How Layers Of Rocks Help Predict Their Age

When rocks occur as undisturbed layers, we find that the lowest layers will be the oldest and topmost layers will be the youngest. Frequently rivers cut incisions into the earth's surface and expose such layers. The Grand Canyon in the U.S.A. is the deepest cleft on the earth's surface. The upper rocks of this canyon are about 200 million years old and contain traces of reptiles, impressions of fern leaves and wings of insects. Halfway down the canyon you find limestone of about 400 million years old which contains the remains of primitive armoured fish. Further down the canyon there are no traces of vertebrates. Three-quarters way down there are no apparent traces of life. Close to the bottom of the canyon the rocks are more than 2 000 million years old. Rocks as old as those of the bottom of the Grand Canyon have been found to contain a fine-grain flint-like substance called chert. Contained in this chert are simple organisms some of which resemble algae filaments others resemble bacteria.