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| One of the most fundamental and awesome geological concepts is that of geologic time. The age of rocks, and thus of the earth, can be determined either relatively, primarily through the use of biostratinomy, or absolutely, by using radioactive dating techniques. The geologic time scale was created long before the advent of radioactive dating. Its creation was based on the recognition that very old rocks do not contain identifiable fossils, and that fossils become abundant in progressively younger rocks at a certain level. | This level became the boundary between the two major intervals of geologic time, it is usually placed at 570 million years ago. The age of rocks containing abundant fossils became known as the Cambrian, and that of older rocks as the Precambrian. The Precambrian is now divided into two intervals, the Archean Eon and the Proterozoic Eon, with the boundary between the two at 2.5 billion years ago. |
| The rest of geologic time, from the Cambrian on, makes up the Phanerozoic Eon, or the "interval of well displayed life". The Phanerozoic Eon is broken down into three main intervals of time, the Paleozoic Era, or "interval of ancient life", the Mesozoic Era, the "interval of middle life", and the Cenozoic Era, the "interval of modern life". | The Mesozoic is sometimes loosely termed the "Age of Dinosaurs" and similarly the Cenozoic is sometimes called the "Age of Mammals". These eras are broken down into smaller intervals called periods, which may contain subdivisons called systems. The periods are in turn broken down into still smaller units of time called epochs. |
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