In any succession of strata not severely deformed, the oldest stratum lies at the bottom, successively younger ones above. This is the basis of the establishment of the relative ages of all strata and their contained fossil (Steno as cited by Dott and Batton 1976).

This diagram starts in the right hand corner (25) and works its way up to the top (23) and then from the base of the left side (22) up (20). The cartoon traces the laying down of strata (25), the development of a cave (24), its collapse (23), filling by strata (22), another cave develops (21) and this finally collapses (20).