THE MESOZOIC ERA
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 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.
REMNANTS OF YOUNGER STRATA—RED BUTTE AND CEDAR MOUNTAIN
(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 great plain high above sea
level. The persistence of Red Butte and Cedar Mountain against time and
the elements is easily explained, moreover, by the hard lava cap of the
former and the protecting layer of pebble-rock on top of the latter.
[Illustration: CEDAR MOUNTAIN]
[Illustration: RED BUTTE]
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 indicate a
shore environment in that section. In the same formation found in Utah
and other places to the north are many seashells of various types.
Gypsum, an indication of arid climate, is also found to a large extent
in the rocks of this group.
THE PETRIFIED FOREST
(SHINARUMP AND CHINLE FORMATIONS; TRIASSIC PERIOD)
The Petrified Forests of northern Arizona and southern Utah were formed
from trees which grew during a period known as the Triassic. The type of
environment in the Painted Desert region during that period makes a very
interesting speculation. The landscape of this ancient country, as
interpreted from the present rock formations found there, was that of a
low, flat area, perhaps a floodplain, where rivers and streams meandered
back and forth, depositing first large pebbles and sand, and later a
great thickness of finer material consisting of sand and much mud. It
was by these streams that a great quantity of logs of evergreen trees
was carried down probably from mountains to the south or southwest. Many
of the tree trunks were worn and battered on the way, and in this region
(from southern Utah to the vicinity of Adamana and Holbrook) they were
buried in great numbers among the depositing sediments. All four of the
Petrified Forests south of Adamana originated in this manner—and
probably represent old log-jams. Nine miles north of Adamana, however,
is the North Sigillaria Forest where the trees are found standing as
they grew.
[Illustration: PETRIFIED FOREST. ARIZONA]
[Illustration: PETRIFIED LOGS OF TRIASSIC AGE]
A large majority of the fossil trees in Petrified Forest National
Monument represent a pine-like species belonging to a group now extinct
in the northern hemisphere. Some individuals of this type have diameters
as great as ten feet and heights of more than 200 feet. Four or five
other varieties of tree occur but they are less common and rather
inconspicuous.
The process by which these trees were changed to stone took place a very
long time after they were buried beneath sands and silts. At a time
probably millions of years later, waters bearing in solution the mineral
silica passed through the rocks of this region. Particle by particle
wood was decayed, and almost simultaneously it was replaced by silica.
The brilliant colors found in much of it are due to small quantities of
iron and manganese which were introduced in a similar manner.
The wicked-looking upper jaw of a phytosaur—relative of the
crocodile—has been discovered buried in one of the colorful mounds of
mud out in the Petrified Forest. In nearby deposits are found still
other fossils including ferns preserved in delicate detail and clamlike
shells. These, together with the petrified logs of evergreen trees, make
the picture very much alive and real to one whose imagination allows him
to review these fascinating early pages in history.
DINOSAUR TRACKS
(WINGATE-KAYENTA; JURASSIC PERIOD)
Dinosaur means “terrible lizard.” Everyone has heard this name and
attaches to it at least a vague conception of a creature of peculiar
shape and tremendous size. In the dim far-away past these giant reptiles
lived, developed and died in many parts of this country, but to most of
us, they are only fantastic creatures in literature—not live or moving
animals.
[Illustration: DINOSAUR TRACKS. PAINTED DESERT]
The reality of the dinosaur should become apparent to those who travel
through the Painted Desert region of Arizona. In that country the grey
and red hills of clay, the cliffs of sand and the many brightly colored
layers of hardened mud represent a part of the ancient landscape in
which lived the “terrible lizards.” Furthermore, life definitely inserts
itself into this picture when a person sees in the rocks the very
three-toed tracks left by the dinosaur. Who can deny the story of life
when he can place his fingers in the foot impressions and measure the
strides of these animals—many of them—both large and small? Such is the
privilege of those who visit the Painted Desert just south of Tuba City.
The tracks of the dinosaurs are found in rocks which apparently were
formed from river deposits of sands and muds. In other rocks closely
associated have been found the bones of dinosaurs, other reptiles, and
animals related to the frog or toad. Some of these creatures apparently
were “largely if not entirely terrestrial in habit” while others
probably lived in either fresh water streams or swamps.
[Illustration: THE PAINTED DESERT]
THE ROCKS OF ZION CANYON AND RAINBOW NATURAL BRIDGE
(NAVAJO SANDSTONE; JURASSIC PERIOD)
The lure of a desert with its drifting sands, its scattered oases, and
its broad extent has ever been great. Today in the Southwest, cut off
from the moist ocean breezes of the west by the lofty Sierra Nevadas and
further isolated by the Rocky Mountains to the east, is America’s great
desert region. Here the winds and the rains are constantly at work
tearing down and sculpturing the great land masses, while in places the
wind is piling up the sand and debris to form dunes and new lands.
Far back in history, probably in what is known as the Jurassic Period,
southwestern United States witnessed conditions of climate and
environment which were perhaps somewhat comparable though probably far
more desert-like than those which exist today. The land which had been
raised in the preceding period to form a great flood-plain remained
above sea level. Lofty mountains were formed to the west and like the
Sierras of today, they robbed the east-travelling winds of their
moisture. These mountains were very high and furnished a vast supply of
sediment which was carried down into the arid basin to the east, worked
and reworked by the wind, and finally deposited as a great layer of
sand.
[Illustration: ZION CANYON. NAVAJO SANDSTONE]
[Illustration: RAINBOW NATURAL BRIDGE. NAVAJO SANDSTONE]
The beautiful Vermilion Cliffs and the White Cliffs of southern Utah
which together form the walls of Zion Canyon, the red sandstone out of
which is carved Rainbow Natural Bridge, and the jagged Echo Cliffs seen
to the northeast of Desert View (Navajo) Point at Grand Canyon are all
monuments of the Jurassic Age. These high walls are of sandstone—formed
from sand steeply piled at varying angles. They have been aptly called
“cliffs of fossil sand dunes.”
While desert winds and a burning sun were playing a prominent part in
the Grand Canyon region, during the Jurassic Age, other sections of the
country were favored with a moist climate and luxuriant vegetation.
Dinosaurs and other reptiles waded about in swamps and developed to
tremendous sizes. The country teemed with life, very different from that
of today, yet none the less interesting.
FROM SEASHELLS TO COAL BEDS
(DAKOTA. MANCOS, MESA VERDE; CRETACEOUS PERIOD)
[Illustration: CRETACEOUS SHELLS FROM SOUTHERN UTAH]
During the last part of the “Age of Dinosaurs,” in that period of
history known as the Cretaceous, much of northeastern Arizona, southern
Utah, and northwestern New Mexico apparently was submerged beneath a
sea. Mollusks abounded, and various other types of water animals swam or
crawled in this region. Their shells were buried and preserved among the
muds and limes which were accumulating, and today many of them are found
in the rocks of this age.
As time went on, the Cretaceous seas were expelled from their basins in
the Southwest. Low-lying coastal plains—two hundred and more miles
wide—with luxuriant vegetation and perhaps swamps, replaced them, and
these new conditions were the means of much coal formation. Great
accumulations of vegetable matter, buried beneath sediments brought down
by streams, eventually formed the many layers of coal which today are
found in various places in Utah, Arizona, and New Mexico. In the last
named state some extensive and valuable coal beds are found in rocks of
this age near Gallup. Impressions of leaves and pieces of fossil wood
are also numerous among these rocks and they indicate that there was a
warm, moist climate during that period in which they grew.
The Cretaceous Period ended with a general change of conditions or a
revolution, the world over. Many of the seas were changed into lands,
new mountains were formed and the dinosaurs and giant reptiles were
largely replaced by mammals and other modern varieties of life.
[Illustration: COAL CANYON. PAINTED DESERT COUNTRY
AERIAL PHOTO COPYRIGHT FRED HARVEY]