Letters from Grand Canyon
In the Beginning: the Rocks

The Rocks of the Grand Canyon area form a great stone book upon which the birth and development of the Colorado River have been written. The story of these rocks defines four magnificent volumes, each covering a specific time and circumstance, which we can read in the rocks today. These circumstances and their consequences have had a major influence on the landscape we see before us; a landscape the River has carved through ceaseless toil. In this Letter we examine these volumes in some detail to reconstruct the great book upon which the River continues to inscribe its story. The volumes describe the building of Grand Canyon's rock layers—long before the Canyon itself was carved.
Volume One: Rocks of the Inner Gorge—Early to Middle Proterozoic Rocks and the Period Before Life
This volume deals with the somber and ominous rocks of the Grand Canyon's Inner Gorge, a place that somehow always conjures in me visions of Dante's Inferno: Lasciate ogni speranza voi che ‘ntrate… “Abandon hope all ye who enter…” In reality, these rocks are not restricted to the Dantesque Inner Gorge but are present throughout the region at depth, forming a basement to the overlying younger rocks.
The time covered by this volume ranges from about 1,400 million years ago (my) to about 1,750 my (1.4 to 1.7 billion years ago). One recently obtained age of 1,840 my is the oldest reported so far. These numbers are refined as more radiometric ages are obtained, but the basic time interval is well established and is part of what geologists designate the Early and Middle Proterozoic. The rocks of the Inner Gorge belong to two families: plutonic—igneous rocks such as granite that were intruded into surrounding rocks while in the molten state, then solidified in place; and metamorphic— preexisting sedimentary, volcanic, or plutonic rocks that have been changed, or metamorphosed, into foliated or banded schist or gneiss by heat, pressure, and deformation.
During the period represented by this volume, the North American continent was busy growing southward and westward by incorporating new rocks at its leading edge. This edge now forms the rocks of the Inner Gorge, which are thus testimony to the most protean expression of Manifest Destiny, or Westward Expansion.
The rocks of the Inner Gorge contain minerals indicating that the temperature and pressure at which they formed were high. Thus, these rocks must have formed under conditions where such temperatures and pressures are found. The traditional view has been that these rocks, after being formed near the continent's edge as shale, sandstone, conglomerate, and lava flows, were deformed into a great mountain chain, much like the Alps or Himalayas of today. Such chains are formed when two rigid continental plates collide and crumple. Thus, the overlying mountain mass would have produced the pressure and temperature indicated by the minerals.
More recently, scientists have proposed that Inner Gorge rocks were instead formed by the collision of a relatively thin and dense oceanic plate, complete with island arcs, with the thicker but less dense continental plate of North America. Collisions of this type are going on today in the western Pacific Ocean and result in subduction—the sliding of a dense oceanic plate under a less dense continental plate. This mechanism generates the temperatures and pressures recorded in the minerals by dragging material to depth rather than by accruing the weight of a mountain over it. The end result is the expansion of a continent through welding of island arcs to its leading edge. So the rocks of the Inner Gorge give us a glimpse of an ancient underworld, though not quite the one envisioned by Dante.
Volume Two: Rock of the Grand Canyon Supergroup— Middle to Late Proterozoic Rocks and The Earliest Evidence of Life
In time, the island-arc collisions of Volume One came to an end. The region was lifted above sea level, and erosion lasting hundreds of millions of years set in wearing down all high areas into a vast plain upon which the events of Volume Two were then written.
These events differ greatly from those of Volume One because they produced mostly sedimentary rocks rather than the igneous and metamorphic ones characteristic of the previous age. Igneous rocks are represented only by the Cardenas basaltic lavas in the eastern Grand Canyon, and the great sills well exposed at Hance Rapid and near the mouth of Tapeats Creek. But the greatest difference is that the compressive forces characteristic of Volume One, and resulting from plate collisions, were replaced by extensional forces.
The extension produced basins in western North America. The basin containing the Grand Canyon region sank about as fast as sediments filled it resulting in an accumulation of sediments nearly three miles thick which record a near-sea-level environment of deposition. This can only occur if the rate at which sediments accumulate matches that at which the basin floor sinks.
The rocks of Volume Two record two things of great importance to the development of life. First, many of these rocks are red, indicating oxidation of iron. This in turn points to free oxygen in the atmosphere, which is indispensable to most life as we know it. This had not been the case earlier, when any life present was anaerobic. Second, the rocks contain fossilized algae, the earliest forms of life known in the Grand Canyon. Many scientists believe that the near-simultaneous appearance of free oxygen and the evolution of aerobic (oxygen-breathing) life is not a coincidence.
When was Volume Two written? This is a difficult question because the volume contains so few rocks that can be dated. We can reasonably estimate that deposition began sometime between 1,200 and 1,300 my. Consequently, Volume Two probably spans the time between about 1,250 and about 850 my—Middle to Late Proterozoic.
Some time after the rocks of Volume Two stopped being deposited (about 850 my), and before the rocks of Volume Three started to be deposited (about 560 my), the older rocks of Volumes One and Two were tilted along great faults, forming ranges and intervening basins possibly similar to those in the Great Basin of today. In due course, this landscape was also planed down by erosion, forming a remarkably smooth surface of regional extent. Owing to the tilting and erosion, the rocks of Volume Two were only preserved as wedges in what had once been the basins. In what had been the ranges, the rocks were stripped off completely, and the erosional surface was underlain by rocks of Volume One. As a consequence this erosional surface, commonly called “The Great Unconformity,” represents a huge interval of time for which we have no rock record. Different amounts of time are missing in different places: where rocks of Volume Two are preserved, 290 my (850 to 560 my) are missing; where such rocks are not preserved and rocks of Volume One are in direct contact with those of Volume Three, 840 my (1,400 to 560 my) are missing. This is half again as much as the entire time between the beginning of deposition of the Paleozoic rocks of Volume Three and today. In other words, the time missing equals one and a half times the period it has taken life as we know it to evolve from its very beginnings.

Volume Three: The Rock of the Canyon's Walls—Paleozoic Rocks and the Explosion of Life
Throughout Paleozoic time, the area of the present Grand Canyon, and indeed of much of the Colorado River, was a region of low relief close to sea level. This position made the area very sensitive to the many sea-level changes that occurred during this time. When the sea level rose, the area was flooded by shallow seas teeming with life, and the seas deposited sediments. The layered walls of Grand Canyon record these events in minute detail. When sea level dropped, the area emerged above water, no sediments were deposited, and some of the sediments that accrued earlier probably were eroded; consequently, much of Paleozoic time is not represented by rocks in the Grand Canyon. Only later in the time interval were non-marine sediments deposited, partly in low-relief plains created and traversed by rivers, partly as dunes, blowing before the wind. Three characteristics of the rocks of Volume Three have a profound influence on the formation and architecture of the Grand Canyon:
1) On the whole, the rocks are resistant to erosion, which encourages the formation of a canyon (rather a valley's gently sloping sides)
2) Softer layers are interspersed with tougher ones, resulting in the distinctive cliff-and-slope topography (soft layers erode into slopes; tough layers form cliffs)
3) The rocks are essentially horizontal, giving rise to the majestic cliff bends that are an essential aspect of the Canyon.
Perhaps the most striking event recorded in Volume Three is the explosion of life. Whereas in Volume Two life forms were few and simple, in Volume Three they became many and complex, conquering sea and land alike.
Volume Three began with the incursion of the sea from the west around 560 my, and ended with the withdrawal of another sea about 250 my. The events that follow lead us more directly toward our main business—the development of the present landscape and the drainage systems that carved it, which will be discussed later.
Volume Four: Rocks Deposited and Removed from Grand Canyon—Mesozoic Rocks and the Age of the Dinosaurs
Whereas Grand Canyon's Paleozoic rocks (Volume Three) record primarily marine environments and only subordinate terrestrial ones, the opposite is true for the rocks of Volume Four, which are primarily terrestrial and span the time from 250 to about 60 my—the Mesozoic. Also included in this volume is a great deformation whose final episodes went on until the Early Tertiary, about 50 my. The rocks of Volume Four, once present at Grand Canyon, were thousands of feet thick, but have since been stripped away. Fortunately, they are preserved over much of the Colorado Plateau, where they form the colorful and unique country of canyons and mesas for which the Plateau is famous.
The characteristic deposits for this time interval are claystone, mudstone and sandstone deposited in low-relief alluvial plains. Even more notable are the great sheets of cross-bedded eolian (wind deposited) sandstone, testimony to vast dune fields that covered the region time and again, and are now represented in great sandstone walls such as those of Zion National Park. Marine deposits are present here and there indicating that the area remained close to sea level for most of this interval.
Near the beginning of Volume Four, great volcanic events occurred beyond the horizon, but close enough that the produces blew or washed into the Grand Canyon region. Upon weathering into colorful clays, now forming the Painted Desert, these materials made for fertile ground, encouraging the growth of great trees whose petrified trunks still litter the ground in abundance. The habitat proved congenial to a variety of land-dwelling animals, including reptiles of all sizes, notably (in the later chapters of Volume Four) the Terrible Lizards so dear to schoolboys and film directors. It is also during this time that certain reptiles took to the air, opening up possibilities for mischief previously only dreamt of. Insects, always more successful than anyone else, had, of course, achieved this feat long before.
The rocks deposited during this time have great impact on today's regional landscape because they consist characteristically of alternations of hard and soft strata. Upon being exposed to erosion, the soft strata erode rapidly, undermining and distressing the overlying hard strata. The result is a succession of great regional cliffs and intervening benches—Chocolate Cliffs, Vermilion Cliffs, White Cliffs, Gray Cliffs, and Pink Cliffs—which collectively form the Grand Staircase; each cliff corresponding to a hard layer, each slope and bench to a soft one.
Near the end of the Volume Four the sea reappears in force, but this time not from the west as it had previously, but from the east, earning the name, “Interior Seaway,” and marking a major period of deformation that brought to an end 500 million years of tranquil stability. In the final chapters of Volume Four, starting in the Late Jurassic (about 140 my), but mostly during the Cretaceous and spilling into the beginning of the following Tertiary, that is to say, between about 70 and 50 my, the Earth grew restless again owing to pressures originating in yet another collision of plates far to the West. On the Colorado Plateau, this took the form of upwarps and folds—the Kaibab Plateau, Echo Cliffs, Circle Cliffs, Comb Ridge, Rapplee Anticline and other picturesquely named features. Overall, the Plateau took the shape of a great saucer, whose upturned western and southern rims consisted of mountain ranges, long gone but still evidenced in their ruin by sheets of debris eroded and shed onto the Plateau. The beginning phases of this deformation also caused warping that allowed the Interior Seaway to flood the land, resulting in deposition of strata such as the somber Mancos Shale. Eventually, the sea retreated, leaving behind residual lakes in low spots. But these lakes, and their influence on the Colorado River, are the subject of the next Letter.
Dr. Ivo Lucchitta
This is the fourth in a series of “Letters from Grand Canyon” by Ivo Lucchitta that will appear in future issues of the bqr.