THE PALEOZOIC ERA
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 accumulations 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 early 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 evidences of primitive 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
(CAMBRIAN PERIOD)
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 sand deposited near shore was
covered by mud and this in turn by lime far out from the beach. Today
this series of sand, mud and lime is found represented in the rocks of
the Tonto Platform in Grand Canyon.
[Illustration: SCENE ON CAMBRIAN SEA FLOOR IN GRAND CANYON REGION
(RECONSTRUCTION. MUSEUM NORTHERN ARIZONA)]
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 trilobites 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.
[Illustration: TRILOBITES PRESERVED IN GREEN SHALE OF GRAND CANYON WALLS
(ONE-EIGHTH NATURAL SIZE)]
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 the Ordovician, the time when fish first appeared in the seas,
and the Silurian, the time when millipeds and scorpions 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
(DEVONIAN PERIOD)
[Illustration: A FRESH WATER FISH OF DEVONIAN AGE]
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 limestone and sandstone
remained. These we find today exposed in the walls of the Grand Canyon
occurring 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)
[Illustration: REDWALL FOSSILS (NATURAL SIZE)]
One of the most prominent and conspicuous features of the Grand Canyon
is the great 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 prospector this formation is known as the Blue Lime; to the
geologist it is the Redwall 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.
[Illustration: HAVASU FALLS IN REDWALL LIMESTONE. GRAND CANYON]
Large amphitheaters, many curving alcoves, caves, and solution tunnels
are all characteristic features of the great Redwall. It is composed of
relatively pure lime so rain and other waters have a 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.
The origin of the Redwall Limestone is as interesting as its form. The
purity of the lime indicates that it was built up in a relatively wide
and quiet sea. Its composition represents a vast accumulation of the
skeletons of ancient plants and animals. Seashells are found in great
numbers, some of them preserved in delicate detail. These and other
forms of ocean life clearly indicate that a great sea connection then
existed between this region and that of western Canada to the north.
TRACKS IN THE SUPAI SANDSTONE
(PERMIAN PERIOD)
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.
[Illustration: TRACKS OF SHORT-LEGGED PREHISTORIC ANIMAL. SUPAI
FORMATION. GRAND CANYON]
When the red beds were accumulating in this region, the climate probably
was more or less arid; the vegetation consisted principally of ferns and
other lowly 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
(PERMIAN PERIOD)
[Illustration: LANDSCAPE AT TIME HERMIT SHALE WAS FORMING
(RECONSTRUCTION. MUSEUM NORTHERN ARIZONA)]
[Illustration: FOSSIL FERN FROM THE HERMIT SHALE (REDUCED)]
Concerning the conditions under which the topmost red formation of the
Grand Canyon (the Hermit Shale) was developed, and the nature of its
origin, we have today a rather definite and interesting picture. A
wealth of fossil plants and a number of tracks of animals have been
found excellently preserved in its muddy layers, and by means of these
and other indications the following conclusions have been drawn.
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
of Grand Canyon. Many of these have not been found elsewhere in the
world, though some were representatives of European plants, and others
had their closest relations in central Asia, India, Australia, Africa,
and South America. This fossil flora consists principally of ferns and
small cone-bearing plants, all of which were 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
(PERMIAN PERIOD)
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 slopes 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.
[Illustration: WIND-BLOWN SAND. COCONINO FORMATION. GRAND CANYON]
[Illustration: TRACKS OF PRIMITIVE FOUR-FOOTED ANIMALS, COCONINO
SANDSTONE. GRAND CANYON
PHOTO BY E. W. ENSOR]
WARM SEAS FROM THE WEST—THE TOROWEAP AND KAIBAB FORMATIONS
(PERMIAN PERIOD)
[Illustration: SEASHELLS FROM KAIBAB LIMESTONE (REDUCED)]
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 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.
SEA LILY OR CRINOID (REDUCED)
HORN CORAL (NATURAL SIZE)
During the early stages of the period when these marine animals lived
and multiplied in the region, a great body of salt water extended 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.
FOSSIL SHARK’S TOOTH (NATURAL SIZE)
[Illustration: ROCKS OF THE FOURTH ERA
The Painted Desert and Zion Canyon Region]