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.
Along Bright Angel Canyon and in several other places in the Grand Canyon, rocks of Algonkian age, representing accumulations of sediments several thousand feet in thickness, are found. Below and to the north of Desert View (southeast of Cape Royal on the North Rim) they form the open floor of the Canyon. Everywhere the most conspicuous layer of this series is a mud rock of brilliant vermilion color. However, the rocks also include a conglomerate or pebble layer, a dark limestone formed principally by plants, and a purple quartzite made by the consolidation of the grains of a sandstone.
The Algonkian rocks of the Grand Canyon region were bent and broken into mountains at an early date. In many places sloping layers showing the steep angle at which they were tilted are easily visible, even from the Canyon rim. Folded areas and strata which have been shattered are also conspicuous features here and there. The mountains which they formed, however, are now missing for they were worn away in large measure by slow erosion. Today only remnants—small hills on a general level surface—remain in the lower parts of Grand Canyon to tell the story.
The rocks of Algonkian age are roughly estimated to be at least six or seven hundred million years old, yet from all indications they were formed under conditions of climate not unlike those of far later periods of history. In several parts of the world traces of great ice sheets—glaciers which scratched and eroded the surface—are found preserved in Algonkian rocks. In other places, including the Grand Canyon, ancient flows of lava are found where they gushed out upon the surface of an old land mass. Among the rocks below Desert View (Navajo) Point and bordering on the Colorado River may readily be seen several black cliffs formed by the volcanic activity of this early age.
The brilliant red shales of Algonkian age found in the lower parts of the Grand Canyon were formed as muds, accumulated probably by large rivers. In these muds are found preserved great quantities of ripple marks, indications of changing currents, also the moulds of salt crystals, and large shrinkage cracks resulting from a very hot sun. In brief, these criteria point toward a hot and probably arid climate in this region during that chapter of history.
OLDEST KNOWN LIFE
(THE ALGONKIAN ERA)
The oldest forms of life represented in rocks of Grand Canyon are found in strata of the Second, or Algonkian, Era. Certain layers of limestone showing peculiar structural patterns on their surfaces are interpreted as being the reefs built up through the activities of primitive one-celled plants known as algae. Similar structures are being formed today by plants of this type. Near Harper’s Ferry, West Virginia, for example, algae are building up limestone layers almost identical to the fossil ones found in the Algonkian rocks of Grand Canyon. In this connection, it is interesting to note that because of this similarity of the present to the past, the reality of the ancient plant structures was recognized a few years ago. They were discovered at a place in the Grand Canyon just west of the mouth of Bright Angel Creek.
Rocks of the Third Era in the Grand Canyon
- Rocks of the Fourth Era (Red Butte)
- Rim of Grand Canyon
- a~ Sandy limestone formed beneath the sea. In it are found corals, sponges, sharks’ teeth and many sea shells.
- Kaibab Formation 300 ft. Permian Age
- b~ Red sandstone and gray limestone. Formed at bottom of a shallow sea.
- Toroweap Formation 250 ft. Permian Age
- c~ Wind-blown sand which was piled in dunes. Contains the tracks of many kinds of primitive reptiles or amphibians.
- Coconino Sandstone 300 ft. Permian Age
- d~ River mud in which are preserved many impressions of ferns and cone-bearing plants, insect wings, raindrop pits and tracks of salamander-like animals.
- Hermit Shale 300 ft. Permian Age
- e~ Ancient flood-plain deposits of sand and mud containing impressions of fern-like plants and tracks of land animals.
- Supai Formation 800 ft. Permian Age
- f~ Sea deposits containing remains of shells, fish, sea-lillies and related forms of life.
- Redwall Limestone 550 ft. Mississippian Age
- g~ Accumulations of sandy lime in which are preserved the remains of some of the earliest fish.
- Temple Butte Limestone Devonian Age
- h~ Sand, muds, and limes representing ancient beach which was gradually covered with water until finally the sediments were at a considerable depth beneath the sea. The earliest definite traces of animal life in the canyon found here.
- Tonto Rocks Cambrian Age
- i and j rocks of the First and Second Era
