 Stratigraphy provides a relative ordering (ordinal ranking) of the CRBG layers.Absolute dates, subject to a statistical uncertainty, are determined through radiometric dating using isotope ratios such as 40Ar/39Ar dating, which can be used to identify the date of solidifying basalt.In the CRBG deposits 40Ar, which is produced by 40K decay, only accumulates after the melt solidifies.