Law of Superposition

It is a general law, based on which, the geochronology is established: in the superposition of any sedimented strata (including the extrusive rock), when not coverred or overturned by the later movement, the youngest strata shall be at the top of the superposition, while the oldest strata shall be at the bottom of the superposition. Older strata is coverred with gradually young strata in succession. In other word, each layer of strata is younger than its underlying one and older than its overlying one. This law was proposed firstly by Steno in 1669.