Grand Canyon: Solving Earth's Grandest Puzzle

Natural History, Sept, 2005 by Laurence A. Marschall

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

There are many passages in his classic, Tertiary History of the Grand Canon District, published in 1882, in which the geologist Clarence Edward Dutton abandoned the customary reserve of the scientist to describe what lay before him:

Reaching the extreme verge the packs are cast off, and sitting up on the edge we contemplate the most sublime and awe-inspiring spectacle in the world.

Geologically, however, the Grand Canyon is also one of the most perplexing spectacles in the world. Dutton knew that, in principle, a record of its past lies exposed in the layers of rocks that so marvelously pattern its walls. But just how to read that record has been a matter of continuous debate ever since John Wesley Powell led the first expedition through the deep gorges of the Colorado River in the summer of 1869.

Visitors nowadays get a general picture of the accepted scientific explanation from guidebooks and park rangers. The canyon's upper sedimentary layers, they will tell you, were laid down in a series of lakes or shallow seas that covered the region for hundreds of millions of years. They will also tell you that, surprisingly, the canyon itself is much younger than its rocks, dating back only five or six million years. Once a meandering river, the Colorado carved a canyon when the entire region that is now northern Arizona, Colorado, and Utah was uplifted by tectonic forces to form the so-called Colorado plateau. As the land rose, the rushing Colorado cut down through it, just as a rotary saw blade cuts a narrow kerf into a log that is being lifted up into a sawmill.

As the river cut through the first 3,000 feet of rock, the canyon walls eroded and slumped into the rushing waters. Each collapse exposed a new wall, leading to cycles of erosion and collapse that opened a wide expanse of spectacular vistas, some measuring fifteen miles from rim to rim. But when the river encountered resistant igneous rocks, it cut more slowly and laboriously, leaving an inner canyon so narrow that, from many of the rim overlooks, the river below cannot be seen at all.

James Lawrence Powell (no relation to John Wesley), a former director of the Natural History Museum of Los Angeles County, ably recounts how explorers and geologists slowly came to understand that sequence of events. Many of the puzzles they initially faced arose from the nascent state of geology in the 1800s. Mechanisms of erosion were so dimly understood that some geologists found it hard to believe a river could cut through a mile of rock in any reasonable length of time. And what is more, the global plate motions that led to volcanism and uplift in the West were not even dreamed of when the Canyon was first discovered. Thus it seemed possible at first that the Colorado had cut down through a preexisting plateau, rather than slicing through the land as it rose.

A broad consensus about the origins of the Grand Canyon emerged as geology became more sophisticated, but like so much of science, the devil remains in the details. When it comes to the exact origin of this or that stretch of the gorge, for instance, substantial disagreement persists to this day. Modern geologists worry about conflicting evidence that the age of the canyon varies along its course, older in the upstream parts than it is downstream. Is it possible that, long ago, the Colorado River flowed through what is now the northeastern part of the canyon, but exited via the southeast and into the Gulf of Mexico? Or did it flow out the northwest, through a region now called the Kaibab Plateau and off into Utah and points west? Or is it possible that parts of the Colorado flowed underground for a while?

No book can adequately convey the grandeur of the canyon, but Powell's well crafted account makes one appreciate just how it came to be so grand.

LAURENCE A. MARSCHALL is the W.K.T. Sahm professor of physics at Gettysburg College in Pennsylvania. He is the 2005 winner of the Education prize of the American Astronomical Society.