One of the most difficult concepts for people to comprehend when they encounter geology is the immense time frame that geologists use. To a geologist a thousand years is an instant. Most geologic changes and events take place over millions of years. The exception would be the rare cataclysmic event such as a meteorite impact. Through their training, geologists learn to think in terms of millions of years. The age of the earth is approximately 4.6 billion years, a number that is essentially incomprehensible to the vast majority of people. 

Through the eighteenth and nineteenth centuries geologists became aware of the necessity for very long periods of time to explain the vast change that their research was finding. Geologists developed a Relative Time Scale and tried to estimate through various means the actual lengths of time and the age of the earth. This relative time scale is based on the study of rock layers (stratigraphy) and the study of fossils (paleontology) and is still in use today. The strange sounding names: Ordovician, Cretaceous, Quaternary, etc., are still used by geologists and refer not just to rocks and fossils but to periods of geologic history. 

The discovery of radioactivity in the 1890s was quickly put to use by geologists to create an absolute time scale. It was important to be able to tell the age of a rock layer in terms of millions of years rather than knowing that this rock is older than that rock. Radioactive elements within the rocks themselves provided a built-in clock. This natural clock works because the rate of radioactive decay or change is a constant. Geologists quickly learned to read these clocks and put absolute ages on the periods of the Geologic Time Scale.