Before it was possible to get these absolute ages, geologists set up the geologic time scale, based on relative ages . In this time scale, the 4.6 billion years of earth history have been divided into units based on major geological or biological events. One major break falls between the Precambrian and the Cambrian. In rocks from the Precambrian, fossils are scarce, not only because life was scarce but also because most creatures had no bones or shells. An explosion in the numbers and types of fossils defines the start of the Cambrian age.
Mass extinctions define other major breaks in the geologic time scale.
Smaller time units were defined based on smaller events. Most of the important rocks in the Grand Canyon date from the Paleozoic, from 570 to 245 million years ago. Paleozoic life, at least as we know it from fossils, was dominated by animals with shells, not animals with backbones like the reptiles and mammals of the Mesozoic and Cenozoic. Much of what is known about what happened in the Paleozoic, especially in North America, is known from rocks in the Grand Canyon. We can use these rock layers to determine the geologic history of northern Arizona.