The Geologic Time Scale is a representation of time based on Earth’s rock record. Using a combination of geochronology (a scientific branch of geology that determines the age of rocks) and chrono stratigraphy (the process of relating strata (rock layers) to time). Before these methods early paleontologist used the fauna (animals) and flora (plants) to determine units. The Geologic Time Scale is to geologist and paleontologist what the periodic table is to chemist and physicist.