Nicolaus Steno (1669) developed two basic geologic principles: 1. Sedimentary rocks are laid down in a horizontal manner, and 2.more recent rock was deposited on top of older rock. James Hutton (1795), and Charles Lyell in the early 1800s indicated that natural geologic processes were uniform in frequency and magnitude throughout time. This idea became known as the "principle of uniformitarianism". Smith (1815) produced a geologic map of England in which he developed the cocept that fossils have a very definite order in which they are found in sediments. This principle led to the use fossils to define increments within a relative time scale. 

Most radioactive isotopes have short half-lives and lose their radioactivity within a few years or less. However, some isotopes decay slowly, and some of these are used as geologic clocks. The parent isotopes and corresponding daughter products most commonly used to determine the ages of rocks are provided below:

The Theory of Continental Drift was first proposed by Alfred Wegener (1910). The theory was challenged yet widely ignored. The theory was lost in obscurity for half a century due to the lack of a plausible mechanism for the drift. With the discovery of sea-floor spreading in the late 1950's, the theory was rediscovered and became known as the Theory of Plate Tectonics. Further efforts have led to the development of time series maps describing Continental Drift since the Precambrian. The history of the earth has been divided into a hierarchical set of divisions for describing geological time.