INTRODUCTION
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 logs 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 not only looks down through tremendous space, but also through time, glimpsing the record of vast ages, 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 evidence of life has been found. 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 plants, 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 last 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.
CHAPTER I
THE ARCHEAN ERA
ROCKS OF FIRST ERA. INNER GORGE OF GRAND CANYON
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 the Rockies, 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 folded them. The rocks themselves were greatly compressed and heated, with the result that complete recrystallization 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 to which they were subjected. 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.
BLOCK DIAGRAMS OF EVENTS OF FIRST ERA
1 Sands and Muds accumulated to form rock strata.
2 The strata were folded into high mountains. Heat and pressure changed both structure and composition of the lower rocks during the folding.
3 Molten masses were forced up into cracks from below. Upon cooling they formed lavas on the surface and granites beneath.
4 During a tremendously long period of time the rivers and rains slowly widened the valleys and wore down the ancient mountains to a great plain near sea level.
As yet no definite traces of either plant or animal life have been found in rocks of the Archean age in Grand Canyon. 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.
GRANITE IN GRAND CANYON
(THE ARCHEAN ERA)
Within the black, crystalline rocks of the Inner Gorge may be seen many large 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. It is by the application of this same principle that crystal forming is prevented in the manufacture of common glass.
The large size of the crystals forming the granite that fills cracks and fissures of the Inner Gorge at Grand Canyon indicates the considerable depth at which it was formed and is further evidence of the great mountains that existed in this region during the first era in geologic history.
CHAPTER II
THE ALGONKIAN ERA
TILTED ROCKS OF THE SECOND ERA. GRAND CANYON
Rocks formed during the second great era of the earth’s history are distinctive in several respects. They are not highly altered or completely 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.