The Grand Question

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Science  16 Sep 2005:
Vol. 309, Issue 5742, pp. 1818-1819
DOI: 10.1126/science.1116363

Grand Canyon

Solving Earth's Grandest Puzzle

by James Lawrence Powell

Pi Press, New York, 2005. 317 pp. $27.95, C$38.95. ISBN 0-13-147989-X.

For eight miles from the Milk Spring we continue to cross hills and valleys, then follow a low swale shaded by giant pines with trunks three to four feet in thickness. The banks are a parterre of flowers. On yonder hillside, beneath one of these kingly trees, is a spot which seems to glow with an unwonted wealth of floral beauty. It is scarcely a hundred yards distant; let us pluck a bouquet from it. We ride up the slope.

The earth suddenly sinks at our feet to illimitable depths. In an instant, in the twinkling of an eye, the awful scene is before us.

—Clarence E. Dutton

Tertiary History of the Grand Canyon (1)

No adults forget their first experience of walking from the relatively flat, forested plateaus of the Coconino or the Kaibab to the Grand Canyon's rim. Suddenly, they are confronted by a vast space of quiet wind and successions of gray, white, and red cliffs and benches that drop thousands of feet beneath them. Somewhere at the bottom of the canyon, an inner gorge holds a river they cannot see. The Grand Canyon is Earth's greatest celebration of geology. Whether tourist or earth scientist, at some point, we ask, “How did this happen?”

Depending on what guide book we read or what ranger talk we listen to, we might learn that the Colorado River is perhaps 30 million years old, having established its course through the Intermountain West prior to regional uplift. And that it then cut its canyons by maintaining its grade as the surrounding landscape rose, by processes known as antecedence (1, 2) or superposition (3). We might learn that complicated changes in stream course took place in the western Grand Canyon about 18 million years ago, but that the river's course was well established by that time further upstream (4). We might learn that the Colorado River once flowed up the modern canyon of the Little Colorado River and perhaps toward the Gulf of Mexico (5). We might learn that the Grand Canyon is less than 5 million years old (6).

“The awful scene is before us.”

Part (“Looking East”) of William Henry Holmes's meticulously detailed Panorama from Point Sublime, from the elephant folio-size Atlas that accompanied Dutton's Tertiary History.

In Grand Canyon: Solving Earth's Grandest Puzzle, James Lawrence Powell attempts to guide us through more than a century of “geologizing,” eventually leading us to the modern interpretation of the Colorado River's age and how the river formed its Grand Canyon. In doing so, Powell (a geologist now directing the National Physical Science Consortium) sketches for the lay reader episodes in the evolution of the science of geology, its methods, and its paradigms.

It is difficult to tell the story of the geological investigation of the Grand Canyon without also telling stories about the geologists themselves. The author does so extensively in describing the work of John Wesley Powell and his associates. This early part of the book is, unfortunately, less comprehensive and less elegant than other accounts focused solely on the topic (7-9). Some of the writing is trite, “the Mississippi can just keep rollin' along”; oversimplified, such as remarks about the three men who left the first Powell expedition; or inexact, “Kanab Canyon…reveals itself…as a nearly vertical, mile-high, but relatively thin, canyon.” Kanab Canyon is narrow, but it is not “thin.” Nonetheless, the introductory chapters with their descriptions of the late-19th-century expeditions to the Grand Canyon region offer readers with varied backgrounds a common footing.

James Powell describes the thought and work of the early geologists, who made profound deductions from simple observations of rock strata and topography. He also provides a feeling for the intellectual context of their times. While Darwin was developing his explanation of evolution, geologists were debunking the notion that a Great Flood formed Earth's topography a few thousand years ago.

The author moves on to describe the observations of later generations of geologists, including the observations and reasoning that completely revised the early explanations of how the Grand Canyon formed. To do so, researchers have had to look away from the Canyon itself to the oldest deposits of the Colorado River downstream from the Grand Canyon and the evidence of the most recent time when the Colorado River did not exist at key localities. They have had to examine igneous rocks, weigh the significance of radiometric dates, and consider the timing and spatial implications of plate tectonics. The evidence is scattered and incompletely preserved, and geologists today are still unclear about details of the Colorado River's development. But these gaps are relatively minor—a few million years of missing evidence here or there, the absence of unambiguous evidence of the fate of the Colorado River prior to the establishment of the river's course in westernmost Grand Canyon. Powell reviews the evidence for concluding that different parts of the Colorado River system are of different ages, that the Grand Canyon is about 5 million years old, and that its establishment is ultimately tied to the opening of the Gulf of California and the movement of Earth's tectonic plates. He also offers readers a taste of modern speculation and the uncertainties surrounding these generally accepted notions.

Grand Canyon will be enjoyed by anyone who is curious about how geologists think, piece together disparate information, and assemble explanations. Until a time machine is invented, we will never know for sure how the Grand Canyon formed. Nonetheless, we do know that the rocks forming the canyon walls are of immense age and that the cliffs and slopes exposing those rocks are features of the last instants of their history.

In telling the Canyon's story, Powell provides an honest and open description of geological detective work and the rethinking of ideas. At a time when the National Park Service sells a book describing a creationist explanation of the Grand Canyon's formation little different than the ideas from which modern geology emerged more than 150 years ago, the book reminds us of the timeless contrast between the methods of modern natural science and the power of myth.


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