The first geologic time scales were based on the natural breaks within the rock layers, which were thought to be evidence of worldwide mountain building events. Scientists soon discovered that not all mountain building events affected the entire Earth but were instead usually limited to a single continent (or a part of a continent) during a specific interval of time.

Today, the majority of divisions are most often delineated by a major geologic or life form event. For example, at the end of the Permian Period (at the beginning of the Triassic Period), a major catastrophic event occurred on Earth, wiping out close to 90 percent of all species on land and in water.

This also is the demarcation between the Paleozoic and Mesozoic Eras. The smaller divisions (most often epochs) are usually divided by more “minor” changes in life forms or events. For example, the division between the Pleistocene and Holocene Epochs (Quaternary Period) signaled the end of the ice ages about 10,000 years ago.