 Consequently, as the picture of the overall rock record emerged, and discontinuities and similarities in one place were cross-correlated to those in others, it became useful to subdivide the overall geologic record into a series of component sub-sections representing different sized groups of layers within known geologic time, from the shortest time span stage to the largest thickest strata eonothem and time spans eon.Concurrent work in other natural science fields required a time continuum be defined, and earth scientists decided to coordinate the system of rock layers and their identification criteria with that of the geologic time scale.This gives the pairing between the physical layers of the left column and the time units of the center column in the table at right.