Nicholas Steno, a Danish physician living in Italy in 1669 proposed that
the Earth’s strata accumulated with three basic principles. Steno
pointed out obvious, but overlooked principles of sediment accumulation.
They included the Principle of Original Horizontality, Principle
of Superposition, and Principle of Original Continuity.
If sediments accumulate in a large basin, the laws of gravity will
deposit the beds, horizontal to the surface of the Earth. Beds can
"pinch out" along the sides of the basin as in the figure
The Principle of Superposition states that in a sequence of
sedimentary rock layers, the bottom layers are older than the top
layers. The bottom layers were deposited first. In the figure below A is
the oldest bed and G is the youngest.
The Principal of Original Continuity states that the beds can
be traced over a long interval if the basins were open. For instance,
Bed F can be traced continuously to the smaller basin in the figure
below. The other beds below F can then be correlated to Beds A-E.