Scientists created the geologic time scale based on fossil evidence. It divides Earth’s history into blocks of time with each block separated by important events, such as the disappearance of a species of fossil from the rock record.
In the geologic time scale, the youngest ages are on the top and the oldest on the bottom.
The time scale is based upon relative times, therefore there aren’t any specific times listed with each era. The timescale is divided into eons, each eon into eras, each era into periods, and each period into epochs.