The geologic time scale is the “calendar” for events in Earth history. It subdivides all time into named units of abstract time called—in descending order of duration—eons, eras, periods, epochs, and ages. Our geologic time scale was constructed to visually show the duration of each time unit. This was done by making a linear time line on the left side of the time columns. Thicker units such as the Proterozoic were longer in duration than thinner units such as the Cenozoic.