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A Grand Enigma

Robert Webb

Grand Canyon: Solving the Earth's Grandest Puzzle. James Lawrence Powell. Pi Press, 2005. $27.95.

The Grand Canyon in Arizona is not the world's deepest canyon, nor is it the widest, but it is an icon of erosion—of the ability of rivers to shape landscapes and transport sediments from mountains to the sea. This gash carved by the Colorado River across the southern Colorado Plateau invites the controversy swirling about it, which is generated on the one hand by geologists eager to unravel the record of erosional processes so prominently on display, and on the other hand by creationists wanting to exploit the canyon to further their explanations of the Earth's origins. James Lawrence Powell's Grand Canyon, although not flawless, belongs on the shelf of any reader serious about understanding not just scientific explanations of the canyon's origin but the entire Colorado Plateau and its geological history.

The book is reasonably well written and provides an unusually lucid history of geological thinking, particularly with respect to river geomorphology and the origin of landforms. The account is marred by repetition, however, and the treatment of John Wesley Powell's explorations and ideas is almost as circuitous as the path of the Green River in Labyrinth Canyon. If the author's intention was to revisit Major Powell and his legacy within the context of the canyon's origins, the book is an abject failure. That John Wesley Powell overcame his humble academic background to make large contributions to geological understanding of rivers and landscapes is not news, nor is his political acumen or his ability to judge scientific talent. Given that exemplary biographies of Powell already exist—Wallace Stegner's Beyond the Hundredth Meridian (1954) and Donald Worster's River Running West (2000)—the first third of this story could have been told in less space.

Grand Canyon shines in its discussion of Powell's protégés: Grove Karl Gilbert and Clarence Edward Dutton, and those who followed in the 20th century. The perspectives of Gilbert (that these are landforms inexorably ground by natural machinery) and Dutton (geomorphology as poetry) are eloquently presented in this book. The progress of scientific thought is illustrated by its successes and failures; even stalwarts such as Gilbert got it wrong on occasion, as when he concluded that Meteor Crater in northern Arizona had been created by steam volcanism rather than meteorite impact.

The 1964 McKee Report...Click to Enlarge Image

The evolution of thought from the first conference on Grand Canyon geology in 1964 to the second one in 2000 is well explained. The 1964 report, spearheaded by legendary Grand Canyon geologist Edwin McKee, concluded that a river cut upstream through the landscape, working its way northward from the Gulf of California and then eroding through the Grand Wash Cliffs, where it linked up with the streams of the Hualapai drainage system, setting the location that the Grand Canyon would later occupy. Eventually, the headward erosion breached the Kaibab Plateau and captured the ancestral upper Colorado River, an action known as stream piracy. Nowadays this is one of the less likely of several possible explanations as to the origins of the canyon, all of which focus on the impediment that the Kaibab Plateau posed and the means by which the river surmounted it.

Most participants in the 2000 conference supported a relatively young age (4 to 5 million years) for the Grand Canyon on the basis of both upstream and downstream evidence. Although many diverse ideas remain about how the canyon formed, two stand out. One holds that, long before the Grand Canyon existed, the ancestral Colorado River breached the Kaibab Plateau, turned 90 degrees and flowed north to northwest. Headward erosion from the Grand Wash Cliffs ultimately intercepted the river, which then downcut to its present depth. If this is what happened, there should be Colorado River gravels northwest of the canyon, but none have yet been found. So this explanation lacks some critical evidentiary support.

An intriguing alternative theory holds that the Kaibab Plateau served as a natural dam, forming a large lake to the east covering much of the present-day Navajo Indian Reservation. Breaching of that dam, either through overtopping or by dissolving the Paleozoic limestones to form pipelike channels, developed a large enough sustained flow to carve the canyon from upstream. Again the critical evidence supporting this idea is missing, perhaps having been washed downstream over the past 4 to 5 million years.

Geological reasoning and technological innovation have combined to hone questions about the canyon's origin, even as answers have become more elusive. This book showcases the development of the geosciences by focusing on one massive landform and the history of its interpretation; seldom has this history been presented in such a readable and entertaining form.

His choice of subtitle notwithstanding, Powell has written a book that shows that the Grand Canyon, with its tantalizing testimony concerning Earth's history, remains an unsolved puzzle, despite a wealth of specific knowledge about its origins. Interpreting the reasons for missing rock is a perilous endeavor, and it may be that so much evidence of the geological history of the Colorado River is gone that a consensus answer as to the origin of the Grand Canyon will never emerge. Powell's eloquent retrospective on the development of geological thought reminds us of one fundamental truth: Scientists disprove hypotheses rather than prove them. The present-day theories reviewed in this book may well be discredited in the future.

The belief of some geologists that there is a final answer to the question of how the Grand Canyon came to exist contrasts sharply with the attitude of most practitioners that scientific understanding is always subject to revision as new evidence accumulates. As the author notes, those who wrote the 1964 McKee report succumbed to the perceived need for a conclusion about the canyon's origin instead of embracing the uncertainty. This book abundantly shows that the elegant scientific knowledge developed by those attempting to extract the secrets of the canyon's origin is itself the prize. As the saying goes, sometimes the journey is better than the destination.

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