Anything cutting a layer is younger than the layer. There is a succession of simple to more complex life forms from older to younger rocks. These fossils can be used as index fossils to determine the relative age of rock units. In other words, a vertical succession of strata is formed by the lateral migration of adjacent environments. Systems and Periods have the same names as given in the standard geologic time scale.
Periods refer to time, systems to rocks.
The names should never be used alone.