John K. Reed and Michael J. Oard*
Part I of this series described the modern stratigraphic method of cyclostratigraphy, which is linked to astrochronology. The latter uses properties of Earth’s orbital mechanics to develop an absolute chronometer back through time. Cyclostratigraphy uses properties of sedimentary rocks as proxies for these orbital cycles to connect the astronomical “clock” to the sedimentary record. In this part of the series, we trace the historical development of these methods, their increasing influence on the geologic timescale, and the implications for a more gradualistic view of uniformitarianism.