Edwin D. McKee

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.

CEDAR MOUNTAIN

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.

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.

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.