LANDSCAPES, NEW AND OLD, IN THE GRAND CANYON NATIONAL PARK
EVER since the days when my youthful imagination was fired by pictures of the extinct animals and ancient landscapes referred to in the preceding chapter, I have been interested in these old animals and landscapes. To this interest I added that which I found in modern landscapes when, in the course of my work as a geologist of the United States Geological Survey, I had opportunity to see many of those American landscapes which in all parts of the world are regarded with wonder and admiration.
A Personally Conducted Tour
In the first chapter of this book an invitation was extended to consider with me some of the great natural scenic wonders of America. Our personally conducted tour of modern landscapes and of scenes of long ago, reconstructed in imagination from the rocks, will start at America's greatest scene, the Grand Canyon of Colorado River. Here, as at other places to be visited, we shall first scan the landscape beneath our eyes and then proceed to reconstruct for mental vision the landscapes of bygone ages.
The Grand Canyon is selected as our point of departure because it exhibits the rocks of the earliest geologic periods. Other parks will be visited in such order that the geologic periods will be considered in order from oldest to youngest.
The geologic record consists mainly of sedimentary bedsthat is, beds deposited in water (Plate XIV). If a place could be found where sediments had been deposited continuously from the beginning of deposition to the present day and the beds thus formed were cut vertically so that their edges could be viewed from the side, the cut would show an unbroken geologic column. This column might be subdivided and its parts named and described so as to form a table of geologic time. But over large areas periods of uplift and erosion intervened between periods of deposition. Every such interruption in deposition produces what geologists term an unconformity. At many places the time divisions shown in the complete geologic column may be absent in whole or in part and be represented by lines of unconformity.
Each geologic age had its peculiar and characteristic forms of animal and plant life. As time progressed, the surface of the earth was changed. Through inconceivably long periods highlands were worn down and lowlands were built up, rivers changed their course, swamps were formed where dry land had been, lands were subdivided, and seas changed their places.
As the changes progressed, plants and animals were obliged to change their habits or perish. This necessity of adaptation to new conditions gave to each age its characteristic forms of life. Many of these forms are preserved as fossils, and from them something of the life of each age is known.
The length of the geologic ages can only be estimated. No reliable measure of time in years can be given. Many estimates have been made by men who have given the subject special attention and therefore are best qualified to judge. But the estimates should not be taken too precisely. At best they are only relative approximations to the truth.
The plateaus of central-western America, in which the Grand Canyon is cut, extend from Wyoming southwestward into Arizona and from central Colorado westward into Utah. This region is one of rugged mountains and tablelands that rise from one to two miles above the sea, and the canyons are cut deep into a varied assortment of gorgeously colored rocks. These rocks are carved by rain and stream (Pl. XV), frost and wind, into countless forms, which for variety of charm and beauty of color challenge the attention of the world.
Variety of Scenes
The variety of scenes is still further increased by volcanic activity, both ancient and recent. At some places quantities of molten matter were poured out. This molten matter hardened into resistant rock and protected the softer rocks beneath it, which elsewhere were worn away. Hence in some places the high plateaus are covered with hard volcanic rock. In other places the older rocks were broken and molten rock was forced into the cracks of the fractured mass. In still other places streams of molten rock poured into the canyons and interfered with the normal flow of the streams.
Since 1869, when Major Powell and his intrepid companions made their daring exploration of the Colorado River, travelers, with one accord, have given the Grand Canyon an exalted place among the wonders of the world.
This chasm, a mile deep and ten miles or more wide, presents the reciprocal of the ordinary mountain landscape. It is the mountain upside down, or outside in. In place of a solid mass of rock which terminates upward in a narrow crest, the observer at the rim gazes down into an empty void, which terminates below in a narrow troughthe exact opposite of the crested ridge. The canyon suggests a mold in which mountain ranges might be cast.
In appearance the canyon is strange, memorable, unforgetable. It is ornately decorated and gorgeously adorned. Color is king. It is beyond easy comprehension or description. We struggle in vain for adequate words and expressions. Had the cradle of the race been near Grand Canyon, words and figures of speech might have come into being that would appropriately express the vigor and the expansive magnificence it exhibits. But as it is we may put away words and similes. There is nothing comparable to it. Grand Canyon is unspeakable!
The Grand Canyon, now admired by multitudes, was probably viewed by a white man for the first time when Cardenas gazed into its marvelous depths in the year 1540, probably near what is now called El Tovar. Most sightseers view it from nearly the same placethat is, from a point in northern Arizona where it may be reached by rail from the south. But this most advantageous place affords, it is generally thought, by no means the most glorious view.
The northern rim of the canyon is about 1,000 feet higher than the southern rim, and the most marvelous panoramas are to be seen from such vantage places as Point Sublime. But until recently the approach to the northern rim has been difficult. Although the superb views repay every effort they cost to the few who are fortunate enough to reach the northern rim, most travelers have been content with the view from the south. Here the gently undulating surface of the forest-covered Coconino Plateau, over which the canyon is approached, extends unbroken to the rim, where it gives place to rock walls that reach abruptly down into the great depths below.
This sudden change from the ordinary nearly level surface of the plateau to the precipitous faces of barren rock, descending, cliff after cliff, in seemingly endless succession down into the heart of the earth, is awe-inspiring to some and startling to others. Some are drawn to the rim as iron is drawn to the magnet, there to gaze in silent fascination; others become garrulous, uttering many an exclamation of wonder or amazement. Still others shrink back in timidity lest some invisible power draw them over the rim.
It is little wonder that the imaginative aborigines, who had no knowledge of the processes by which the great gorge was formed and no appreciation of its meaning, peopled the canyon with grotesque and diabolical spirits!
Let not the traveler think to take in Grand Canyon at a glance! Facts and impressions worth the gleaning require time. Long contemplation of the grand ensemble of stupendous mesas and buttes (Plate XVI), tributary gorges and amphitheaters, causes the eye to tire and the mind to flag. The observer can no more sense the Grand Canyon in an hour than he can gain in an hour the information to be obtained in a great museum.
If you have learned to admire the subdued natural scenery of New England, with its mild colors, its restful halftones, and its hazy vistas, lay that admiration reverently aside before you approach the Grand Canyon!
If you have learned to love the forested Adirondacks and their verdure-clothed valleys, or the rounded forms of the Appalachian Mountains, forget them before reaching the Grand Canyon, for such scenes are unknown here.
Even if you have learned to love the great symmetrical masses of granite in the Rocky Mountains or have seen the majestic monarchs of other mountain systems, with snowy crowns and cloudy halos, lay the mental picture resolutely aside before stepping out on the rim of Grand Canyon.
The canyon is different! In place of the rounded, graceful curves of eastern landscapes we see hard, angular, rocky forms, which stand out in striking boldnessforms that may at first seem grotesque and repellent rather than attractive. Here, in place of subdued colors gently blending in a hazy atmosphere, are brilliant, blazing colors, which, although somewhat mellowed by varying atmospheric effects, seem at first to carry with them something of the hardness of the rocks from which they emanate, for verdure is so scarce in the canyon walls as to be negligible and the atmosphere is too clear to give the softer atmospheric effects which the eastern artist strives to reproduce on canvas.
The Grand Canyon is not to be directly compared with other natural scenes. As the observer studies the great panorama from some point of vantage he may gradually learn to realize that what at first seemed harsh has a peculiar beauty and symmetry, and that each separate form in the great architectural ensemble has an attractiveness all its own.
Need of Study
The author who would write with intelligent enthusiasm of Grand Canyon must dwell in its presence long enough to gain some intimacy with it. The observer who fails at first glance to experience a rapturous exaltation may censure for exaggeration the writer who waxes enthusiastic.
It has been truthfully said that objects which disclose their full power and beauty as soon as they are presented to the mind have little of those qualities to disclose. Great innovations, whether in art or literature, in science or in nature, seldom take the world by storm. They must be understood before they can be rightly estimated and must be cultivated before they can be understood. The observer who would appreciate Grand Canyon must take time to understand itand no man has yet exhausted its possibilities.
It is reserved for a favored few to stand on Point Sublime and view the wide landscape spread before them. There are many other almost equally commanding points on the north rim which afford unusually extensive views. These remain chiefly for enjoyment in the future. Vastly more than can be realized in the short time of an ordinary visit may be seen from any one of the many points in the south rim which may be reached easily.
At one of the points within easy walking distance of Grand Canyon Station, such as Hopi Point, we stand on the edge of the Coconino Plateau at an altitude of more than 7,000 feet above sea level and gaze downward over a succession of cliffs so precipitous that they seem to form an almost vertical face of rock terminated 3,500 feet below by a broad shelf preserved n the side of the canyon by a layer of hard rock and known as the Tonto Platform.
Jutting out from these cliffs and rising shelf on shelf above this platform are long, narrow tongues composed of brilliantly colored rock. Below this platform more than a thousand feet of ancient crystalline rock is exposed in the narrow sinuous trench called Granite Gorge, at the bottom of which flows Colorado River.
As seen from the rim the stream appears so minute and so feeble that an observer untrained in physiographic processes may receive with incredulity the statement that this little thread-like stream has carved out the enormous chasm which a whole chain of mountains would scarcely fill. But those who have intimate knowledge of this river know that it is a turbulent stream of great erosive power. One who stands on its brink in time of flood has no difficulty in understanding that the river dug the canyon.
One who is ascending a mountain passes through the same climatic zones that he would traverse in going northward at a low altitude. A traveler ascending one of the high mountains near the equator passes in succession from a tropical region through a temperate zone to an arctic zone at the top and encounters corresponding changes in plant and animal life. The reverse experience is felt by one who descends into Grand Canyon, for the cold-temperate climate at the rim changes to an almost tropical climate at the bottom.
The surface of the Kaibab Plateau, which lies north of the canyon in Arizona, reaches altitudes nearly 8,000 feet above sea level and has a climate comparable perhaps to the northern part of the Great Lakes region. Here at some places winter snow accumulates to a depth of 10 feet. Forests of yellow pine cover the uplands, and Engelmann's spruce covers the slopes. Here and there are groves of tangled scrub oak. Thickets of aspens, whose tall, silvery trunks are surmounted by scant tufts of tremulous leaves, are to be seen in many places. The constant nervous agitation of these leaves gives rise to the name quaking asp, by which the tree is commonly known. So richly is the turf covered with flowering plants in summer that 600 species have been collected by one man in a single season.
The surface of the Coconino Plateau, south of the canyon, is about a thousand feet lower than that of the Kaibab, and, because of its lower altitude it has a milder climate and a different flora. Here the snows of winter rarely last a week, and a few of the summer days are unpleasantly warm. Juniper and pinon, or nut pine, are common. Some of the forest trees are dwarfed, and many of them are richly decorated with mistletoe, whose light green and yellowish shades contrast strongly with the dark green of the pines. In the more open spaces between the groves of forest trees are found such desert vegetation as the bunched sagebrush, "Mormon tea," cactus, and "mescal."
The next climatic zone in descending order is that of the Esplanade, an irregular sloping shelf about a thousand feet below the south rim of the canyon, which is much more conspicuous a few miles farther downstream than it is near Hopi Point. Here the climate is notably milder than that on the plateau above and the flora takes on an aspect more nearly like that of a desert. There is a commingling of the plants of the temperate zone above and the more nearly tropical zone below.
The Tonto Platform
On descending to the Tonto Platform, about 3,500 feet below the south rim, a climate is found so mild that a fall of snow is almost unknown and the plants are chiefly those of an arid desert, such as "cats-claw," "red bud," and other small trees, and many species of cactus. The plants exhibit the spiny exterior, the aroma, the small leaf surface, the water-storing devices, and the somber hues of a desert flora.
The Inner Gorge
But it is in the inner gorge that a tropical climate is found. Here winter weather is rare and the summer sun makes the bottom of the canyon a veritable inferno. The winds at times are like blasts from a furnace. One of the most vivid recollections of my stay at Grand Canyon is a trip one winter's day on Bright Angel Trail. From the river, where we were uncomfortably warm, we passed upward through days of ideal weather until we reached a point a few hundred feet below the top. There we encountered snow, and finally we reached the rim in a driving storm, which had spread several inches of snow over the plateau during our absence.
How the Canyon Was Made
One of the first questions that arises in the mind of the observer is, How was Grand Canyon formed? Many an answer has been givensome plausible, some curiously ingenious, some picturesque, and some consisting only of poetic fancies that have no basis in fact.
Major Powell gives the Indians' conception of its origin as follows:
Long ago there was a great and wise chief, who mourned the death of his wife and would not be comforted until Ta-vwoats, one of the Indian gods, came to him and told him she was in a happier land, and offered to take him there, that he might see for himself, if upon his return he would cease to mourn. The great chief promised. Then Ta-vwoats made a trail through the mountains that intervene between that beautiful land, the balmy region in the great west, and this, the desert home of the poor Nu'-ma.
This trail was the canyon gorge of the Colorado. Through it Ta-vwoats led him; and when they had returned the deity exacted from the chief a promise that he would tell no one of the joys of that land, lest, through discontent with the circumstances of this world, they should desire to go to heaven. Then he rolled a river into the gorge, a mad, raging stream, that should engulf any who might attempt to enter it.
Little less fantastic, and lacking the saving poetic qualities of the aborigines' account, is the explanation of the Canyon as a great crack on the earth's crust formed by an earthquake.
Competent judges have but one explanation. The chasm is a product of erosion. Colorado River and its tributaries, aided by rain, frost, wind, and chemical action, have been at work age after age, cutting away the rock and carrying the waste material to or toward the sea.
As the process of erosion, or the wasting of the land, will here be referred to many times, a brief picture may be drawn to show how its results are accomplished. This is especially appropriate here because the canyon is the most conspicuous product of erosion to be found in all the world and because the river is cutting away the rock at the present time probably as fast as it ever did.
Great results may be accomplished by forces so insignificant that they escape the notice of unobservant persons. Some who have not given the subject special thought may read with incredulity the statement that great mountains are carved out largely by rain water. But those who understand the processes by which a landscape is shaped have long recognized the fact that rain is one of the principal means of erosion. Each raindrop picks up a tiny load of mud or sand, and the myriad drops collect to form rivulets, which join a river that carries great loads of mud to or toward the sea.
The late John Murray has shown that each year water equivalent in volume to many cubic miles falls on the earth as rain and that it carries into the sea about 2,735,000,000 tons of solid matter.
But probably work equal to that performed by water falling as rain is done by means of other agents. Water has been called a universal solvent. In spite of frequent references to "pure water" there is no such thing on the face of the earth. Water always carries mineral matter in solution. If it contains certain kinds in sufficient quantity it may be called a mineral water, but in reality the water of every spring, every well, and every stream holds mineral matter in solution.
The raindrop on its way from the cloud to the earth absorbs oxygen, carbon dioxide, and other gases, and on penetrating the soil takes up vegetable acids and various other ingredients. It carries these into the minute crevices of the rock, where chemical changes take place. The water is not so much the cause of the chemical action as it is the carrier of the chemical agents. Some of the newly formed compounds are easily soluble and are quickly taken into solution by the water; others are less soluble. Thus, through the agency of rain water, parts of the rocks are dissolved out and other parts are left, these parts being held together so loosely that the rock finally crumbles. The consideration of chemical solution and its results might be expanded into volumes, for it embraces many important processes, such as the production of soil, the formation of minerals, and the deposition and concentration of metalliferous ores.
The decay of rocks seems more conspicuous on lowlands and on gentle slopes than on highlands or on steep slopes such as those in Grand Canyon, because the removal of waste products goes on less vigorously there than on highlands. Also, where the rocks are bare, as in the steep walls of the canyon, relatively little water finds its way into them. Most of the rain escapes as surface run-off.
On the other hand, in regions where water may freeze at night and the ice melt by day, the water that enters the cracks and pores of the rock becomes an effective agent of destruction. As the water expands on freezing, the rock is fractured. A crack thus formed may be filled with water, which on freezing enlarges the crack. This may occur again and again until the loosened masses are finally pushed to positions where they fall when released by the melting ice. The fall of masses released in this way from exposed ledges is common in high mountains and in steep walls of canyons, where, after a night of frost, the morning sun weakens the hold of the ice on the loosened masses.
The chemical work of solutions in disintegrating the rocks and the mechanical work of frost in breaking them up would have little effect in shaping a landscape if the products of their activity were not carried away. A relatively small amount of rock waste is carried away by wind; most of it is carried away by streams, which receive solutions of mineral matter and the smaller fragments of rock and transport them to the sea. The small particles remain suspended for a time, rendering the water turbid, and they may be carried for great distances down the stream before they find temporary lodgment in places along its course. The larger pieces of broken rock, such as boulders, pebbles, and sand, take shorter journeys. They are moved farther and farther downstream until they are ground to powder. Also in the course of their passage they scour the rocks they pass over.
The streams do actual cutting by rolling pebbles along the bottom and by scouring their beds with the mud and sand they hold in suspension, but the mechanical work done by ordinary streams is subordinate to their work of transportation. In streams where muddy water is whirled swiftly through rock channels, such as Colorado River in its passage through Grand Canyon, the cutting is at a maximum. In streams where the water is clear the abrasive work is negligible.
Ordinarily streams are inspected when their water is clear and they are doing little work. They do their cutting principally during times of flood, when the water is muddy. Particles of mud and grains of sand are carried downstream in endless procession, each particle doing its bit toward cutting away the rock.
Some observers who gaze into the Grand Canyon are content with the enigmatical statement that the chasm was "gnawed out by the tooth of time." Others are interested to know that the teeth which did the gnawing were the grains of sand that are set in the jaws of every flood.
Major Powell's Exploration
Major Powell, who conducted the famous exploration of the Grand Canyon, had implicit faith in the ability of these minute grains to accomplish great results. Many have marveled at the temerity shown by him and his daring associates in entering a canyon whose walls at many places rise sheer from the water, without knowing what falls they were likely to encounter. But Major Powell had observed that the water of the Colorado is always muddy, and he was convinced that great waterfalls are not to be expected in a muddy river. Such falls as may have existed there at one time had long ago been worn down by the myriads of sand grains which had passed over them. It is said that his faith in the efficiency of these minute agents of erosion was such that he staked his life on their ability to destroy falls.
Powell and others who followed him make frequent mention of falls in the canyon of the Colorado, where the river descends rapidly over heaps of boulders. These are rapids, as technically distinguished from falls. There are no cascades or cataracts on the Coloradono place where the river pours with sheer drop over a ledge of rock.
How Mountains are Carried to the Sea
But even in a stream which is perpetually muddy, like Colorado River, the abrasive work is subordinate to the work of transportation. Water descends in the form of rain so quietly that we think little of the work it may accomplish. But the gathering of the rain water into myriads of streamlets which bear the small particles of sand and mud to the brook accomplishes enormous tasks: The water of the stronger current of the brook gathers still larger fragments of loose material and carries them to the river on their way to the sea.
Other parts of the rain water enter the rock and dissolve portions of it, and these in time find their way downstream, together with the mud. Every year the streams of the United States carry to the sea about 270,000,000 tons of dissolved mineral matter and 513,000,000 tons of suspended matter, such as silt and mud.
The rate at which streams work is influenced by many conditions, such as the velocity of flow, the character of the country they drain, and the climate, which determines whether the flow is constant, as in streams of a moist country, or intermittent, as in streams of a desert. On the average the surface of North America is lowered by erosion at the rate of one foot in about 9,000 years. If this erosive action of the streams of the United States could have been concentrated on the Isthmus of Panama it would have dug in about 73 days the canal which required 10 years' work with the most powerful appliances yet devised by man.
Although the energy of the river has been used chiefly in cutting downward into the rocks and in carving the plateau into a vast labyrinth of gorges, in any one of which the Niagara gorge would seem lost, there will probably come a time when the energy of the river will be expended chiefly in widening rather than in deepening the canyonwhen the vast panorama of buttressed mesas and ornately decorated pinnacles will be swept away. It cannot cut downward below the level of its mouth, and a time may come, as it has come to many an older stream, when it can no longer cut downward. But it can and will continue to cut away its banks, undermine the canyon walls, and carry the débris to the sea. If this process is not interrupted the whole plateau will be swept away at some date in the inconceivably long time to come.
Will the process continue? Who can tell? During geologic ages long past the leveling process in this same region was carried to an extreme at least four times, and each time a plain of unknown extent was formed near sea level.
On the other hand, movement of a large part of the earth's crust or a variety of other causes may change the program, or even reverse the river's activity and cause it to deposit sediment and build up its bed, rather than to deepen its course and broaden it. At some places along its lower course the Colorado is filling its channel, which was once deeper than it is now, and during two periods of its history it filled its valley with sand and gravel to depths of hundreds of feet.
Power of Running Water
The fact that the river carries great loads of gravel and boulders was indelibly impressed on my mind several years ago, while I was engaged in work in northern Arizona. In addition to the numerous exploratory excursions overland it was my good fortune to take one by boat down the river from the mouth of the Grand Canyon to Yuma.
While I was at the mouth of the canyon, where the river emerges like a huge mill race from between the mile-high walls, the water began to rise and to increase the velocity of its flow. This increase disturbed the boulders that had lodged along the river's course and made them resume their interrupted journey toward the sea. The river here is perhaps 300 feet wide and 25 feet deep and is confined in a trough of solid rock. The gravel and boulders are washed through this trough with terrific force to find temporary lodgment in the more open spaces below the mouth of the canyon.
My attention was attracted by a peculiar muffled rumbling, which I did not at first understand. My thought at once turned to the sudden floods in Arizona with which I was familiar, where the dry sandy bed of a wash may suddenly and without obvious cause become a raging torrent; the cause, a sudden downpour of rain at some distant point hours before, may have been forgotten or may have been beyond the range of vision.
I did not then know the Colorado as well as I know it now, and, fearing a sudden rise in the water, my boatmen hastily secured the boat and we climbed up the banks for safety. I made my way along a shelf of rock into a narrow part of the gorge and there discovered the cause of our alarm. The swift current was rolling boulders through the rock trough.
My shelf of rock was perhaps 10 feet above the water and the boulders in the turbid flood below were crashing and grinding against the rock walls. Reason told me that I was perfectly safethat thousands of floods had rolled boulders through the trough without injury to the shelf on which I stood. But of what use is reason at such a time? Precaution (or perhaps it should be called fear) argued that "some time this shelf will fall, and maybe that time is now!" A score of reasons were marshalled why I should leave that particular shelf forthwith.
The Colorado River within the Grand Canyon averages about 300 feet in width and 30 feet in depth, and has a mean velocity of about 2 miles an hour. The drainage from large parts of five States swells the volume of its flow. And what may be said of the volume of rock waste that its muddy waters have transported to the sea? The volume might be computed but the figures would be incomprehensible.
Unmeasured quantities of mud and sand have been washed down its course from the Rocky Mountains, for it rises in the heart of these mountains in the Rocky Mountain National Park. Unmeasured quantities of mud and sand have found their way from the mountains and plains of Wyoming down its rocky trough. Thicknesses of rock measured by thousands of feet have been removed from the plateaus of Utah, Colorado, New Mexico, and Arizona, and the products have passed in endless procession down the river to the sea.
From thousands of square miles thousands of feet of rock have been removed by the river. What has become of it all? Those who have crossed the built-up plains and the filled valleys of Arizona and southeastern California have seen some of it, for these level plains occupy the site of old valleys filled to the brim and overflowing with gravel, sand, and mud from mountains, plains, and plateaus hundreds of miles away.
Those who have traveled over the Southern Pacific railroad in southern California, where this road crosses the broad delta which the Colorado built out across the Gulf of California so far that the north end of the depression now occupied by Salton Sea was completely cut off from the southern part, have seen some of it. But what of the rest? As well ask what has become of a life that is spent! It has passed down the river, it has done its bit toward accomplishing a great task, and has vanishedsomewhere.
Rocks of Grand Canyon
It has been rightly said that the Grand Canyon offers the most instructive exhibit of geology in the world. This is true because, in cutting the great trench through the plateau, the river has exposed to view in profile rocks more than a mile thick, ranging in age from the oldest (the Archean) to the youngest and representing nearly all of the great periods of geologic time. Expressed in another way, the time represented by the rocks which the eye takes in at a glance is many thousands of times greater than the popularly accepted duration of the life of the human race.
In the walls of the canyon are seen the edges of a succession of sedimentary rocks lying layer upon layer, each of uniform thickness and character, extending as far as the eye can reach and nearly horizontal in position, like sheets of cardboard laid one upon another.
These layers of rock differ in kind and in character. Some are composed of sand and shale, others of limestone. Some are red, a color due to the presence of oxide of iron, or iron rust. Others are green or gray or white. The softer layers break down readily and form slopes, the harder ones resist erosion and stand out as cliffs. The rim is composed of hard limestone about 600 feet thick, which contains fossil sea shells.
Rocks like individuals, may be conveniently referred to by name. This limestone has been named Kaibab and was christened for Kaibab Plateau, which it caps. The next lower layer is a 300-foot gray, conspicuously cross-bedded sandstone, the Coconino, which forms cliffs and appears as a narrow light-colored band near the top of the canyon walls. Next below is a group of rocks consisting of layers of red sandstone separated by layers of soft red shale. It is called the Supai formation, which is 1200 to 1400 feet thick. The soft layers break down readily and form sloping surfaces; the hard layers form benches.
The next thick layer of rocks, in order downward, is the hard Redwall limestone, the most conspicuous cliffmaker in Grand Canyon. Between this limestone and the granite below are the soft, shaly beds of the Tonto group, which have been worn back to form the Tonto platform. The lowest layer of this group is a hard brown pebbly sandstone, which makes the floor of the platform.
The oldest rocks exposed at the bottom of the chasm are the ancient crystallines called Archean. In some places, but not in all, between these oldest rocks and those of the Tonto group there are remnants of a great series of sediments which probably represents as much or even more of the earth's history than all the other rocks above put together. And yet most of this great series of beds was worn away before the sediments of the Tonto group were laid down. The contact of these younger rocks with the older where the intermediate rocks were removed is known as an unconformity.
Ancient Lands and Unconformities
The sediments of the rocks exposed in the walls of the canyon were not laid down continuously. At some places long periods of time elapsed after one layer was finished before the overlying layer was begun. Some of these intervals were so long that mountain systems were formed and later worn away before sediments were again laid down there. For the history of events during this interval we must look in other parts of the earth.
The scenes in the plateau region, the most remarkable of which are found in Grand Canyon, are results of a long series of events, which extend far back into the dim ages before life was begun. Hence it may be said that the older rocks of the canyon have witnessed the development of the earth from a mass that was "without form and void"at least without the external form that it now hasand void of any of the forms of plant life that now beautify it and of the animals that inhabit it.
These older rocks were buried by sand, mud, and limy ooze, much of which was deposited in sea water during the time when the plateau region, now between one and two miles high, was below sea level. At other times the region was above water and the deposits were partly, and in places wholly, cut away by wave and stream. The parts which escaped destruction constitute the record from which the history of the region may be deciphered.
Geology and the Interpretation of Natural Scenery
This is not the place for a detailed account of the many events in the growth and development of the canyon region. It is rather a place to point the way to delve further into the great store of information which here invites the student who is interested in natural objects.
The interpretation of a landscape depends on a knowledge of the forces which produced it. As the rocks exposed in the canyon country range in age from the very youngest to the very oldest known on the face of the earth, an adequate interpretation of the scenery here may involve a knowledge of the whole range of historical geology, as well as a knowledge of physiography, or the science which teaches how landscapes are formed. For in geologic history one is dealing with a long succession of landscapes, a moving picture of the ages, in which the scene changes from time to time. Mountains rise out of the sea; are immediately attacked by the agents of erosion; are sculptured into peak and ridge, gorge and valley; are worn down and leveled; and finally are again covered by the sea. The canyon region was thus elevated, worn down, and resubmerged again and again in the course of its history.
Last Updated: 31-Dec-2009