class="hst">Devonian Age

CHAPTER III
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.

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.

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)

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)

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.

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