Geologic time period is a way of measuring the Earth's history, which is divided into four main divisions: EPOCH, EON, ERA, and Period. An EPOCH is the smallest division of geologic time and is typically millions of years long. An EON is the largest division of geologic time and is typically hundreds of millions of years long. An ERA is a subdivision of an EON and is typically tens of millions of years long. Finally, a Period is a subdivision of an ERA and is typically millions of years long.