Grand Canyon – The Great Denundation

IT is matter of common knowledge that the general reader does not care to have his story interrupted by too much information, scientific or otherwise. He looks for entertainment rather than instruction, and at the Canyon is perhaps quite willing to forego geology except in elementary and homceopathic doses. But geology here is more or less compulsory because it is everywhere in evidence, and everywhere important. It is the one spot on earth where certain rock strata may be read as in a book. It is not necessary to apologize for opening the book. The geological story is interesting in itself and is its own excuse for being.

The cutting out of the Canyon is the end of the story, not the beginning. It happened in late times—possibly the Tertiary Period—and is geologically considered a recent occurrence, though no one knows how many scores of centuries ago it first started. The process of cutting is still going on. The River continues to deepen, and in the ages to come the Inner Gorge may be cut down nearly to sea-level or the Plateau Country may subside, and perhaps the Canyon itself may then turn into a fiord where still blue waters will lie under purple rocks and the rushing River will have cut back far into Utah or Wyoming. But that is a possible sequel to the story, at which we have not yet arrived.

We are of necessity greatly impressed with this latter-day cutting of the Canyon because of its colossal scale. It seems an erosion of proportions such as the world has never experienced elsewhere, and yet it should be stated at once that as compared with what preceded it the great chasm is a mere scratch in the shell—a minor affair. Before ever the Canyon was started this Plateau Country was swept by a denudation of vast extent. Over an area of about fourteen thousand square miles the whole surface was planed off and the beds of five geological periods disappeared from the top. At the Rim where you stand, under your feet, are layers of Carboniferous rocks, and upon these rocks were once upbuilded strata upon strata of sedimental rocks belonging to the Permian, the Triassic, the Jurassic, the Cretaceous, the Eocene. Ten thou-sand feet of them were once over your head. Many centuries ago they were cut out and swept away in what has been called “the Great Denudation.” That sounds like a statement put forth to make people catch their breath, but it is susceptible of proof, as we shall presently see.

The rock strata seem to be laid down about the globe very much as the layers or various skins encompass an onion, though, of course, with no such regularity or uniformity. In places certain strata are missing, were perhaps never laid down. How deep down the distance before the strata end is a matter of some speculation, but at present geology contents itself with something like twelve periods, each made up of many layers or beds The normal and sequential appearance of these beds is often greatly disturbed or broken by accidents of upheaval and subsidence, flood and fusion. It is so here. The onion has had a number of layers gouged or washed out of it. Of twelve geological periods at one time existent, only six are now to be seen at the Canyon, and two of these appear only in remnants and fragments. There is plain evidence on every hand of a great disturbance—a great denudation.

When we look across the Canyon from El Tovar and see strata of the Kaibab and Coconino running along the North Rim corresponding to the strata on our side of the River, we cannot doubt that all the strata once extended across the Canyon and were somehow broken through, cut out and carried away by the River. The likeness holds good for a longer view—for a view of a hundred miles to the north. For up in Utah there still exists a higher Rim—the broken face-walls of a greater Canyon—the strata of which were once spread over the whole Plateau Country. The higher Rim in Utah now appears in the form of huge cliffs with upright walls. At long intervals other cliffs descend from them in a great geological stairway, as Dutton has put it—descend from the highest to the lowest, and stretch out in their descent from the lofty plateaus of Utah to the depressed basins of Central Arizona. We must come down the stairway to the Grand Canyon to realize the successive steps—to realize that each step in turn has been a temporary water-line during successive periods of the Great Denudation. The lines of the cliffs are still there.

The Markagunt Plateau, just over the southern Utah boundary-line, is eleven thousand feet above sea-level. The Rim at El Tovar is nearly seven thousand feet. Some four thousand feet of the lost strata are accounted for in that difference of elevation, and the remainder is explained by the dip of the strata to the northeast which carries the strata down instead of up. The strata at El Tovar, it will be remembered, are Carboniferous, but on the surface of the Markagunt they are Eocene—that is, five geological periods later on, or, as we have stated it, about ten thousand feet of strata higher up.

The top layers of the Markagunt are uniformly bedded, lie flat and regular, and are composed of lake marls and fresh-water deposits. The face-walls, made by the Great Denudation, now appear as abrupt cliffs, and the southern line of them along the bases of the Markagunt and Paunsagunt Plateaus are now known as the Pink Cliffs. They are some eight hundred feet high, are rather square-edged like pilasters, and run on for many miles like an enormous broken colonnade. Their color is a brilliant rose-red, which varies under different lights, and in general gives a highly spectacular appearance to the face-walls. The Eocene ends at the foot of these cliffs and is not met with again in moving south across the Grand Canyon country until we reach New Mexico. The cliffs themselves make the first, the top riser, of the geological stair-way we have imagined.

The platform below the Pink Cliffs and extending out from under them—the lower platform stretching away to the south—is made up of Cretaceous rock. The layers are of yellow sandstone and clay. This is the second step in the stairway, but not so abrupt a one as that of the Eocene. The tread of the step is not very broad directly south of the Pink Cliffs, and when the riser is reached it is not very high, but this Cretaceous platform extends east-ward into the Kaiparowits Plateau and southward across Glen Canyon upon the Painted Desert, where it appears as the high mesas lying back of Echo Cliffs. They may be seen readily enough from Desert View (Navaho Point). The mesa and the fiat-topped butte seem more characteristic of the Cretaceous than the long file of uniform cliffs, though the latter do appear in striking form and color in the Paria Valley.

The third step down is from the Cretaceous to the Jurassic. The latter is made up of red shales that lie upon a massive thousand-foot bed of white sandstone. The sandstone shows in simple bold cliffs cut through into detached buttes in places. The cliffs appear without taluses, as though rising abruptly from an under platform. This simplicity is, of course, subject to some variation, and fantastic traceries occasionally appear in the Jurassic. The most notable appearance of this riser is in the White Cliffs of the Virgin. South of the Markagunt and Paunsagunt Plateaus the exposures of the Jurassic are very grand. It extends eastward under the Kaiparowits Plateau, crosses Glen Canyon, and appears over on the Painted Desert beyond Echo Cliffs and beneath the Cretaceous. But nothing of this nor of the Cretaceous or Eocene appears in or around the Grand Canyon.

The Jurassic and the Triassic are somewhat con-fused in their exposures, but it may be generally accepted that the Triassic is the fourth step down. The great stairway of terraces leading to the Can-yon has no more splendid riser than that of the Triassic as shown in the celebrated Vermilion Cliffs between the Kanab and the Virgin. The color of the shales and sandstones is peculiarly brilliant in the Valley of the Virgin. Near Pipe Spring these cliffs of the Triassic have a height of between fifteen hundred and two thousand feet. They appear as the main escarpment or face of Echo Cliffs over on the Painted Desert, where they often show with great color-splendor when struck by the long shafts of . the setting sun. But again no trace of the Triassic shows in or around the Grand Canyon. Echo Cliffs is the nearest outlier.

After the Triassic the next platform to which we step down is the Permian. This is the series originally laid down upon the Carboniferous—the top series at El Tovar. The Permian is a distinct ter-race, but not such a typical step as the other formations. In fact, the platform which shows the Permian also reveals much of the Triassic. The Permian beds are thin and of impure limestone, with deep colors in belts of purple, violet, lavender, dark red, Indian red. They reach down from the Triassic upon the Kanab and Kaibab Plateaus directly north of the Grand Canyon. They appear again at the foot of Echo Cliffs on the Painted Desert. Remnants or remains of them are still seen about the Grand Canyon as survivors of the series—the hard cores that have resisted the Great Denudation, and now appear in butte form, standing isolated upon a Carboniferous foundation. Cedar Mountain, a few miles from Desert View, on the Painted Desert, is an example. Red Butte, which one sees from the railway coming up to the Canyon, is another, and Mounts Trumbull and Logan, over to the north-west, are a third. The Great Denudation washed them into butte form, but could not entirely destroy them.

All this Plateau Country was once under the sea —was laid down in horizontal beds as a sea-floor. In the fulness of time it rose, or was pushed up by lateral pressure, and for many centuries was probably a shallow inland sea. As it gradually rose above water it became a low alluvial plain, and its drainage system was then established by rivers that possibly still exist as the Colorado, the Virgin, the Paria. The rivers continued to hold their courses notwithstanding certain inequalities and deformations that afterward came into existence. They were perhaps older than the deformations and continued to run their ways in spite of the deformations, against the strata, against geological faults, against dips. They do so yet.

The plateau continued to rise and at the same time to lose from its upper surface by erosion. The so-called Great Denudation extended south from the Utah plateaus to the southern deserts of Arizona. The geological steps do not stop with the Carboniferous at the Canyon Rim but continue to descend at the south until, on the lower deserts, almost at sea-level, we meet with the most ancient forms of the Archaean. The denudation took place over a vast area and probably carried on through many thousands of years.

And so it came about that the cutting of the Grand Canyon itself was an after-happening. For all its vast proportions the Canyon was merely a drainage ditch in the bottom of the huge basin originally hollowed out by flowing waters. It has since undergone some minor changes, but evidently the Great Denudation was never approached in far-reaching results. That was the climacteric happening—the supreme event.

As for the remaining geological strata or steps in the stairway, we do not need to follow them across Arizona to the sea. By trailing down into the Can-yon we shall find the earliest of all, the old Archaean, in the Inner Gorge where the River runs (Plate 5), and on the way down we shall meet with the inter-mediate strata in the walls.