadors estimated it at six feet. Three of the lightest and most agile men attempted to descend to the river, but after clambering down for most of a day, they got only about one-third of the way and gave up. Rock spires that from the rim had appeared no taller than a man turned out to be higher than the Great Tower of Seville. In 1604, the next expedition to arrive, led by Juan de Oñate, Governor of New Mexico, found a muddy, silt-laden river and named it el Rîo Colorado, "the Red River." The stream turned out to be the Little Colorado, not the main stem, the first of many confusions and disagreements over the name of the river. Most of two centuries would elapse before another Spanish explorer, the intrepid Father Garcés, passed by on his way to the Hopi villages. Offering neither gold nor souls, the country of the Big Cañon was too vast and barren for the Spaniards to comprehend. They saw no reason to remain or return.
The first American to see the cañon, Army Lieutenant Joseph Christmas Ives, agreed. He declared it "altogether valueless," adding that "Ours has been the first and will undoubtedly be the last, party of whites to visit the locality. It seems intended by nature that the Colorado River along the greater portion of its lonely and majestic way, shall be forever unvisited and undisturbed." Not until John Strong Newberry, the first geologist to arrive, did anyone begin to appreciate the Big Cañon. He wrote, "The Colorado plateau is to the geologist a paradise. Nowhere on the earth’s surface, so far as we know, are the secrets of its structure so fully revealed as here." The next geologist on the scene, John Wesley Powell, concurred, saying, "The grand cañon of the Colorado will give the best geological section on the continent." The third, the dry, Euclidian Grove Karl Gilbert, said: "The Plateau province offers valuable matter in an advantageous manner"—for him, praise indeed. The scenery and geology inspired Clarence Dutton, the one Grand Canyon geologist who also qualified as a poet, to write, "It would be difficult to find anywhere else in the world a spot yielding so much subject matter for the contemplation of the geologist; certainly there is none situated in the midst of such dramatic and inspiring surroundings."
The four geologists were among the greatest of the nineteenth century, or, for that matter, any century. Collectively, they redefined the science of geology and gave it a distinctly American cast, while on the other side of the Atlantic, Charles Darwin was redefining biology and giving it a rather British complexion. The Colorado Plateau was a fount of scientific insight for these American scientists in the way the Galapagos Islands proved to be Darwin’s land of inspiration. The plateau provided raw, variegated rock unobscured by vegetation or glacial drift; layer cake bedding with few faults and folds; incised canyons to provide the essential, but usually scarce, third dimension. The pioneer geologists had good reason to believe that they would soon decipher the geologic history of the Plateau and its rivers and learn lessons that would apply everywhere. How astounded they would have been to find that more than a century-and-a-half after Newberry’s arrival, scientists still debated the history of the Colorado River and the origin of the Grand Canyon. Indeed, a few months into the twenty-first century, seventy-seven geologists spent a week on the rim of the Grand Canyon arguing their favorite subject all day and into the night. By the time the symposium ended, not only had they failed to reach consensus, but more theories than ever were on the table. Surprisingly, what had seemed to the pioneers to be an easy geologic puzzle to solve proved just the opposite.
Though the rock exposures of the Colorado Plateau are nakedly displayed and appear simple to understand, in fact the geologic history of the Colorado River and its canyons turns out to be deceptively complicated, vastly more so than the history of that ideal American river: the Mississippi. So fond are we of our longest and widest river that it has earned a set of respectful nicknames: Father of Waters, Old Man River, and Big River. In commerce, the Mississippi is our most important river; in music, history, and literature, our most celebrated. The Mississippi is the epitome of a river—the end to which, it is natural to think, all rivers head.
On its long journey, the Mississippi travels more or less directly south from the Twin Cities to salt water, there to deposit its sediment load and construct its elaborate birds-foot delta. The Mississippi not only is navigable its entire length, it is comprehensible. Already a sizable stream when it leaves the Minnesota lake country, the Mississippi grows steadily larger as other streams, some great in their own right, pay tribute. But the river never changes in any fundamental way. Why should it? It has no mountain ranges to avoid, no canyons into which to plunge, no quirks of geology to accommodate. The Mississippi can just keep rollin’ along, leaving those seeking white-water adventure, or a river that can tell them more about how a continent and its rivers evolve, to look elsewhere. In flood it is dangerous and has cost many lives and billions of dollars in property damage, yet even then the Father of Waters remains like the streams with which we are familiar, only bigger, and, at those times, much more frightening.
Visitors to the Colorado River and the Grand Canyon recognize at once that they have come upon a "valley" and a river fundamentally different from the Mississippi and the other familiar streams of the eastern United States. The first glimpse of the giant chasm is such a shock that, years later, most of us can recapture the emotion it inspired. For some, we feel as we did at momentous times in our lives: the birth of a child, the death of a parent, the assassination of a president, or the end of a war. Ever after, we remember the instant when, traveling north through a beautiful pine forest, we arrived at Grand Canyon National Park and approached the rim. At first we could not see the Grand Canyon itself, for unlike a mountain range, or even the Mississippi River valley, the canyon remains almost entirely hidden until one is right on top of it. Then suddenly, without warning, the land fell away and there, where it had been all along, spread before us an unimaginably wide and deep chasm. Nothing we had ever seen—no other river, and no photograph or film of the Grand Canyon—truly prepared us for the sight. Years and decades later, we can bring back the sense of disbelief that we felt and remember how quickly it passed into awe. Those of us who are not expert photographers soon understood that our puny efforts and modest equipment could not do the scene justice. After a few perfunctory shots, we put away our cameras and simply gazed.
Inevitably, a few decide to hike down the Bright Angel or Kaibab trails to get a closer look. Like Coronado’s men, most are in for a rude surprise. Just as the apparent simplicity of Grand Canyon geology tempted the pioneer geologists, so the easy downhill trip tempts a hiker to go farther and farther—why not at least to the edge of the Tonto Plateau? It is on beginning the return hike that the immensity of the Grand Canyon comes home. Still, by resting at strategic stops, determined not to require rescue by mule or helicopter, the hiker eventually regains the plateau, exhausted, proud, and with a new appreciation of the magnitude of the Grand Canyon.
After recovering from the shock of seeing the canyon for the first time, questions occur. Why is the Grand Canyon so different from other river valleys, even other western canyons? What caused it? Why is it located here and nowhere else? In a flash of inspiration, untold numbers of visitors have realized that the answer has become obvious to them! Some rare, terrible force ripped the earth’s surface apart and provided a channel for the tiny stream a mile below. First came the chasm, then the river. But those who espouse this theory ought not forget the saying, "For every difficult question, there is an answer that is clear and simple and wrong."
The geologists of two hundred years ago, decades before any had seen the Grand Canyon, endorsed the simple answer. They believed that all valleys had been pre-created for the rivers that flow in them. These devout men accepted the Biblical account of earth history and thought that the turbulent waters of Noah’s flood had excavated valleys, into which the water then naturally flowed. As late as the 1890s, even Clarence King, first Director of the U.S. Geological Survey, believed that valleys came before the rivers that lie in them. Today some claim that not only did the canyon come before the river, the entire history of the Grand Canyon—indeed, the entire history of the earth—compresses into only the last 10,000 years. But two centuries of progress have taught that, because rivers have the deep time of geology at their disposal, they can and do carve their own valleys.
The history of any valley or any canyon, no matter how grand, is inseparable from the history of the river that occupies it. The uppermost headwaters of the Colorado River lie in the Wind River Mountains of Wyoming, where its longest tributary, the Green River, rises. From there to the delta in the Gulf of California, the Green and Colorado together run for 1,750 miles. The Grand Canyon, impressive as are its depth and width, measures only about one-sixth of that total. The section of the river in the Grand Canyon is thus but one part of the whole, albeit the most important part. To understand the canyon, we have to first understand the river. The river is the parent, the canyon the child.
By the 1860s, Indians, explorers, mountain men, and Army surveyors had traveled each of the major rivers in the United States, save one: the Colorado. Trappers knew the upper Green, the site of their annual rendezvous in the 1830s, perhaps better than they remembered their homes back east. Spaniards, Major Ives, and others had explored the Colorado River from its mouth in the Gulf of California past Fort Yuma and on up to the vicinity of present-day Las Vegas (see Figure 15, page 216 for the geography of the lower Colorado River). But in between the map was blank. At the border between Wyoming and Utah, the river disappeared through an ominous rock portal; a thousand miles downstream, it debouched from a great canyon. What lay in between no one knew, for in that long stretch explorers had reached the Colorado River at only a handful of places. Even as late as the mid-1860s, not just the river, but the entire Colorado Plateau, as large as several eastern states combined, remained unexplored.
Those who did set out to explore the Colorado Plateau had no maps and almost no word of mouth. If any had gone before, they had not lived to tell the tale, much less bring back a map. Among the many inventions of the late twentieth century, none would have been of more use to the pioneer geologists and explorers than the map of the Colorado River and the Colorado Plateau, Figure 1. Whether the map would have reassured or terrified the pioneer explorers, it would certainly have revealed a river completely different from the Mississippi and its eastern kin.
Had the earliest plateau geologists had such a map, they could have traced the Green and Colorado Rivers all the way to the Gulf of California. Figure 1 and Figure 2 on page 15 track the river’s journey starting in the alpine lakes of the Wind River Range, from whence the Green River descends across the high plains of Wyoming to the eponymous town on today’s Interstate 80. From there it continues southward to the Utah-Wyoming border, where lies the Uinta Range, with peaks reaching 13,000 feet. In the nineteenth century, at this point the river entered a fabulous, vermilion-walled canyon, which Powell named the Flaming Gorge. Today that name is attached to the man-made lake that occupies the valley. Nearly alone among the mountains of the Western Hemisphere, the Uintas trend east-west, placing the range directly athwart the path of the southwarding river. The Green heads straight for the mountain range; then, as if losing its nerve, it swerves ninety degrees to the east and runs for fifty miles parallel to the foot of the mountains through an elongated natural park. By traveling only a few miles farther east, the Green could have avoided the Uintas altogether, but instead it suddenly swings back south and, through the menacing Gate of Lodore, enters a canyon incised into the heart of the mountains.
After twenty miles of white water, the Green exits Lodore and comes out into a valley only to enter another canyon, this one cut right through the center of the aptly named Split Mountain. From there the river passes through Desolation Canyon, then Gray Canyon, cut into the Book and Roan Cliffs, respectively. Downstream, the (formerly named) Grand River enters from the east, and the direction of the (now) Colorado River swings a bit to the west. Cataract Canyon comes next, and after that, Glen Canyon, now submerged under Lake Powell. As the river crosses the Arizona line, below the Glen Canyon Dam, it begins to swing back south again, passing on its right the mouth of the Paria River and the plateau of the same name. It flows straight through Marble Canyon, heading for the Kaibab Plateau, the highest obstacle in its path after the Uinta Range. Figure 8, page 188, shows this section at a larger scale.
Although from the map it appears that the river could have bent less steeply by following a route south through the valley of the present-day Little Colorado River, that route is uphill. Instead, the river cuts across the nose of the Kaibab Plateau and then swings northwest. Thirty miles later, the Colorado makes another sharp bend back to the southwest, placing it on a collision course with the next high plateau, the Shivwits. But the river shifts direction and runs straight beside the Shivwits Plateau for thirty miles, then bends around it and resumes its northwesterly course. At the Grand Wash Cliffs, which we will meet repeatedly (see Figure 8, page 188 and Figure 11, page 199), the Colorado leaves the Grand Canyon and flows out into the Basin and Range province and modern Lake Mead. Here it turns ninety degrees south and now, finally, like the Mississippi, proceeds for several hundred miles with little deviation, traveling most of the way in a single large valley, until at last it reaches the Gulf of California (see Figure 15, page 216). There it has built a delta and today, if thirsty irrigators and desert cities had not impounded and extracted all its water, would continue to do so.
To a geologist, nothing beats a map. When maps of sea floor topography became available in the late 1940s, for the first time scientists could see the enormous faults; the gargantuan, world-encircling, undersea mountain ranges; and the abyssal trenches that scar the seafloor. By the 1960s, to deny that something terrible had happened in the ocean basins, something on the scale of the continents themselves, had become impossible. Geologists could no longer dismiss as absurd the idea that continents had drifted: the seafloor appeared to retain the very tracks of their ponderous passage. In the same way, the modern map that we have been following reveals that the Colorado River has had anything but the simple history of the Mississippi. The Colorado crosses three distinct geologic terrains: the Colorado Plateau, Canyon Country, and the Basin and Range Province. It cuts deep canyons in some mountain ranges and goes around, or nearly around, others; changes direction unpredictably; meanders here and runs straight there; and generally behaves without apparent rhyme or reason.
The beauty of the Grand Canyon and the unparalleled exposure of its rocks have tempted generations of geologists. The unexpected complexity of the Canyon’s geologic history confounded even the best of them, though gradually and collectively they have been able to put boundaries around the possible explanations. The pioneers had been sure that by studying the Colorado River and the Grand Canyon, they would learn lessons that eastern rivers could never teach them, lessons that would apply everywhere. They were right, but the actual lessons turned out to be quite different from those they expected. As we will see, those lessons have even deeper implications for earth history. One hundred and forty years of studying Grand Canyon geology have paid off, but in ways that have surprised even the best geologists.
The myth of the "scientific method" makes it seem that science is so logical—indeed, inevitable—that serendipity has no place. But in reality, it’s just the opposite. And in geology, at least, some of the most important discoveries have come about serendipitously. And the most important of all were, strange to say, counterintuitive. Take one recent advance: when we observe the quiet night sky, we see the planets move in their predictable orbits and the moon pass through its familiar phases; save for the rare flash of a shooting star, nothing much seems to happen. Until the space age, we had no reason to suspect that our solar system was born in colossal, random violence and that since then, the impact of asteroids and comets has been its most fundamental process. Indeed, only in the last two decades have we discovered that Homo sapiens might exist only because, through a roll of cosmic dice, one errant comet or asteroid, out of the thousands that fly through space, happened to strike the Yucatan peninsula sixty-five million years ago and exterminate the dinosaurs and seventy percent of all living species.
In the same way, a rafter floating down the Colorado has no reason to suspect the power with which geologic time endows rivers. Who would guess that they engage in a vicious competition for territory that causes the more energetic streams to capture the waters of the less energetic? That only the fittest survive to carve deep canyons? The pioneer geologists of the Grand Canyon, outstanding as they were for their day, could not have imagined just how far the lessons of the Big Cañon would extend. They had no way to know that, by studying the imprisoned rivers, vast erosion, and dramatic uplift of the Colorado Plateau, they would uncover the underpinnings of a scientific revolution a century ahead. But such is the way of science.
Like a raft trip on the Colorado’s white water, following the efforts of five generations of geologists to understand the river and its Grand Canyon stimulates our thinking, but, we soon recognize, requires more concentration and effort than does a riverboat cruise on the mighty Mississippi. The protagonist of this story had canoed the Father of Waters and other eastern streams for hundreds of miles. Yet those streams failed to teach him the lessons that the canyons of the Colorado brought home almost from the moment he launched onto their waters.
It is obvious that if the geologists of the nineteenth century had correctly understood the origin and history of the Grand Canyon, someone would have written a book like this one more than a century ago. The only remaining reason for geologists to hold a conference on the canyon in the year 2000 would have been for self-congratulation. Instead, the meeting was necessary because even today, geologists continue to try to answer what has turned out to be one of the most complicated questions they ever asked: what caused the Grand Canyon? Inevitably, in tracing the work of the many geologists who have attempted to answer the question, we are going to have to follow trails that at first look promising, but which in the end peter out. As the geologists have had to do, we will then back up and follow another trail. The obligation of the author is to mark the journey clearly and to play fair by not trumping up theories beyond what they deserve.
In only a few hundred years, science has made astounding progress. Geology and paleontology, two of its oldest disciplines, teamed at the start of the scientific revolution in the mid-1600s when a conflicted anatomist discovered a shark’s tooth inside a rock. Paleontology later provided the evidence for Darwin’s theory of evolution; geology eventually established the deep time that he required. As we will see, the trail from the Grand Canyon reaches all the way to the revolutionary theory of plate tectonics. And today, planetary scientists even search for evidence of plate tectonics on other bodies in the solar system. The pattern of observation, theory, experiment, and discovery continue as the magic carpet of science flies on.