The geologic time scale is a way of putting Earth’s vast history into an orderly fashion, giving a better perspective of events. At the turn of the nineteenth century, William Smith (1769–1839), an English canal engineer, observed that certain types of rocks, along with certain groups of fossils, always occurred in a predictable order in relation to each other. In 1815, he published a map of England and Wales geology, establishing a practical system of stratigraphy, or the study of geologic history layer-by-layer. Simply put, Smith proposed that the lowest rocks in a cliff or quarry are the oldest, while the highest are the youngest.

By observing fossils and rock type in the various layers, it was possible to correlate the rocks at one location with those at other locations. Smith’s work, combined with the first discoveries of dinosaur fossils in the early 1800s, led to a framework that scientists still use today to divide Earth’s long history into the geologic time scale, with its various, arbitrary divisions of time including eras, periods, and epochs. Established between 1820 and 1870, the time divisions are a relative means of dating; that is, rocks and fossils are dated relative to each other as to which are older and younger. It was not until radiometric dating was invented in the 1920s that absolute dates were applied to rocks and fossils—and to the geologic time scale.