The Geology of Grand Canyon
A selection from "Ancient Landscapes of the Grand Canyon Region, The geology of Grand Canyon, Zion, Bryce, Petrified Forest, and Painted Desert"
By Edwin D. McKee, Park
Naturalist, Grand Canyon National Park
PROBABLY no place in the world of similar area has recorded a more complete or a more interesting resume' of the earth's history than has the high plateau country of northern Arizona and southern Utah. Although many great events and some long intervals of time are not represented by the formations of this region, yet of the five major chapters or eras into which all of time has been divided by geologists, at least some parts of each have left their traces in this area.
Whether on the brink of the mighty Grand Canyon, among beautiful fogs of the Petrified Forest, or beneath the lofty walls of Zion-the "Rainbow of the Desert"-one looks upon rocks which are not alone curious or colorful, but which are also records of the past inscribed and illustrated in an intensely interesting manner, in one place is seen the sand of ancient dunes, in another the border of an early sea, or perhaps the floodplain of mighty rivers, and in all of these remain the unmistakable evidences of life-plants and animals preserved to make a reality of the living, moving past. Everywhere are found the evidences of those great processes of Nature-erosion of the high country, land formation in the low country, and mighty crustal movements slowly raising or lowering the land in both.
From the rim of Grand Canyon one looks not only down a few hundred feet, a half mile, or even a mile in distance, but one looks back in history through vast periods, measurable not in centuries but in millions and even hundreds of millions of years. There In the bottom of that mighty chasm are found rocks formed during the first and oldest era-rocks in which the original structure has been entirely modified by great heat and pressure and in which no traces of life are known to occur. There in the Grand Canyon are also seen two other great series of rocks, those of the second era which are partially altered and which contain earliest traces of life, and those of the succeeding era in which are preserved primitive animals of many types. Rocks of the fourth great era-the age of dinosaurs-lend color to the Painted Desert, and to the sheer walls of Zion Canyon. Beautiful little Bryce Canyon to the north boasts of some of the most recently formed rocks in the region-those of the fifth and fast era, the age of mammals. The great volcanic mountains and the marvelous features of erosion, such as the canyons and the desert cliffs, are also developments of this most recent chapter. In brief, the Grand Canyon region affords some wonderfully interesting glimpses of ancient landscapes during many different parts of the earth's history, and these make the past a moving, living thing.
East/West Cross-Section Of The Grand Canyon Region
THE EARTH'S OLDEST ROCKS
(THE ARCHEAN ERA)
LOOKING into the depths of Grand Canyon from any point within the Bright Angel section, one is Immediately impressed by the narrow V-shaped gorge cut in the black rocks at the bottom. This is popularly termed the Granite or Inner Gorge. Within its walls one Is in another world, both scenically and geologically. Their steep, bare sides, whose surfaces are chaotic in the extreme, have a history-long and complex. The rocks of which they are formed-some of the oldest known today on the surface of the earth-partially tell the story of the first great era in geologic history.
Other rocks of this, the Archean age, are found in New England, in the Adirondacks of New York, and to a very great extent in eastern Canada. In the last named place they contain valuable deposits of. iron, nickel, cobalt, and copper. Rocks which probably also correspond in age occur in Scandinavia, Brazil, China, India. and Central Africa.
At the Grand Canyon, although we are impressed by the depth of the dark Archean rocks, beneath the plateau surface approximately a mile, yet we marvel even more when we contemplate their great age and the important series of events whose history they partially record. Built up originally as great horizontal deposits of sand and mud, they were bent by mighty crustal movements until high mountains, probably comparable to the present Alps, were formed. Pressures from the northwest and southeast apparently foided them. The rocks themselves were greatly compressed and heated, with the result that compile recrystalllzation and the development of a banded structure were brought about. The present vertical attitude of these ancient beds, together with their dense crystalline character, is evidence of the great depth at which they were formed and of the extreme pressures of the overlying masses. In brief, the rocks that we see today in the Canyon bottom represent merely the roots of once lofty mountains, and the flat surface .cut on these rocks is an old plain that resulted from the wearing down of high country in this region.
As yet no definite traces of either plant or animal life have been found in rocks of the Archean age. Though various forms of life may have existed then, and may have been preserved in the original rocks, their record has since been entirely removed by those extreme pressures which altered even the composition and structure of the rocks themselves.
Within the black, crystalline rocks of the Inner Gorge may be seen many arge streaks, bands or irregular masses of a lighter color. From the Canyon rim these appear white, but from nearby they are usually pink. These light colored rocks are granites with a coarse crystalline texture.
Granites derive their name from their granular texture. They are formed by the slow cooling of molten masses that have been forced into older rocks from the earth's Interior. From a similar source are formed lavas and volcanic ash, but these flow out or are ejected on the surface of the earth where they cool so rapidly that no crystals form. Exceptionally fast cooling or chilling of molten masses, moreover, forms volcanic glass or obsidian, and it is by the application of this same principal that crystal forming is prevented in the manufacture of common glass.
The unusually large crystals of the granite which occurs in the cracks and fissures of the inner Gorge at Grand Canyon indicate the tremendous depth at which It was formed and are further evidence of the great mountains that existed in this region during the first era in geologic history.
THE ALGONKIAN ERA
ROCKS formed during the second great era of the earth's history are distinctive throughout- the world in several respects. They are not highly altered or completeiy changed in form and structure as are those of the oldest era, but are largely free from such changes and, for the most part, similar to rocks which are seen in the process of formation today. Furthermore, they are known to contain definite traces of plant life, though no certain forms of animal life have yet been found in them. They represent a period probably as great as all of subsequent time.
Along Bright Angel Canyon and in several other places in the Grand Canyon, rocks of Algonkian age, representing accumulations of sediments several thousand feet in thickness, are found. Below and to the north of Desert View (southeast of Cape Royal on the North Rim) they form the open floor of the Canyon. Everywhere the most conspicuous stratum of this series is a mud rock of brilliant vermilion color, however the rocks also include a conglomerate or pebble layer, a dark limestone formed principally by plants, and a purple quartzite made by the consolidation of the grains of a sandstone.
FORMATION OF MOUNTAINS
(THE ALGONKIAN ERA)
The Algonkian rocks of the Grand Canyon region were bent and broken into mountains at an early date. In many places sloping layers showing the steep angle at which they were tilted are easily visible, even from the Canyon rim. Folded areas and strata which have been shattered are also conspicuous features here and there. The mountains which they formed, however, are now missing for they were worn away in large measure by slow erosion. Today only remnants-small hills on a general level surface-remain in the lower parts of Grand Canyon to tell the story.
(THE ALGONKIAN ERA)
The rocks of Algonkian age are roughly estimated to be at least six or seven hundred million years old, yet from all Indications they were formed under conditions of climate not unlike those of far later periods of history. In several parts of the world traces of great ice sheets-glaciers which scrarched and eroded the surface -are found preserved in Algonkian rocks. In other places, including the Grand Canyon, ancient flows of lava are found where they gushed out upon the surface of an old land mass. Among the rocks below Desert View (Navajo) Point and bordering on the Colorado River may readily be seen several black cliffs formed by the volcanic activity of this early age.
The brilliant red shales of Algonkian age found in the lower parts of the Grand Canyon were formed as muds, accumulated probably by large rivers- In these muds are found preserved great quantities of ripple marks, indications of changing currents, also the moulds of salt crytals, and large shrinkage cracks resulting from a very hot sun. In brief, these criteria point toward a hot and probably arid climate in this region during that chapter of history.
OLDEST KNOWN LIFE
(THE ALGONKIAN ERA)
The first appearance of life on the earth so far as now known was in the Second, or Algonkian, Era. In rocks of this age, numerous layers of limestone are found, and many of these layers represent great reefs built up by the action of primitive one-celled plants l;nown as aigas. These are the oldest definite traces of life. Similar plants are found living today. Near Harper's Ferry, West Virginia, they are building up limestone structures almost identical to the fossil ones found in the Algonkian rocks of Grand Canyon. In this connection, It is interesting to note that because of this similarity of the present to the past, the reaii+y of the ancient plant structures was recognized a few years ago. They were discovered at a place in the Grand Canyon Just west of the mouth of Bright Angel Creek.
IT WAS during the third or middle chapter in the earth's history that all of the apparently horizontal, upper layers In the Grand Canyon walls were formed. As will be seen in the succeeding pages, some of these rocks are sandstones formed from the sands of early beaches or sand dune areas, others are shales-the hardened muds of ancient river deltas-and still others are limestones built up by accumulallons of plant and animal remains on sea bottoms. All are rocks formed by the deposition of sediments by wind and water during vast intervals of time. In them have been hidden and preserved many forms of life. Seashells, footprints, fern impressions, and various other traces of eariy plants and animals remain to tell the story of these ancient times. It is of special interest to note that in rocks formed during the earliest part of this chapter are found the first definite traces of animal life, that in other rocks of this chapter have been found the earliest evidences of fish, and that in the most recent rocks of this group occur the traces of early reptiles, insects, ferns, and cone-bearing plants. In the walls of Grand Canyon examples of all of these fossils have been found, and these will be described in detail in the succeeding pages.
FIRST ANIMAL LIFE-THE TONTO ROCKS
Great highlands which were formed in the Grand Canyon region during the Second Era of history were afterwards gradually worn away by erosion until near the start of the next era a flat, almost featureless plain existed. Here and there, however, isolated hills of dark, crystalline rocks of the First Era stood above the general surface, as seen opposite Yaki point. In other places, such as to the west of where Bright Angel Creek now flows, small mountains of red Algonkian rocks (Second Era) remained. Around and against these, sediments were then deposited. Pebbles and sands accumulated, forming a thick layer which today appears as the brown sandstone rim of the Inner Gorge. These represent the first deposits of the Third Era. But the sea was encroaching upon the land during this period, and gradually the sands of the beach were covered by the muds of shallow water, and these in turn by limes of a deeper sea. Today this series of sands, muds and limes is found represented in the rocks of the Tonto Platform in Grand Canyon.
Along the Tonto Trail a few hundred yards east of Indian Gardens numerous primitive sea animals have been found buried and preserved in layers of thin shale. Many of these are creatures with rounded shells smaller than the nail of a person's little finger, others are animals related to the snail, and still others are crab-like creatures known as trilobites. The triiobites undoubtedly were the rulers of that age for they excelled not only In numbers but in size. Some specimens from Grand Canyon have measured over three inches in length. Despite this size, however, the trilobites and their associates from the Tonto Platform represent some of the earliest known forms of animal life.
TRILOBITE FROM TONTO PLATFORM. GRAND CANYON
THE MISSING PERIODS OF THE THIRD ERA
(ORDOVICIAN AND SILURIAN PERIODS)
The geologist has found that two long periods of history are lacking In the great succession of ages represented by the strata in the Grand Canyon walls. These missing periods which belong to the Third Era are known as t-he Ordovician, the time when armoured fish were dominant in the seas, and the Silurian, the time when lung fish developed and scorplans became our first air breathers. These ages immediately followed the Cambrian and involved millions of years. The absence of the first of them is explained by some geologists as the result of its rocks having been completely worn away at a later time. It seems more probable, however, that the Grand Canyon region was above sea level during these two ages so that no sediments were accumulated and consequently no rocks formed.
THE AGE OF FISH
A FRESH-WATER FISH OF DEVONIAN AGE
(FROM RECONSTRUCTION BY PROF. PATTEN)
During that period of geologic time commonly known as the "age of fish," sands and limes were accumulated on the surface of the Grand Canyon region filling in old river channels and burying the bodies of fish and other animals. The deposits formed at this time were later eroded to a large extent. The surface of the land was worn and washed away, until finally only isolated patches or pockets of the lime and sandstone remained. These we find today exposed in the wails of the Grand Canyon occuring just at the base of the great Redwall cliff in about- fifteen different localities.
Although fish were rulers of the age during Devonian times, they were of primitive types and apparently depended for defense upon bony skin armour rather than upon speed. The plates and scales of fresh-water fish have been found preserved in the lavender rocks of this age in the Grand Canyon.
SEA LIFE FROM THE NORTH-THE REDWALL LIMESTONE
(MISSISSIPPIAN OR LOWER CARBONIFEROUS PERIOD)
One of the most prominent and conspicuous features of the Grand Canyon is the greal red cliff of limestone about midway in its walls. This cliff is the highest in the Canyon -averaging about 550 feet in the area of Bright Angel Canyon. In most places it is almost vertical, and in some it even overhangs to such an extent that a visitor once aptly said, "The Washington Monument might be placed beneath it and kept out of the rain."
To the prospecior this formation is known as the Blue Lime; to the geologist it is the Redwoll Limestone. Both are correct. Actually the rock is a rather pure limestone of a grey or bluish color, but in most places where seen, its surface has been stained a bright red by iron oxides from above. It appears throughout the Grand Canyon as a wide band or ribbon of red.
Large amphitheaters, many curving alcoves, caves, and solution funnels are all characterisic features of the great Redwall. It is composed of relatively pure lime so rain and other waters have s chemical action upon it-they leach and dissolve it. Waters all tend to drain toward curving centers, and so increase this curving. Everywhere the rounding off of corners takes place. Thus has the graceful form of the Red-wall been brought about.
The origin of the Redwall Limestone is as Interesting as its structure. The purity of the lime indicates that it was formed in a relatively wide and quiet sea. Its composition represents a vast accumulation of the skeletons of ancient plants and animals. Sea-shells are found in great numbers, some of them very beautifully preserved. These and other forms of ocean life clearly indicate that a great sea connection then existed between this reqion and that of western Canada to the north.
TRACKS IN THE SUPAI SANDSTONE
During that period in geological history known as the Permian, when some of the beds of soft coal in eastern America were being formed, a large area in northern Arizona was receiving red sediments from the east, probably carried by rivers from the granitic highlands of that region. Today these sediments appear in the Grand Canyon walls as alternating layers of red sandstone and shale immediately above the great Redwall. They are almost a thousand feet in thickness.
When the red beds were accumulating in this region, the climate probably was more or less arid: the vegetation consisted principally of ferns and related plants: and the animal life included a group of large but primitive four-footed creatures. Numerous tracks of the latter, preserved in the walls of Grand Canyon, have provided one of the most interesting discoveries of recent years. Some of these footprints are several inches in length, and the number of toes varies between three and five. They show no close relationship to the tracks of other localities, and apparently represent a fauna new to North America.
LANDSCAPES OF THE HERMIT SHALE
Concerning the conditions under which the topmost red stratum of the Grand Canyon (the Hermit Shale) was formed, and the means of its formation, we have today a rather definite and interesting picture. A wealth of fossil plants and a number of tracks of animals have been found beautifully preserved in its muddy layers, and by means of these and other indications the following conclusions have been drown.
The Hermit Shale represents accumulations of mud and fine sandy material deposited probably by streams flowing from the northeast. Here and there are found evidences of pools and arroyos with wavy ripple marks on their borders and a thin film of shiny slime covering the surface. The trails of worms, the footprints of small salamander-like animals, and the fronds of ferns, mostly mascerated or wilted, are found delicately preserved in this slime. Raindrop impressions, the molds of salt crystals and numerous sun-cracks also add to the picture. This region has been described by Dr. David White as "the scene of showers, burning sun, hailstorms, occasional torrents and periods of drought and drying up of pools" during Hermit times.
Thirty-five species of plants are at present known from the Hermit Shale formation of Grand Canyon. Many of these are otherwise unknown throughout the world, though some ere representatives of European plants, and others have their closest relations In Central Asia, India, Australia, Africa, and South America. This flora consists principally of ferns and small cone-bearing plants, all of which are relatively dwarfed in size and appear less dense than those of corresponding age found in eastern America. They apparently indicate a semi-arid climate with long dry seasons, for the absence of moist-climate and swamp-loving types is noticeable.
Several insect wings have been found in the Hermit Shale, one of which was four inches in length. Numerous footprints of vertebrate animals have also been found, and undoubtedly represent an interesting fauna.
WIND-BLOWN SAND-THE COCONINO
The light-colored formation which appears as a conspicuous ribbon-like band around the upper part of the Grand Canyon has long presented a puzzle concerning its origin. The grains of white sand of which it is composed apparently were deposited at steep angles, for the many and varied dopes which were formed may be readily seen today on the surface of the rock. These slopes were probably once the lee sides of sand dunes deposited by winds in an area bordering the sea. We find the only traces of life in this formation represented by the trails of ancient worms and insects, and by the foot-prints of early lizard- or salamander-like creatures. Already the tracks of some 27 species of animals have been discovered In this sandstone within the Grand Canyon, though strangely enough no bones have yet been located.
Along both sides of Grand Canyon at the top, two buff and gray layers of limestone stand out as massive cliffs separated by a tree-covered slope. The upper of these limestones forms the plateau surface and may be seen for a great distance in every direction. Both layers were formed as the result- of vast accumulations of organic and sandy materials on sea bottoms, and in places they are composed largely of the remains of marine life-shells, corals and sponges. The teeth of sharks have also been found in the upper limestone.
During the early stages of the period when these marine animals lived and multiplied in the region, a great body of salt water extend over its surface from far to the west, remained briefly, then retreated from the area. Soon, however, marine waters advanced once more and another sea was formed with its shoreline extending eastward even beyond the region in which we now find Grand Canyon. Evidences of the second and larger sea are found beyond Flagstaff to the south, in the Painted Desert to the east, and almost to Zion Canyon to the north.
The presence of corals and sharks teeth not only indicates that this region was covered on more than one occasion by marine waters, but also suggests that these seas were warm and shallow. This is estimated to have been some 200 million years ago.
THE Fourth Chapter of the earth's history is commonly known as "the age of dinosaurs." Large reptiles were the dominating forms of life all over the world during this age. Landscapes and types of climate varied considerably, and in the Grand Canyon region they were changed completely several times. During some periods ocean bodies covered the country; at others desert winds piled up dunes on the surface. Again this region was the flood plain of rivers, where pebbles, mud and great logs of pine were washed in and deposited. At still other times coal was formed in some quantity. All of these interesting features of the Fourth Chapter will be briefly treated in the pages following. Their records as found at Zion Canyon to the north, in the Painted Desert to the east, and at the Petrified Forest to the south are such that a visitor to the region can scarcely help but marvel and wonder at their meaning.
MOENKOPI FORMATION; TRIASSIC PERIOD
Two isolated hills of unusual interest rise above the plateau surface near Grand Canyon. Looking east from Desert View (Navajo) Point one of these, a flat-topped mesa called Cedar Mountain, may be seen. The other, known as Red Butte, is a rounded hill about fifteen miles to the south of Grand Canyon village. The most interesting feature of these hills is found in the fact that they are composed for the most part of red sandstones and shales which once formed a continuous layer over this entire plateau region. These same rocks are found throughout southern Utah to the north, and in the Painted Desert to the east. Except at Red Butte and Cedar Mountain they have been completely stripped off and eroded away from the vicinity of Grand Canyon. The time involved in this erosion was tremendous and the consequences widespread. As a result the present flat plateau surface was formed, a qreat plain at an elevation close to sea level. The persistence of Red Butte and Cedar Mountain against time and the elements is easily explained, however, by the hard lava cap of the former and the protecting layer of pebble-rock on top of the latter.
The red sandstones and shales found in Red Butte and Cedar Mountain were formed from sands and muds accumulated during the early part of the Triassic Age - the beginning of the fourth great era of history. Near Flagstaff at the southernmost limit of the formation have been found many tracks and trails left by small crawling animals which probably indicate an old shoreline in that section. In the same formation found in Utah and other places to the north are many seashells of various types. Gypsum, en indication of and climate, is also found to a large extent in the rocks of this group.
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