Stephen J. Gould  argued that one of the greatest scientific realizations was the concept of “deep time,” a phrase coined by John McPhee to encompass the breadth of geological time. We rely on measurements of geological time to provide rates of geological and geophysical processes and a chronology for the evolution of the Earth. Geological measurements of deep time rely on a limited number of radiometric ages that provide the only true clocks. Our chronometer is the geological time scale, and it is based on these ages and interpolations between them. The most precise ruler for these interpolations is provided by minor variations in the Earth's orbit, which control solar insolation and global climate changes; this “astronomical pacemaker” has provided a precise chronology (one with 5,000-year resolution) for the past few million years. It has proven difficult to extend this astronomical time scale to the geological record older than about 5 m.y. However, for the Late Cretaceous to Pleistocene (approximately the last 150 m.y.), another ruler is available: reversals of the Earth's magnetic field provide a Geomagnetic Polarity Time Scale (GPTS) that serves as a framework for a limited number of radiometric ages.